Article

The Evolutionary Significance of Mountain Sheep Horns

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Abstract

The hypothesis was tested, that horns of male mountain sheep evolved as display organs. Seven predictions were made from this hypothesis, of which six were investigated and verified. These are: Rams have a distinct horn display behavior; rams can distinguish horn sizes; large horned individuals dominated smaller horned rams irrespective of the latter's age; horn size conveyed a priori dominance to its bearer; large horned rams enjoyed a reproductive advantage. The prediction, that loss of much horn leads to a loss in dominance was not investigated. The above hypothesis in conjunction with the observation that breeding rams show higher mortality than nonbreeding rams, predicted that rams with good horn growth die younger than rams with poor horn growth. The prediction was verified. The horns of rams are not only important as weapons and shields in combat, but also as the major dominance-rank determinance, and as visual dominance-rank symbols.

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... Rupicaprini are not adapted to withstand, nor do they engage in, the furious head-on colli.sions of male caprines. Instead, horn-Iockmg and head-to-head pushing occur, along with some body-butting (Couturier, 1938;Geist, 1964;1966b). Such behavior is considered to be ancestral to the patterns observed in the Caprini (Geist, 1966b). ...
... Both contestants back off a good distance (up to 30 ft in Ovis canadensis- Welles and Welles, 1961), charge, and collide at full tilt. The closing velocity is greatest in the large sheep (Ovis canadensis and O. ammon), and these animals complete the charge on their hind legs (Mills, 1937;Murie, 1944;Walthers, 1961;Welles and Welles, 1961;Walker et al., 1964;Geist, 1966a;1966b). In the smaller sheep (Ovis aries, Scott, 1960;pers. ...
... Photographic evidence (Haas, 1959;Welles and Welles, 1961;Geist, 1966a;1966b;Grzimek, 1966;and pers. obs.) and observations of damage to the horn sheaths indicate that male caprines hold the head at an angle that ranges from 90°(perpendicular to the ground) to about 135°( tucked under the neck) when butting. ...
... However, the number bighorn sheep fell dramatically in during the early 20 th century as a result of hunting, disease, and competition with livestock (Buechner 1960). Bighorn sheep occupy extreme environments including steep terrain, high elevation, low temperatures and deep snow during winter (Geist 1966 ). They have a polygynous mating system with intense malemale competition for mates (Hogg 1984), and male reproductive success increases with the size of horns (Coltman et al. 2002) used in physical combat and dominance interactions associated with competition for access to mates (Geist 1971). ...
... Bighorn sheep rams must make heavy energetic investments during the development of horns that can comprise 8-12% of body mass in older rams (Geist 1966; Geist 1971) (Fig. 1). These massive horns strongly affect male reproductive success and play a crucial role in the mating system of bighorn sheep by functioning in physical competitions for social dominance and mates (Geist 1966; Geist 1971). ...
... Bighorn sheep rams must make heavy energetic investments during the development of horns that can comprise 8-12% of body mass in older rams (Geist 1966; Geist 1971) (Fig. 1). These massive horns strongly affect male reproductive success and play a crucial role in the mating system of bighorn sheep by functioning in physical competitions for social dominance and mates (Geist 1966; Geist 1971). However, the associated energetic cost means that selection might favour individuals with alleles conferring small horns due to increased survival probability. ...
Article
The identification of genes influencing fitness is central to our understanding of the genetic basis of adaptation and how it shapes phenotypic variation in wild populations. Here, we used whole genome resequencing of wild Rocky Mountain bighorn sheep (Ovis canadensis) to > 50 fold coverage to identify 2.8 million SNPs and genomic regions bearing signatures of directional selection (i.e., selective sweeps). A comparison of SNP diversity between the X chromosome and the autosomes indicated that bighorn males had a dramatically reduced long term effective population size compared to females. This likely reflects a long history of intense sexual selection mediated by male-male competition for mates. Selective sweep scans based on heterozygosity and nucleotide diversity revealed evidence for a selective sweep shared across multiple populations at RXFP2, a gene that strongly affects horn size in domestic ungulates. The massive horns carried by bighorn rams appear to have evolved in part via strong positive selection at RXFP2. We identified evidence for selection within individual populations at genes affecting early body growth and cellular response to hypoxia; however these must be interpreted more cautiously as genetic drift is strong within local populations and may have caused false positives. These results represent a rare example of strong genomic signatures of selection identified at genes with known function in wild populations of a non-model species. Our results also showcase the value of reference genome assemblies from agricultural or model species for studies of the genomic basis of adaptation in closely related wild taxa. This article is protected by copyright. All rights reserved.
... Patches of ornamentation have been suggested to function as badges of status in a wide range of taxa, including insects (Tibbetts and Dale, 2004), reptiles (Whiting et al., 2003) and birds (Senar, 2006). The status signalling hypothesis was originally proposed to explain variation in the size of mountain sheep horns (Beninde, 1937;Geist, 1966), but the hypothesis has become increasingly important in the study of variability in plumage ornamentation in birds (Rohwer, 1975;Senar, 2006). Among the many bird species studied (Santos et al., 2011), the house sparrow (Passer domesticus) has become the classic textbook example of status signalling (Andersson, 1994;Searcy and Nowicki, 2005;Senar, 2006;Davies et al., 2012). ...
Article
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The status signalling hypothesis aims to explain within-species variation in ornamentation by suggesting that some ornaments signal dominance status. Here, we use multilevel meta-analytic models to challenge the textbook example of this hypothesis, the black bib of male house sparrows (Passer domesticus). We conducted a systematic review, and obtained primary data from published and unpublished studies to test whether dominance rank is positively associated with bib size across studies. Contrary to previous studies, the overall effect size (i.e. meta-analytic mean) was small and uncertain. Furthermore, we found several biases in the literature that further question the support available for the status signalling hypothesis. We discuss several explanations including pleiotropic, population- and context-dependent effects. Our findings call for reconsidering this established textbook example in evolutionary and behavioural ecology, and should stimulate renewed interest in understanding within-species variation in ornamental traits.
... Thus, when the sex ratio is equal, mitochondrial markers have an effective population one quarter that of microsatellite markers (Birky et al. 1983). However, this disparity is offset, to some extent, by the highly polygynous mating structure of bighorn sheep in which most adult ewes reproduce each year while generally only the most dominant rams successfully mate (Geist 1966, Geist 1971, Coltman et al. 2005. ...
... Second, horns may have evolved different function in males and females . Female horns may have evolved as antipredator weaponry to protect young (Geist, 1966), whereas male horns may have evolved under sexual selection. Male horns are indeed considered to be very important for reproductive success in sexually dimorphic ungulates like ibex (CluttonBrock et aI., 1982; Geist, 1974). ...
