Ecological causes for the evolution of sexual dimorphism: a review of the evidence

Can sexual dimorphism evolve because of ecological differences between the sexes? Although several examples of this phenomenon are well known from studies on birds, the idea has often been dismissed as lacking general applicability. This dismissal does not stem from contradictory data so much as from the difficulties inherent in testing the hypothesis, and its apparent lack of parsimony, in comparison to the alternative explanation of sexual selection. The only unequivocal evidence for the evolution of sexual dimorphism through intersexual niche partitioning would be disproportionate dimorphism in trophic structures (e.g., mouthparts). This criterion offers a minimum estimate of the importance of ecological causes for dimorphism, because it may fail to identify most cases. A review of published literature reveals examples of sexually dimorphic trophic structures in most animal phyla. Many of these examples seem to be attributable to sexual selection, but others reflect adaptations for niche divergence between the sexes. For example, dwarf non-feeding males without functional mouthparts have evolved independently in many taxa. In other cases, males and females differ in trophic structures apparently because of differences in diets. Such divergence may often reflect specific nutritional requirements for reproduction in females, or extreme (sexually selected?) differences between males and females in habitats or body sizes. Ecological competition between the sexes may be responsible for intersexual niche divergence in some cases, but the independent evolution of foraging specializations by each sex may be of more general importance. If ecological causation for dimorphism can be demonstrated in so many cases, despite the inadequacies of the available criteria, the degree of sexual size dimorphism in many other animal species may well also have been influenced by ecological factors. Hence, it may be premature to dismiss this hypothesis, despite the difficulty of testing it.     

Why are American mink sexually dimorphic? A role for niche separation

American mink are highly sexually dimorphic, with males being up to twice the size of females. Sexual dimorphism may arise for several reasons, including intra- or inter-sexual selection, inter-sexual competition, or divergent reproductive roles. Whether or not dimorphism arises from competition, a degree of niche separation is expected in dimorphic species. Sexual divergence in feeding niche has been reported for many species, including mink. This is likely to be manifested in a greater degree of dimorphism in those structures, such as teeth, that are used for the acquisition of prey. We tested the hypothesis that teeth and other trophic structures of male mink would be significantly larger than those of females, after controlling for underlying skeletal size differences. Canine and carnassial teeth, and several skull dimensions, were larger than predicted in males. There is good evidence that sexual dimorphism in mink trophic apparati is greater than predicted from allometry. We examined the development of dimorphism in various features with age and found that it was not consistent. Several trophic features were dimorphic amongst juveniles, and the degree of dimorphism remained relatively constant with age. Dimorphism in canines, and in relative body mass, was less apparent amongst juveniles and increased with increasing age. We discuss our results in the light of contemporary theories on the evolution and maintenance of sexual size dimorphism and argue that niche separation as a result of dimorphism in trophic features, while probably not the driving force behind sexual size dimorphism, may play a role in its maintenance.     

Seasonal sexual segregation in two Thalassarche albatross species: competitive exclusion,reproductive role specialization or foraging niche divergence?

Sexual segregation by micro- or macrohabitat is common in birds, and usually attributed to size-mediated dominance and exclusion of females by larger males, trophic niche divergence or reproductive role specialization. Our study of black-browed albatrosses, Thalassarche melanophrys, and grey-headed albatrosses, T. chrysostoma, revealed an exceptional degree of sexual segregation during incubation, with largely mutually exclusive core foraging ranges for each sex in both species. Spatial segregation was not apparent during brood-guard or post-guard chick rearing, when adults are constrained to feed close to colonies, providing no evidence for dominance-related competitive exclusion at the macrohabitat level. A comprehensive morphometric comparison indicated considerable species and sexual dimorphism in wing area and wing loading that corresponded, both within and between species, to broad-scale habitat preferences relating to wind strength. We suggest that seasonal sexual segregation in these two species is attributable to niche divergence mediated by differences in flight performance. Such sexual segregation may also have implications for conservation in relation to sex-specific overlap with commercial fisheries.     

