Larger Sex Research Articles

Sex-specific growth and morphometric sexing of chicks of the Black-headed Gull (Chroicocephalus ridibundus)

In birds, sexual size dimorphism often becomes evident as early as chicks’ growth stage. A larger sex usually has an advantage in food competition among siblings. In the smaller sex, the lesser ultimate size can be compensated by faster growth or earlier fledging. As a consequence, sex differences can manifest themselves not only in body size but also in the body condition or the rate of weight gain. This phenomenon provides additional opportunities for sexing of chicks by discriminant analysis of growth parameters. Our work deals with the influence of sex on the growth of Black-headed Gull chicks. Using logistic models, we examined growth patterns of 80 males and 76 females from hatching to departure from breeding colonies during three seasons. In males, the total head length, tarsus length, and body mass were greater than those in females from the first days of life and reached large asymptotic values. Tarsus length was a better predictor of sex than head length or body mass was. The sex of chicks did not have a significant effect on global growth rates; however, compared to males, females had a lower body condition (the difference between observed and expected chick body mass) at age 8 days and gained less weight between 21 and 28 days. Using these differences, we developed a discriminant function that allowed to correctly predict the sex of 88.5% of Black-headed Gull chicks before fledging.

Sex predicts response to novelty and problem-solving in a wild bird with female-biased sexual dimorphism.

A wide range of animals, including a number of bird, fish, mammal and reptile species, show sex differences in cognitive tests. Hardly anything is known, however, about whether and how sex-specific non-cognitive factors (e.g. response to novelty) affect the expression of cognition in the wild. We used a series of learning and problem-solving tasks in wild breeding skuas, a species in which females are the larger sex (female-biased sexual size dimorphism). We also evaluated the birds' response to novelty (novel objects) before and after the tasks were administered. Both sexes performed equally well in learning (Discrimination-learning task) and re-learning (Reversal-learning task) food associations with colour and spatial cues, but female skuas outperformed males in problem-solving tasks (String-pulling task, Box-opening task). Females were also less neophobic than males: they were faster at accepting a food reward in novel situations. Better female performance may not imply higher cognition per se. Sex-specific size differences may translate into less or more neophobic behavioural types, which, in turn, predict females' problem-solving success and response to novelty. Species with female-biased sexual dimorphism may present a useful model to assess the interactions between sex, non-cognitive factors and cognition in the wild.

Individual flexibility in group foraging behaviour of reef manta rays (Mobula alfredi)

Flexibility in animal foraging strategies can increase overall feeding efficiency for individuals. For example, group foraging can increase the efficiency of resource exploitation; conversely solo foraging can reduce intraspecific competition, particularly at low resource densities. The cost–benefit trade-off of such flexibility is likely to differ within and among individuals. Reef manta rays (Mobula alfredi) are large filter-feeding elasmobranchs that often aggregate to feed on ephemeral upwellings of zooplankton. Over three years in the Maldives, we free-dived to film 3106 foraging events involving 343 individually identifiable M. alfredi. Individuals fed either solo or in groups with a clear leader plus between one and eight followers. M. alfredi were significantly more likely to forage in groups than solo at high just prior to high tide and when aggregations were larger. Within aggregations, individuals foraged in larger groups when more food was available, and when the overall aggregations were relatively large suggesting that foraging in large groups was more beneficial when food is abundant, and the costs of intraspecific competition were outweighed by the efficiency resulting from group foraging strategies. Females, the larger sex, were more likely to lead foraging groups than males. The high within-individual variance (over 70%), suggested individuals were unpredictable across all foraging behaviours, thus individual M. alfredi cannot be classified into foraging types or specialists. Instead, each individual was capable of considerable behavioural flexibility, as predicted for a species reliant on spatially and temporally ephemeral resources.Significance statementReef manta rays (Mobula alfredi), listed as “Vulnerable” on the IUCN Red List, are at risk from targeted and by-catch fisheries due to their slow life history and aggregative behaviour. M. alfredi feed together in aggregations on short-lived glut of microscopic zooplankton. Over three years in the Maldives, we filmed 3106 foraging events involving 343 individually identifiable M. alfredi. Manta rays were more likely to forage in groups than solo just prior to high tide and when aggregations were larger, attracted by the influx of zooplankton. Foraging groups included more individuals when plankton was more abundant. However, individuals flipped between solo and group foraging and did not specialise. Foraging groups were most often led by females, the larger sex. Individuals were very flexible in how they foraged, which makes sense for a species that relies on a food source that varies enormously in when, where and for how long it is available. Understanding manta ray foraging behaviour will help conservation management efforts and predict their responses to climate change.

