Variation in female mate choice and mating success is affected by sex ratio experienced during early life

Sex-specific recruitment rates contribute to male-biased sex ratio in Adélie penguins

Sex-related differences in vital rates that drive population change reflect the basic life history of a species. However, for visually monomorphic bird species, determining the effect of sex on demographics can be a challenge. In this study, we investigated the effect of sex on apparent survival, recruitment, and breeding propensity in the Adélie penguin (Pygoscelis adeliae), a monochromatic, slightly size dimorphic species with known age, known sex, and known breeding history data collected during 1996-2019 (n = 2127 birds) from three breeding colonies on Ross Island, Antarctica. Using a multistate capture-mark-recapture maximum-likelihood model, we estimated apparent survival (S ^ ), recapture (resighting) probability (p ^ ), and the probability of transitioning among breeding states and moving between colonies (ψ ^ ; colony-specific non-juvenile pre-breeders, breeders, and non-breeders). Survival rate varied by breeding status and colony, but not sex, and pre-breeders had higher survival rates than breeders and non-breeders. Females had a higher probability of recruiting into the breeding population each year and may enter the breeding pool at younger ages. In contrast, both sexes had the same probability of breeding from year to year once they had recruited. Although we detected no direct sex effects on survival, the variation in recruitment probability and age-at-first reproduction, along with lower survival rates of breeders compared to pre-breeders, likely leads to shorter lifespans for females. This is supported by our findings of a male-biased mean adult sex ratio (ASR) of 1.4 males for every female (x ^ proportion of males = 0.57, SD = 0.07) across all colonies and years in this metapopulation. Our study illustrates how important it can be to disentangle sex-related variation in population vital rates, particularly for species with complex life histories and demographic dynamics.

Within-population variation in female mating preference affects the opportunity for sexual selection and the evolution of male traits, but things are not as simple as expected

Females from the same population usually have phenotypic variation in their mating preferences. However, the effects of this within-population variation on the sexual selection acting on males are still unclear. We used individual-based models to explore how within-population variation in female preference (i.e. which male trait value is preferred) and preference strength (i.e. how strong the preference is) affects the opportunity for sexual selection (I_s ) and the evolution of a sexually selected male trait. We found the highest I_s values when females had high variation in preference and an open-ended preference function. The lowest I_s occurred when the magnitude of variation in female preference and male trait value were the same and preference function was closed. Male trait exaggeration was higher when there was high within-population variation in preference and females had an open-ended preference function. Also, higher male trait variation was maintained by high variation in preference, but only for a closed preference function. Thus, we found that only within-population variation in female preference, not in preference strength, influences the opportunity for sexual selection and the evolution of sexually selected male traits. Moreover, we found that the shape of the preference function (i.e. open-ended or closed) and the magnitude of within-population variation in female preference compared to male trait variation also influences the I_s and consequently the evolution of male traits.

Analysis of the territorial vocalization ritual of the common pheasant Phasianus colchicus

The aim of the study was to analyze the ritual of territorial vocalization of pheasant cocks. The research aim was to determine whether the duration of the ritual of the territorial vocalization of pheasant cocks depends on the season of the year or whether it is an individual feature. The work was performed based on the analysis of film recordings of 7 male pheasants recorded in the Bystrzyca Valley in Lublin (Poland). Several-minute recordings were made from April 19 to June 27, 2018 in the morning (around 6: 00-8: 00). The characteristic for the species ritual vocalization were divided into four phases. The study concluded that the average duration of individual phases of ritual vocalization in individual birds varies and is an individual value. The number flaps of the wings before and after the vocalization varies depending on the cock and is bird-specific, and depends to a small extent on the month of observation.

Size dimorphism and sexual segregation in pheasants: tests of three competing hypotheses

Fine scale sexual segregation outside of the mating season is common in sexually dimorphic and polygamous species, particularly in ungulates. A number of hypotheses predict sexual segregation but these are often contradictory with no agreement as to a common cause, perhaps because they are species specific. We explicitly tested three of these hypotheses which are commonly linked by a dependence on sexual dimorphism for animals which exhibit fine-scale sexual segregation; the Predation Risk Hypothesis, the Forage Selection Hypothesis, and the Activity Budget Hypothesis, in a single system the pheasant, Phasianus colchicus; a large, sedentary bird that is predominantly terrestrial and therefore analogous to ungulates rather than many avian species which sexually segregate. Over four years we reared 2,400 individually tagged pheasants from one day old and after a period of 8–10 weeks we released them into the wild. We then followed the birds for 7 months, during the period that they sexually segregate, determined their fate and collected behavioural and morphological measures pertinent to the hypotheses. Pheasants are sexually dimorphic during the entire period that they sexually segregate in the wild; males are larger than females in both body size and gut measurements. However, this did not influence predation risk and predation rates (as predicted by the Predation Risk Hypothesis), diet choice (as predicted by the Forage Selection Hypothesis), or the amount of time spent foraging, resting or walking (as predicted by the Activity Budget Hypothesis). We conclude that adult sexual size dimorphism is not responsible for sexual segregation in the pheasant in the wild. Instead, we consider that segregation may be mediated by other, perhaps social, factors. We highlight the importance of studies on a wide range of taxa to help further the knowledge of sexual segregation.