Article
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We studied long-term cohort effects on chest girth and horn length in a recently established population of alpine ibex (Capra ibex ibex). Environmental conditions of the year of birth affected chest girth and first-annual increment of horns of males but did not affect chest girth and horns of females. Females compensated for a slow horn growth during their Ist year of life, whereas males did not. Level of polygyny of the species and environmental conditions experienced by the population could account for the occurrence of long-term cohort effects in male growth and its absence in female growth. Abundance of food resources throughout the study period allowed females to show compensatory growth. However, evolutionary constraints on growth that may exist in males of polygynous species may have prevented males from showing compensatory growth.
... This population has been the of a major review (Keast 1977). Niche differences be-subject of studies by several workers (Geist 1966, tween the sexes have been documented in such di-1968, 1971, Petocz 1973, Shackleton 1973, Stelfox verse groups as vascular plants, fish, reptiles, birds, 1976, Geist and Petocz 1977 ) whose publications conand mammals (see Shank 1979 for a partial review). tain full descriptions of the study area and the local Geist and Petocz (1977) showed that Rocky Moun-sheep population. ...
Article
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In Banff National Park, rams remained primarily on the north end of the study area while ewes, lambs and yearlings occupied primarily the south end. The southern part of the range was characterized by relatively more Bromus, Carex, Koeleria, Poa, Oxytropis and Juniperus and less Festuca, Fragaria, Hedysarum, Arctostaphylos and Rosa than the northern portion. The southern part was typified by less soil, more xeric conditions, lower plant biomass and apparently overgrazed conditions. Ewes, lambs and yearlings had diets more similar to each other than to that of adult rams. Class effects in multivariate statistical analysis were significant; the major univariate differences were that non-rams ate more Carex and less Festuca than did rams. This class effect was wholly attributable to the fact that rams and non-rams segregate onto separate portions of the range which contain different availabilities of forage plants. It is unlikely that the spatial separation arose from diet preferences but rather from some unrelated factor. -from Author
... In polygynous mammals, body size is often a major determinant of male–male competition and larger adult males have high reproductive success (e.g., elephant seals [Mirounga angustirostris], Haley et al. 1994; brown bears [Ursus arctos], Zedrosser et al. 2007). In many large herbivores, male mating success also increases with size of horns, antlers, and tusks that evolved as intraspecific weapons (Geist 1966, Andersson 1994). Recent studies found that in ungulates, siring success increases with a male's age, social rank, and body or weapon size (Pemberton et al. 1992, Coltman et al. 2002, Mainguy et al. 2009). ...
Article
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ABSTRACT  In ungulates, big males with large weapons typically outcompete other males over access to estrous females. In many species, rapid early growth leads to large adult mass and weapon size. We compared males in one hunted and one protected population of Alpine chamois (Rupicapra rupicapra) to examine the relationship between horn length and body mass. We assessed whether early development and hunter selectivity affected age-specific patterns of body and horn size and whether sport hunting could be an artificial selection pressure favoring smaller horns. Adult horn length was mostly independent of body mass. For adult males, the coefficient of variation of horn length (0.06) was <50% of that for body mass (0.16), suggesting that horn length presents a lower potential for selection and may be less important for male mating success than is body mass. Surprisingly, early development did not affect adult mass because of apparent compensatory growth. We found few differences in body and horn size between hunted and protected populations, suggesting the absence of strong effects of hunting on male phenotype. If horn length has a limited role in male reproductive success, hunter selectivity for males with longer horns is unlikely to lead to an artificial selective pressure on horn size. These results imply that the potential evolutionary effects of selective hunting depend on how the characteristics selected by hunters affect individual reproductive success.
... With the 30% harvest rate of legal rams typical of Ram Mountain added to natural age-specific mortality, a ram legal at age 4 yr has about a 15% chance of surviving to rut at age 7 yr, while a ram not legal until 8 yr (that only faces natural mortality) has about a 64% chance of surviving over the same period (Jorgenson et al. 1997, Loison et al. 1999). Recent research on both bighorn sheep and Alpine ibex (Capra ibex) casts doubt over the hypothesis that horn growth is negatively correlated with longevity in either species (Geist 1966; 1971). Instead, the relationship appears to be positive, as one may expect if large-horned males were high-quality individuals (von Hardenberg et al. 2004). ...
Article
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Male bighorn sheep (Ovis canadensis) complete about 80% of horn growth by age 5, yet horn size appears to play little or no role in their mating success until they are 6-8 yr old. Only the most dominant rams, typically 8 yr and older, can tend estrous ewes. Subordinate rams use alternative mating tactics whose success appears independent of their horn size. Rams with fast-growing horns may become 'legal' to harvest a few years before those large horns lead to higher mating success. If hunting pressure is high, rams with rapidly growing horns will have lower lifetime mating success than rams with slow- growing horns that do not become legal until an older age. Because ram horn size is inheritable, harvest of rams with rapidly growing horns may favor genetically small- horned rams. We documented this phenomenon at Ram Mountain, where rams with horns of 4/5 curl or greater were 'legal' and hunting by Alberta residents was unrestricted, leading to an average harvest rate of about 30% of 'legal' rams. Because traits that affect horn size in rams are genetically correlated with fitness-related traits in ewes, selective hunting may have affected the demographic performance of the population. The selective effects of trophy hunting should increase with hunting pressure and decrease with immigration of rams from protected areas. BIENN. SYMP. NORTH. WILD SHEEP AND GOAT COUNC. 15: 42-49
... Antlers and horns appear to have evolved as social adaptations (e.g., Espmark 1964a Espmark , 1964b Espmark , 1971 Geist 1966a Geist , 1966b Goss 1983; Bubenik 1990). In Rangifer, unlike other members of the cervid family, animals of both sexes grow antlers. ...
Article
The frequency of occurrence of female reindeer and caribou (Rangifer tarandus) without antlers was recorded in the field in southern Norway, Svalbard, Iceland, eastern and western Greenland, and Newfoundland. Additional data were retrieved from the literature. The study showed that antlerless Rangifer females occur throughout their habitat, though they are generally more common among woodland populations than among tundra or alpine populations. Antlerless males are extremely rare. Within subspecies, the frequency of occurrence of antlerless females varies with geographical location: 5 – 47% in Svalbard reindeer, 21 – 79% in western Greenland caribou, and 12 – 92% in Newfoundland woodland caribou. Within the same population the frequency has changed over time, as at Snøhetta and Hardangervidda in southern Norway and possibly also in Newfoundland (the Interior herd). Among tundra reindeer in southern Norway there appears to be a relationship between habitat quality, body size or physical condition, and antler status. Antlerless females are few or absent in populations in prime physical condition and common in populations with animals in poor condition.