Does sexual segregation occur during the nonbreeding period? A comparative analysis in spatial and feeding ecology of three Calonectris shearwaters

Sexual segregation (SS) is widespread among animal taxa, with males and females segregated in distribution, behavior, or feeding ecology but so far, most studies on birds have focused on the breeding period. Outside this period, the relevance of segregation and the potential drivers of its persistence remain elusive, especially in the marine environment, where animals can disperse over vast areas and are not easily observed. We evaluated the degree of SS in spatio‐temporal distribution and phenology, at‐sea behavior, and feeding ecology during the nonbreeding period among three closely related shearwaters: Scopoli's, Cory's, and Cape Verde shearwaters (Calonectris diomedea, C. borealis, and C. edwardsii, respectively). We tracked 179 birds (92 males and 87 females) from 2008 to 2013 using geolocation‐immersion loggers and collected the 13th secondary remige (molted in winter) for stable isotope analyses as a proxy of trophic level and diet. The global nonbreeding distribution did not differ between sexes for the three species, but one specific nonbreeding area was visited only by males. Cory's shearwater males remained in areas closer to the colony in a larger proportion compared to females and returned earlier to the colony, probably to defend their nests. Males presented a slightly lower nocturnal flying activity and slightly (but consistently) higher isotopic values of δ¹³C and δ¹⁵N compared to females. These differences suggest subtle sexual differences in diet and a slightly higher trophic level in males, but the extent to which sexual dimorphism in bill size can determine them remains unclear. Our study showed that SS in ecological niche in seabirds can persist year‐round consistently but at a different extent when comparing the breeding and nonbreeding periods. Based on our findings, we propose that SS in these seabird species might have its origin in an ecological specialization derived from the different roles of males and females during reproduction, rather than from social dominance during the nonbreeding period.     

Sexual size dimorphism in shorebirds, gulls, and alcids: the influence of sexual and natural selection

Abstract. Charadrii (shorebirds, gulls, and alcids) have an unusual diversity in their sexual size dimorphism, ranging from monomorphism to either male-biased or female-biased dimorphism. We use comparative analyses to investigate whether this variation relates to sexual selection through competition for mates or natural selection through different use of resources by males and females. As predicted by sexual selection theory, we found that in taxa with socially polygynous mating systems, males were relatively larger than females compared with less polygynous species. Furthermore, evolution toward socially polyandrous mating systems was correlated with decreases in relative male size. These patterns depend on the kinds of courtship displays performed by males. In taxa with acrobatic flight displays, males are relatively smaller than in taxa in which courtship involves simple flights or displays from the ground. This result remains significant when the relationship with mating system is controlled statistically, thereby explaining the enigma of why males are often smaller than females in socially monogamous species. We did not find evidence that evolutionary changes in sexual dimorphism relate to niche division on the breeding grounds. In particular, biparental species did not have greater dimorphism in bill lengths than uniparental species, contrary to the hypothesis that selection for ecological divergence on the breeding grounds has been important as a general explanation for patterns of bill dimorphism. Taken together, these results strongly suggest that sexual selection has had a major influence on sexual size dimorphism in Charadrii, whereas divergence in the use of feeding resources while breeding was not supported by our analyses.     

Seasonal dimorphism in the horny bills of sparrows

Bill size is often viewed as a species‐specific adaptation for feeding, but it sometimes varies between sexes, suggesting that sexual selection or intersexual competition may also be important. Hypotheses to explain sexual dimorphism in avian bill size include divergence in feeding niche or thermoregulatory demands, intrasexual selection based on increased competition among males, or female preference. Birds also show seasonal changes in bill size due to shifts in the balance between growth rate and wear, which may be due to diet or endogenous rhythms in growth. Insight into the function of dimorphism can be gained using the novel approach of digital x‐ray imaging of museum skins to examine the degree to which the skeletal core or the rhamphotheca contribute to overall dimorphism. The rhamphotheca is ever‐growing and ever‐wearing, varying in size throughout life; whereas the skeletal core shows determinant growth. Because tidal marsh sparrows are more dimorphic in bill size than related taxa, we selected two marsh taxa to investigate dimorphism and seasonality in the size of the overall bill, the skeletal core, and the rhamphotheca. Bill size varied by sex and season, with males having larger bills than females, and bill size increasing from nonbreeding to breeding season more in males. Skeletal bill size varied with season, but not sex. The rhamphotheca varied primarily with sex; males had a larger rhamphotheca (corrected for skeletal bill size), which showed a greater seasonal increase than females. The rhamphotheca, rather than the skeletal bill, was responsible for sexual dimorphism in overall bill size, which was particularly well developed in the breeding season. The size of the rhamphotheca may be a condition‐based character that is shaped by sexual selection. These results are consistent with the evidence that bill size is influenced by sexual selection as well as trophic ecology.     

