Facultative Adjustment of Brood Sex Ratio in Response to Indirect Manipulation of Behaviour

ArticleinEthology 115(11):1057 - 1065 · September 2009with 12 Reads
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Abstract
Sex allocation theory states that parents should adjust their offspring sex ratio according to the expected fitness returns from sons and daughters. Several recent studies indicate that such adaptive manipulation of offspring sex ratio is achievable, and that it may be influenced by e.g. morphological characters. Here we manipulate behaviour through interspecific cross-fostering of great tits (Parus major) and blue tits (Cyanistes caeruleus), and investigate its effect on the offspring sex ratio of adults that were themselves cross-fostered as chicks. The experience of being raised by a different species has previously been shown to result in aberrant species assortative behaviour and song, and a lowered dominance status during winter. Brood sex ratios of conspecifically breeding pairs with and without cross-fostered members were compared. Broods with at least one cross-fostered parent contained significantly more males than did control broods. Sex of cross-fostered parents did not influence the brood sex ratio. We conclude that female great tits and blue tits seem to be able to adjust the sex ratio of their broods, and that changes in their own or their partners’ behaviour may elicit such adjustments.

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    Sex-allocation theory predicts that females paired to attractive males should bias the brood sex ratio towards male offspring, as these would inherit the attractiveness of their father. We studied sex allocation based on male ornamentation in blue tits. Brood sex ratios varied with male UV coloration in an age-dependent manner. For juvenile males, the proportion of sons increased with increasing UV ornamentation, which is in agreement with previous findings from a Swedish population. However, the relationship between UV ornamentation and brood sex ratio was reversed for adult males, with females paired to less UV-ornamented adult males producing more sons. This pattern fits with the observation that, in our population, less UV-ornamented adult males sire the majority of extra-pair young. To test the causality of the association between brood sex ratio and male coloration, we experimentally manipulated crown colour largely within the natural range. We created two groups of males: one with higher and one with lower UV reflectance, UV(+) and UV(−), respectively. Contrary to our expectations, there was no significant treatment effect. However, in UV(−), but not UV(+) males, the proportion of sons was negatively correlated with male coloration before manipulation. This suggests that the UV(−) treatment caused males that were more UV ornamented to decline more in attractiveness, as shown in a similar experiment in Sweden. However, given that correlational patterns differ between these populations, similarities in experimental results should not be taken as evidence for consistent patterns of adaptive sex allocation in this species.
  • Article
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    Previous studies have shown that some female black-capped chickadees (Poecile atricapillus) solicit copulations from males that rank higher in winter flocks than their social mates, and extra-pair paternity in nests occurs commonly enough to be considered a potential female mating tactic. This study uses blood samples collected in 1992–1995 from 58 families of black-capped chickadees to test whether females with extra-pair offspring have chosen extra-pair sires higher in social rank than their mates. Paternity was assessed with multilocus DNA fingerprinting in 1992–1994 nests and with microsatellite and single-locus minisatellite DNA typing in 1995 nests. Seventeen of 58 nests (29.3%) contained young genetically mismatched with their social father. In 11 of 15 cases where the identity of the extra-pair male was known, the extra-pair male was dominant to the social father. Using data from 29 nests located in 1994 and 1995 for which we had the most data on relative ranks of males, high-ranking males had greater realized reproductive success than low-ranking males as a result of extra-pair fertilizations. There was no significant difference between the number of nests containing extra-pair young of females mated to low-ranked versus high-ranked males. Two nests in 1995 contained young either genetically mismatched with both social parents (intraspecific brood parasitism) or, in one nest, genetically mismatched with the social mother but not the social father (quasi-parasitism). The implications of female strategies acquiring genetic benefits through extra-pair copulations are discussed.
  • Article
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    The costs and benefits of bird song are likely to vary among species, and different singing patterns may reflect differences in reproductive strategies. We compared temporal patterns of singing activity in two songbird species, the blue tit (Cyanistes caeruleus) and the great tit (Parus major). The two species live side by side year round, and they have similar breeding ecology and similar rates of extra-pair paternity. However, they differ in two aspects of reproductive strategy that may have an influence on song output: blue tits are facultatively polygynous and have a fairly short breeding season with almost no second broods, whereas great tits are socially monogamous but more commonly raise second broods. We found that great tit males continued singing at high levels during the egg-laying and incubation periods, while monogamously paired blue tit males strongly reduced singing activity after the first days of egg-laying by their female. Since males of both species sang much more intensely shortly before sunrise than after sunrise, at midday or in the evening, this difference was most conspicuous at dawn. No differences in singing activity were found within species when testing for male age. We suggest that in contrast to blue tits, great tit males continued singing after egg-laying to defend the territory and to encourage the female for a possible second brood.