... Horn-like structures of Cervidae and Bovidae are among the most spectacular examples of secondary sexual characteristics (Geist 1966a, Gould 1974, Wilson and Mittermeier 2011), and likely evolved primarily for male-male combat, including display related to such interactions (Geist 1966bGeist , 1971 CluttonBrock 1982; Goss 1983; Bubenik and Bubenik 1990). Antlers typically are branched structures composed of bone that are grown and shed on an annual basis, whereas true horns are permanent structures with sheaths composed of cornified epidermal cells usually arranged around a bony core (Goss 1983). ...
Article
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Hunting remains the cornerstone of the North American model of wildlife conservation and management. Nevertheless, research has indicated the potential for hunting to adversely influence size of horn-like structures of some ungulates. In polygynous ungulates, mating success of males is strongly correlated with body size and size of horn-like structures; consequently, sexual selection has favored the development of large horns and antlers. Horn-like structures are biologically important and are of great cultural interest, both of which highlight the need to identify long-term trends in size of those structures, and understand the underlying mechanisms responsible for such trends. We evaluated trends in horn and antler size of trophy males (individuals exhibiting exceptionally large horns or antlers) recorded from 1900 to 2008 in Records of North American Big Game, which comprised >22,000 records among 25 trophy categories encompassing the geographic extent of species occupying North America. The long-term and broad-scale nature of those data neutralized localized effects of climate and population dynamics, making it possible to detect meaningful changes in size of horn-like structures among trophy males over the past century; however, ages of individual specimens were not available, which prevented us from evaluating age-class specific changes in size. Therefore, we used a weight-of-evidence approach based on differences among trophy categories in life-history characteristics, geographic distribution, morphological attributes, and harvest regimes to discriminate among competing hypotheses for explaining long-term trends in horn and antler size of trophy ungulates, and provide directions for future research. These hypotheses were young male age structure caused by intensive harvest of males (H1), genetic change as a result of selective male harvest (H2), a sociological effect (H3), effects of climate (H4), and habitat alteration (H5). Although the number of entries per decade has increased for most trophy categories, trends in size of horn-like structures were negative and significant for 11 of 17 antlered categories and 3 of 8 horned categories. Mean predicted declines during 1950–2008 were 1.87% and 0.68% for categories of trophy antlers and horns, respectively. Our results were not consistent with a sociological effect (H3), nutritional limitation imposed by climate (H4), or habitat alteration (H5) as potential explanations for long-term trends in size of trophies. In contrast, our results were consistent with a harvest-based explanation. Two of the 3 species that experienced the most conservative harvest regimes in North America (i.e., bighorn sheep [Ovis canadensis] and bison [Bison bison]) did not exhibit a significant, long-term trend in horn size. In addition, horn size of pronghorn (Antilocapra americana), which are capable of attaining peak horn size by 2–3 years of age, increased significantly over the past century. Both of those results provide support for the intensive-harvest hypothesis, which predicts that harvest of males has gradually shifted age structure towards younger, and thus smaller, males. The absence of a significant trend for mountain goats (Oreamnos americanus), which are difficult to accurately judge size of horns in the field, provided some support for the selective-harvest hypothesis. One other prediction that followed from the selective-harvest hypothesis was not supported; horned game were not more susceptible to reductions in size. A harvest-induced reduction in age structure can increase the number of males that are harvested prior to attaining peak horn or antler size, whereas genetic change imposed by selective harvest may be less likely to occur in free-ranging populations when other factors, such as age and nutrition, can override genetic potential for size. Long-term trends in the size of trophy horn-like structures provide the incentive to evaluate the appropriateness of the current harvest paradigm, wherein harvest is focused largely on males; although the lack of information on age of specimens prevented us from rigorously differentiating among causal mechanisms. Disentangling potential mechanisms underpinning long-term trends in horn and antler size is a daunting task, but one that is worthy of additional research focused on elucidating the relative influence of nutrition and effects (both demographic and genetic) of harvest.
... To function as indicators of status these signals should be variable and correlate with social rank and/or physical characteristics that relate to fighting ability, for example size (Rohwer 1975). In addition, they should also be used by other individuals to assess dominance prior to aggressive interactions (Geist 1966). ...
Article
The purpose of this study was to examine the relationship between crowing and dominance using domestic roosters,Gallus gallus domesticus. Dominant males crowed significantly more often than subordinate males and often attacked subordinates that crowed. Dominants also produced crows that were higher in frequency than subordinates. In addition, dominant males spent more time near the speaker when crows of dominant males were played than they did when crows of subordinate males were played. Neither subordinate males nor females responded to tapes from males of either status. These results suggest that both crow rate and quality vary with male status and that dominants can and do discriminate between males using crow quality as a cue. Crowing by roosters could thus potentially function as a signal of status.
... Horn growth in rams is related to allozyme heterozygosity in Rocky Mountain bighorn sheep Ouis canadensis canadensis (Fitzsimmons 1992, Fitzsimmons et al. 1995. Horns of males are used in displays, and as weapons and shock absorbers in collision fighting (Geist 1966). Horn size is important when males are struggling to establish their positions in the breeding hierarchy (Geist 1971, Hogg 1987. ...
Article
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Regular development of morphology is challenged by any environmental influence that draines energy from developing individuals. For half a century biologist have recognized that developmental stability, measured as the capability to regulate development of normal morphological structures, is influenced by genetic variation. This review considers the influence of enzyme polymorphism upon developmental stability. Empirical studies in a wide variety of animals have reported morphological variability and bilateral assymmetry to decrase with the heterozygosity of enzyme polymorphism. A controversy focuses on the question of whether the enzymes are neutral markers that either reveal variation in levels of inbreeding or are in linkage disequilibrium with genes directly influencing development. Another controversy focuses on whether the relationships between enzyme heterozygosity and development stability, most commonly reported in poikilotherms, will also be found homeotherms. These controversies are addressed by considering recent empirical studies of enzyme polymorphism and developmental stability.
... Growth of a tail carries energetic costs that can be expressed as reduced body growth or reproductive output (Ballinger & Tinkle 1979; Dial & Fitzpatrick 1981). Therefore, the tail as a status-signalling badge perhaps should be better viewed as intermediate between the true badges of birds and the more expensive and thus potentially more honest status signals like horn size in mountain sheep (Geist 1966) or croak pitch in anurans (Davies & Halliday 1978; Ryan 1980 ...