Vive le difference! Sexual dimorphism and adaptive patterns in lizards of the genus Anolis

The repeated, convergent evolution of body shape and microhabitat use in Greater Antillean lizards of the genus Anolis (anoles) provides compelling evidence of the importance of microhabitat specialization in shaping morphology. Interestingly, sexual dimorphism is also extensive, with males and females differing in body size as well as in shape. It is important to note that the components of shape analyzed in these studies is related to locomotion and are size-adjusted, including: relative limb and body lengths and mass of the body. Numbers of lamellae were also used and these do not vary with size. Furthermore, dimorphism in both size and shape differs by habitat type. Thus, does functionally-relevant sexual dimorphism imply that one sex is the "ecological" sex, with the other being maladapted to it's environment? Alternatively, sexual dimorphism may interact with adaptive diversification. Different classes of individuals within a species may act as separate ecological units if they play ecologically different roles. Here, I reanalyze a data set of morphological data for 15 species of Puerto Rican and Jamaican Anolis, which represent two largely independent adaptive radiations of lizards. I test for concordance between size and shape dimorphism and microhabitat (ecomorph) type, and for "parallel" patterns of sexual dimorphism among species. I integrate these results and, in the light of previous research, evaluate the relative influence that larger-scale ecological patterns have on sexual dimorphism, as well as the influence of sexual dimorphism on community structuring. I conclude that the presence of ecologically-relevant dimorphism may in fact increase the adaptive diversity present within a community.     

Evolution of sexual dimorphism in bill size and shape of hermit hummingbirds (Phaethornithinae): a role for ecological causation

Unambiguous examples of ecological causation of sexual dimorphism are rare, and the best evidence involves sexual differences in trophic morphology. We show that moderate female-biased sexual dimorphism in bill curvature is the ancestral condition in hermit hummingbirds (Phaethornithinae), and that it is greatly amplified in species such as Glaucis hirsutus and Phaethornis guy, where bills of females are 60 per cent more curved than bills of males. In contrast, bill curvature dimorphism is lost or reduced in a lineage of short-billed hermit species and in specialist Eutoxeres sicklebill hermits. In the hermits, males tend to be larger than females in the majority of species, although size dimorphism is typically small. Consistent with earlier studies of hummingbird feeding performance, both raw regressions of traits and phylogenetic independent contrasts supported the prediction that dimorphism in bill curvature of hermits is associated with longer bills. Some evidence indicates that differences between sexes of hermit hummingbirds are associated with differences in the use of food plants. We suggest that some hermit hummingbirds provide model organisms for studies of ecological causation of sexual dimorphism because their sexual dimorphism in bill curvature provides a diagnostic clue for the food plants that need to be monitored for studies of sexual differences in resource use.     

Selection on females can create 'larger males'

In many bird and mammal species, males are significantly larger than females. The prevailing explanation for larger-sized males is that sexual selection drives increased male size. In addition, researchers commonly assume that the extent of dimorphism indicates the strength of selection for increased size in males. Here, through reconstruction of ancestral morphology for males and females of one large avian clade we present data that contradict this assumption and illustrate that selection for decreased female size explains 'male-biased' dimorphism ca. 50% of the time. Our findings are also inconsistent with ecological niche partitioning between the sexes and increased breeding benefits from reduced female size as general explanations for the evolution of size dimorphism within the clade. We conclude that it is incorrect to assume sexual dimorphism results from a single selective factor, such as directional sexual selection on increased male size. Rather, we suggest that the selective forces leading to sexual dimorphism may vary between species and should be tested on a case-by-case basis using a phylogenetic approach.     

Sexual size dimorphism in greater mouse‐eared bat Myotis myotis (Chiroptera: Vespertilionidae) from a Mediterranean region

Although sexual size dimorphism (SSD) is common among mammals, there is no clear explanation for its maintenance in nature. Bats are one of the few groups of mammals where reverse SSD appears. In this group, the size of individuals may have very important ecological consequences related with flight. In this study, we examine sexual dimorphism in the wing measurements of 195 adult individuals (141 males and 54 females) of the greater mouse‐eared bat Myotis myotis in the south‐east of the Iberian Peninsula. We also investigated size differences between paired and single males in a swarming roost. The results indicate that there are significant differences in the wing measurements between sexes, females being bigger than males in this respect. While no significant differences in the wing measurements of paired and single males were observed, significant differences were found in their relative weight and fitness, single males being significantly heavier and having a better physical condition. We discuss the implications of SSD for the female of M. myotis in terms of reproductive advantages, trophic niche segregation and a greater ability to move, which may favour gene flow between populations.     