Sexual dimorphism and ecomorphology of the Berthold’s bush anole (Polychrus gutturosus) in Costa Rica

Abstract Morphological variation present within a species is the result of sexual or natural selection and determines the ecological relationships (i.e., ecomorphology) and the degree of sexual dimorphism. For most lizard species, either males or females are the larger sex, suggesting that selection operates differently on each sex. Thus, differences in morphology can determine the habitat use made by males and females. Here we use the Berthold’s bush anole (Polychrus gutturosus), a rare arboreal, diurnal lizard, to examine whether morphological and ecological measures differ among sexes and whether such potential differences emerge as a result of resource partitioning. We measured the morphology of wild individuals from one population and from preserved specimens collected in Costa Rica. We collected data on nine morphological measurements, nocturnal body and air temperatures, and sleeping perch height. We measured 45 wild individuals (22 males and 23 females) and 17 preserved specimens (6 males and 11 females). We found a female-biased body size, and male-biased tail length, head dimensions, and limb lengths. We also observed differences between males and females in body color and the size and shape of femoral pores, although these differences require quantification. We did not find sex differences in sleeping perch height or nocturnal body temperature. Our results demonstrate that resource partitioning does not explain differences among sexes, and that female-biased body size in P. gutturosus might reflect selection operating on increased fecundity.

When Rensch meets Foster: insular gigantism may reduce sexual dimorphism in anurans

According to the Island rule, insular populations exhibit gigantism in small species and dwarfism in large species. These contexts offer opportunities to test for complementary aspects pertaining to the evolution of body size, and particularly sexual size dimorphism (SSD). ‘Rensch's rule' states that SSD should vary with increasing body size, depending on the larger sex. As a consequence, it is expected that dwarfism or gigantism occurring in insular populations should influence the magnitude of SSD. Using anuran amphibians as a study system (because most anuran species express a female‐biased SSD and a marked lability in size), we investigated our hypothesis both in a specific comparison of continental and insular populations of a widespread coastal amphibian, and using a large‐scale analysis across anurans (10 species across 62 sites). Both datasets yielded similar results, with increasing body size in insular populations reducing the magnitude of SSD through an increase of male body size, as expected by Rensch's rule. Detailed data on insular populations considering both sexes are scarce, and future studies are required to complement the existing literature in order to test for the validity of our hypothesis at a wider scale and to infer the mechanistic causes of size variations, which remain unknown. In addition, further investigations are required to explore the consequences of insularity on the magnitude of SSD including various taxa (e.g. mammals, birds, lizards and snakes) in which the Island rule has found support, and considering taxa in which males are the larger sex.Keywords: amphibians, body size, insularity, Island rule, Rensch's rule, sexual dimorphism

Ecological and evolutionary trends of body size in Pristimantis frogs, the world's most diverse vertebrate genus

Body size is a key organismal trait. However, the environmental and evolutionary factors that drive body size patterns at the interspecific level remain unclear. Here, we explored these relationships between phenotype-environment using neotropical frogs of Pristimantis, the world’s most diverse vertebrate genus. We analyzed: (a) whether this group follows the Rensch’s rule, a trend of sexual size dimorphism (SSD) to increase with size when males are the larger sex; (b) whether environmental constraints have influenced body size variation; and (c) how the rates of body size evolution have varied over time. Analyses were based on two information sources, the first one including body sizes of ~ 85% (495 species) of known species in the genus, and a second one incorporating molecular phylogenetic information for 257 species. Our results showed that all Pristimantis species exhibited marked SSD but did not follow Rensch’s rule. We found that the models that best explained body size in males, females, and SSD contained environmental variations in temperature, precipitation, and elevation as predictors. In turn, body size has evolved toward an optimum, with a decelerating rate of evolution differentiated between the large Pristimantis clades.

