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# Sexual selection is not the origin of long necks in giraffes

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## Abstract

The evolutionary origin of the long neck of giraffes is enigmatic. One theory (the ‘sexual selection’ theory) is that their shape evolved because males use their necks and heads to achieve sexual dominance. Support for this theory would be that males invest more in neck and head growth than do females. We have investigated this hypothesis in 17 male and 21 female giraffes with body masses ranging from juvenile to mature animals, by measuring head mass, neck mass, neck and leg length and the neck length to leg length ratio. We found no significant differences in any of these dimensions between males and females of the same mass, although mature males, whose body mass is significantly (50%) greater than that of mature females, do have significantly heavier (but not longer) necks and heavier heads than mature females. We conclude that morphological differences between males and females are minimal, that differences that do exist can be accounted for by the larger final mass of males and that sexual selection is not the origin of a long neck in giraffes.

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... Thus, positive allometry is more likely for male giraffe under sexual selection than for females and this was verified for 77 Namibian giraffe (Simmons & Scheepers, 1996). This hypothesis has been re-assessed recently using a Zimbabwean sample (Mitchell, van Sittert, Skinner, 2009, n = 17 males and 21 females). The allometric relationship is given by ...
... Slopes derived previously using RMA regression for males (k= 1.35) and females (k= 0.97) were similar (Simmons & Scheepers, 1996). Mitchell et al. (2009) reported k=1.13 for males and k= 1.06 for females in their smaller Zimbabwean sample. While Mitchell et al. (2009) interpreted their results as evidence for positive allometry in both sexes, they reported neither confidence intervals or significance levels for k nor differences between the sexes. ...
... Mitchell et al. (2009) reported k=1.13 for males and k= 1.06 for females in their smaller Zimbabwean sample. While Mitchell et al. (2009) interpreted their results as evidence for positive allometry in both sexes, they reported neither confidence intervals or significance levels for k nor differences between the sexes. The k= 1.06 reported for Zimbabwean females (Mitchell et al., 2009) appears to reveal isometry, not positive allometry, because it lies close to 1.00 and within confidence limits from the Namibian dataset; thus the two populations probably do not differ significantly. ...
Article
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Recent years have witnessed a resurgence in tests of the evolution and origin of the great height and long neck of the giraffe Giraffa camelopardalis. The two main hypotheses are (1) long necks evolved through competition with other browsers allowing giraffe to feed above them (&apos;competing browsers&apos; hypothesis); or (2) the necks evolved for direct use in intra-sexual combat to gain access to oestrous females (&apos;necks-for-sex&apos; hypothesis). Here, we review recent developments and their relative contribution in explaining giraffe evolution. Trends from Zimbabwean giraffes show positive allometry for male necks and isometry for female necks relative to body mass, while comparative analyses of the cervical versus the total vertebral column of the giraffe, okapi and fossil giraffe suggest selection specifically on neck length rather than on overall height. Both support the necks-for-sex idea. Neither study, however, allows us to refute one of the two ideas. We suggest new approaches for quantifying the relative importance of the two hypotheses. A direct analysis of selection pressure on neck length via survival and reproduction should clarify the mechanism maintaining the trait, while we predict that short robust ossicones should have arisen concurrently with incipient neck elongation if sexual selection was the main selective driver. The main challenge for the competing browser hypothesis is to explain why giraffe have remained about 2 m taller than their tallest competitors for over 1 Myr, whereas the sexual selection hypothesis cannot provide an adaptive explanation for the long neck of female giraffe. We conclude that probably both mechanisms have contributed to the evolution and maintenance of the long neck, and their relative importance can be clarified further.
... Extant giraffes (Giraffa camelopardalis) appeared~1 million years ago (Mya) having evolved over a period of~15 My via Canthumeryx, Giraffokeryx, Paleotragus sp., Samotherium sp. and Bohlinia (Churcher, 1978;Hamilton, 1978;Geraads, 1986;Mitchell and Skinner, 2003). The neck length of Canthumeryx was~550 mm Palaeotragus germaini and Samotherium (boisseri)~850 mm (Badlangana et al., 2009), Bohlinia~1500 mm and in extant giraffes up to 2200 mm long (Mitchell et al., 2009). Similarly, leg length increased from 800 mm in Canthumeryx to 1560 mm in Paleotragus sp to 1740 mm in Samotherium sp and 2100 mm in adult extant giraffes (Colbert, 1938;Mitchell et al., 2009;van Sittert et al., 2015). ...
... The neck length of Canthumeryx was~550 mm Palaeotragus germaini and Samotherium (boisseri)~850 mm (Badlangana et al., 2009), Bohlinia~1500 mm and in extant giraffes up to 2200 mm long (Mitchell et al., 2009). Similarly, leg length increased from 800 mm in Canthumeryx to 1560 mm in Paleotragus sp to 1740 mm in Samotherium sp and 2100 mm in adult extant giraffes (Colbert, 1938;Mitchell et al., 2009;van Sittert et al., 2015). Thus, during their evolution natural selection favoured progressive leg elongation of~3 fold and a neck~4-fold longer than it was in Canthumeryx to produce their familiar tall, slender, dolichomorphic shape. ...
... However, females show no preference for any specific male (Bercovitch et al., 2006) and there is no sexual jealousy between males when they are testing to see if a female is in oestrus (Innis, 1958). Moreover, sexual selection implies sexual dimorphism and higher mortality (Darwin, 1874), but long necks are not associated with higher mortality and there is no sexual dimorphism in neck mass or length or head mass (Mitchell et al., 2009(Mitchell et al., , 2013a. Brownlee (1963) suggested two other hypotheses. ...
Article
One of several hypotheses for the evolution of the shape of giraffes is that it evolved to maximize heat loss via a high surface area to mass ratio. We calculated the surface area (SA) of the head, neck, trunk and upper legs, and the lower legs in 60 giraffes of both sexes and a body mass range of 141–1358 kg. No sex differences were found for giraffes of equivalent body mass. Relative surface area (cm² kg⁻¹ body mass) declined from 145 in juvenile giraffes to 90 in adults. Average total body SA was 7.3 ± 2.5 m² (range 2.2–11.7), which is not significantly different to that of mammals of equivalent mass. The extra area of the neck and legs was offset by smaller trunk area. However, the narrow diameters of the neck and lower legs enhance the rate of convective and evaporative heat loss and reduce the incident solar radiant heat load when giraffe face the sun, a behaviour supplemented by seeking shade if it is available. We have concluded that giraffes do not have an unusually large SA for their mass, but their shape confers other thermoregulatory benefits that have advantages for survival in the arid habitat they prefer.
... The other hypothesis for why giraffe developed such a long neck was that bulls with longer necks had a sexual advantage as they use their necks in fighting for females [11], however, a study on a population of Zimbabwean giraffe (Giraffa camelopardalis giraffe; [12]) found evidence of limited sexual dimorphisms of neck and leg length, and head and neck mass, and those that were found could be explained as generic male-female differences, as occur in most species. However, evidence to the contrary was noted in a Namibian giraffe population (G. ...
... The only significant differences between any of the body measurements (Table 1), between the two sexes, were those of the forelegs. The measurements from shoulder to hoof and knee to hoof were both significantly longer in the males than the females, this seems to be supported by what Mitchell et al. [12] reported, although they did not report age in their study. They did; however, report that for males and females, when increasing from 100 kg to 1100 kg live weight, males had a 2.01-fold increase in foreleg length, while females only had a 1.69-fold increase. ...
... The lack of any other significant sexual dimorphisms in length, including the neck, is supported by what Hall-Martin [4] reports on giraffe of this age. Mitchell [12] found that, contrary to what Simmons and Scheepers [11] reported on males having larger necks for sexual advantages, when correcting for body weight, males and females had no significant differences in neck length, which agrees with the findings of this study, that, despite the lack of sexual dimorphism in length, there was a dimorphism in weight. ...
