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.     

It takes two: Seasonal variation in sexually dimorphic weaponry results from divergent changes in males and females

Sexually dimorphic weaponry often results from intrasexual selection, and weapon size can vary seasonally when costs of bearing the weapon exceed the benefits outside of the reproductive season. Weapons can also be favored in competition over nonreproductive resources such as food or shelter, and if such nonreproductive competition occurs year‐round, weapons may be less likely to vary seasonally. In snapping shrimp (Alpheus angulosus), both sexes have an enlarged snapping claw (a potentially deadly weapon), and males of many species have larger claws than females, although females are more aggressive. This contrasting sexual dimorphism (larger weaponry in males, higher aggression in females) raises the question of whether weaponry and aggression are favored by the same mechanisms in males and females. We used field data to determine whether either sex shows seasonal variation in claw size such as described above. We found sexual dimorphism increased during the reproductive season due to opposing changes in both male and female claw size. Males had larger claws during the reproductive season than during the nonreproductive season, a pattern consistent with sexual selection. Females, however, had larger claws during the nonreproductive season than during the reproductive season—a previously unknown pattern of variation in weapon size. The observed changes in female weapon size suggest a trade‐off between claw growth and reproduction in the reproductive season, with investment in claw growth primarily in the nonreproductive season. Sexually dimorphic weaponry in snapping shrimp, then, varies seasonally due to sex differences in seasonal patterns of investment in claw growth, suggesting claws may be advantageous for both sexes but in different contexts. Thus, understanding sexual dimorphisms through the lens of one sex yields an incomplete understanding of the factors favoring their evolution.     

Effects of Sexual Dimorphism and Landscape Composition on the Trophic Behavior of Greater Prairie-Chicken

Partitioning of ecological niche is expected in lekking species that show marked sexual size dimorphism as a consequence of sex-specific ecological constraints. However, niche partitioning is uncertain in species with moderate sexual dimorphism. In addition, the ecological niche of a species may also be affected by landscape composition; particularly, agricultural fragmentation may greatly influence the trophic behavior of herbivores. We studied trophic niche variation in Greater Prairie-Chickens (Tympanuchus cupido), a grouse species that shows moderate sex-dimorphism. Greater Prairie-Chickens are native to tallgrass prairies of North America, although populations persist in less natural mosaics of cropland and native habitats. We used stable isotope analysis of carbon and nitrogen in blood, claws and feathers to assess seasonal differences in trophic niche breadth and individual specialization between male and female Greater Prairie-Chickens, and between birds living in continuous and fragmented landscapes. We found that females showed broader niches and higher individual specialization than males, especially in winter and autumn. However, differences between females and males were smaller in spring when birds converge at leks, suggesting that females and males may exhibit similar feeding behaviors during the lekking period. In addition, we found that birds living in native prairies showed greater annual trophic variability than conspecifics in agricultural mosaic landscapes. Native habitats may provide greater dietary diversity, resulting in greater diversity of feeding strategies.     

The moults of captive Scottish ptarmigan (Lagopus mutus)

The moults of captive Scottish ptarmigan were studied at Banchory, north-east Scotland from December 1968 to February 1971. In the autumn moult (June to September) which included the primaries, cock ptarmigan moulted earlier and more completely than hens. In the winter moult (September to February) hens moulted earlier and both sexes moulted more completely than in spring. In the spring moult (February to June) cocks moulted more rapidly to begin with but by mid-April hens had caught up and thereafter moulted at least as rapidly as cocks. When kept indoors at slightly higher temperatures ptarmigan grew more pigmented feathers during the winter moult. In a colder winter the birds became whiter than in a milder one. First-winter ptarmigan completed the winter moult later than older birds. Birds from the Cairnwell hills had more dark feathers in winter than those from the eastern Cairngorms. There was no correlation between the start or finish of egg-laying and moulting.     

Molecular Evidence for Gender Differences in the Migratory Behaviour of a Small Seabird