Influence of microhabitat, fecundity, and parental care on the evolution of sexual size dimorphism in Caribbean Eleutherodactylus frogs.

Rensch's rule suggests that sexual size dimorphism (SSD) increases with species size when males are the larger sex, whereas it decreases when females are the larger sex. However, the process responsible for this pattern remains obscure. SSD can result from sexual selection, such as intrasexual competition for access to mates, or from natural selection, due to resource partitioning or fecundity selection. We studied SSD in Caribbean Eleutherodactylus frogs using phylogenetic comparative methods to investigate the influence of microhabitat, fecundity, and parental care. Our results show that in Caribbean Eleutherodactylus females tend to be larger and, contrary to Rensch's rule, dimorphism increases with species size. SSD was not related to microhabitat use. However, SSD was positively correlated with fecundity, mediated by a greater increase in female size. SSD was also influenced by parental care, suggesting that male care promotes larger male size and reduces the female bias in SSD. As suggested for other anurans, female-biased SSD in Caribbean Eleutherodactylus results from fecundity selection, although the magnitude is countered by increased male size in species with paternal care. Our results highlight the importance of considering various selective forces that may act in concert to influence the evolution of SSD.

Sexual selection for flight performance in hummingbirds

AbstractAmong size-dimorphic animals, a few clades such as hummingbirds show “reversed” sexual size dimorphism: females tend to be the larger sex. What selects for this pattern? Sexual selection for flight performance could drive the evolution of smaller, more agile males, either for male-male combat or female choice for aerial courtship displays. Alternately, natural selection can select for female fecundity (e.g., egg size influences female body size), or sex differences in foraging niche could favor body size differences. The sexual selection hypotheses predict that dimorphism extends to other aspects of flight morphology (e.g., flight muscle size) whereas the natural selection hypotheses predict that male and female flight morphologies are isometric, and the niche differentiation hypothesis predicts that bill dimorphism is correlated with size dimorphism. We tested these predictions through phylogenetic comparative analyses of flight morphology, wingbeat frequency, and courtship behaviors, focused on 30 species within the “bee” hummingbird clade (tribe Mellisugini). There is no correlation between bill morphology and dimorphism. Relative to females, males tend to be smaller, have proportionately shorter wings and higher hovering wingbeat frequencies, but also longer keels and larger flight muscles. Male wingbeat frequencies are greatly elevated during aerial displays, and the species with the greatest wingbeat frequencies have the greatest dimorphism. Of the four hypotheses for dimorphism, the data best support the hypothesis that female choice for courtship displays has selected for aerial agility and small size in male hummingbirds.

Applying computer vision to digitised natural history collections for climate change research: Temperature‐size responses in British butterflies

Abstract Natural history collections are invaluable resources for understanding biotic response to global change. Museums around the world are currently imaging specimens, capturing specimen data and making them freely available online. In parallel to the digitisation effort, there have been great advancements in computer vision: the computer trained automated recognition/detection, and measurement of features in digital images. Applying computer vision to digitised natural history collections has the potential to greatly accelerate the use of these collections for biotic response to global change research. In this paper, we apply computer vision to a very large, digitised collection to test hypotheses in an established area of biotic response to climate change research: temperature‐size responses. We develop a computer vision pipeline (Mothra) and apply it to the NHM collection of British butterflies (>180,000 imaged specimens). Mothra automatically detects the specimen and other objects in the image, sets the scale, measures wing features (e.g. forewing length), determines the orientation of the specimen (pinned ventrally or dorsally) and identifies the sex. We pair these measurements and specimen collection data with temperature records for 17,726 specimens across a subset of 24 species to test how adult size varies with temperature during the immature stages of species. We also assess patterns of sexual size dimorphism across species and families for 32 species trained for automated sex ID. Mothra accurately measures the forewing lengths of butterfly specimens compared to manual measurements and accurately determines the sex of specimens, with females as the larger sex in most species. An increase in adult body size with warmer monthly temperatures during the late larval stages is the most common temperature‐size response. These results confirm suspected patterns and support hypotheses based on recent studies using a smaller dataset of manually measured specimens. We show that computer vision can be a powerful tool to efficiently and accurately extract phenotypic data from a very large collection of digital natural history collections. In the future, computer vision will become widely applied to digital collections to advance ecological and evolutionary research and to accelerate their use to investigate biotic response to global change.