Article
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Various body measurements and commercial carcass yields of relatively young (2½–6 yrs old) giraffe (Giraffa camelopardalis angolensis) were investigated to quantify the effect of sex there upon. Eight male and eight female giraffe were culled by standard practice in Namibia, where body and horn measurements were taken, before the carcasses were dressed. There were no significant differences between the mean dead weights of the two sexes (bulls = 691.1 kg; cows = 636.5 kg; p = 0.096), the only body measurements found to differ significantly were those of the forelegs, with the shoulder to hoof (p = 0.046) and the knee to hoof (p = 0.025) both being significantly longer in the bulls. The horn measurements were all found to be significantly larger in the bulls than the cows even at this young age. The neck weight as a percentage of the carcass weight was found to be significantly heavier for the bulls compared to the cows, however, the back percentage values were significantly heavier in the cows than the bulls. There was a strong positive correlation between the body weight and most of the body lengths, as well as between most of the individual body measurements. The giraffe used had an average age of 3.7 years old, and had therefore not yet reached their growth plateau, which may be why sex had no influence on most of the body measurements recorded.
... The extraordinary size and shape of the giraffe (Giraffa camelopardalis) invites study as to the specialisation of its cardiovascular, respiratory, thermoregulatory, nervous and musculoskeletal systems (Badlangana et al. 2007;McMahon 1975;Mitchell & Skinner 2003Thompson 1917;Van Schalkwyk et al. 2004). Additionally, because of the huge functional demands such a long neck places on these systems, the giraffe is often used when describing evolutionary processes, although there is still no consensus on the long neck's adaptive advantages (Brownlee 1963;Darwin 1888;Mitchell et al. 2009a;Pincher 1949;Simmons & Scheepers 1996). Considering the long neck's functional and evolutionary interest, there are still very few studies on the giraffe vertebral column. ...
... 1.09 compared with ca. 0.72 in the foetus; (Mitchell et al. 2009a), the neck and head would extend beyond the forelegs during parturition, which may pose an increased risk of elbow flexion and ultimately dystocia. The mechanism that triggers differential elongation of the cervical vertebrae postnatally is unknown and needs further investigation. ...
... The finding that sex did not play a role in the scaling of vertebral body length supports the recent finding that sexual selection is not the origin of the long neck of the giraffe (Mitchell et al. 2009a). Although the spinous processes of T1 and T2 of all the males above 1000 kg were longer than would be predicted by our allometric equations, this is probably a biomechanical response resulting from a need to support the significantly heavier heads in male giraffes (Mitchell et al. 2009a(Mitchell et al. , 2013a, as the thoracic spinous processes are integral in supporting the neck and head. ...
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... These dimensional data were calibrated by the body height of a true-to-scale contour of a human depicted in the same figure 1 of Vidal et al. (2020) given to be 1.70 m. With all this, we estimated that the giraffe contour in figure 1 of Vidal et al. (2020) represents a female specimen with an overall body length ("total length" (Mitchell et al., 2009)) of about 4.30 m and, thus (van Sittert et al., 2010, tab. 1), weight of about 800 kg. ...
... 1), weight of about 800 kg. For a 800 kg female, a cross-check using the "total height" (Mitchell et al., 2009) Dimensions in the frontal plane of a giraffe are not documented in the literature. As an alternative, photos from the internet provided us with a rough guess of neck and head widths: we estimated the ratio of frontal width to sagittal depth values of the neck being two to three, which enabled us to approximate the neck geometry by a frustum with an elliptic base area. ...
... Biology Open • Accepted manuscript by guest on January 2, 2021 http://bio.biologists.org/ Downloaded from Geometrical dimensions of an adult (female) giraffe with an overall body length ("total length" (Mitchell et al., 2009): sum of tail, neck and head lengths, plus distance from tail base to withers) of about 4.30 m and weight of 800 kg, respectively, with the dimensions given in metres. The neck length is 1.55 m, and the "total height" (Mitchell et al., 2009) is the withers height (2.45 m) plus neck length (1.55 m) plus half of the head height (0.34 m), i.e. 4.17 m. ...
Article
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In humans, compressive stress on intervertebral discs is commonly deployed as a measurand for assessing the loads that act within the spine. Examining this physical quantity is crucially beneficial: the intradiscal pressure can be directly measured in vivo in humans, and is immediately related to compressive stress . Hence, measured intradiscal pressure data are utterly useful for validating such biomechanical animal models that have the spine incorporated, and can, thus, compute compressive stress values. Here, we utilise human intradiscal pressure data to verify the predictions of a reductionist spine model, which has in fact only one joint degree of freedom. We calculate the pulling force of one lumped anatomical structure that acts past this (intervertebral) joint at the base of the spine—lumbar in hominins, cervical in giraffes—to compensate the torque that is induced by the weight of all masses located cranially to the base. Given morphometric estimates of the human and australopith trunks, respectively, and the giraffe's neck, as well as the respective structures’ lever arms and disc areas, we predict, for all three species, the compressive stress on the intervertebral disc at the spine base, while systematically varying the angular orientation of the species’ spinal columns with respect to gravity. The comparison between these species demonstrates that hominin everyday compressive disc stresses are lower than such in big quadrupedal animals. Within each species, erecting the spine from being bent forward by, for example, thirty degrees to fully upright posture reduces the compressive disc stress roughly to a third. We conclude that erecting the spine immediately allows to carry extra loads of the order of body weight, and yet the compressive disc stress is lower than in a moderately forward-bent posture with none extra load.
... As giraffe tallness is brought about by elongation of neck and limbs, we expected similar scaling patterns with regard to length in both these body regions. Therefore, we anticipated that giraffe leg elongation exponents will be higher postnatally than in the fetus and that there will be a lack of sexual dimorphism in lengthening, as sexual selection did not seem to play a major role in the evolution of tallness in this animal (Mitchell et al., 2009;van Sittert et al., 2010). Because of the relative slenderness and length of the giraffe limb bones (and similar to the okapi data presented by Kilbourne and Makovicky, 2012), we expected limb bones to become increasingly gracile throughout ontogeny. ...
... We did not expect to find sexual dimorphism between equivalent body mass giraffes, and we did not find any. Our expectation was based on the lack of dimorphism in giraffe neck length and cervical vertebrae growth patterns (Mitchell et al., 2009(Mitchell et al., , 2013 Van Sittert Page 30 of 43 van Sittert et al., 2010). The results we report here further confirm that sexual selection has not played a major (if any) role in the evolution of tallness in giraffes. ...
... Furthermore, data on artiodactyls comparable to this study are still very scant, and conclusion should be drawn with caution. Data from Carrier ( being approximately twice their length at birth compared to a three-fold increase in neck length (Mitchell et al., 2009). Giraffes thus have relatively longer limbs and shorter necks per unit body mass as juveniles, and the remarkable length of limb bones per body mass seems to be established in utero already. ...
Article
Giraffes have remarkably long and slender limb bones, but it is unknown how they grow with regard to body mass, sex, and neck length. In this study, we measured the length, mediolateral (ML) diameter, craniocaudal (CC) diameter and circumference of the humerus, radius, metacarpus, femur, tibia, and metatarsus in 10 fetuses, 21 females, and 23 males of known body masses. Allometric exponents were determined and compared. We found the average bone length increased from 340 ± 50 mm at birth to 700 ± 120 mm at maturity, while average diameters increased from 30 ± 3 to 70 ± 11 mm. Fetal bones increased with positive allometry in length (relative to body mass) and in diameter (relative to body mass and length). In postnatal giraffes bone lengths and diameters increased iso- or negatively allometric relative to increases in body mass, except for the humerus CC diameter which increased with positive allometry. Humerus circumference also increased with positive allometry, that of the radius and tibia isometrically and the femur and metapodials with negative allometry. Relative to increases in bone length, both the humerus and femur widened with positive allometry. In the distal limb bones, ML diameters increased isometrically (radius, metacarpus) or positively allometric (tibia, metatarsus) while the corresponding CC widths increased with negative allometry and isometrically, respectively. Except for the humerus and femur, exponents were not significantly different between corresponding front and hind limb segments. We concluded that the patterns of bone growth in males and females are identical. In fetuses, the growth of the appendicular skeleton is faster than it is after birth which is a pattern opposite to that reported for the neck. Allometric exponents seemed unremarkable compared to the few species described previously, and pointed to the importance of neck elongation rather than leg elongation during evolution. Nevertheless, the front limb bones and especially the humerus may show adaptation to behaviors such as drinking posture.