Molecular sexing revealed an unexpectedly strong female bias in the sex ratio of pre-breeding European Storm Petrels (Hydrobates pelagicus), attracted to playback of conspecific calls during their northwards migration past SW Europe. This bias was consistent across seven years, ranging from 80.8% to 89.7% female (mean annual sex ratio ± SD = 85.5% female ±4.1%). The sex ratio did not differ significantly from unity (i.e., 50% female) among (i) Storm Petrel chicks at a breeding colony in NW France, (ii) adults found dead on beaches in Southern Portugal, (iii) breeding birds attending nest burrows in the UK, captured by hand, and (iv) adults captured near a breeding colony in the UK using copies of the same sound recordings as used in Southern Europe, indicating that females are not inherently more strongly attracted to playback calls than males. A morphological discriminant function analysis failed to provide a good separation of the sexes, showing the importance of molecular sexing for this species. We found no sex difference in the seasonal or nocturnal timing of migration past Southern Europe, but there was a significant tendency for birds to be caught in sex-specific aggregations. The preponderance of females captured in Southern Europe suggests that the sexes may differ in migration route or in their colony-prospecting behaviour during migration, at sites far away from their natal colonies. Such differences in migration behaviour between males and females are poorly understood but have implications for the vulnerability of seabirds to pollution and environmental change at sea during the non-breeding season.     

Sex, size and colour in a semi-terrestrial crab, Heloecius cordiformis (H. Milne Edwards, 1837)

We investigated the relationship between sex, size and colour in the little studied Australian endemic semaphore crab, Heloecius cordiformis, and related it to the crabs' social system with the aim of identifying the potential signalling function of claw colour.Equal sampling of crabs from all size classes revealed a strong relationship between sex, size and claw colour. Purple-clawed males were larger and had larger claws than pink-, orange- or green-clawed males. Male claws showed positive allometric growth: relative to body size, purple-clawed males had larger claws. The largest females had pink claws; the few with purple claws were no larger than immature green-clawed crabs. Female claws grow isometrically with the body so the relative claw size did not differ among the female colour classes. Quantitative measurements of claw colour revealed spectral differences between these subjectively described colours. The purple claws typical of large males also contrasted more strongly against the mudflat background than the other colours.Heloecius copulate outside female-owned burrows and probably within male-owned burrows. The male's waving display, in which both claws are raised and lowered, may feature in both mating strategies: as a territorial display and to attract wandering females. Large males are competitively superior so size, and potentially colour, are important in territorial disputes and may also feature in mate choice.     

Seasonal and between-sex differences in the diet of Black Grouse Tetrao tetrix

Capsule Dietary differences between sexes and seasons reflected diversity in plant availability and habitat preferences.

Aims To analyse Black Grouse diet through the identification of plant and invertebrate material found in the crop.

Methods Crops were dissected and the content categorized into plant species and components (i.e. leaf, stem, flower, fruit and seed). Multivariate analysis was used to test for seasonal, sex- or location-related differences in the diet.

Results Plant fragments accounted for 98% of the diet and included 53 plant species or taxa. Invertebrates made up the remaining 2%. Diet varied significantly between seasons and sexes. Both sexes ate more ericaceous shrubs in autumn and winter, with females eating more than males. The plant parts eaten varied seasonally. In summer, fruits, flowers and seeds were favoured over leaves, which dominated in winter.

Conclusions With few trees, birds were reliant upon Heather in autumn and winter.     

Breeding site fidelity in willow ptarmigan: the influence of previous reproductive success and familiarity with partner and territory

Summary Breeding site fidelity is high in willow ptarmigan: only 9% of males and 31% of females switched territories between years. Unpaired males were more likely to switch territories than paired males. For paired males, survival of their previous partner and reproductive success in year x did not influence probability of switching in year x+1. A female was more likely to switch territories if her previous partner disappeared. If her partner returned, she had a higher probability of switching if she did not produce chicks the previous year. Most hens moved to the territories of older males, although hens paired with unfamiliar older males did not have higher reproductive success than those paired with yearlings. Individuals that paired with their previous partner laid earlier and produced heavier chicks than those paired with unfamiliar partners. Excluding birds paired with familiar partners, survival and reproductive success in year x+1 was similar for males and females that did or did not switch territories. Males had a higher probability of producing chicks after switching than before, but females were more likely to lose their clutch after switching. For both sexes, birds that switched territories were as successful as the birds that replaced them on their former territories. We conclude that high site fidelity in willow ptarmigan is maintained because of the benefits of pairing with a familiar partner.     

The effect of social dominance on foraging by the Three-toed Woodpecker Picoides tridactylus

During the winter, female Three-toed Woodpeckers Picoides tridactylus , when unaccompanied by the male, foraged at a lower height above the ground and on tree trunks of greater diameter than when foraging together with a male. Among males, however, no such differences were found between those birds foraging in the absence of, or in company with, a female. The niche breadth of the males was less than that of the females, indicating a higher degree of specialization by the males. The niche overlap between the sexes was less when the birds foraged together in pairs, especially as regards foraging height and foraging site diameter. The intersexual segregation of foraging niche recorded for the Three-toed Woodpecker is therefore probably not determined genetically, but is due to social dominance by males which obliges the females to occupy the less preferred niche. Both sexes spent less time being vigilant when foraging together, than when alone.     