Sexual size dimorphism and geographic variation in forearm length of Rafinesque’s Big-eared Bat (Corynorhinus rafinesquii) and Southeastern Myotis (Myotis austroriparius)

Abstract Sexual size dimorphism is common in many taxa and results from various pressures, including competition, reproductive requirements, functional differences, and sexual recognition. For mammals, males are typically the larger sex; however, for vespertilionid bats, females are more often the larger sex. Forearm length, a feature that influences overall wing and body size and is often sexually dimorphic, is a standard morphological measurement taken from bats. Forearm length was measured in two vesper bat species (Corynorhinus rafinesquii and Myotis austroriparius) that co-occur across much of the southeastern United States. Forearm length was greater in females of both species, and females of both species also exhibited regional variation in forearm length. By having a longer forearm and therefore being larger in size, females may be more maneuverable and better equipped to carry young. While this study did not directly investigate the mechanisms behind regional variation in forearm length, it is possible this is the result of variability in habitat types, resources, or thermodynamic constraints. Knowledge of sexually dimorphic characteristics is important for obtaining a general understanding of a species and its morphology.

No sex‐specific differences in the influence of high air temperatures during early development on nestling mass and fledgling survival in the Southern Pied Babbler (Turdoides bicolor)

Sex‐biased mortality in response to environmental adversity during early development occurs in a number of bird species. The three most prominent theories proposed to explain sex‐biased mortality in response to early‐life adversity are that (1) the heterogametic sex (e.g. females in birds), (2) the larger sex (could be male or female depending on species) or (3) the sex with the more costly and complex endocrine system (e.g. males producing higher levels of testosterone) will suffer higher mortality compared with the other sex when exposed to challenging conditions (e.g. suboptimal weather during early development). We tested for sex‐specific differences in the effects of very hot weather on nestling body mass and tarsus length, and survival of fledglings to nutritional independence in the Southern Pied Babbler Turdoides bicolor. The effect of exposure to high air temperatures did not differ by sex but had a consistently negative impact on nestling body mass, nestling tarsus length and fledgling survival, raising concerns about population replacement and the persistence of this species under rapidly advancing climate change.

Concerted evolution of body mass, cell size and metabolic rate among carabid beetles

Alterations in cell number and size are apparently associated with the body mass differences between species and sexes, but we rarely know which of the two mechanisms underlies the observed variance in body mass. We used phylogenetically informed comparisons of males and females of 19 Carabidae beetle species to compare body mass, resting metabolic rate, and cell size in the ommatidia and Malpighian tubules. We found that the larger species or larger sex (males or females, depending on the species) consistently possessed larger cells in the two tissues, indicating organism-wide coordination of cell size changes in different tissues and the contribution of these changes to the origin of evolutionary and sex differences in body mass. The species or sex with larger cells also exhibited lower mass-specific metabolic rates, and the interspecific mass scaling of metabolism was negatively allometric, indicating that large beetles with larger cells spent relatively less energy on maintenance than small beetles. These outcomes also support existing hypotheses about the fitness consequences of cell size changes, postulating that the low surface-to-volume ratio of large cells helps decrease the energetic demand of maintaining ionic gradients across cell membranes. Analyses with and without phylogenetic information yielded similar results, indicating that the observed patterns were not biased by shared ancestry. Overall, we suggest that natural selection does not operate on each trait independently and that the linkages between concerted cell size changes in different tissues, body mass and metabolic rate should thus be viewed as outcomes of correlational selection.