... The story is more interesting than this, however. More recently, the challenge to the standard explanation was accepted, and an alternative to the IFCH hypothesis was proposed and supported (Cameron & Toit, 2007;Mitchell et al., 2009), in contradiction to Simmons and Scheepers (1996). Their hypothesis is that intraspecies competition for food drove the evolution of longer necks. ...
... Interspecific foraging competition hypothesis (IFCH) Use maximum reach of the neck to get food during times of food scarcity. There should be no sex difference in neck length relative to body size (Mitchell et al., 2009). Neither males or females forage at the highest levels, even during food shortage and in competition with other browsers. ...
... But an interspecific arms race could drive directional selection for longer necks. *Simmons and Scheepers (1996);Anderson (1982, 1985); ¥ Cameron and DuToi (2007); § Mitchell et al, (2009) note: EEA = environment of evolutionary adaptiveness. ...
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From: "Evolutionary Studies: Unfolding Darwin’s Roadmap across the Curriculum," edited by Glenn Geher, David Sloan Wilson, Hadassah Head, and Andrew Gallup. Oxford University Press. https://www.oxfordscholarship.com/view/10.1093/oso/9780190624965.001.0001/oso-9780190624965
... Doubt has been cast on the sexual selection hypothesis by analysis of the allometry of the growth of neck and leg length, and head and neck mass in a population of Zimbabwean giraffes (G.c. giraffa) (Mitchell, van Sittert & Skinner, 2009). This analysis found that there were few gender differences and those that existed could be explained as generic male/female differences: most mammalian males have final body dimensions greater than their females. ...
... We analyzed the possibility of differences between the two populations using the data reported by Simmons and Altwegg (2010) for Namibian giraffes and the data we have accumulated on body, neck and head mass in Zimbabwean giraffes (Mitchell, van Sittert & Skinner, 2009). The data for head, neck, and body mass in Namibian giraffes were extracted from the discreet data points identifiable in Figures 1 and 2 in Simmons and Altwegg (2010). ...
... The study we report here set out to identify if sexual dimorphism was present in the heads and necks of male and female giraffes and so test, therefore, whether sexual selection is a possible explanation for the evolution of their long neck. In two previous analyses of this possibility opposite conclusions were drawn based on measurements made in two different populations of giraffes, one in Namibia (Simmons & Scheepers, 1996) and the other in Zimbabwe (Mitchell et al, 2009). The reanalysis we report here of these two populations has led us to conclude that there are no significant differences in growth patterns between the populations or between Namibian and Zimbabwean giraffes except perhaps that Namibian giraffes may be smaller. ...
Article
... Despite its uniquely long neck, it seems that the giraffe could be just a large ruminant adapted to a specific type of grazing (Mitchell et al. 2009). The evolution process distinguished the giraffe from its relatives with an extremely elongated neck, most probably adapting to its shifting environment (Mitchell and Skinner, 2003;van Schalkwyk et al., 2004;Badlangana et al., 2009;Mitchell et al., 2009). ...
... Despite its uniquely long neck, it seems that the giraffe could be just a large ruminant adapted to a specific type of grazing (Mitchell et al. 2009). The evolution process distinguished the giraffe from its relatives with an extremely elongated neck, most probably adapting to its shifting environment (Mitchell and Skinner, 2003;van Schalkwyk et al., 2004;Badlangana et al., 2009;Mitchell et al., 2009). ...
Article
The anatomy of the giraffe (Giraffa camelopardalis Linnaeus, 1758) has been poorly studied, except for the circulatory system. In particular, only a handful of studies have concerned the brain of this species since the first description in 1839. Accordingly, only a very few articles discussing encephalization mentioned the giraffe or used it in their calculations. In this article, we performed a thorough examination of the literature including old and grey, regarding the central nervous system of the giraffe. Furthermore, we examined the brain of 3 giraffes, and calculated the encephalization quotient (EQ) of the species, based on our own data and the values found in the literature. We also revised the pre-existing literature and re-mapped the main sulci based on current comparative interpretation and anatomical nomenclature. Our results were compared to those of other selected significant mammals. The mean brain weight was of 719.9???12.5 g. Our data indicate that the EQ of the giraffe is 0.64 and matches that of the typical ungulate, despite having the largest brain among terrestrial Cetartiodactyla. This emphasizes that the giraffe is a highly specialized mammal, within the limitations of its clad. Anat Rec, 2017. ? 2017 Wiley Periodicals, Inc.
... The singular shape of the giraffe Giraffa camelopardalis has invited many questions about its biology, ecology and evolution. It remains controversial as to how many cervical vertebrae giraffes have (Solounias, 1999), and debate continues as to whether their necks evolved for sexual selection (Simmons and Scheepers, 1996) or not (Mitchell et al., 2009), or to provide a competitive advantage over contemporary herbivores (Cameron and du Toit, 2007). The giraffe's long limbs and neck, sloping back and short body give it a distinctive gait (Powell, 1984). ...
... This is in contrast to the horse where the neck is only modestly rotated forwards from its terrestrial inclination (compare Fig. 1a with Fig. 2). From the study of the body mass proportions of a cull of giraffes from Zimbabwe (Mitchell et al., 2009), the mass of the head+neck of males relative to total body mass was found to be 10.8 70.4%. The density of the model head and neck were set to 850 gm/l, and a measure of the accuracy of this assignment is that the combined mass of the head and neck, 174.2 kg, represents 10.8% of the total body mass -a value identical to that found for the wild animals. ...
Article
Giraffes (Giraffa camelopardalis) are often stated to be unable to swim, and while few observations supporting this have ever been offered, we sought to test the hypothesis that giraffes exhibited a body shape or density unsuited for locomotion in water. We assessed the floating capability of giraffes by simulating their buoyancy with a three-dimensional mathematical/computational model. A similar model of a horse (Equus caballus) was used as a control, and its floating behaviour replicates the observed orientations of immersed horses. The floating giraffe model has its neck sub-horizontal, and the animal would struggle to keep its head clear of the water surface. Using an isometrically scaled-down giraffe model with a total mass equal to that of the horse, the giraffe's proportionally larger limbs have much higher rotational inertias than do those of horses, and their wetted surface areas are 13.5% greater relative to that of the horse, thus making rapid swimming motions more strenuous. The mean density of the giraffe model (960 gm/l) is also higher than that of the horse (930 gm/l), and closer to that causing negative buoyancy (1000 gm/l). A swimming giraffe - forced into a posture where the neck is sub-horizontal and with a thorax that is pulled downwards by the large fore limbs - would not be able to move the neck and limbs synchronously as giraffes do when moving on land, possibly further hampering the animal's ability to move its limbs effectively underwater. We found that a full-sized, adult giraffe will become buoyant in water deeper than 2.8m. While it is not impossible for giraffes to swim, we speculate that they would perform poorly compared to other mammals and are hence likely to avoid swimming if possible.
... These results are certainly consistent with giraffe necks being sexually selected. However, Mitchell, van Sittert & Skinner (2009) failed to find any evidence for a difference in neck sizes of male and female giraffes of the same body size and demonstrated positive allometry in the necks of both male and female giraffes. A key aspect of the study by Mitchell et al. (2009) is that they measured neck length directly, while Simmons & Scheepers (1996) inferred neck length from other measurements which had originally been taken to investigate different aspects of giraffe biology. ...
... However, Mitchell, van Sittert & Skinner (2009) failed to find any evidence for a difference in neck sizes of male and female giraffes of the same body size and demonstrated positive allometry in the necks of both male and female giraffes. A key aspect of the study by Mitchell et al. (2009) is that they measured neck length directly, while Simmons & Scheepers (1996) inferred neck length from other measurements which had originally been taken to investigate different aspects of giraffe biology. So there appears to be no convincing evidence for (or against) sexual selection in the evolution of giraffe necks. ...