Sexual differences in locomotor performance in Tropidurus catalanensis lizards (Squamata: Tropiduridae) – body shape,size and limb musculature explain variation between males and females

Sexual dimorphism (SD) is the evolutionary outcome of selection acting differently on males and females. Several studies describe sexual differences in body size, although other morphological traits might be allometric between sexes and imply functional consequences. Here we test whether morphological differences between sexes in size and shape in the lizard Tropidurus catalanensis explain variation in performance of four locomotor traits. Our results show that males are larger than females and also exhibit longer limbs, longer muscles and larger muscle cross‐sectional areas, while females have longer trunks and more sharped anterior claws; males outperform females in all locomotor performances measured. Sexual differences in sprinting and climbing is related with body size, and climbing performance is also explained by limb lengths, by differences in lengths and cross‐sectional areas of specific muscles, and by interlimb distances. Between‐sex differences in exertion are also related to SD, despite associations with sharper posterior claws that are independent of sex. Grasping performance, however, is associated with some muscle and morphological parameters that are not sexually dimorphic. Together our results suggest that morphology might be under sexual selection in T. catalanensis, given that better locomotor performance likely favours male lizards in typical activities of this polygenic species, such as territory defence and female acquisition. Moreover, the longer trunks that characterize females may confer more space to accommodate eggs. On the other hand, territory defence by males probably increases their exposure to predators, resulting in a synergistic effect of sexual and natural selection in the evolution of SD in T. catalanensis.     

Seasonal changes in glomerular filtration rate in Antechinus stuartii (Marsupialia: Dasyuridae)

The small marsupial Antechinus stuartii experiences a synchronised life cycle that culminates in complete male mortality (within 3 weeks) following the 1 week mating period in mid-August (late winter). There are pronounced physiological changes in male A. stuartii over the life cycle and renal function was assessed for correlation with these changes. Glomerular filtration rate and urine and plasma electrolytes were determined in male and female A. stuartii in February, May, July and August. Females showed little change in glomerular filtration rate, except for pre-mating values in August which decreased. In contrast, glomerular filtration rate of males decreased significantly in July and August. Plasma sodium and chloride levels were higher in males than females and were higher in animals in July and August than in February and May. Plasma potassium levels dropped in both males and females in July and August. Plasma osmolality was higher in animals in February compared to animals from May and August. However, there were no significant sex or seasonal differences in urine electrolytes, although urea concentration was higher in females than males. Urine osmolality was higher in both sexes in July and August. There were no significant differences in total excretory rates of sodium, potassium or chloride between sexes or between seasons. Many of the alterations in renal function are correlated with known physiological and hormonal profiles in A. stuartii. This is the first observation of seasonal changes in glomerular filtration rate that are unrelated to dietary and water stresses. Accepted: 8 September 1997     

Winter segregation of migrant European robins Erithacus rubecula in relation to sex, age and size

Birds often show some form of social segregation during winter, both at large geographical scales (a consequence of differential migration) and at the regional or local level, when comparing different habitats or micro-habitats. However, our understanding of the mechanisms underlying such patterns is still poor. These issues have been rarely investigated in migratory Old-World passerines, particularly with respect to differences between the sexes. In this study, we show that female European robins Erithacus rubecula (sexed by molecular techniques) greatly outnumber males in southern Iberia, which confirms that this species is a differential migrant with a strong latitudinal segregation of the sexes. Furthermore, sex, age and body size influence the habitat distribution of robins in winter. Subordinate birds (females, juveniles and small individuals) were generally more common in habitats with a greater shrub development, and comparatively scarce in woodlands with relatively little undergrowth. Birds wintering in woodlands were in better condition (assessed by breast-muscle scoring) than birds wintering in shrubland. These results are consistent with the hypothesis that proposes that social dominance, mediated by differences in size and experience, is important in determining the habitat segregation of sex and age classes. The alternative hypothesis (habitat specialization), although not specifically supported by our findings, cannot be ruled out on the basis of the available evidence.     