Causes of secondary sexual differences in plants — Evidence from extreme leaf dimorphism in Leucadendron (Proteaceae)

In extreme cases leaves in male plants of the dioecious genus Leucadendron (Proteaceae) are up to an order of magnitude smaller than female leaves. This secondary sexual dimorphism (SSD) in leaf size has previously been suggested to be due to intra-male sexual selection, leading to an increase in male allocation to reproduction in dimorphic species. After critically evaluating previous data provided to support this hypothesis, I suggest on both theoretical grounds and on re-analysis that this argument is unlikely and unsupported. Leaf size dimorphism could theoretically evolve directly due to disruptive ecological selection between genders, leading to niche dimorphism either within or between habitats. I test this ecological causation hypothesis by providing data on specific leaf area (sla) and water use efficiency (δ ¹³C) of leaves from males and females of several Leucadendron species. Results confirm the expectation of minimal gender differences. I argue that leaf dimorphism is a consequence of selection on flower size and architecture.     

Variation in the diet of the viperine snake Natvix mama in relation to prey availability

The diet of the viperine snake was compared with food availability in the Ebro Delta, a wetland largely occupied by rice fields, in 1990 and 1991. Snake selection of prey type and size was studied seasonally and by snake group: males, females and immature snakes. Overall, feeding activity (percentage of individuals with prey and number of prey per stomach) increased with food availability. Diet analysis showed that viperine snakes mainly foraged on the green frog Rana perezi (adults and tadpoles) and the carp Cyprinus earpio. Conversely, viperine snakes rejected the mosquito fish Gambusia holbroki which is the most abundant species in autumn, when Natrix maura has a low feeding activity. Statistical comparisons between viperine snake diet and prey availability showed that males selected small carp, immature snakes selected tadpoles and, in spring, females selected frogs. The selection of small carp by males may reflect a sexual divergence of trophic niche related to sexual size dimorphism, as females are larger than males. As tadpoles are presumably easier to catch than fish, tadpole selection by immature individuals may reflect variance in capture abilities. In spring, the selection of frogs by females overlapped with vitellogenesis, suggesting that females compensate for the cost of reproduction by selecting green frogs, which have a greater biomass and higher energy content than fish. Carps eaten in spring were smaller than in summer. Moreover, in summer viperine snakes selected smaller carp than the available mean size. This divergent tendency between carp size selection and carp size availability reveals how seasonal diet shifts in prey size selection may be a response to an increase in prey size.     

Sexual differences in head form and diet in a population of Mexican lance‐headed rattlesnakes,Crotalus polystictus

Sexual dimorphism of phenotypic traits associated with resource use is common in animals, and may result from niche divergence between sexes. Snakes have become widely used in studies of the ecological basis of sexual dimorphism because they are gape‐limited predators and their head morphology is likely to be a direct indicator of the size and shape of prey consumed. We examined sexual dimorphism of body size and head morphology, as well as sexual differences in diet, in a population of Mexican lance‐headed rattlesnakes, Crotalus polystictus, from the State of México, Mexico. The maximum snout–vent length of males was greater than that of females by 21%. Males had relatively larger heads, and differed from females in head shape after removing the effects of head size. In addition, male rattlesnakes showed positive allometry in head shape: head width was amplified, whereas snout length was truncated with increased head size. By contrast, our data did not provide clear evidence of allometry in head shape of females. Adults of both males and females ate predominately mice and voles; however, males also consumed a greater proportion of larger mammalian species, and fewer small prey species. The differences in diet correspond with dimorphism in head morphology, and provide evidence of intersexual niche divergence in the study population. However, because the sexes overlapped greatly in diet, we hypothesize that diet and head dimorphisms in C. polystictus are likely related to different selection pressures in each sex arising from pre‐existing body size differences rather than from character displacement for reducing intersexual competition. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 633–640.     

Mating systems, sperm competition, and the evolution of sexual dimorphism in birds

Comparative analyses suggest that a variety of factors influence the evolution of sexual dimorphism in birds. We analyzed the relative importance of social mating system and sperm competition to sexual differences in plumage and body size (mass and tail and wing length) of more than 1,000 species of birds from throughout the world. In these analyses we controlled for phylogeny and a variety of ecological and life-history variables. We used testis size (corrected for total body mass) as an index of sperm competition in each species, because testis size is correlated with levels of extrapair paternity and is available for a large number of species. In contrast to recent studies, we found strong and consistent effects of social mating system on most forms of dimorphism. Social mating system strongly influenced dimorphism in plumage, body mass, and wing length and had some effect on dimorphism in tail length. Sexual dimorphism was relatively greater in species with polygynous or lekking than monogamous mating systems. This was true when we used both species and phylogenetically independent contrasts for analysis. Relative testis size was also related positively to dimorphism in tail and wing length, but in most analyses it was a poorer predictor of plumage dimorphism than social mating system. There was no association between relative testis size and mass dimorphism. Geographic region and life history were also associated with the four types of dimorphism, although their influence varied between the different types of dimorphism. Although there is much interest in the effects of sperm competition on sexual dimorphism, we suggest that traditional explanations based on social mating systems are better predictors of dimorphism in birds.     