Article
There has been recent discussion about the evolutionary pressures underlying the long necks of extant giraffes and extinct sauropod dinosaurs. Here we summarise these debates and place them in a wider taxonomic context. We consider the evolution of long necks across a wide range of (both living and extinct) taxa and ask whether there has been a common selective factor or whether each case has a separate explanation. We conclude that in most cases long necks can be explained in terms of foraging requirements, and that alternative explanations in terms of sexual selection, thermoregulation and predation pressure are not as well supported. Specifically, in giraffe, tortoises, and perhaps sauropods there is likely to have been selection for high browsing. It the last case there may also have been selection for reaching otherwise inaccessible aquatic plants or for increasing the energetic efficiency of low browsing. For camels, wading birds and ratites, original selection was likely for increased leg length, with correlated selection for a longer neck to allow feeding and drinking at or near substrate level. For fish-eating long-necked birds and plesiosaurs a small head at the end of a long neck allows fast acceleration of the mouth to allow capture of elusive prey. A swan's long neck allows access to benthic vegetation, for vultures the long neck allows reaching deep into a carcass. Geese may be an unusual case where anti-predator vigilance is important, but so may be energetically efficient low browsing. The one group for which we feel unable to draw firm conclusions are the pterosaurs, this is in keeping with the current uncertainty about the biology of this group. Despite foraging emerging as a dominant theme in selection for long necks, for almost every taxonomic group we have identified useful empirical work that would increase understanding of the selective costs and benefits of a long neck.
... As shown in Section 3.1, the stability criterion is applicable for the parameters of a quadruped robot listed in Table 1. In this section, the applicability of the stability criterion is verified for several animals including cat, grey hound, lion, and horse, as their body weights vary from small to large [23][24][25][26][27][28]. The corresponding parameters are listed in Table 2. ...
... Parameters of animals and the initial values of simulation[23][24][25][26][27][28]. ...
Article
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Some imbalance and balance postures of a passive quadruped robot with a simplified mathematical model are studied. Through analyzing the influence of the touchdown angle of the rear leg on the posture of the trunk during the flight phase, the stability criterion is concluded: the closer are the twomoments which are the zero time of the pitching angle and the peak time of the center of mass, the better is the stability of the trunk posture during the flight phase. Additionally, the validity of the stability criterion is verified for the cat, greyhound, lion, racehorse, basset hound, and giraffe. Furthermore, the stability criterion is also applicable when the center of the mass of body is shifted. Based on the stability criterion, the necessary and sufficient condition of the galloping stability for the quadruped robot is proposed to attain a controlled thrust. The control strategy is designed by an optimization dichotomy algorithm for seeking the zero point of the balance condition.Through the control results, it is demonstrated that the imbalance posture of the trunk could be stabilized by adjusting the stiffness of four legs.
... Secondly, the giraffes were weighed piecemeal as described previously (e.g. Mitchell et al., 2009). The body mass used in our analyses was the mean of the mass calculated from the relevant gender specific equation and the mass obtained by piecemeal weighing. ...
... For the subset of 9 other giraffes it was 181 to 1396 kg (3 females, 6 males; Table 2A). Body mass and age are related and we estimated the age range of these giraffes to be six months to greater than 10 years (Mitchell et al., 2009). ...
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We have measured rumen-complex (rumen, reticulum, omasum, abomasum) and intestine (small and large combined) mass in 32 wild giraffes of both sexes with body masses ranging from 289 -1441kg, and parotid gland mass, tongue length and mass, masseter and mandible mass in 9 other giraffes ranging in body mass from 181 to 1396kg. We have estimated metabolic and energy production rates, feed intake and home range size. Interspecific analysis of mature ruminants show that components of the digestive system increase linearly (Mb(1)) or positively allometric (Mb(>1)) with body mass while variables associated with feed intake scale with metabolic rate (Mb(.75)). Conversely, in giraffes ontogenetic increases in rumen-complex mass were negatively allometric (Mb(<1)), and increases in intestine mass, parotid gland mass, masseter mass, and mandible mass were isometric (Mb(1)). The relative masseter muscle mass (0.14% of Mb) and the relative parotid mass (0.03% of Mb) are smaller than in other ruminants. Increases in tongue length scale with head length(0.72) and Mb(.32) and tongue mass with Mb(.69). Absolute mass of the gastrointestinal tract increased throughout growth but its relative mass declined from 20% to 15% of Mb. Rumen-complex fermentation provides ca43% of daily energy needs, large intestine fermentation 24% and 33% by digestion of soluble carbohydrates, proteins, and lipids. Dry matter intake (kg) was 2.4 % of body mass in juveniles and 1.6% in adults. Energy requirements increased from 35Mj/day to 190 Mj/day. Browse production rate sustains a core home range of 2.2- 11.8 km(2). Copyright © 2015. Published by Elsevier Inc.
... Studies of sexual selection have tended to concentrate on obvious morphological dimorphisms such as crests, horns, antlers, or weapons (Bonduriansky, 2007). However, many traits that show no obvious sexual dimorphism might nevertheless still be under sexual selection, although it can be difficult to tease apart the different potential selective pressures, e.g. the elongated necks of male and female giraffes (Giraffa camelopardalis) (Simmons & Scheepers, 1996;Mitchell, Van Sittert & Skinner, 2009). ...
Article
Studies of sexual selection have tended to concentrate on obvious morphological dimorphisms such as crests, horns, antlers, and other physical displays or weapons; however, traits that show no obvious sexual dimorphism may nevertheless still be under sexual selection. Sexual selection theory generally predicts positive allometry for sexually selected traits. When fighting, male kangaroos use their forelimbs to clasp and hold their opponent and, standing on their tail, bring up their hind legs to kick their opponent. This action requires substantial strength and balance. We examined allometry of forelimb musculature in male and female western grey kangaroos (Macropus fuliginosus) to determine whether selection through male–male competition is associated with sex differences in muscle development. Forelimbs of males are more exaggerated than in females, with relatively greater muscle mass in males than the equivalent muscles in females. Furthermore, while muscles generally showed isometric growth in female forelimbs, every muscle demonstrated positive allometry in males. The significant positive allometry in male forelimb musculature, particularly those muscles most likely involved in male–male combat (a group of muscles involved in grasping: shoulder adduction, elbow flexion; and pulling: arm retraction, elbow flexion), clearly suggests that this musculature is subject to sexual selection. In addition to contributing to locomotion, the forelimbs of male kangaroos can also act as a signal, a weapon, and help in clasping,features that would contribute towards their importance as a sexually selected trait. Males would therefore benefit from well-developed musculature of the arms and upper body during competition for mates.
... The physical attribute that is most characteristic of giraffes is their long neck. As increases in both eye size and neck length correlate with increases in body mass (Mitchell et al. 2009; this study), our data suggest that neck elongation is accompanied by changes in visual anatomy that are associated with good vision. In other words it seems as if giraffes have co-evolved good vision and a periscope-like ability to see above tree level as advantageous adaptations for the open woodland savanna environment they inhabit: these two characteristics provide an early warning system that enhances predator avoidance, and provide a means of communication between widely separated conspecifics. ...
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Giraffe are thought to have excellent vision.We measured eye size, orbit orientation and retina surface area in 27 giraffes of both sexes ranging in age from neonates to mature adults (>10 yrs), to assess how it changes with growth, whether their eye anatomy correlates with their apparently excellent vision and lifestyle, and we have compared our findings with those for other large mammals to assess whether giraffe eye anatomy is unique.We found that giraffe eye volume increases from 33 cmat birth to approximately 65 cmin adults. The focal (axial) length increases from c. 40 to 48 mm in adults and retina surface area from c. 3000 mmat birth to 4320 mmin adults. The orbital axis angle at birth is c. 73&Deg; and the horizontal visual field mainly monocular and panoramic.With age the axis angle becomes more acute to c. 50&Deg; in adults and the visual field more binocular, changes that occur concurrently with increasing neck length. These results show that the giraffe eye and retinal surface area are larger than in all other ungulates, and their visual fields more binocular, attributes which are consistent with the idea that they have excellent vision.
... Mateus, Maidment & Christiansen (2009) applied Senter's criteria to the long-necked stegosaur Miragaia longicollum, but with inconclusive results. However, recent work on both giraffes (Cameron & du Toit, 2007;Mitchell, van Sittert & Skinner, 2009) and sauropods (Christian & Dzemski, 2007;Taylor, Wedel & Naish, 2009) challenges Senter's (2006) analyses and the assumptions underlying them. Also of relevance is the recent resurgence in work on sexual selection in non-avian dinosaurs: this research has involved the reassessment and reanalysis of 'conventional' hypotheses in the light of new data and new techniques (e.g. ...