Differences in social preference between the sexes during ontogeny drive segregation in a precocial species

Abstract

Hypotheses for why animals sexually segregate typically rely on adult traits, such as differences in sexual roles causing differential habitat preferences, or size dimorphism inducing differences in diet or behaviour. However, segregation can occur in juveniles before such roles or size dimorphism is well established. In young humans, leading hypotheses suggest that (1) sexes differ in their activity and the synchronisation of behaviour causes segregation and (2) sexes separate in order to learn and maximise future reproductive roles. We reared pheasants, Phasianus colchicus, from hatching in the absence of adults in a controlled environment. Females aggregated with their own sex from hatching, whereas males initially exhibited random association, but segregation became pronounced with age. The increase in segregation corresponded to an increase in sexual size dimorphism. By standardising habitat availability and diet and by removing predation risk, we could disregard the Predation Risk and the Forage Selection Hypotheses operating at this age. Activity budgets did not differ between the sexes, providing no support for the Behavioural Synchrony or the Activity Budget Hypotheses. Both sexes preferentially approached groups of unfamiliar, same-sex birds in binary choice tests, providing support for the Social Preference Hypothesis. Females may segregate to avoid male aggression. Sexual segregation may become established early in development, especially in precocial species, such as pheasants. A clear understanding of ontogenetic factors is essential to further our understanding of adult assortment patterns. Assortment by sex may not be inherent, but rather emerge as a consequence of social interactions early in life.

Significance statement

Hypotheses pertaining to the force driving sexual segregation typically rely on adult traits, such as size dimorphism or differences in sexual roles. However, in some species, animals segregate as juveniles, so that most hypotheses previously invoked to explain sexual segregation in adults are irrelevant. We reared pheasants, Phasianus colchicus, from hatching and monitored multiple aspects of the chicks’ life history in an effort to determine what causes sexual segregation. Females aggregate with their own sex from hatching, whereas males initially have a more random association, but segregation becomes pronounced as both sexes got older, coinciding with greater sexual dimorphism. We controlled for influences of predation risk and dietary/habitat choice and found that activity budgets did not differ between the sexes. Instead, we found that both sexes preferred their own sex when presented with a binary choice, providing evidence that social preference could drive sexual segregation in pheasants.

Early-Life Social Isolation Influences Mouse Ultrasonic Vocalizations during Male-Male Social Encounters

Early-life social isolation has profound effects on adult social competence. This is often expressed as increased aggression or inappropriate displays of courtship-related behaviors. The social incompetence exhibited by isolated animals could be in part due to an altered ability to participate in communicatory exchanges. House mice (Mus musculus) present an excellent model for exploring this idea, because social isolation has a well-established influence on their social behavior, and mice engage in communication via multiple sensory modalities. Here, we tested the prediction that social isolation during early life would influence ultrasonic vocalizations (USVs) emitted by adult male mice during same-sex social encounters. Starting at three weeks of age, male mice were housed individually or in social groups of four males for five weeks, after which they were placed in one of three types of paired social encounters. Pair types consisted of: two individually housed males, two socially housed males, or an individually housed and a socially housed male (“mixed” pairs). Vocal behavior (USVs) and non-vocal behaviors were recorded from these 15-minute social interactions. Pairs of mice consisting of at least one individually housed male emitted more and longer USVs, with a greater proportional use of USVs containing frequency jumps and 50-kHz components. Individually housed males in the mixed social pairs exhibited increased levels of mounting behavior towards the socially housed males. Mounting in these pairs was positively correlated with increased number and duration of USVs as well as increased proportional use of spectrally more complex USVs. These findings demonstrate that USVs are part of the suite of social behaviors influenced by early-life social isolation, and suggest that altered vocal communication following isolation reflects reduced social competence.