WHY SHOULD LEK-BREEDERS BE MONOMORPHIC?

Approximately one-quarter of all lek-breeding bird species are sexually monomorphic. Understanding the significance, if any, of this exception to the usual correlation between sexual selection and dimorphism requires detailed data on the mating systems of both monomorphic and dimorphic species. The capuchinbird (Perissocephalus tricolor) is a sexually monomorphic, lek-breeding member of the cotinga family. I studied the social and sexual behavior of this species, and compared it with the Guianan cock-of-the-rock (Rupicola rupicola), a dimorphic, lekking member of the same family. Male–male competition in capuchinbirds involved direct contests for dominance, rather than territorial displays as in classic lek species. In each year, one dominant individual was able to control the most desired display site on the 8-male lek, and was the only male that copulated. In contrast to dimorphic lek birds, female as well as male capuchinbirds engaged in frequent and intense aggression at the lek, and both males and females engaged in sexual mimicry. I suggest that plumage monomorphism in lek birds has evolved as a result of social competition affecting both sexes. This hypothesis accounts for the exaggerated plumage characters shared by males and females in capuchinbirds and a number of other monomorphic lek birds. The evolution of plumage can best be analyzed as an arms race, in which the balance of selective forces acting on each sex can produce a variety of equilibrium states, ranging from sexual indistinguishability to extreme dimorphism.     

Intersexual trophic niche partitioning in an ant-eating spider (Araneae: Zodariidae)

Background

Divergence in trophic niche between the sexes may function to reduce competition between the sexes (“intersexual niche partitioning hypothesis”), or may be result from differential selection among the sexes on maximizing reproductive output (“sexual selection hypothesis”). The latter may lead to higher energy demands in females driven by fecundity selection, while males invest in mate searching. We tested predictions of the two hypotheses underlying intersexual trophic niche partitioning in a natural population of spiders. Zodarion jozefienae spiders specialize on Messor barbarus ants that are polymorphic in body size and hence comprise potential trophic niches for the spider, making this system well-suited to study intersexual trophic niche partitioning.

Methodology/Principal Findings

Comparative analysis of trophic morphology (the chelicerae) and body size of males, females and juveniles demonstrated highly female biased SSD (Sexual Size Dimorphism) in body size, body weight, and in the size of chelicerae, the latter arising from sex-specific growth patterns in trophic morphology. In the field, female spiders actively selected ant sub-castes that were larger than the average prey size, and larger than ants captured by juveniles and males. Female fecundity was highly positively correlated with female body mass, which reflects foraging success during the adult stage. Females in laboratory experiments preferred the large ant sub-castes and displayed higher capture efficiency. In contrast, males occupied a different trophic niche and showed reduced foraging effort and reduced prey capture and feeding efficiency compared with females and juveniles.

Conclusions/Significance

Our data indicate that female-biased dimorphism in trophic morphology and body size correlate with sex-specific reproductive strategies. We propose that intersexual trophic niche partitioning is shaped primarily by fecundity selection in females, and results from sex-differences in the route to successful reproduction where females are selected to maximize energy intake and fecundity, while males switch from foraging to invest in mating effort.     

Does size dimorphism reduce competition between sexes? The diet of male and female pine martens at local and wider geographical scales