Article
It has recently been argued that the elongate necks of sauropod dinosaurs evolved primarily through selection for their use as sexual and dominance signals, and not as an adaptation for accessing a large ‘feeding envelope’ as traditionally thought. Here we explore this idea and show that all six arguments that have been advanced in support of the sexual selection hypothesis are flawed: there is no evidence for sexual dimorphism in the necks of sauropods; neither is there any evidence that they were used in dominance displays; long necks provided significant survival benefits in allowing high browsing and energetically efficient grazing; their fitness cost was likely less than has been assumed; their positive allometry through ontogeny is uninformative given that ontogenetic allometry is common in animals; apparent lack of correlation between neck and leg length across phylogeny is illusory due to over-representation of mamenchisaurids in a previously analysed dataset, and in any case is not informative as the unique morphology of sauropod necks suggests they, rather than legs, may have been cheaper to elongate when evolving increased vertical reach. In no speciose, morphologically varied, long-lived tetrapod clade has sexual selection consistently acted on a single part of the body, and it is unlikely that Sauropoda is the exception to this. In summary, there is no convincing evidence that sexual selection was the primary force driving the evolution of sauropod necks. While a subsidiary role for sexual selection cannot be discounted, the traditional hypothesis that sauropod necks evolved primarily due to the feeding benefits that they conferred is, by comparison, far better supported.
... The skeletons were analysed to establish body mass, height and age. We have established in a separate study on culled giraffes (Van Sittert et al., 2010) that there are robust allometric relationships between the length of bones (mm) and body mass (kg) in giraffes , between their height (measured from the sole of the front foot to the occipital crest) and body mass [Mitchell et al., 2009]) and between height and age (Dagg & Foster, 1976; Pellew, 1983b). The marker bone we used to determine body mass was cervical vertebra 7 (C7). ...
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Darwin’s theory for the evolution of the long neck of giraffes is that height confers access to browse free of competition from smaller browsers. The theory predicts that survivors of a drought will be the tallest animals in a population. All studies so far have tested this hypothesis by analysis of feeding patterns and behaviour.We have studied it by analysing the demography of deaths in a drought. Using skeletal material from 26 giraffes that died as a result of a drought in southeastern Zimbabwe in 2008, we established the body mass, height, and age of the dead giraffes using allometric equations developed from culled animals. Typical giraffe populations consist of 55% adults (>6 years old), 15% young adults (3–6 years old), 15% juveniles (1–3 years old), and 15% neonates (<1 year old). Skeletons came from 54% adults, 14% young adults, and 32% juveniles. No neonatal skeletons were found. More juveniles died than expected because they have to compete with other browsers for nutrients. Most adult deaths occurred in the tallest and largest males because their daily requirements for browse are highest and could not be met by the amounts available at any level. Thus the survivors of this drought were young adults, a finding contrary to the predictions of Darwin’s feeding hypothesis.
... The dominance hierarchy has been described for male giraffes only [34] and has not been deeply analysed. The theory that the long neck of a giraffe evolved to gain dominance in sexual encounters [43] has recently been abandoned [44,45]. ...
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Wild giraffes live in extensive groups in the fission fusion system, maintaining long social distances and loose social bonds. Within these groups, resources are widely distributed, agonistic encounters are scarce and the dominance hierarchy was reported in males only, while never deeply analysed. In captivity, the possibility to maintain inter-individual distances is limited and part of the resources is not evenly distributed. Consequently, we suggest that agonistic encounters should be more frequent, leading to the establishment of the dominance hierarchy. Based on the differences in resource-holding potential, we suggested that the rank of an individual would be affected by age and sex. Based on hypotheses of prior ownership, we tested whether rank was positively affected by the time spent in a herd and whether it was stable in adult females, which were present long-term in the same herd. We originally monitored four herds of Rothschild giraffes (Giraffa camelopardalis rothschildii) in Dvůr Králové zoo (n = 8), Liberec zoo (n = 6), and two herds in Prague zoo: Prague 1 (n = 8) and Prague 2 (n = 9). The Prague 1 and Prague 2 herds were then combined and the resulting fifth herd was observed over three consecutive years (2009, 2010, and 2011) (n = 14, 13, and 14, respectively). We revealed a significantly linear hierarchy in Dvůr Králové, Prague 2 and in the combined herd in Prague. Rank was significantly affected by age in all herds; older individuals dominated the younger ones. In females, rank was positively affected by the time spent in the herd and adult females in Prague maintained their rank during three consecutive years. This study represents the first analysis of the dominance hierarchy in the captive giraffe, and discusses the behavioural flexibility of the social structure in response to monopolisable resources in a captive environment.
... Photogrammetric measurements were very close to actual lengths (mean difference = 4.1 cm, SD = 4.1). We measured and assigned age class at first capture for a subset of 500 giraffe from all sites with observed neck lengths using allometric equations for neck length and total height in Mitchell et al. (2009) andvan Sittert et al. (2010), along with total height at age data from Pellew (1983). We performed survival analysis for known-age giraffe and computed an age curve which we used to interpolate subadult survival for all sites based on site-specific calf and adult survival estimates (Supporting Information S5C). ...
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... Females rarely partake in this behaviour, but when they do it is only with males, so it should not be selective in females. The evidence for this mechanism of sexual selection is weak as there are no great differences between male and female giraffe necks, when scaled for gender body size [17,[19][20][21]. ...
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Giraffe (Giraffa camelopardalis), with their long neck and legs, are unique amongst mammals. How these features evolved is a matter of conjecture. The two leading ideas are the high browse and the sexual-selection hypotheses. While both explain many of the characteristics and the behaviour of giraffe, neither is fully supported by the available evidence. The extended viewing horizon afforded by increased height and a need to maintain horizon vigilance, as a mechanism favouring the evolution of increased height is reviewed. In giraffe, vigilance of predators whilst feeding and drinking are important survival factors, as is the ability to interact with immediate herd members, young and male suitors. The evidence regarding giraffe vigilance behaviour is sparse and suggests that over-vigilance has a negative cost, serving as a distraction to feeding. In woodland savannah, increased height allows giraffe to see further, allowing each giraffe to increase the distance between its neighbours while browsing. Increased height allows the giraffe to see the early approach of predators, as well as bull males. It is postulated that the wider panorama afforded by an increase in height and longer neck has improved survival via allowing giraffe to browse safely over wider areas, decreasing competition within groups and with other herbivores.
... The physical attribute that is most characteristic of giraffes is their long neck. As increases in both eye size and neck length correlate with increases in body mass (Mitchell et al. 2009; this study), our data suggest that neck elongation is accompanied by changes in visual anatomy that are associated with good vision. In other words it seems as if giraffes have co-evolved good vision and a periscope-like ability to see above tree level as advantageous adaptations for the open woodland savanna environment they inhabit: these two characteristics provide an early warning system that enhances predator avoidance, and provide a means of communication between widely separated conspecifics. ...
Article
Giraffe are thought to have excellent vision. We measured eye size, orbit orientation and retina surface area in 27 giraffes of both sexes ranging in age from neonates to mature adults (>10 yrs), to assess how it changes with growth, whether their eye anatomy correlates with their apparently excellent vision and lifestyle, and we have compared our findings with those for other large mammals to assess whether giraffe eye anatomy is unique. We found that giraffe eye volume increases from 33 cm³ at birth to approximately 65 cm³ in adults. The focal (axial) length increases from c. 40 to 48 mm in adults and retina surface area from c. 3000 mm² at birth to 4320 mm² in adults. The orbital axis angle at birth is c. 73° and the horizontal visual field mainly monocular and panoramic. With age the axis angle becomes more acute to c. 50° in adults and the visual field more binocular, changes that occur concurrently with increasing neck length. These results show that the giraffe eye and retinal surface area are larger than in all other ungulates, and their visual fields more binocular, attributes which are consistent with the idea that they have excellent vision.
... Furthermore, in some species, the long neck contributes to thermoregulation and dominance during male competition (Simmons and Scheepers 1996;Senter 2007;Ward et al. 2008). There have been many discussions about the evolutionary pressures underlying neck elongation in these species (Parrish 2006;Dzemski and Christian 2007;Sander and Clauss 2008;Mitchell et al. 2009;Stevens 2013), but mechanisms behind elongation of the cervical series are little understood. ...
