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 statementHypotheses 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.

Young chicks quickly lose their spontaneous preference to aggregate with females

“…Our results show that soon after hatching (Experiment 1) chicks discriminate between sexes and preferentially approach female peers. This spontaneous preference is partially in line with the observations of Whiteside et al (2017), who found that in the first week, female pheasants chicks preferred to associate with their own sex peers, while male pheasant chicks exhibited a random assortment. At hatching, chicks appear sexually monomorphic, and in line with this we did not find significant differences in the weight of males and females.…”

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

“…However, we did not observe differences in crop content when considering a suite of common food items. Similarity in dietary preference was also observed in pheasants when tested as chicks in captivity and presented with a choice of a variety of natural and man-made food items ( Whiteside et al, 2017 ). Dietary difference between sexes often occurs, or become more pronounced, in periods leading up to nesting and incubation ( Lewis et al, 2002 ; Nisbet, 1997 ).…”

“…Such segregation persists until early March ( Hill & Ridley, 1987 ; Hill & Robertson, 1988 ) when harems of females visit and eventually join territory-holding males, which likely reduces their harassment by other males ( Ridley & Hill, 1987 ) and allows females to decrease their vigilance levels and so increase time spent foraging ( Whiteside, Langley & Madden, 2016 ), until they independently start to nest and incubate their eggs ( Taber, 1949 ). Pheasants become sexually dimorphic by three weeks of age ( Whiteside et al, 2017 ), and chicks in captivity (<8 weeks old) exhibit preference for their own sex ( Whiteside et al, 2017 ). As adults, males have highly conspicuous plumage and are 40% larger than the cryptic females ( Wittzell, 1991 ).…”

“…In the wild, sexual segregation of adults was observed at both a spatial and temporal scale that could not be explained by crude measures of habitat structure, although the homogenous nature of the study and reliance on data from supplementary feeding sites meant that fine scale differences in habitat structure may have not been captured ( Whiteside et al, 2018 ). During early life when the sexes differ little in their size, juvenile pheasants reared in captivity in an environment that controlled for habitat selection and diet, albeit under unnaturally high numbers but at a sex ratio analogous to that observed in the wild, exhibited strong preferences for same-sex individuals in binary choices which may drive segregation ( Whiteside et al, 2017 ). We suggested that at this stage, females aggregate with other females in response to male aggression.…”

“…If the Activity Budget Hypothesis operates then we expect that during the periods of sexual segregation male and female pheasants will differ in their behaviour, specifically state behaviours known to influence sexual segregation in ungulates (e.g., foraging, locomotion or resting time ( Ruckstuhl, 1998 )). Male pheasant chicks in captivity were more aggressive than females and this aggression provides potential mechanisms driving segregation ( Whiteside et al, 2017 ). However, pheasants in the wild exhibit little aggression during the periods that they sexually segregate, with male-male aggression rising from the start of the breeding season and peaking mid-breeding season ( Ridley, 1983 ).…”