Sexual,Seasonal and Group Size Differences in the Allocation of Time between Vigilance and Feeding in the Greater Rhea,Rhea americana

We studied the effect of sex and group size on the proportion of time a greater rhea, Rhea americana, allocates to vigilance and feeding during the breeding and the non-breeding seasons. We analysed 175 records of focal animals that were feeding alone or in groups of 2 to 26 birds. In both seasons, males spent more time in vigilance and less time in feeding than females. Both sexes spent more time in vigilance and less time in feeding during the breeding season. Sexual and seasonal differences in vigilance were the result of different mechanisms. Males had shorter feeding bouts than females but there were no sexual differences in the length of the vigilance bouts. On the contrary, seasonal differences were the result of males and females having longer vigilance bouts during the breeding season but there were no seasonal differences in the length of the feeding bouts. During the non-breeding season, individual vigilance was higher in rheas foraging alone than in groups. In this case, solitary birds had longer vigilance and shorter feeding bouts than birds foraging in groups. We discuss the possible effect of intragroup competition and food availability on the allocation of time between feeding and vigilance in this species.     

Quantifying shape and ecology in avian pedal claws: The relationship between the bony core and keratinous sheath

Terrestrial tetrapods use their claws to interact with their environments in a plethora of ways. Birds in particular have developed a diversity of claw shapes since they are often not bound to terrestrial locomotion and have heterogeneous body masses ranging several orders of magnitude. Numerous previous studies have hypothesized a connection between pedal claw shape and ecological mode in birds, yet have generated conflicting results, spanning from clear ecological groupings based on claw shape to a complete overlap of ecological modes. The majority of these studies have relied on traditional morphometric arc measurements of keratinous sheaths and have variably accounted for likely confounding factors such as body mass and phylogenetic relatedness. To better address the hypothesized relationship between ecology and claw shape in birds, we collected 580 radiographs allowing visualization of the bony core and keratinous sheath shape in 21 avian orders. Geometric morphometrics was used to quantify bony core and keratinous sheath shape and was compared to results using traditional arc measurements. Neither approach significantly separates bird claws into coarse ecological categories after integrating body size and phylogenetic relatedness; however, some separation between ecological groups is evident and we find a gradual shift from the claw shape of ground‐dwelling birds to those of predatory birds. Further, the bony claw core and keratinous sheath are significantly correlated, and the degree of functional integration does not differ across ecological groups. Therefore, it is likely possible to compare fossil bony cores with extant keratinous sheaths after applying corrections. Finally, traditional metrics and geometric morphometric shape are significantly, yet loosely correlated. Based on these results, future workers are encouraged to use geometric morphometric approaches to study claw geometry and account for confounding factors such as body size, phylogeny, and individual variation prior to predicting ecology in fossil taxa.     

Antler stiffness in caribou (Rangifer tarandus): testing variation in bone material properties between males and females

Vertebrate bones, including deer antler, often exhibit variation in their mechanical properties that corresponds to differences in the functional demands they encounter. Among deer, antlers are found in both males and females only in caribou (Rangifer tarandus). Several differences between male and female R. tarandus in behavior and physiology might subject their antlers to differing demands, potentially making divergence of antler material properties between the sexes advantageous. Alternatively, antler material properties might not differ between male and female R. tarandus because both sexes are members of the same species, and the properties of their antlers could, therefore, have emerged under similar pressures and constraints through evolution. To test for sexual dimorphism in antler material properties, we compared the stiffness (Young's modulus of elasticity) of antler specimens from male and female caribou using three-point bending tests. Despite behavioral and physiological differences between males and females, stiffness values did not differ significantly between the sexes in caribou, with a mean (+/-S.E.) stiffness of 5.8+/-0.4GPa across all specimens. This value differed by less than 10% from the values published for R. tarandus specimens of unknown sex, verifying the comparability of bone material property data collected across multiple studies, and lending confidence to recent analyses of the evolution of antler stiffness in deer that have drawn on literature data.     

Zebra finches are bolder in an asocial, rather than social, context

Despite an expanding interest in animal personalities, the influence of social interactions and sex differences on individual differences in behaviour remains poorly understood. Using the social zebra finch (Taeniopygia guttata), we tested for behavioural differences in exploration of a novel environment and objects, between individuals of both sexes in relation to a social context; the presence of three male companions, three female companions or no companion birds. We predicted that the presence of conspecific companions should result in focal birds reacting to novelty by exploring more extensively because the companion birds contribute to anti-predator vigilance behaviour and because social isolation often causes behavioural inhibition in social species. We found that exploratory behaviour of focal individuals was significantly reduced in the presence of conspecific companions, irrespective of the companion's sex. Moreover, we found a weak trend towards females being more exploratory than males, irrespective of the social context. These results demonstrate the importance of considering the social context in animal personality studies and of exploring sex differences in personalities.     