Sex-specific niche segregation is often used to explain sexual size dimorphism (SSD). However, whether food niche partitioning between sexes occurs as a case of sexual size dimorphism or by other mechanisms, such as behavioural dimorphism or habitat segregation, remains poorly understood. To evaluate the nature and extent of food-niche differentiation between sexes in a solitary predator I examined variation in the diet of male and female pine martensMartes martes Linnaeus, 1758 in years of high and low rodent abundance. Small mammals were the most important prey for pine martens in years of both low and high rodent abundance (occurring in more than 49% of scats). Birds, invertebrates and plant material were relatively common food items in summer diet, whereas ungulate carcasses were often consumed in autumn—winter. In general, males consumed more ungulate carcasses, plant material, amphibians and reptiles than did females, whereas females preyed more on squirrels and birds than males. There was significant seasonally dependent, between-sex variation in the occurrence of shrews, small rodents, other mammals, birds and invertebrates in marten diet. Whereas the occurrence of bank vole, birds, carcasses and plant material changed between sexes, seasons and years with various rodent abundances, both sexes consumed larger prey and had increased food niche breadth in years of low compared with high rodent abundance. Neither prey size nor food niche breadth were significantly different between males and females. The food-niche overlap between sexes was consistently lower in spring and in years of low rodent abundance. A wider geographical comparison of different marten populations showed that the diet of males and females varied significantly between locations. Females consistently preyed on squirrels and birds, whereas males fed more often on ungulate carcasses and plant material. Local and geographical comparison of male and female diets suggest that food-niche partitioning between male and female pine martens changes across different habitat and food conditions, and is not related to sexual size dimorphism, but rather to behavioural differences between sexes.     

Habitat- and sex-related differences in a small carnivore’s diet in a competitor-free environment

The alien invasive American mink Neovison vison is fully established in the low species richness and competitor-free environment of Iceland. This study documents the diversity as well as seasonal and sexual variation in the diet of mink in Iceland based on stomach contents. Seasonal changes mainly reflected variation in abundance of migratory birds and wood mice Apodemus sylvaticus. In comparison with mink elsewhere in similar habitats, the mink in Iceland consumed more fish and birds and fewer mammals, which is in accordance with local availability. This reinforces evidence of opportunistic foraging. Females generally ate more fish and fewer birds than males and this might be explained by their smaller body size and possible limitation in catching larger birds. Mink in coastal habitats showed greater sexual differences in diet than mink in riparian habitats, probably reflecting less prey diversity in riparian habitats than coastal ones. This study is an input towards explaining the ecological consequences of sexual size dimorphism and supports the hypothesis that generalist species might be successful invaders due to their capability in utilising new and diverse resources. The mink in Iceland can be regarded as a model for a small-bodied semi-aquatic carnivore away from the confounding effects of inter-specific competition.     

Variance‐Sensitive Green Woodhoopoes: A New Explanation for Sex Differences in Foraging?

Studies of cooperatively breeding birds rarely benefit from the extensive research on adaptive foraging behaviour, despite the potential for concepts such as state‐dependent foraging to explain many aspects of behaviour in social groups. For example, sex differences in preferred foraging techniques used by green woodhoopoes, Phoeniculus purpureus, have previously been explained by sexual dimorphism in bill length and the benefits afforded by foraging specialization and niche differentiation within cooperative groups. Contrary to this argument, there were no sex differences in mean foraging success and/or prey size captured when males and females used the same foraging techniques. Subordinates of both sexes did experience lower and more varied foraging success compared with dominants, but probably only as a consequence of competition or inexperience. However, dominant males experienced greater variance in individual foraging success compared with dominant females, and dominant males also experienced greater variances in prey size when using their preferred foraging techniques. Dominant males therefore appeared to specialize in foraging techniques that provided more variable rewards, whilst dominant females consistently chose to minimize variation in reward. Dominant females also experienced less variance in foraging returns when using the same techniques as males, suggesting a possible link with sexual dimorphism in bill length. Partitioning of foraging niches in dominant green woodhoopoes therefore appears to be better explained by sex differences in variance (risk) sensitivity to foraging rewards. We suggest that this kind of detailed analysis of state‐dependent foraging has the potential to explain many of the crucial age and sex differences in behaviour within cooperative groups.     

Absence of protandry in the spring migration of a population of Song Sparrows Melospiza melodia

Early male arrival at breeding sites, or protandry, is thought to have evolved from intrasexual competition among males for access to mates or breeding resources. Males of polygynous species tend to be larger than females and have exaggerated secondary sexual traits. Additionally, such species show a high degree of protandry, suggesting that timing of arrival is sexually selected. Species showing limited sexual dimorphism and showing sexual monochromatism may be expected to show limited early male arrival. However, there are very few studies of migration timing of the sexes in such species because individuals cannot be readily identified to sex in the hand. In this study, we genetically sexed birds and found no evidence for early male arrival, for a population of migratory Song Sparrows Melospiza melodia . For our study population, males and females display limited sexual size dimorphism and are sexually monochromatic which is characteristic of the species. Fat scores for males were inversely associated with timing of arrival, whereas for females, larger-winged birds arrived sooner – suggesting that early migration timing may be selected for in both sexes.