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Long necks have evolved independently in several different taxa, but the processes underlying the evolution of this trait are not yet fully understood. In this study, we examined the skeletal mechanism underlying the neck elongation in the tribe Antilopini (Bovidae, Artiodactyla). We calculated the growth patterns of the cervical vertebrae in the gerenuk (Litocranius walleri), which possesses the longest neck in this tribe, and compared it with those in two related species. The growth rates of the vertebrae were not significantly different between species, suggesting that the long neck of the gerenuk has resulted from the elongation of the cervical vertebrae during the fetal or juvenile stage. The morphology of the cervical vertebrae of gerenuks differed from that of the closely related, relatively long-necked dama gazelle (Nanger dama), with protrusions occurring on the dorsal surface of the ventral arch of the atlas. This implies that gerenuks possess a well-developed transverse ligament of the atlas that functions to hold the dens of the axis against the atlas. We also found that the atlas lies in close proximity to the neural spine of the axis in the gerenuk, suggesting that hyperextension of the atlantoaxial joint is osteologically limited in this species. While foraging on high foliage, gerenuks flex and extend their necks freely in a bipedal posture without moving their entire body. These morphological characteristics peculiar to the gerenuk enhance the rigidity of the atlantoaxial joint and decrease the risk of subluxation of the joint during this unique foraging behavior.
... Similarly, if the two sexes have the same trait but it differs in relative size or development, then one can tentatively assume that sexual selection is involved (e.g. Mitchell, Van Sittert & Skinner, 2009;Simmons & Scheepers, 1996). ...
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Males may use tactics before, during and after mating to increase their reproductive success. With finite energy resources available, theory predicts that there should be a trade-off between investment in pre-copulatory traits (e.g. body size, armaments) and post-copulatory traits (e.g. testes size, spermatogenic efficiency). Western grey kangaroos (Macropus fuliginosus) are found in large, labile mixed-sex groups, in which the males show a dominance hierarchy. Males show indeterminate growth, and will reach up to six times the body mass of females. While the largest males use their size as a reproductive advantage, forelimb musculature further aids male-male contest, female attraction and/or female coercion. Under a trade-off scenario, we therefore predicted that larger, more muscular males would show less investment in sperm competitive traits. Consistent with this prediction, more muscular males showed decreased spermatozoa velocity. However, muscularity was also positively correlated with mass of two pairs of bulbourethral accessory glands, as well as mass of the penis and its muscles of erection. Seasonal changes in muscularity and accessory gland masses were also evident. Male kangaroos therefore invest in multiple reproductive traits on which selection can work.
... Indeed, it was observed that maintaining a longer neck requires more nutrients, which puts longer-necked giraffes at risk during a food shortage and contradicts the first hypothesis (Mitchell et al. 2010). The objection to the second hypothesis is that it fails to explain why female giraffes also have long necks (Mitchell et al. 2009). However, none of the above objections matter for the selection through social behaviour, which suggests that the long neck emerged as a feature genetically linked to certain successful change in giraffe's behaviour that then became a part of the new phenotype of a separate population (in the next chapter we will discuss the importance of genetic linkage in more detail). ...
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Giraffa sivalensis occurred during the Plio-Pleistocene period and probably represents the terminal species of the genus in Southern Asia. The holotype is an almost perfectly preserved cervical vertebra of disputed anatomical location. Although there is also uncertainty regarding this animal’s size, other specimens that have been assigned to this species include fragments of two humeri, a radius, metacarpi and teeth. Here we estimate neck length, leg length and body mass using interspecific and, unusually, ontogenetic allometry of extant giraffe skeletal parameters. The appropriateness of each equation to estimate body mass was evaluated by calculating the prediction error incurred in both extant giraffes (G. camelopardalis) and okapis (Okapia johnstoni). It followed that the equations with the lowest prediction error in both species were considered robust enough to use in G. sivalensis. The size of G. sivalensis, based on the holotype, is proposed as 400 kg (range 228 kg–575 kg), with a neck length of approximately 147 cm and a height of 390 cm. The molar lengths of tooth specimens considered agree with this size estimate. The humerus was the most appropriate long bone to establish body mass, which estimates a heavier animal of ca 790 kg. The discrepancy with the vertebral body weight estimate might indicate sexual dimorphism. Radial and metacarpal specimens estimate G. sivalensis to be as heavy as extant giraffes. This may indicate that the radius and metacarpus are unsuitable for body mass predictions in Giraffa spp. Alternatively, certain long bones may have belonged to another long legged giraffid that occurred during the same period and locality as G. sivalensis. We have concluded that if sexual dimorphism was present then males would have been about twice the size of females. If sexual dimorphism was not present and all bones were correctly attributed to this species, then G. sivalensis had a slender neck with a relatively stocky body.
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Background Numerous factors like continuous habitat reduction or fragmentation for free-ranging giraffes (Giraffa camelopardalis) as well as e.g. suboptimal housing conditions for animals in captivity might lead to behavioural alterations as part of the overall adaptation process to the changing living conditions. In order to facilitate current and future studies on giraffe behaviour, a comprehensive ethogram was compiled based on existing literature, as well as observations on giraffes in the wild (Hwange National Park, Zimbabwe; Entabeni Game Reserve, South Africa), and in captivity (National Zoological Gardens of South Africa, Pretoria). Findings The resulting ethogram lists 65 different behavioural patterns, which were described and grouped into seven categories: General activities, Abnormal repetitive behaviours, General interactions, Bull-Cow behaviour, Bull-Bull behaviour, Cow-Bull behaviour, Maternal behaviours, and Interactions by calves. The behaviours were further described regarding a presumed purpose, particularly with respect to social interactions and sexual behaviour. Contradictory descriptions from previous studies were considered and discussed in comparison with our own observations. Conclusions This ethogram provides a basis for current and future studies by suggesting a terminology which can be used for harmonizing behavioural observations, thus helping to facilitate comparability of future results. Subsequently, a better understanding of the behavioural ecology of giraffes in the wild as well as in captivity could aid future conservation efforts.
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The neck of Giraffe has excellent characteristics as a long robot mechanism. The neck of giraffe can move violently represented in necking, a motion striking necks each other. Further, in a necking motion, a neck of a giraffe wards off impacts and adapts to the shape of the opponent. On the other hand, conventional robot mechanisms have hardly achieved both powerfulness and kinematic and dynamic flexibility to the opponent. In order to apply these excellent characteristics of necks of a giraffe to robot mechanisms, we developed a half-size musculoskeletal robot prototype based on anatomical knowledge of the neck of giraffes and the skeletal shape of an actual giraffe. The musculoskeletal robot protype is actuated by thin McKibben pneumatic artificial muscles, which bent flexibly, and we confirmed adaptability to an external force.
Article
As mammalian cervical vertebral count is almost always limited to seven, the vertebral column of the giraffe (Giraffa camelopardalis) provides an interesting study on scaling and adaptation to shape in light of these constraints. We have defined and described the growth rates of the lengths, widths, and heights of the vertebrae from fetal through neonatal life to maturity. We found that the disproportionate elongation of the cervical vertebrae is not a fetal process but occurs after birth, and that each cervical (C2-C7) vertebrae elongates at the same rate. C7 is able to specialize toward elongation as its function has been shifted to T1. We concluded that T1 is a transitional vertebra whose scaling exponent and length is between that of the cervical and thoracic series. Despite its transitional nature, T1 is still regarded as thoracic, as it possesses an articulating rib that attaches to the sternum. The other dimensions taken (width, height, and spinous process length) show that giraffe vertebral morphology exhibit adaptations to biomechanical strain, and we have underlined the importance of the thoracic spinous processes in supporting the head and neck.