It is not always about body size: evidence of Rensch's rule in a male weapon.

In many species, sexual dimorphism increases with body size when males are the larger sex but decreases when females are the larger sex, a macro-evolutionary pattern known as Rensch's rule (RR). Although empirical studies usually focus exclusively on body size, Rensch's original proposal included sexual differences in other traits, such as ornaments and weapons. Here, we used a clade of harvestmen to investigate whether two traits follow RR: body size and length of the fourth pair of legs (legs IV), which are used as weapons in male-male fights. We found that males were slightly smaller than females and body size did not follow RR, whereas legs IV were much longer in males and followed RR. We propose that sexual selection might be stronger on legs IV length than on body size in males, and we discuss the potential role of condition dependence in the emergence of RR.

Sex identification of the ashy-throated parrotbill (Paradoxornis alphonsianus): A species heavily hunted for bird fighting in Guizhou Province, China

The ashy-throated parrotbill ( Paradoxornis alphonsianus) is a sexually monomorphic species with high abundance in Southwest China, which has been widely used as a fighting bird across Guizhou Province, leading it to become one of the most coveted and heavily hunted wild birds in the region at present. Information on the sexes is a fundamental requirement for a wide variety of avian studies. From a conservation perspective, knowledge necessary for quick sexing of this species should be important, as the determination of sex contributes to the understanding of which sexes are used for fighting. Our goal was to develop a quick method that can be used to identify sex of the ashy-throated parrotbill in the field. Seven body traits were measured and compared between the sexes among 124 individual ashy-throated parrotbills, with sex determined by molecular techniques. Data revealed that the male is the larger sex, with significantly greater measurements than the female in bill length, wing length, and middle claw length. The univariate discriminant function based on bill length featured the highest identification accuracy (67.7%). The larger body size of males may have evolved by sexual selection, but additional data are needed to test this hypothesis. This study found that male and female ashy-throated parrotbills are divergent in size, although further efforts are required for a discriminant function with more robust accuracy.

Brood sex ratio modulates the effects of extra food on parental effort and sibling competition in a sexually dimorphic raptor

Conflicts are inherent to family systems and may occur at three levels. First, each parent benefits if its mate takes the greater share of parental investment. Second, offspring try to manipulate their parents into devolving more resources than it is optimal for them. Third, siblings compete for resources. Food availability can affect the dynamics of each level of interaction. By means of a food supplementation experiment, we assessed how the initial availability of extra food during breeding affects later parental effort, sibling competition, and parent-offspring interactions in a small dimorphic raptor, the lesser kestrel (Falco naumanni). Being the larger sex, female nestlings are likely to have higher energy requirements. Female-biased broods had a higher rate of aggressive interactions and were fed more frequently in control nests. Food was not shared evenly among broodmates, and daughters tended to receive more feedings than sons. In young broods, parents controlled food allocation by entering the nest, whereas in older broods, offspring controlled food allocation by monopolizing the nest site entrance. Extra food induced male parents to reduce the rate of feedings delivered by entering the nest. Additionally, extra food improved nestling growth in male-biased broods, leading to an increase in the frequency of parental feedings and aggressive interactions, likely due to faster growth rates. These findings reveal a key effect of brood sex ratio in determining family interactions in a species with reverse sexual size dimorphism and suggest that all levels of conflict between family members should be considered simultaneously when investigating the evolution of parental care. Breeding poses energetic costs on parents, and environmental resources are usually limited. To successfully breed, parents coordinate their efforts and manage the allocation of resources to offspring, while offspring communicate their needs. Conflict can arise among family members: each parent benefits if the other takes on most of the workload, offspring ask for more care than what parents can provide, and siblings compete for food. Food availability affects these interaction levels separately, but they have rarely been integrated. By providing extra food to lesser kestrel (Falco naumanni) nests, we show that parental provisioning behaviour and sibling competition are simultaneously affected by initial resource availability during breeding. Although lesser kestrel nestlings show only moderate sex differences in body size (and therefore energy requirements), the effect of extra food on family interactions primarily depended on the sex ratio of the brood.