Article
Individuals of many lizard species lose tails to escape predation, but incur later costs, including reduced social status. Since size relates strongly to social rank in lizards, experiments were devised to separate effects of reduced size from other aspects of tail loss on social status following loss of tail. Tail removal from dominant subadult side-blotched lizards, Uta stansburiana, lowered social status in dyadic encounters. Artificial restoration of tail length restored social status in females but not males, largely due to the dimorphic response of subordinate lizards. Subordinate females displayed the same social relation to former dominant opponents with restored tails, whereas they increased aggression towards former dominant opponents without restored tails. Subordinate males responded equally to both kinds of former dominant opponents. Females, but not males, appear to use the tail as a status-signalling badge, or indicator of resource-holding potential. Females may assume an alternate social role once the tail is lost.
... Most of these studies were prompted by the seminal contribution of Zahavi (1975), whose work firmly established what is called 'handicap principle'. These signals include the development of bright plumage and long flamboyant tails by male peacock to attract desirable mates(Grafen 1990), the use of costly begging calls by baby birds to convey their feeling of hunger to their parents (Godfray 1991 and), prey deterring a predator by showing off his ability to run to safety (Yachi 1995) and the display of aggressive intention and fighting ability of animals when contesting food, territories, mates or other resources ( Zahavi 1975; Geist 1966). In our model, signaling is only excessively costly for dishonest politicians to participate in politics though participation costs are zero for their honest counterparts, at least for the range of costs identified. ...
... Intrasexual aggression typically differs between the sexes, with males coming into conflict over breeding opportunities and females over resources such as food and space. Horns are seldom used against predators and primarily for defending the offspring, in intrasexual competition for mates, or in social interactions by both sexes (Geist, 1966Geist, , 1967 Alvarez, 1990; Locati and Lovari, 1990). Moreover, predatory birds do not attack their victims directly at the throat. ...
... Horns are epidermal keratinous appendages used by males of many ungulate species for intra-sexual competition and fighting during the rut. In addition to their function in combat, these secondary sexual characteristics probably serve as signals of male vigour used by females to select mates (Geist 1966Geist , 1991). von Hardenberg et al. (2007) even suggest that longhorned Alpine ibex (Capra ibex ibex) males are of better genetic quality than those with shorter horns. ...
Article
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Studies on seasonal horn development and the endocrine mechanism regulating its pattern in wild ruminants are scarce. The aim of this paper was to study the influence of photoperiod and prolactin (PRL) on horn growth in two wild ruminant species: the European mouflon and the Iberian ibex. Eighteen male ibexes and 13 mouflon rams, maintained in captivity, were divided into three groups: a control group, kept under a natural photoperiod (latitude, 40°25′ N); a long-day group, exposed to an artificial photoperiod of 15-h light and 9-h darkness; and a group treated with bromocriptine (BCR; 10 mg twice weekly during spring and summer) to induce hypoprolactinaemia. Horn length growth (HLG) was recorded weekly for 18 months; plasma PRL concentrations were measured twice monthly by radioimmunoassay. In the ibexes of the long-day group, the period of strong horn development during spring–summer was significantly reduced by 2 months compared with the controls. In the mouflons of the long-day group, this same period was significantly increased by 9 months. In the BCR-treated animals, hypoprolactinaemia was observed in both species, but HLG was the same as in the corresponding controls. The present results suggest that the seasonal pattern of horn growth of wild ruminants is primarily modulated by photoperiod in a species-dependent manner. The persistence of resurgence of horn growth during spring in the BCR-induced hypoprolactinaemic animals of both species suggests that annual variations in blood PRL concentration have no effect on seasonal variation in horn growth.
... Horns are epidermal keratinous appendages used by males of many ungulate species for intra-sexual competition and fighting during the rut. In addition to their function in combat, these secondary sexual characteristics probably serve as signals of male vigour used by females to select mates (Geist 1966Geist , 1991). von Hardenberg et al. (2007) even suggest that longhorned Alpine ibex (Capra ibex ibex) males are of better genetic quality than those with shorter horns. ...
... Morphometric analyses are becoming increasingly common in biological studies to quantify and investigate biological shape (Adams, Rohlf & Slice, 2013; Bookstein, 1985; Bookstein, 1997; Fabre et al., 2013; Klingenberg, 1996; Klingenberg, 1998; Mitteroecker et al., 2013; Rohlf, 1990; Samuels, Meachen & Sakai, 2013; Zelditch, Sheets & Fink, 2003; Zelditch et al., 2004). One of the key uses of morphometric methods in both neontology and palaeobiology is as a more objective and repeatable means to quantify patterns of shape change and and, likewise, secondary sexual characters are often positively allometric (Geist, 1966; Geist, 1968; Simmons & Tomkins, 1996). Based on this, features which show strong positive allometric growth within extinct clades may be potential display structures, and under sexual selection (Brown, Russell & Ryan, 2009; Dodson, 1975c; Evans, 2007; Goodwin et al., 2006; Goodwin & Horner, 2004; Gould, 1973; Gould, 1974; Hone, Naish & Cuthill, 2011; Horner & Goodwin, 2009; Knell & Sampson, 2011; Padian & Horner, 2011; Sampson, Ryan & Tanke, 1997; Tomkins et al., 2010). ...
Article
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Quantitative morphometric analyses, particularly ontogenetic allometry, are common methods used in quantifying shape, and changes therein, in both extinct and extant organisms. Due to incompleteness and the potential for restricted sample sizes in the fossil record, palaeobiological analyses of allometry may encounter higher rates of error. Differences in sample size between fossil and extant studies and any resulting effects on allometric analyses have not been thoroughly investigated, and a logical lower threshold to sample size is not clear. Here we show that studies based on fossil datasets have smaller sample sizes than those based on extant taxa. A similar pattern between vertebrates and invertebrates indicates this is not a problem unique to either group, but common to both. We investigate the relationship between sample size, ontogenetic allometric relationship and statistical power using an empirical dataset of skull measurements of modern Alligator mississippiensis. Across a variety of subsampling techniques, used to simulate different taphonomic and/or sampling effects, smaller sample sizes gave less reliable and more variable results, often with the result that allometric relationships will go undetected due to Type II error (failure to reject the null hypothesis). This may result in a false impression of fewer instances of positive/negative allometric growth in fossils compared to living organisms. These limitations are not restricted to fossil data and are equally applicable to allometric analyses of rare extant taxa. No mathematically derived minimum sample size for ontogenetic allometric studies is found; rather results of isometry (but not necessarily allometry) should not be viewed with confidence at small sample sizes.
... The suggestion that successful males with large horns, tusks or antlers suffer high natural mortality as a consequence of greater mating effort (Geist 1966), although solidly grounded in sexual selection theory, has received little empirical support. When hunting mortality is not considered, horn length has either no effect or a slight positive effect on the survival of adult males, with the possible exception of the very oldest males in some species (Bonenfant et al. 2009, Toïgo et al. 2013). ...
