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

Abstract

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.

Spatial segregation in a sexually dimorphic central place forager: Competitive exclusion or niche divergence?

“…It is, however, often challenging to derive the causes of sexual segregation and there is little consensus on the evolutionary mechanisms driving this segregation (Giery & Layman, 2019; Ruckstuhl & Neuhaus, 2006; Wearmouth & Sims, 2008). Difficulties in testing potentially relevant hypotheses to explain sex‐specific segregation have often led to multiple possible evolutionary drivers being proposed (Alves et al., 2013; Bonenfant et al., 2004; Loe et al., 2006; Whiteside et al., 2018). This ambiguity can at least partly be explained by resource availability, a fundamental component of competition (Courbin et al., 2018), often being poorly understood, particularly in the marine environment (Lehodey et al., 2010).…”

Spatial cognitive ability is associated with transitory movement speed but not straightness during the early stages of exploration

“…We fitted birds with tracking tags attached with a backpack harness that comprised elastic wing-straps threaded through heat-shrink tubing. Tags weighed 22 g, which was a mean of 2.63% of released body mass (range = 2.00–3.67%), although birds were expected to continue to grow meaning that after a couple of months, tags were expected to weigh between 1.1% and 2.3% of adult body mass [ 48 ]. We released the pheasants into a 4000 m 2 enclosure (hereafter the release pen) within a small woodland on North Wyke farm.…”

Response learning confounds assays of inhibitory control on detour tasks

“…Birds in poor body condition (low scores) were considered to be more food-motivated than birds in good body condition (high scores). As male pheasants are larger than female pheasants (Whiteside et al 2018), differences in growth rates may lead to motivational differences, and we have previously found these to differentially influence participation on cognitive tests (van Horik et al 2017). We, therefore, used plumage features to visually identify the sex of each individual at 10 weeks old.…”