Problems of compensative fisheries: increasing the adaptive potential of broad whitefish Coregonus nasus (Pallas) in early ontogenesis by weak magnetic fields. 2. Histological analysis

In the second part of the article, we performed a cytological and histological analysis of the important functional systems of the control and experimental batches of juvenile broad whitefish as a result of a production experiment at the Sobskiy fish hatchery (settlement Kharp, YaNAO). The experimental batch was processed by the employees of Telos-Siberia LLC using generators of ultra-weak pulsed magnetic fields at the final stages of embryogenesis. We studied the state of internal organs, control and experimental juveniles of this species in the postembryonic period. During the cytological and histological analysis, no pathological changes in the internal organs were revealed. But the treated larvae showed a longer development of the nervous system; in larvae and fry, an intensive course of gameto- and gonadogenesis was noted. A significant increase in the number of primary gonocytes in larvae, larger sex glands and numerous gonocytes in fry has been shown. Also, the experimental fry underwent anatomical and cytological differentiation of the gonads in the direction of the ovaries earlier than in the control. At the end of the rearing period before being released into the Ob for feeding, the number of germ cells in experimental females significantly exceeded the corresponding number in control individuals. The data obtained allow us to conclude that superweak pulsed magnetic fields accelerate the development of the reproductive system, increase the reproductive potential of an individual and do not have a negative effect on the development of somatic systems.

Bigger Males, Bigger Females? Pigeons’ Sexual Size Dimorphism

Sexual dimorphism, defined as phenotypic differences between males and females, is a common phenomenon in animals. In this line, Rensch’s rule states that sexual size dimorphism increases with increasing body size when the male is the larger sex and decreases with increasing average body size when the female is the larger sex. Domesticated animals offer excellent opportunities for testing predictions of functional explanations of Rensch’s theory. Pigeon breeds encounters many different functional purposes and selective constraints, which could influence strongly their morphology. The aim of this paper is to examine, for first time, Rensch’s rule among domestic pigeons. It was compiled a database of 12 quantitative traits (body weight, body height, beak thickness, beak length, neck length, neck thickness, wing length, rump width, tail length, tarsus length, tarsus thickness and middle toe length) for males and females of 11 different domestic pigeon breeds: Bangladesh Indigenous, Racing Homer, Turkish Tumbler, Indian Lotan, Kokah, Mookee, Indian Fantail, Bokhara Trumpeter, Bombai, Lahore and Hungarian Giant House; Rock Pigeon (Columba livia) was also considered as wild relative for comparative purposes. Comparative results between males and females showed that only body weight, wing length and neck thickness were consistent with Rensch’s rule. The rest of trait did not present correlations. Among domestic pigeons, there can appear different expressions of dimorphism according to each trait, so it must be considered that Rensch’s rule vary when considering other traits than body weight.

Son or daughter, it does not matter: brood parasites do not adjust offspring sex based on their own or host quality

Sex allocation theory posits that parents should adjust offspring sex ratio based on the costs and benefits associated with the production of either sex in a given context. Maternal condition should influence sex ratios when it has sex‐specific impacts on offspring fitness (Trivers–Willard hypothesis) or when rearing costs differ between sons and daughters (cost of reproduction hypothesis). In sexually size-dimorphic species, mothers in good condition are predicted to produce an excess of offspring of the larger sex, whereas mothers in poor condition the opposite. Brood parasites constitute an exciting model for testing sex allocation theory as parasitic females are freed from rearing costs while these costs are covered by the hosts. Here, we investigate the effect of maternal and host quality (both expressed as egg volume and blue-green chroma) on offspring sex allocation in the common cuckoo (Cuculus canorus) parasitizing the great reed warbler (Acrocephalus arundinaceus). We found no significant effect of parasite egg-laying date and maternal or host quality on sex ratio. One explanation may be that parasitic females, instead of investing in differential sex allocation, invest in securing egg acceptance by the host, by laying non-randomly within a host population to match the appearance of host clutches. Alternatively, male and female parasite eggs do not differ in size suggesting that their production bears comparable costs. This, together with the fact that the whole parental care is covered by the hosts, supports the previous findings that the common cuckoo does not adjust offspring sex ratio.