Article
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Potential evolutionary consequences of selective hunting of mammals are controversial because of limited evidence and important socio-economic impacts. Several ecological and management variables facilitate evolutionary responses to selection for horn, tusk or antler size, including strong selective hunting pressure; harvest of males with large horns, tusks or antlers before they can breed; unavailable or ineffective sources of unselected immigrants; and age-dependent relationships between horn, tusk or antler size and male mating success. Plastic responses of male horns, tusks and antlers to environment are probably more common than evolutionary changes. Evidence for evolutionary effects of selective hunting is strong for large mammals where biological characteristics and hunting regulations combine to favour them.
... Moreover, although the length of the structure is often related to dominance (Jennings et al. 2006, see Figure 14.2), the contention that these structures serve to reduce fighting by facilitating assessment (e.g. Geist 1966a, Lincoln 1972, Barrette & Vandal 1986) has received only limited support (e.g. Wahlström 1994, Jennings et al. 2006). ...
... Most of these studies were prompted by the seminal contribution of Zahavi (1975), whose work firmly established what is called 'handicap principle'. These signals include the development of bright plumage and long flamboyant tails by male peacock to attract desirable mates (Grafen 1990), the use of costly begging calls by baby birds to convey their feeling of hunger to their parents (Godfray, 1995), prey deterring a predator by showing off his ability to run to safety (Yachi 1995) and the display of aggressive intention and fighting ability of animals when contesting food, territories, mates or other resources (Zahavi 1975; Geist 1966). In this model, signaling is only excessively costly for dishonest politicians but cheap for their honest counterparts, at least for the range of costs identified. ...
Conference Paper
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This essay, having identified the failure of political leadership as the bane of economic development and democracy in Africa, has outlined mechanisms for selecting high quality leaders into public office. In the first stage of the screening process, certain elements of costs are imposed on the intending politicians so as to deter the entry of dishonest politicians into politics. This is accomplished via the inverse relationship between the degree of honest and the costs of participation. The unique thing is that separating equilibrium that makes participation costs zero for honest individuals and heavy for their dishonest counterparts could in principle be obtained. Interestingly the mechanisms that generate information about the past of intending politicians also impose heavy costs on groups which supply this information.
... Some signals, such as sexually selected traits, convey information about the reproductive fitness of the sender. Weapons (such as antlers of a male deer) represent traits that involve high energetic expenses, but have high payoffs in life-and-death battles (Geist 1966). Similarly, sexual ornamentation (such as plumage on a peacock) also provides information regarding the sender's ability to pay nutritional, energetic, and physiological costs (Veiga & Puerta 1996, Keyser & Hill 1999, Viljugrein 1997). ...
Thesis
Status badges, such as bird plumage colors, are important parts of animal communication; they mediate intra- as well as intersexual interactions. Reliability of avian plumage badges is thought to be maintained by selective pressures, including social punishment. Costs, benefits, and resultant fitness tradeoffs are thought to maintain reliable status badges as evolutionarily stable signals. We tested this hypothesis during two breeding seasons (summers 2008-2009) in a population of Mountain White-crowned Sparrows (Zonotrichia leucophrys oriantha; MWCS) in Colorado (USA). Both sexes of this species possess a black and white striped crown that mediates interactions between juvenile and adult birds. Crown whiteness, expressed as the percentage of the crown that consists of white feathers, varies widely across individual sparrows. To test whether the reliability (the consistent transfer of information relating signal design and content) of crown whiteness in males is maintained by social punishment, we examined cost and benefits associated with experimentally manipulated phenotypes. We predicted status badge related tradeoffs to sender condition and social interactions. We conducted a series of territory intrusions / call playbacks using male sparrow decoys with manipulated crowns to measure social punishment costs. Resident MWCS males received two simulated territorial intrusions by the same mounted decoy (once with a ???white-enhanced??? and once with a ???white-reduced??? crown treatment; these were presented in random order on different days). Males responded with significantly increased aggression when presented with white-enhanced decoys. In a parallel experiment we tested the prediction that crown whiteness is (1.) associated with sender condition and variation in levels of corticosterone and (2.) under sexual selection. That is, we predicted that males with whiter crowns would gain reproductive benefits (measured as number of fledglings). In these experiments we experimentally enlarged or reduced the proportion of whiteness in males??? crown feathers. Crown manipulations had no significant effect on baseline, nor on post-stress series corticosterone levels. While there was a trend for more offspring in males with white-enhanced crowns, this relationship was not ii significant due to small sample sizes of recovered nests. Our data provide support for the social punishment hypothesis, yet show no significant relationship between stress response and crown characteristics. Social costs are therefore at least partially responsible for maintaining the reliability of crown whiteness as a status badge in male MWCS.
... Moreover, territorial males have to patrol and mark the territory they defend using glandular secretions, which is likely to be costly. Males of territorial species have been reported to be more often subject to injuries and exhaustion than males of harem species that only fight exceptionally (Geist 1966). In the same way, allocating to sexually selected traits such as horns or antlers (Bro-Jørgensen 2007;Plard et al. 2011;Lemaître et al. 2014c) should influence the onset or rate of senescence because of inherent trade-offs in energy allocation between reproduction and maintenance (Williams 1966). ...
Article
In most mammals, both sexes display different survival patterns, often involving faster senescence in males. Being under intense sexual competition to secure mating opportunities, males of polygynous species allocate resources to costly behaviors and conspicuous sexual traits, which might explain these observed differences in longevity and senescence patterns. However, comparative studies performed to date have led to conflicting results. We aimed to resolve this problem by first reviewing case studies of the relationship between the strength of sexual selection and age-specific survival metrics. Then, we performed a comprehensive comparative analysis to test whether such relationships occur among species of captive ruminants. We found that the strength of sexual selection negatively influenced the onset of actuarial senescence in males, with males senescing earlier in polygynous than in monogamous species, which led to reduced male longevity in polygynous species. Moreover, males of territorial species senesced earlier but slower, and have a shorter longevity than males of species displaying other mating tactics. We detected little influence of the strength of sexual selection on the rate of actuarial senescence. Our findings demonstrate that the onset of actuarial senescence, rather than its rate, is a side effect of physiological mechanisms linked to sexual selection, and potentially accounts for observed differences in longevity. This article is protected by copyright. All rights reserved.
... In addition, Mao et al. did not further discuss the effect of sinus in modeling study. In some literature [27, 28], the idea of frontal sinuses as " shock absorbers " was raised and repeated and even used. As a result, the strain, stress, and other injury parameters would probably be influenced. ...