  • Article
    Species recognition may be learned through imprinting early in life. Imprinting has normally been studied under highly unnatural conditions in the laboratory. We tested whether species recognition mediated through imprinting is individually modifiable in a field setting where great tits, Parus major, have been artificially cross-fostered to blue tits, Cyanistes caeruleus, and vice versa. We have shown previously that cross-fostered birds have deviant species recognition, in terms of both mate choice and aggressive responses towards rivals. Natural interactions among conspecifics and heterospecifics are common in these populations, potentially giving cross-fostered birds scope for relearning their species identity. We tested whether species recognition may change with experience during adulthood by comparing the aggressive response of cross-fostered birds and controls of different ages towards caged intruders. When breeding, cross-fostered birds responded aggressively towards same-sex individuals of their heterospecific foster species, while unmanipulated controls responded mainly towards conspecifics. We found that the aggressive response decreased with age at similar rates in both treatments and in both species. Moreover, there was no effect of age on the relative response towards conspecifics and heterospecifics in either treatment. Hence, we found no evidence that the species recognition behaviour towards same-sex individuals is shifted towards conspecifics with age in interspecifically cross-fostered birds. We conclude that species recognition is irreversible once it has been established in free-living great tits and blue tits. This is the first study to investigate the stability of species recognition in the field.
  • Article
    Female birds have repeatedly been reported to adjust the primary sex ratio of their offspring to environmental, social and physiological cues. However, the mechanism behind sex adjustment remains unknown. It has been suggested that maternal hormones may constitute an important mediator in this mechanism, as androgen levels differ between eggs bearing male and female embryos. To evaluate whether the level of maternal androgens affects the offspring sex ratio, we injected female zebra finches, Taeniopygia guttata, with testosterone during egg laying. The sex ratio of eggs laid after testosterone administration became significantly male biased, compared to eggs laid by control females that received a vehicle injection. However, sons of testosterone-treated females suffered lower hatching success. In contrast, daughters seemed to benefit from elevated androgen level in terms of future survival prospects. The opposite effects on male and female offspring may constitute an important constraint on maternal androgen allocation to the eggs and reduce the benefits of biasing the sex ratio towards males by increasing the testosterone level.
  • Article
    Through a cross-fostering experiment, we studied song learning of blue tit, Cyanistes caeruleus, and great tit, Parus major, males reared by heterospecific parents. This was done in the wild, and so potential song tutors, territorial neighbours, potential mates and other social factors were all natural and not affected by the treatment. Normally, the song repertoires of the two species are completely discrete. However, the cross-fostered great tit males altered their song in several aspects, including repertoire size and composition, and temporal and frequency parameters, thus becoming intermediate between the normal songs of the two species. For the cross-fostered blue tit song, only repertoire composition was affected. However, an analysis of repertoire size in both species proved this to be larger in cross-fostered than in control males. This increase in repertoire size shows that repertoire size is influenced by social conditions in these species and is not strictly constrained by memory capacity. Furthermore, several of the cross-fostered great tit males included trilled songs in their repertoires. In these two species, trilled songs have been regarded as unique to the blue tit. In conclusion, our results show the importance of early social experiences for song learning in these wild tits.
  • Article
    The proximate physiological mechanisms producing the parental ability to vary offspring sex ratio in many vertebrates remain elusive. Recently, high concentrations of maternal testosterone and glucose and low concentrations of maternal corticosterone have been suggested to explain male bias in offspring sex ratio. We examined how these factors affect secondary offspring sex ratio in nondomesticated field voles, Microtus agrestis, while controlling for maternal age, testosterone level of the male and body condition of both the female and the male. We found that females with high preconception serum testosterone and glucose levels produced a male-biased litter, whereas there was no association between maternal corticosterone level and litter sex ratio. Older females produced a bias towards sons, but neither their body condition nor paternal testosterone level correlated with litter sex ratio. Finally, females mated with a high body-condition male tended to deliver a male-biased litter. Our results suggest that several physiological traits of the mother may simultaneously be related to offspring sex ratio in mammals.