Sex-Related Migration Distances in the Dimorphic Eurasian Bittern Botaurus stellaris Breeding in the Netherlands

In many bird species intraspecific variation in migration strategies is related to sex or size. The Eurasian Bittern Botaurus stellaris is a sexually size-dimorphic heron species with a vast breeding and wintering range spanning a range of climates. Ringing data show that Bitterns from northern populations migrate westwards or southwards over thousands of kilometres, while Bitterns from southern or temperate breeding areas migrate much shorter distances or remain resident. So far, any differences in the migrations of males and females have remained unstudied. In temperate climates, relatively benign winters alternate with the occasional harsh winter, and under these conditions, males as the larger sex might take the risk to stay rather than show seasonal migration. In the years 2010–2012 we equipped three females and three males from breeding areas in The Netherlands with a tracking device, recording their movements over periods of 2–5 years. All three males and one female stayed within or near the breeding area during the non-breeding season, but two females moved to distant wintering sites. One female migrated 4900 km to The Gambia providing the first direct evidence for trans-Saharan migration in this species. Another female migrated 700 km to winter in Devon, UK, for five consecutive years. The Bitterns were site-faithful to their breeding area and, mostly, to their wintering areas as well, although one male and one female progressively wintered closer to the breeding area in three successive years. Our results suggest that larger males from The Netherlands, a breeding area with fluctuating winter conditions, are predominantly resident, while the females are partial migrants with individually different and flexible strategies.

Low food abundance prior to breeding results in female-biased sex allocation in Tengmalm’s Owl (Aegolius funerus)

Parents can enhance their fitness by favouring that sex whose reproductive value is expected to be highest. In species in which females are the larger sex with potentially greater fitness returns, one can assume that parents should bias their investment toward daughters to increase their daughters’ reproductive value (i.e. age-specific expectation of all present and future offspring) and, thereby, indirectly increase their own inclusive fitness. In the study reported here, we investigated sex allocation in Tengmalm’s Owl (Aegolius funereus), a bird species with a pronounced female-biased sexual size dimorphism in which females are larger than the males. Assuming that parental investment would have the greatest effect on the fitness of larger daughters, we hypothesized that daughters should be favoured in good conditions and sons in poor conditions. Our study was conducted in the Czech Republic over seven breeding seasons (2006–2012). In total, 205 nestlings from 52 nests were sexed. The mean nestling sex ratio, 48.5 ± 4.6% (± standard error), did not depart from parity, and we did not identify any variable to be related to it. However, we did find that at fledging (1) the body mass of female offspring was approximately 8% heavier than that of male offspring, and (2) surprisingly, the body mass of female offspring tended to increase with decreasing prey abundance in the autumn, i.e. prior to breeding. One possible explanation of this “carry-over effect” is that parents increased their investment toward daughters to maximize their daughters’ survival and reproductive value in a poor environment. This explanation could be supported by the sex-specific effect of the adult’s condition on reproductive success. Whereas the number of fledglings tended to increase with increasing age of the mother, it also increased with decreasing wing length of the father. These results indicate that in the Tengmalm’s Owl large body size may be mainly important for female reproduction, while male reproductive success is independent of body size or is even associated with small body size. To the best of our knowledge, our study is the first to have found such a positive, relatively long-lasting, sex-specific carry-over effect of pre-breeding prey abundance on the condition of female offspring in a bird of prey species.