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The objective of this study is to analyze the biomechanical effects of sinuses in the skull on the facial impact response. Two models were built, where one had sinuses and the other had none. The models were verified using cadaver test data, including impacts to frontal bone, zygomatic bone, and maxillae. In the maxilla and zygoma impact, sinuses were found to have no significant effect on the global distribution of stress or stiffness of facial bones, and the influence was limited in local area. In forehead impact, the sinuses significantly affected the distribution of stress and strain in the skull due to its location in facial bones. The result shows that if the sinus is far away from the location of impact, its effect on the overall response of skull could be ignored. In addition, the distance between the region of interest and sinuses is another important parameter when studying the local effect of sinuses.
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The opponent-independent cost game is an animal contest in which the bigger and more heavily-armed opponent wins a disputed resource without significant fighting costs. A strategy is a choice of investment level in armament. Increasing armament is assumed to have fitness costs that are unrelated to contests; i.e. the cost of an individual's investment in arms is independent of the strategy played by an opponent.Previous work with this model showed that no ESS exists if a strategy prescribes an arms level exactly. This is equivalent to the notion that there is no environmental variation in the arms level attained by a given strategy. If environmental variation is introduced, a pure ESS can generally exist. A strategy is assumed to prescribe an exact investment cost, but this is translated into a probability distribution of arms levels attained, rather than an exact arms level. Increasing investment increases the mean of the arms level distribution. The ESS investment level depends both on how environmental factors distribute arms levels, and on the shape of the cost function (i.e. on the way that costs increase with investment); in some instances there is no ESS. Two types of model are investigated; in one fitness is additive (benefit-cost), in the other it is multiplicative (benefit × survivorship). The multiplicative model is likely to apply to the case where contests are between males for access to females. Here the ESS investment level (an ESS degree of risk that a male sustains as a result of armament) increases as fewer individuals guard the available resources. Thus sexual size dimorphism (male/female size) and relative male armament should increase as harem size increases. The ESS investment level will also be highest if most individuals are small and poorly armed, as would often be the case where size increases throughout life.The model can be applied to coevolutionary arms races between two classes of opponent, such as prey and predator, or parent and offspring. Here the ESS is likely to be a pair of ESS arms levels, one for each class of opponent.
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An ethogram of 22 behavior patterns is described. Larger and older sea lions (Eumetopias jubata) are dominant to smaller and younger ones. Males are more socially involved, and their behavior becomes more complex than that of females.Adult male and female behavior is less vigorous than that of subadult males. Territories where adult males and females are concentrated are more tranquil than areas where subadult males aggregate. Since females avoid areas of activity and harassment by sexually mature subadult males, they group in territories which act as refuges. This contributes to the spacial organization of the colony.The behavior of subadult males results in social disruption, whereas social stability is accommodated by adult males and females who are more stationary and so maintain prolonged social relationships.Non-pupping colonies are spacially organized similar to pupping colonies except for the relative proportion of age–sex classes. Socially, non-pupping colonies are less organized than pupping colonies. Social organization in E. jubata is promoted by the tranquil or energy-conserving behavior and ability of adults and inhibited by the behavior of subadult males.
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This paper discusses behavioral postures, calls, and sequences of the New Zealand fur seal (Arctocephalus forsteri) recorded from 1967 to 1969. A. forsteri was decimated by sealers prior to the early 1900s and although numbers have increased over the past 50 years, it remains one of the least known pinnipeds. Behavior was studied in a breeding and in a nonbreeding colony. Numbers in the breeding colony did not show marked seasonal change but there were changes in proportions of the sexes and various age classes present. the annual cycle is described insofar as it is known at present. Much of the agonistic behavior of A. forsteri is ritualized and six associated postures and three calls are described. the full neck display is one of the most important postures and appears to function as a threat by which males are able to assess another’s status without fighting. No data on behavior at the peak reproductive period, mid-November to late December, are available. the behavioral sequence by which a female locates her pup appears stereotyped. Vocalization and sight are important initially and olfactory recognition of the pup by the female gives final confirmation. Observations on locomotion, reactions to temperature, and reaction to humans are included.
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Dominant and subordinate Harris' sparrows usually coexist in the same flock and signal their social status by a distinctive plumage badge. We investigated advantages of subordination and costs of dominance in an effort to understand both why dominants and subordinates coexist when groups are not composed of relatives and why an inexpensive signal of high status should not be mimicked by poor fighters. Dominants and subordinates do not seem adapted to different microhabitats determined by seed density. Instead, we hypothesize that they coexist in a manner analogous to shepherds and sheep with dominants defending space in good habitats for subordinates and subordinates being used as food finders by dominants. In support of this shepherds hypothesis dominants displaced other dominants more frequently than they displaced subordinates when neither bird was feeding, but dominants displaced any lower ranked birds from caches of seed. This difference in aggression at and away from small and hidden patches of concentrated food suggests that a cost of dominance is fighting with other dominants and that a benefit of subordination is not having to fight with dominants for space in good habitats. Showing this helps explain why subordinates signal their subordination by a plumage badge. Curiously, among nonfeeding birds subordinates attack subordinates more frequently than do dominants. This indirect interference competition among subordinate Harris' sparrows could be for the privilege of being "sheep" of dominant Harris' sparrows or for the privilege of being "shepherds" to other lower ranked species of the community. This competition among subordinates (and, by extrapolation, within plumage or status ranks) helps explain why Harris' sparrows are fairly continuously variable in coloration rather than dichromatic with respect to dominance status.
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Sutural complexity (the degree of interdigitation) of 13 cranial sutures was compared between male and female wild sheep (Ovis orientalis) to investigate a morphological feature that is potentially important with respect to stress transmission in the skulls of males during fighting. Most facial sutures (four of six) were not sexually dimorphic, but two sutures, the maxillojugal and jugolacrimal, had greater complexity in males than in females, suggesting that significant forces may be transmitted through the facial region of rams, most likely during horn clashing. Most of the braincase sutures (five of seven) were more complex in males than in females, and different factors appear to underlie this sexual dimorphism. In females, increased complexity of sutures during ontogeny was predicted best by variables measuring growth of the skull, brain or face, while in males, changes in complexity were predicted best by variables representing mechanical loading and frontal bone growth.