  • Article
    Nestling mass, wing length and tarsus length were inversely related to brood size. Hatching data also affected nestling growth, but its effect differed between years. Intrabrood variability in nestling size increased with brood size. Nestling survival was inversely related to brood size. Small nestlings suffered higher mortality than large ones. Fledgling survival to autumn and to the following breeding season was also inversely related to brood size; fledglings from reduced broods survived better than those from control broods, which in turn survived better than fledglings from enlarged broods. This resulted in the most productive category being the control broods. Survival of fledglings increased with their size as nestlings and decreased with hatching date. Proportionally more males survived in enlarged than in reduced broods. Dispersal distance of juveniles was not affected by brood size, size as nestling or hatching date; it was affected by sex. -from Authors
  • Article
    IN many birds, females copulate with males other than their social mate, resulting in extra-pair fertilizations (EPFs)1-7. It is still unknown, however, why females seek EPFs7,8. In one study, males that accounted for most EPFs had higher survival6, but neither the characteristics revealing male quality nor the benefits accruing to females selecting attractive males were identified. Great reed warblers, Acrocephalus arundinaceus, are socially polygynous, and females base their mate choice on territory quality9 and song-repertoire size10, both of which predict harem size and reproductive success11,12. By DNA fingerprinting13, we demonstrate that female great reed warblers obtain EPFs from neighbouring males with larger song repertoires than their social mate. In addition, the relative post-fledging survival of offspring was positively correlated with their genetical fathers' song repertoire size. These data support the hypothesis that females, by engaging in extra-pair fertilizations, seek genetic benefits for their offspring7,8.
  • Article
    The recent development of simple, DNA-based methods for the determination of an individual's sex will make possible large-scale studies of sex allocation and the consequence of gender in birds. Birds provide ideal systems for studying these questions in vertebrates, as so much is known about their biology and determinants of fitness. Until recently, however, little quantitative work has been possible because of the difficulty in determining gender in most cases. Recent studies suggest that biased sex allocation be more widespread in birds than has been realized.
  • Article
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    Sex allocation theory proposes that parents should bias the sex ratio of their offspring if the reproductive value of one sex is greater than that of the other. In the monogamous blue tit (Parus caeruleus), males have a greater variance in reproductive success than females, and high-quality males have higher reproductive success than high-quality females due to extrapair paternity. Consequently, females mating with attractive males are expected to produce broods biased toward sons, as sons benefit more than daughters from inheriting their father's characteristics. Song and plumage color in birds are secondary sexual characters indicating male quality and involved in female choice. We used these male sexual traits in blue tits to investigate adaptive sex ratio manipulation by females. We did not find any relationship between male color ornamentation and brood sex ratio, contrary to previous studies. On the other hand, the length of the strophe bout (i.e., the mean number of strophes per strophe bout) of fathers was positively related with the proportion of sons in their broods. The length of the strophe bout is supposed to reflect male quality in terms of neuromuscular performance. We further showed that sons produced in experimentally enlarged broods had shorter strophe bouts than sons raised in reduced broods. These results are consistent with the hypothesis that females adjust the sex ratio of their broods in response to the phenotype of their mate. Copyright 2006.
  • Article
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    When fitness benefits of investment in sons and daughters differ, animals are predicted to manipulate the sex ratio of their offspring. Sex ratio manipulation occurs in many taxa, but the mechanisms underlying the phenomenon in vertebrates remain largely unknown. Factors favoring skewed sex ratios, such as reduced maternal condition or food availability, also induce elevated corticosteroids. Recent experimental studies support a causal relationship between corticosteroids and sex ratio. Evidence of a natural correlation between maternal corticosteroids and offspring sex ratio has been lacking, however. Without such evidence, the importance of corticosteroids in influencing sex ratios in natural populations was unknown. We measured baseline corticosteroids in 19 free-ranging female white-crowned sparrows (Zonotrichia leucophrys) and the sex ratios of their offspring. Females with high corticosteroids produced more daughters than females with low hormone levels. We then conducted a controlled, field-based experiment investigating the effects of moderately increased maternal corticosteroids on offspring sex ratios to determine if the observed correlation reflects a causal relationship between maternal corticosteroids and offspring sex ratio. Hormone-implanted females produced more female embryos than control females. These findings provide the first evidence of a natural correlation between maternal corticosteroids and offspring sex ratios in free-ranging birds, and the first experimental evidence of a causal link between moderate increases in corticosteroids and biased primary sex ratios. Copyright 2007, Oxford University Press.