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A classic example of extreme morphological adaptation to the environment is the neck of the giraffe (Giraffa camelopardalis), a trait that most biologists since Darwin have attributed to competition with other mammalian browsers. However, in searching for present-day evidence for the maintenance of the long neck, we find that during the dry season (when feeding competition should be most intense) giraffe generally feed from low shrubs, not tall trees; females spend over 50% of their time feeding with their necks horizontal; both sexes feed faster and most often with their necks bent; and other sympatric browsers show little foraging height partitioning. Each result suggests that long necks did not evolve specifically for feeding at higher levers. Isometric scaling of neck-to-leg ratios from the okapi Okapia johnstoni indicates that giraffe neck length has increased proportionately more than leg length-an unexpected and physiologically costly method of gaining height. We thus find little critical support for the Darwinian feeding competition idea. Ne suggest a novel alternative: increased neck length has a sexually selected origin. Males fight for dominance and access to females in a unique way: by clubbing opponents with well-armored heads on long necks. Injury and death during intrasexual combat is not uncommon, and larger-necked males are dominant and gain the greatest access to estrous females. Males' necks and skulls are not only larger and more armored than those of females' (which do not fight), but they also continue growing with age. Larger males also exhibit positive allometry, a prediction of sexually selected characters, investing relatively more in massive necks than smaller males. Despite being larger, males also incur higher predation costs than females. We conclude that sexual selection has been overlooked as a possible explanation for the giraffe's long neck, and on present evidence it provides abetter explanation than one of natural selection via feeding competition.
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Vigilance in ungulates is considered to have a predominantly antipredator function, with the frequency and duration of scans per individual decreasing with increasing group size. Social influences on vigilance scans have been overlooked in studies on ungulates, although studies in primates and birds show that conspecific scans are important determinants of vigilance behaviour. We investigated group size effects in giraffes and examined social influences on their scanning behaviour, as well as the influence of feeding posture. We found that group size has little effect on scanning behaviour in either bulls or cows, which may be attributable to our inability to measure a group as perceived by a giraffe. Time spent scanning by lone cows did not differ from that of cows in any other group type, but time spent scanning by bulls when alone was less than that in groups. The presence of calves in a group did not influence scanning behaviour. Predation risk does not appear to be a significant modifier of vigilance behaviour, although a constant level of antipredator vigilance is probably maintained. Social factors were a significant modifier of vigilance scanning. Bulls scanned the most when they were in groups with larger bulls, and least when they were with smaller bulls. A similar pattern was seen with nearest-neighbour identity, and the identity of individuals within 10 m of a focal animal. Cows were significantly more vigilant when an adult bull was close, or was the nearest neighbour. Finally, vigilance advantages have been postulated as a determinant of sexual segregation in giraffe foraging heights but we found that the posture associated with high foraging heights imposes a vigilance cost, not an advantage. We therefore conclude that differential vigilance requirements are not a determinant of feeding height segregation between giraffe bulls and cows.
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Predator-prey relationships amongst the larger mammals of the Kruger National Park
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With their vertically elongated body form, giraffes generally feed above the level of other browsers within the savanna browsing guild, despite having access to foliage at lower levels. They ingest more leaf mass per bite when foraging high in the tree, perhaps because smaller, more selective browsers deplete shoots at lower levels or because trees differentially allocate resources to promote shoot growth in the upper canopy. We erected exclosures around individual Acacia nigrescens trees in the greater Kruger ecosystem, South Africa. After a complete growing season, we found no differences in leaf biomass per shoot across height zones in excluded trees but significant differences in control trees. We conclude that giraffes preferentially browse at high levels in the canopy to avoid competition with smaller browsers. Our findings are analogous with those from studies of grazing guilds and demonstrate that resource partitioning can be driven by competition when smaller foragers displace larger foragers from shared resources. This provides the first experimental support for the classic evolutionary hypothesis that vertical elongation of the giraffe body is an outcome of competition within the browsing ungulate guild.
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Previous references to “necking” behaviour, and the main features of the study area are briefly outlined. “Necking” behaviour in giraffe takes place only in all male herds. When the animals are in a head to head posture the intensity is either high or low, but when animals take up a head to tail posture the actions are always of high intensity and appear to have greater sexual significance. The significance of “necking” is discussed, and it is suggested that these ritualized actions form an important sexuo‐social bonding mechanism whereby a hierarchy is created amongst the males,and movement between strictly bachelor and mixed herds helps to maintain contact between the sexesin this polygamous mammal.
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The crude protein of the leaves of Acacia spp. is generally higher than in those of other species and they are therefore a better food source. The leaves of preferred plants show the following seasonal trends: a decrease in moisture and protein content from January until August/September when they again increase; conversely, the two carbohydrate fractions, crude fibre and N-free extract, increase during the corresponding period until August/September, whereafter they decrease again. Leaf moisture content was always greater during the night than during the day. This may be important for supplementing water gain in times of drought.-from Authors
Article
Data from 44 adult giraffes (Giraffa camelopardalis) showed that leg weight as a percentage of body weight is sufficiently constant to provide regression equations useful for estimating body or carcass weight. Data from 53 giraffes of various ages show that reliable estimates of body weight can be made from linear body measurements, regardless of sex or age. The best for field use are linear regressions between body weight and chest girth; however, shoulder height is usually the only measurement obtained from predator kills and this can also be used. Measures of the volume of the animals such as length × $\text{girth}^{2}$ and log transformations give superior correlations.
Article
The origin, phylogeny, and evolution of modem giraffes (Giraffa camelopardalis) is obscure. We review here the literature and conclude that the proximate ancestors of modern giraffes probably evolved in southern central Europe about 8 million years ago (Mya). These ancestors appear to have arisen from the gelocid ancestral assemblage of 20–25 Mya via the family Palaeomerycidae. From the palaeomerycids arose the Antilocaprinae (Pronghorns) via the subfamily Dromomerycinae, and two subfamilies of giraffids, the Climacoceratidae and Canthumerycidae. The terminal genus of the Climacoceratid line was the now extinct massive giraffid Sivatherium sp. The Canthumerycids gave rise to the okapi and giraffes via the intermediate forms of Giraffokeryx, Palaeotragus sp. (of which the okapi is the extant form), Samotherium sp. and Bohlinia sp. All of which are extinct. Stimulated by climate change, progeny of Bohlinia entered China and north India, evolved into typical Giraffa species and became extinct there about 4 Mya. Similarly, following their preferred habitat, African Giraffa entered Africa via Ethiopia about 7 Mya. Here, seemingly unaffected by the climate changes occurring to the east and causing extinction of its Asian counterparts, Giraffa radiated into several sequential and coeval species culminating with the evolution of G. camelopardalis in East Africa from where it dispersed to its modern range. Fossils of G. camelopardalis appear about 1 Mya in East Africa.The underlying stimulus for Giraffa evolution seems to have been the vegetation change that began about 8 Mya, from the prevalent forest (C3) biome to a savannah/woodland/shrub (C4) biome. Giraffa's success as a genus is attributed to its great height and unique coat markings. Its height is a consequence of elongation of all seven cervical vertebrae and of the lower more than the upper limb bones. Advantages conferred by its height include protection from predation, increased vigilance, and in males sexual dominance and access to nutrients. Its coat colourings are highly hereditable and provide protection from predation by camouflage, especially in the young. As giraffe are unable to sweat and pant, the patches may also act as thermal windows and may have an important thermoregulatory function.
Article
Specimens of Climacoceras africanus are described from Maboko, Kenya. The new species Climacoceras gentryi is established on the basis of ossicones, mandibles, and upper and lower dentitions from Fort Ternan and Baringo, Kenya. By interpretation of its lower canines Climacoceras is identified as a giraffoid and is placed in the new family Climacoceridae. Canthumeryx sirtensis is identified from Muruarot and Rusinga, Kenya. A dentition and associated partial skeleton of this species are described. The teeth agree closely with specimens of the same species from Gebel Zelten, Libya. Zarafa zelteni from Gebel Zelten is synonymized with Canthumeryx sirtensis. Again on the basis of its lower canines Canthumeryx is identified as a giraffoid and is placed in the new family Canthumerycidae. Specimens of Palaeotragus primaevus are described from Baringo, Kenya. This material includes a cranium with the ossicones, skull roof, occipital and basicranial regions preserved. Palaeotragus primaevus specimens from Fort Ternan are used in this description and some of these are redescribed. The relations of the giraffoids are assessed by methods of phylogenetic systematics. Palaeomeryx, Prolibytherium and Propalaeoryx are excluded from the Giraffoidea as their lower canines are not known. The Palaeotraginae is shown to be an invalid polyphyletic grouping and the genus Palaeotragus is also shown to be polyphyletic. Palaeotragus microdon is probably synonymous with Palaeotragus rouenii and the three species Palaeotragus rouenii (P. microdon), Palaeotragus coelophrys and Palaeotragus quadricornis are retained in the genus Palaeotragus. It is suggested that 'Palaeotragus' expectans and 'Palaeotragus' decipiens are closely related to Samotherium. Palaeotragus primaevus is probably synonymous with Palaeotragus tungurensis and this species is closely related to the giraffines. With slight changes the subfamilies Sivatheriinae and Giraffinae are valid monophyletic groups. Hydaspitherium is synonymized with Bramatherium and the Sivatheriinae includes the genera Giraffokeryx, Birgerbohlinia, Bramatherium and Sivatherium while the Giraffinae includes the genera Honanotherium, Bohlinia and Giraffa and the species 'Palaeotragus' tungurensis (P. primaevus). Okapia is identified as the sister-group of the other giraffids. Triceromeryx is the sister-group of the Giraffidae. Canthumeryx is the sister-group of Triceromeryx plus the Giraffidae while Climacoceras is the sister-group of the other giraffoids.