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Objective: To characterize grossly, histologically, and via computed tomography (CT) the appearance of intrascleral cartilage, bone, or both in domestic goats with otherwise normal eyes and to correlate this with age, sex, and breed. Animals studied: Sixty-eight domestic goats (89 eyes). Procedures: Forty-nine formalin-fixed globes from 38 goats underwent high-resolution CT, and gross and light microscopic examination. An additional 40 eyes from 30 goats underwent light microscopy only. Age, breed, and sex of affected goats were retrieved from medical records. Results: Considering all methods of evaluation collectively, cartilage was detected in 42% of eyes (44% of goats) and bone in 11% of eyes (12% of goats); bone was never seen without cartilage. Goats in which bone, cartilage, or both were detected ranged from 0.25 to 13 (median = 3.5) years of age, represented 11 of 12 breeds of the study population, and had a male:female ratio of 11:19. Bone was detected in the eyes of significantly more males (n = 8) than females (n = 2). No sex predilection was noted for cartilage alone. Histology revealed intrascleral chondrocyte-like cells, hyaline cartilage, and islands of lamellar bone. Some regions of bone had central, adipose-rich, marrow-like cavities. CT localized mineralized tissue as adjacent to or partially surrounding the optic nerve head. Conclusions: This is the first report of intrascleral bone or cartilage in a normal goat and of intrascleral bone in an otherwise normal mammal. The high prevalence of intrascleral cartilage and bone in this study suggests that this finding is normal and likely represents an adaptation in goats.
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Bovid horns consist of a highly vascularized bone core covered by a keratin sheath which seems to offer limited resistance to heat flow. Based on dynamic cooling curves measured for inverted horns filled with warm water, we developed estimates of the thermal conductance of keratin and the coefficients of convective heat transfer at the water-to-sheath and the sheath-to-air boundaries to allow us to quantify heat flux through the horn sheath. Coupled with measurements of the internal and external horn dimensions, we constructed a simplified conceptual model of sheaths from 68 horns of 14 bovid species to test the prediction that the horns of temperate bovid species offer greater resistance to heat flux than the horns of tropical bovids. The specific heat capacity of the keratin sheath was 1.53 ± 0.07 (SD) J g-1°C-1. The coefficient of conductive heat transfer for keratin was 6.30 x 10-3 ± 0.30 x 10-3 (SD) W cm-1°C-1. We estimated the coefficients of convective heat transfer at the water-to-sheath and the sheath-to-air interfaces to be 8.79 x 10-3 ± 5.20 x 10-3 W cm-2°C-1 and 2.49 x 10-3 ± 1.98 x 10-3 W cm-2°C-1, respectively. A reduction in the size of the bone core and overlying vascular bed and an increase in the thickness of the keratin sheath in temperate bovids acted to reduce the surface area through which heat was lost to the environment. Because the surface-specific thermal conductances of temperate sheaths were lower than those of tropical sheaths, we estimate that a temperate bovid having horns of the same length and external surface as a tropical bovid would experience only 75.7% of the heat loss when facing a thermal gradient of 20°C. We argue that differences in horn morphology between temperate and tropical Bovidae appear to have evolved as adaptations to restrict heat loss in the former while facilitating heat loss in the latter group.
Chapter
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Large horns or antlers require a high energy allocation to produce and carry both physiological and social reproductive costs. Following the principle of energy allocation that implies trade-offs among fitness components, growing large weapons early in life should thus reduce future growth and survival. Evidence for such costs is ambiguous, however, partly because individual heterogeneity can counterbalance trade-offs. Individuals with larger horns or antlers may be of better quality and thus have a greater capacity to survive. We investigated trade-offs between male early horn growth and future horn growth, baseline mortality, onset of actuarial senescence, and rate of ageing in an Alpine ibex (Capra ibex ibex) population. Horn growth of males in early life was positively correlated to their horn length throughout their entire life. Cohort variation and individual heterogeneity both accounted for among-individual variation in horn length, suggesting both long-lasting effects of early life conditions and individual-specific horn growth trajectories. Early horn growth did not influence annual survival until 12 years of age, indicating that males do not invest in horn growth at survival costs over most of their lifetime. However, males with fast-growing horns early in life tended to have lower survival at very old ages. Individual heterogeneity, along with the particular life-history tactic of male ibex (weak participation to the rut until an old age after which they burn out in high mating investment), are likely to explain why the expected trade-off between horn growth and survival does not show up, at least until very old ages.
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Social behavior of male Pacific walruses. Odobenus rosmarus (L.), summering (i.e. outside the breeding season) on an Alaskan hauling ground is described. Social interaction on land is mostly agonistic. Visual presentation of tusks and striking with tusks feature prominently in most agonistic interactions: vocal communication occurs in a minority of them. Agonistic interactions are analyzed in R- and Q-approaches with MIN1SSA, a fully non-metric multidimensional scaling procedure programmed in the Guttman-Lingoes series. Large body size and long tusks characterize dominant walruses. Dominants are most frequently aggressive and threatening, and least frequently exhibit submissive, defensive, protective, and avoidance behavior. Subordinate walruses show the opposite trend. Walruses are bullies; individuals strongly disadvantaged in body size or tusk length, or both, receive numerous strikes and visual threats. Walruses tend to initiate agonistic interactions with smaller individuals. Walrus tusks are importa...
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Social behavior of the aoudad was studied in a free-ranging population during the rutting season. A total of 22 sexual and aggressive behavioral patterns was recorded among members of the various sex-age classes. The largest males (Class III) appropriated estrus females and courted them. Such females were defended by the dominant male in a group. Intensive combat among rival males, though rare, ensued when a dominant was challenged. Sparring and clashing were the predominant patterns used in such fights. Once a dominant established superiority over other members of the group he effectively chased away other males from an estrus female by horn-threatening or rushing at them. Class III males engaged in most sexual encounters with the females and possibly sired a large number of them. In the absence of Class III males in a group Class II rams actively participated in courtship. During courtship, the dominant ram maintained close contact with an estrus female and found little time to feed. Prior to mounting the male displayed ritualized patterns at the female by twisting and crouching at her repeatedly. Such behavior apparently seemed to concentrate the female’s attention to the male and led to copulation. The majority of behavioral patterns displayed by aoudads are common among sheep and goats with the exception of the crouch and stand not recorded among other members of the Caprini tribe. In aggressive behavior the aoudad seems to display forms which are similar to primitive sheep species such as neck-fighting, the tendency to wrestle the opponent down and clashing while on all four feet. Based on its behavioral characteristics the aoudad seems to occupy an intermediate position between the two main Caprini genera.
