Variance‐Sensitive Green Woodhoopoes: A New Explanation for Sex Differences in Foraging?

Abstract

Studies of cooperatively breeding birds rarely benefit from the extensive research on adaptive foraging behaviour, despite the potential for concepts such as state‐dependent foraging to explain many aspects of behaviour in social groups. For example, sex differences in preferred foraging techniques used by green woodhoopoes, Phoeniculus purpureus, have previously been explained by sexual dimorphism in bill length and the benefits afforded by foraging specialization and niche differentiation within cooperative groups. Contrary to this argument, there were no sex differences in mean foraging success and/or prey size captured when males and females used the same foraging techniques. Subordinates of both sexes did experience lower and more varied foraging success compared with dominants, but probably only as a consequence of competition or inexperience. However, dominant males experienced greater variance in individual foraging success compared with dominant females, and dominant males also experienced greater variances in prey size when using their preferred foraging techniques. Dominant males therefore appeared to specialize in foraging techniques that provided more variable rewards, whilst dominant females consistently chose to minimize variation in reward. Dominant females also experienced less variance in foraging returns when using the same techniques as males, suggesting a possible link with sexual dimorphism in bill length. Partitioning of foraging niches in dominant green woodhoopoes therefore appears to be better explained by sex differences in variance (risk) sensitivity to foraging rewards. We suggest that this kind of detailed analysis of state‐dependent foraging has the potential to explain many of the crucial age and sex differences in behaviour within cooperative groups.

Adaptive strategies for managing uncertainty may explain personality‐related differences in behavioural plasticity

“…In one study in Siberian jays Perisoreus infaustus , breeders tended to be variance‐prone while subordinates tended to be variance‐averse (Ratikainen et al 2010). Another study reported that female green woodhoopoes Phoeniculus purpureus were more variance‐averse than males (Wright and Radford 2010). These differences in preference or aversion for variance were attributed to differences in energetic requirements between classes of individuals (subordinates vs dominants, and males vs females respectively), which ultimately affect the shape of the utility function (Ratikainen et al 2010, Wright and Radford 2010).…”

Bet-hedging across generations can affect the evolution of variance-sensitive strategies within generations

“…In Model 2, we introduced the mechanism that traditional variance-sensitivity literature assumes will favor variance-proneness: a sigmoid utility function relating individual energetic state to fitness and the predictions regarding variance sensitivity from the energy budget rule (Caraco 1981;Stephens 1981;Bednekoff 1996;Wright and Radford 2010;Kacelnik and El Mouden 2013). In this model, fitness correlations among individuals can come from two sources: correlations in environmental conditions determining energetic state (ge), and correlations in payoffs for individuals playing the variable strategy (gr).…”

“…Conversely, if the organism's utility function is concave (decelerating), being varianceaverse and preferring less variable sources of resource gain provides higher expected fitness (see Fig. 1 in Wright and Radford 2010). These predictions from the energy budget rule (Caraco 1981;Stephens 1981;Bednekoff 1996) have largely been supported in experimental studies of foraging decisions in a range of animal species (Shafir 2000) and recently even plants (Dener et al 2016), but results are inconsistent, indicating that there are still unresolved issues in this paradigm (Kacelnik and El Mouden 2013).…”

“…For example, the fitness benefits of a food resource are expected to increase exponentially early on when there is a real danger of starvation, and they will flatten out towards an asymptote due to diminishing returns of additional resource gains when the animal becomes satiated (see Fig. 1 in Wright and Radford 2010). This is a general property of some threshold level of resources being needed to complete a certain task, be it surviving a cold night, undergoing migration, achieving a certain social dominance rank, attracting a mate or successfully raising offspring (Real and Caraco 1986;Bednekoff 1996;Kacelnik and Bateson 1997;Hurly 2003;Ratikainen et al 2010).…”

The ecological economics of kleptoparasitism: pay‐offs from self‐foraging versus kleptoparasitism

“…Alternatively, males may engage in kleptoparasitism more frequently than females because they suffer less from gaining more variable energy returns associated with kleptoparasitism, (e.g. female egg production requires consistent energy returns) (Wright & Radford ) or because they have larger energy requirements than females that force them to adopt a more risky foraging tactic with higher energy returns (Barnard & Brown ). Selection may also have operated on drongos to partition foraging niches between the sexes (Radford & du Plessis ), as pairs occupy the same territory throughout the year and may therefore compete for food (Hockey, Dean & Ryan ).…”