Article
Osteophagia, variable serum calcium (Ca) and phosphorus (P) concentration, high serum alkaline phosphatase activity, a high growth rate, and a large skeletal mass, all suggest that Ca and P requirements and availability are finely balanced in giraffes. The mineral content of some marker bones in skeletons obtained from adult male giraffes (browsers) and adult male African buffaloes (grazers of similar body mass) were compared to assess the idea of critical Ca and P balance. Our results show that the P concentration of plasma varies more than Ca concentration, and that the Ca content of giraffe bones (0.196 +/- 0.01 g/g) and buffalo (0.202 +/- 0.006 g/g) varies more than P content (0.095 +/- 0.002 in both). The average Ca and P content of the bones analysed was similar in both species (Ca = similar to 20.0%, P = similar to 9.5%). Giraffe skeletons, however, contain three times more Ca and P than do buffalo skeletons. This translates into a 1.5-2.0 fold higher Ca requirement for giraffes, with which they seem to cope effectively by selection for Ca-rich, dicotyledonous, browse. Sources of P to meet requirements are not obvious and a seasonal deficiency of P is a more likely cause of observed osteophagia than Ca deficiency. Giraffe rib P density, the best measure of P balance, of 0.142 +/- 0.01 g/cm(3) is, however, above the deficiency threshold. Bone mineral content (Ca and P) correlates well with bone density and shows only slight differences between adult males of the two species.
Article
In a year's study of the giraffe population of the Arusha National Park we identified 462 individuals. The cumulative first sightings of individuals were fitted to a logistic model that gave an estimated total population of 471. Immatures accounted for only 24% of the population. Twenty-two calves were born during the period of study.The giraffe of the Park are divided into ‘North’ (more precisely northeastern) and ‘South’ (southeastern) subpopulations, and although there is no barrier between them, only 3% of the population (all bulls) were ever seen in both the northern and southern parts of the Park. The northern region is further divided into four areas, each with its own assemblage of individuals, which were seldom seen elsewhere. Bulls and male juveniles were seen less often relative to their numbers than were cows and female juveniles. This indicates that they spend a greater proportion of the time in the forest than do the females.A partial list of plants eaten by giraffe in the Park is given. Acacias are an insignificant component of the diet.Calves were usually accompanied by their mothers; typically they did not lie out. Each calf was usually seen in the company of the same few individual peers. Calves spent a higher percentage of the time with calves than with their own mothers, they were closer to one another than to their mothers, and there was much physical contact between them.Bulls of each age class occurred less frequently with individuals of their own class and more frequently with bulls of other age calsses than would be expected. Young bulls were frequently seen in the company of other bulls, cows and immatures, and they often engaged in sparring. Older bulls were more solitary, tended to avoid other bulls in the presence of cows and immatures, and instead of sparring they tested and courted cows.The inadequate recruitment of this senescent population is attributed mainly to low birth rate and, in the South, high mortality in the first year. A decline in numbers is anticipated.
Article
The immensely long neck of a sauropod is one of the most familiar and striking of anatomical specializations among dinosaurs. Here, I use recently collected neontological and paleontological information to test the predictions of two competing hypotheses proposed to explain the significance of the long neck. According to the traditional hypothesis, neck elongation in sauropods increased feeding height, thereby reducing competition with contemporaries for food. According to the other hypothesis, which is advanced for the first time here, neck elongation in sauropods was driven by sexual selection. Available data match the predictions of the sexual selection hypothesis and contradict the predictions of the feeding competition hypothesis. It is therefore more plausible that increases in sauropod neck lengths were driven by sexual selection than by competition for foliage.
Article
The growth of the foetus of the southern giraffe is discussed. Gestation period is calculated as 457 days being the mean of 48 observations of other workers. Birthmass is taken as 102 kg from measurements on wild giraffe. This is almost twice as great as some published figures for captive giraffe. However, subspecies also differ. Foetal growth followed a typical “J” shaped curve when plotted from the mass of 24 foetuses. The increase in vertebral column length followed a straight line. Crown/rump measurements could not always be accurately taken.
Article
The giraffe of Nairobi National Park, Kenya have been studied for one year. Each animal seen has been photographed from the left side and the pattern on the neck used to recognize each individual. While the pattern may become darker with age, it does not significantly change in detail even over many years. At present 65 adult male, 72 adult female and 30 immature giraffe can be recognized. Movements of individuals are described as well as associations with other individuals. The 3ex ratio of the giraffe is discussed in relation to that found in the plains ungulates. The population of giraffe in the Park is considered to be particularly high and the relation of giraffe to the vegetation is discussed briefly.
Article
Previous references to “necking” behaviour, and the main features of the study area are briefly outlined. “Necking” behaviour in giraffe takes place only in all male herds. When the animals are in a head to head posture the intensity is either high or low, but when animals take up a head to tail posture the actions are always of high intensity and appear to have greater sexual significance. The significance of “necking” is discussed, and it is suggested that these ritualized actions form an important sexuo-social bonding mechanism whereby a hierarchy is created amongst the males,and movement between strictly bachelor and mixed herds helps to maintain contact between the sexesin this polygamous mammal.
Article
Using a mechanical model of the giraffe neck and head circulation consisting of a rigid, ascending, carotid' limb, a cranial' circulation that could be rigid or collapsible, and a descending, jugular' limb that also could be rigid or collapsible, we have analyzed the origin of the high arterial and venous pressures in giraffe, and whether blood flow is assisted by a siphon. When the tubes were rigid and the jugular' limb exit was lower than the carotid' limb entrance a siphon operated, carotid' hydrostatic pressures became more negative, and flow was 3.3 l min–1 but ceased when the cranial' and jugular' limbs were collapsible or when thejugular' limb was opened to the atmosphere. Pumping water through the model produced positive pressures in the carotid' limb similar to those found in giraffe. Applying an external tissue' pressure to the jugular' tube during pump flow produced the typical pressures found in the jugular vein in giraffe. Constriction of the lowest, jugular cuff', portion of the jugular' limb showed that the cuff may augment the orthostatic reflex during head raising. Except when all tubes were rigid, pressures were unaffected by a siphon. We conclude that mean arterial blood pressure in giraffes is a consequence of the hydrostatic pressure generated by the column of blood in the neck, that tissue pressure around the collapsible jugular vein produces the known jugular pressures, and that a siphon does not assist flow through the cranial circulation.
Tall tales or how the giraffe got its neck
• R E Simmons
Simmons, R.E. (2008). Tall tales, or how the giraffe got its neck. Africa Geographic, March, 32-39.
Carcass composition of the giraffe, Giraffa camelopardalis giraffa
• A J Hall-Martin
• M Von La Chevallerie
• J D Skinner
Hall-Martin, A.J., von la Chevallerie, M. & Skinner, J.D. (1977). Carcass composition of the giraffe, Giraffa camelopardalis giraffa. S. Afr. J. Anim. Sci. 7, 55-64.
The Zoological Society of London 286 Giraffe evolutionary morphology G
• Mitchell
Journal of Zoology 278 (2009) 281–286 c 2009 The Authors. Journal compilation c 2009 The Zoological Society of London 286 Giraffe evolutionary morphology G. Mitchell et al.
The giraffe of Nairobi national park
• Foster J.B.