大山雀身体大小的性二态及其季节变化

动物中普遍存在雌雄个体身体大小的性二态现象。了解近缘种之间身体大小性二态现象的差异，可为深入探讨身体大小性二态现象的潜在驱动机制提供证据。国外对欧亚大山雀（Parus major）的研究发现，其喙长、跗跖长、翅长等 6 项身体大小指标存在着明显的性二态，且喙长的性二态存在季节间差异。大山雀（P. cinereus）曾被作为欧亚大山雀的一个亚种，其形态和行为与欧亚大山雀存在着诸多相似之处。为探讨大山雀是否也存在身体大小性二态及季节性差异，本研究分析了 2018 至 2020 年间在河南董寨国家级自然保护区捕捉的 226 只（雌性 96 只和雄性 130 只）大山雀的喙长、头喙长、跗跖长、翅长、尾长和体长这 6 项体征指标的两性差异及其季节变化。结果显示，大山雀上述 6 项身体大小指标均存在不同程度的性二态现象，且雄性个体仅喙长与雌性的差异不显著，其余 5 项指标均显著大于雌性。此外，身体大小指标的两性差异不随季节显著变化，但两性的跗跖长在秋季均显著短于冬季和繁殖季，尾长在繁殖季均显著长于秋季和冬季。上述结果表明，大山雀身体大小的性二态及其季节性差异与欧亚大山雀并不完全相似。无论其身体大小存在性二态和季节变化的原因，还是其与欧亚大山雀在身体大小性二态模式上的差别，都有待今后进一步的研究。     

Response properties of the auditory telencephalon in songbirds change with recent experience and season

The caudomedial nidopallium (NCM) is a telencephalic auditory area that is selectively activated by conspecific vocalizations in zebra finches and canaries. We recently demonstrated that temporal and spectral dynamics of auditory tuning in NCM differ between these species [1]. In order to determine whether these differences reflect recent experience, we exposed separate groups of each species and sex to different housing conditions. Adult birds were housed either in an aviary with conspecifics (NORM), with heterospecifics (canary subjects in a zebra finch aviary, and vice versa: (CROSS)), or in isolation (ISO) for 9 days prior to testing. We then recorded extracellular multi-unit electrophysiological responses to simple pure tone stimuli (250-5000 Hz) in awake birds from each group and analyzed auditory tuning width using methods from our earlier studies. Relative to NORM birds, tuning was narrower in CROSS birds, and wider in ISO birds. The trend was greater in canaries, especially females. The date of recording was also included as a covariate in ANCOVAs that analyzed a larger set of the canary data, including data from birds tested outside of the breeding season, and treated housing condition and sex as independent variables. These tests show that tuning width was narrower early in the year and broader later. This effect was most pronounced in CROSS males. The degree of the short-term neural plasticity described here differs across sexes and species, and may reflect differences in NCM's anatomical and functional organization related to species differences in song characteristics, adult plasticity and/or social factors. More generally, NCM tuning is labile and may be modulated by recent experience to reflect the auditory processing required for behavioral adaptation to the current acoustic, social or seasonal context.     

Seasonal sex ratios and the evolution of temperature-dependent sex determination in oviparous lizards

Although the adaptive significance of temperature-dependent sex determination (TSD) remains a puzzle, recent models implicate a seasonal bias in offspring sex production that translates into sex-specific fitness benefits later in life. Sex-specific emergence has been linked to fitness gains in some fish, birds and reptiles, but field data supporting the occurrence of a seasonal pattern of sex ratios in oviparous lizards are lacking. We tested the hypothesis that patterns of nest site selection and seasonal temperature changes combine to inhibit the materialization of sex-biased hatching times in a population of water dragons (Intellagama lesueurii). As predicted, a seasonal increase in air and nest temperatures resulted in a sex bias by nesting date; male-producing clutches were laid 17.8 days sooner than female-producing clutches, on average. However, the seasonal ramping of nest temperatures also caused shorter relative incubation periods in the later, all-female clutches. As a consequence of this developmental ‘catch-up’, the mean hatching date for male-producing nests preceded the mean hatching date for female-producing nests by only 7.2 days. We suggest that a contracted distribution of hatching dates compared to the distribution of oviposition dates represents a general pattern for oviparous reptiles in seasonal climates, which in TSD species may largely offset the temporal disparity in nesting dates between the sexes. Although data are needed for other TSD species, such minor age differences between male and female hatchlings may not translate into fitness differences later in life, an assumption of some models for the evolution and maintenance of TSD.