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The titanothere horn, which underwent extraordinary evolutionary growth relative to other skeletal dimensions, served as a device to protect the head and neck region against injury during butting, which was probably chiefly intraspecific in nature. In terms of morphological and behavioral traits related to butting, titanotheres are to rhinos within the Perissodactyla as sheep are to goats within the Caprini. Rhinos and goats have sharp horns and do not commonly engage in severe intraspecific combat. Advanced titanotheres display adaptations for head-on ramming analogous to those of sheep. Growth of the titanothere horn apparently served a compound function. First, it provided a broad, solid surface of impact distal to vulnerable portions of the facial region. Second, it brought the force vector of impact toward horizontal alignment with the occipital condyles, reducing the likelihood of neck damage; a slight offset probably remained in titanotheres, as in sheep, allowing the nuchal ligament to absorb some of the impulse and reduce the maximum force of impact. And third, by producing two divergent flanges, it formed a self-righting device that reduced the possibility of lateral twisting of the neck and may also have prolonged the duration and lessened the maximum force of impact. Large-horned titanothere species, like large sheep, possibly used their horns in part for display, to reduce the frequency of physical combat, but this function is questionable because titanotheres had small and presumably weak eyes. By analogy with sheep, sexual selection probably produced the butting adaptations of titanotheres, and it probably also produced the marked increase in body size. Frequency and severity of injury may or may not have decreased in titanothere evolution, depending on how behavior changed and how forces of impact and strength of resisting structures scaled with size, but any argument that relative growth of the horn was nonfunctional or deleterious is totally unjustified.
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The significance of the placement of bird species along a winter-plumage-variability axis is hypothesized to be causally related to the social system through which access to resources is controlled. When individuals compete for resources through territorial systems a single plumage type should be advantageous; when individuals compete through dominance behavior, variations in plumage which serve to signal dominance status are increasingly advantageous as flock stability declines. The following predictions from this theory were confirmed: (1) Winter flocking species are more variable than winter territorial species. (2) Three measures of flock stability (migratory status, nocturnal versus diurnal migrants, and frequency of distress screams) all showed plumage variability to be greater when the stability of individual memberships in flocks was lower. (3) Individual plumage differences signal dominance status in Harris sparrows. A further prediction that individuals similar in appearance would fight more than very different individuals was not confirmed in Harris sparrows. This prediction was based on two important assumptions: (1) that observed fights were within flocks and (2) that disputes were over position in the hierarchy. A flock cluster analysis served to eliminate the potential difficulty of assumption (1), and the prediction that individuals more alike in appearance should fight most was then strongly rejected. This implied that assumption (2) was not valid. Examination of this assumption showed fighting to be despotic; that is, top dominants concentrate their attacks on the lowest birds which are least likely to fight back. Such a pattern of fighting should give the top dominants the greatest gain in resources for lowest risk and least energy invested in defense of resources. This despotic pattern of fighting contrasts sharply with that reported for mountain sheep where the defensible resource conferring fitness-access to estrus ewes-requires only a stable hierarchy and not the exclusion of subordinates.
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Ornithischia, a diverse clade of herbivorous dinosaurs, has numerous members with structures hypothesized to function in combat. These include the horned ceratopsids, dome-headed pachycephalosaurs, spike-thumbed iguanodonts, tail-clubbed ankylosaurs and spiked stegosaurs, among others. Three main lines of evidence support such inferences: (1) analogy with modern animals; (2) biomechanical analysis and simulation; and (3) paleopathology. The most solid inferences utilize multiple pieces of evidence, although this is hampered by a limited understanding of combat in modern animals.
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Clarifying the emergent fitness associated with sexually selected traits under the current, increasingly anthropogenic selection regimes is important to understand ongoing evolutionary changes in nature and inform the conservation management of endangered species. Several reasons exist why sexual selection may affect extinction risk. Increased risk may result either from inherent trade‐offs between sexually selected traits and viability traits or from selective hunting of sexually selected species. Reduced risk is also possible, for instance if the preference for high‐performing mates characteristic of sexually selected species has beneficial genetic consequences for the population. Here, I show that the threat level of bovid species increases with large male horn size. This is the first time, to my knowledge, that sexually selected weaponry has been shown to increase extinction risk at the interspecific level. However, threat level was unrelated to another trait under sexual selection, sexual body size dimorphism, indicating that the effect of sexual selection on extinction risk depends on trait‐specific interactions with extrinsic factors. The results suggest that the higher threat level of long‐horned species is not primarily due to current trophy hunting practices and rather point to environmentally induced viability costs as a possible main driver. Still, the fact that long‐horned species are known to be preferred by trophy hunters highlights the importance of continuously monitoring trophy hunting practices to assure their long‐term sustainability.
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Narwhal and beluga whales are important species to Arctic ecosystems, including subsistence hunting by Inuit, and little is understood about their mating ecology. Reproductive tract metrics vary across species in relation to mating strategy, and have been used to infer mating ecology. Reproductive tracts from beluga and narwhal were collected between 1997 and 2008 from five beluga stocks and two narwhal stocks across the Canadian Arctic. Tract length for males and females, relative testes mass for males, and tusk length for male narwhal were measured. We assessed variation relative to species, body size, stock, maturity, and season. Significant variation was found in testes mass across month and stock for beluga, and no significant difference between stock or date of harvest for narwhal. Beluga had significantly larger testes relative to body size than narwhal, suggesting they were more promiscuous than narwhal. A significant relationship was found between narwhal tusk length and testes mass, indicating the tusk may be important in female mate choice. No significant differences were found between narwhal and beluga reproductive tract length for males or females. The mating systems suggested for narwhal and belugas by our results mean the two species may respond differently to climate change.
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Horns of bovids are important social organs, their growth is often indicative of population characteristics and habitat quality, but Little is known of the factors affecting their growth in individuals. We studied horn growth of 135 (51 males, 84 females) marked mountain goats (Oreamnos americanus) in Alberta, Canada, for 9 years. In both sexes, horn growth was quadratic during the first 5 years of life and not significant after 5 years of age. Goats completed 93% of horn growth by 3 years of age. Horns of males grew more than those of females during the first 1.5 years of life. Horns of females grew more than those of males during the third year. Although males maintained longer horns than females because of their longer first increment, adult males had shorter horns than females for a given body size. Males had thicker horns than females at all ages, absolutely or relative to body size. Horn growth early in life was correlated negatively with later growth. Annual growth increments of horns of females
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Mountain goats were observed during two rutting seasons. The manifestations of the rut, the agonistic and the courtship behavior are described and interpreted. It is shown that goats have evolved intense threat displays and have reduced fighting to a minimum. Fights are disadvantageous as examinations of wounded males and observations on a wounded male during the rut revealed. Males court females carefully until the agonistic tendencies of the females are depressed. The differences in agonistic behavior of the sexes lead to a social structure in which large, adult males are subdominant to females and yearlings. It was found that the courtship behavior is the antithesis of the males' present threat. Marking behavior may function as an olfactory intimidation mechanism. The significance of the goats' coat pattern is discussed.