  • Article
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    Extrapair paternity (EPP) has proved to be widespread and highly variable among birds and other taxa, including socially monogamous species. A multitude of hypotheses have been put forward to explain this variation, but its occurrence is not fully understood. Male age, social dominance rank, song and breeding density or synchrony have been among the suggested correlates of EPP, but results so far are inconclusive. We interspecifically cross-fostered blue tits (Parus caeruleus) and great tits (Parus major) in the wild, thus manipulating males to exhibit reduced social dominance rank, sing aberrant songs, and consequently be perceived as low-quality males as compared to controls. This allowed us to test if male quality had an influence on loss of paternity. We found no statistically significant differences between cross-fostered and control males of either species, neither with respect to levels of cuckoldry nor proportions of extrapair young (EPY) in the broods. Paternity levels were comparable to other studies on the same species. No effect of density could be detected on levels of EPP either, while an age effect seemed to be present at least in the blue tit, EPY being almost absent in broods of older blue tit males. We conclude that the effects of male quality on paternity loss are minor, if any, in these populations. Copyright 2005.
  • Article
    Empirical studies of sex ratios in birds have been limited due to difficulties in determining offspring sex. Since molecular sexing techniques removed this constraint, the last 5 years has seen a great increase in studies of clutch sex ratio manipulation by female birds. Typically these studies investigate variation in clutch sex ratios across individuals in relation to environmental characteristics or parental traits, and often they find no relationships. In this study we also found that clutch sex ratios did not vary in relation to a number of biological and environmental factors for 238 great tit Parus major nests. However, interesting sex ratio biases were revealed when variation in clutch sex ratios was analyzed within individual females breeding in successive years. There was a significant positive relationship between the change in sex ratio of a female's clutch from one year to the next and the relative body condition of her partner. Females mating with males of higher body condition in year x + 1 produced relatively male-biased sex ratios, and the opposite was true for females mated with lower condition males. Within-individual analysis also allowed investigations of sex ratio in relation to partner change. There was no change in sex ratios of females pairing with the same male; however, females pairing with a new male produced clutches significantly more female biased. Comparisons of clutch sex ratios within individuals may be a powerful method for detecting sex ratio variation, and perhaps female birds may indeed manipulate egg sex but require personal contextual experience for such decisions. Copyright 2002.
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    Social dominance influences the outcome of competitive interactions over limited resources, and may hence be important for individual fitness. Theory thus predicts that its heritability will be low and that non-genetic determinants of dominance should prevail. In this field experiment we reciprocally cross-fostered great tits (Parus major) to blue tits (Parus caeruleus) to investigate the impact of early social experience on dominance status in competition over food during winter. Controlling for potential effects of age, size, sex and site-related dominance, we show that cross-fostered birds of both species were subdominant to conspecific immigrants, while controls originating from unmanipulated broods were dominant to conspecific immigrants. Furthermore, blue tits reared by blue tit parents but with at least one great tit broodmate had lower dominance status relative to conspecific immigrants than did controls. Although great tits generally dominated blue tits, cross-fostered birds of both species initiated marginally more fights against the other species than did their respective controls, suggesting faulty species recognition. Since both social parents and broodmates strongly influence the dominance behavior of offspring later in life, we conclude that social conditions experienced at an early age are crucial for the determination of subsequent social dominance. Copyright 2004.
  • Article
    Sex allocation theory, and its success in predicting sex ratios in such taxa as parasitoid wasps, is often cited as one of the crowning achievements of theoretical evolutionary biology. Its success in some vertebrate taxa, particularly birds, has been more modest. I discuss two reasons for this. First, it is difficult to obtain avian sex ratio data before substantial offspring mortality has occurred. Second, the theory and data required to predict sex allocation patterns (let alone sex ratio patterns) in vertebrates are complex and hard to obtain. Recently developed molecular genetic techniques allowing sex identification from DNA samples have largely solved the first problem and there have been several striking empirical demonstrations of sex ratio biases consistent with sex allocation theory in wild bird populations. Solution of the second problem may come with the incorporation of realistic life history data into models and the use of experimental manipulations to reveal the fitness consequences of allocation strategies. Further data concerning sex ratio variation in taxa such as birds, with chromosomal sex determination, are valuable because they allow the investigation of the role of constraint vs. adaptation in evolution.