Foraging and fat reserves in great tits in a variable environment

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Endogenous timing in competitive interactions among relatives

Most evolutionary game theory models solve for equilibrium levels of some behaviour on the restrictive assumptions that players choose their actions simultaneously, and that a player cannot change its action after observing that of its opponent. An alternative framework is provided by sequential or ‘Stackelberg’ games in which one player commits to a ‘first move’ and the other has an opportunity to observe this move before choosing its response. Recent interest in the economic literature has focused on Stackelberg games which exhibit ‘endogenous timing’, i.e. games in which a leader and a follower arise spontaneously as a consequence of each player attempting to maximize its reward. Here, we provide the first demonstration of endogenous timing in an evolutionary context using a simple model of resource competition (the ‘tug-of-war’ model). We show that whenever two related individuals compete for a share of communal resources, both do best to adopt distinct roles in a sequential game rather than engage in simultaneous competition. Somewhat counterintuitively, the stable solution is for the weaker individual to act as leader and commit to a first move, because this arrangement leads to a lower total effort invested in competition. Endogenous timing offers a new explanation for the spontaneous emergence of leaders and followers in social groups, and highlights the benefits of commitment in social interaction

Evolution of antipredator behavior in an island lizard species, Podarcis erhardii (Reptilia: Lacertidae): The sum of all fears?

Organisms generally have many defenses against predation, yet may lack effective defenses if from populations without predators. Evolutionary theory predicts that "costly" antipredator behaviors will be selected against when predation risk diminishes. We examined antipredator behaviors in Aegean wall lizards, Podarcis erhardii, across an archipelago of land-bridge islands that vary in predator diversity and period of isolation. We examined two defenses, flight initiation distance and tail autotomy. Flight initiation distance generally decreased with declining predator diversity. All predator types had distinctive effects on flight initiation distance with mammals and birds having the largest estimated effects. Rates of autotomy observed in the field were highest on predator-free islands, yet laboratory-induced autotomy increased linearly with overall predator diversity. Against expectation from previous work, tail autotomy was not explained solely by the presence of vipers. Analyses of populations directly isolated from rich predator communities revealed that flight initiation distance decreased with increased duration of isolation in addition to the effects of current predator diversity, whereas tail autotomy could be explained simply by current predator diversity. Although selection against costly defenses should depend on time with reduced threats, different defenses may diminish along different trajectories even within the same predator-prey system. © 2014 The Society for the Study of Evolution

Theoretical models of adaptive energy management in small wintering birds

Many small passerines are resident in forests with very cold winters. Considering their size and the adverse conditions, this is a remarkable feat that requires optimal energy management in several respects, for example regulation of body fat reserves, food hoarding and night-time hypothermia. Besides their beneficial effect on survival, these behaviours also entail various costs. The scenario is complex with many potentially important factors, and this has made ‘the little bird in winter’ a popular topic for theoretic modellers. Many predictions could have been made intuitively, but models have been especially important when many factors interact. Predictions that hardly could have been made without models include: (i) the minimum mortality occurs at the fat level where the marginal values of starvation risk and predation risk are equal; (ii) starvation risk may also decrease when food requirement increases; (iii) mortality from starvation may correlate positively with fat reserves; (iv) the existence of food stores can increase fitness substantially even if the food is not eaten; (v) environmental changes may induce increases or decreases in the level of reserves depending on whether changes are temporary or permanent; and (vi) hoarding can also evolve under seemingly group-selectionistic conditions

State-dependent behaviour and energy expenditure: an experimental study of European robins on winter territories

1. We sought evidence for state-dependent behaviour and energy expenditure by manipulating the body-state of birds. We reviewed three hypotheses that could describe responses to a change in body-state: the Increased Effort; Reduced Effort; and Balanced Effort Hypotheses. 2. Territorial European robins Erithacus rubecula L. were captured at dusk during winter, and chilled or warmed overnight to manipulate body-state. 3. The robins were released onto their territories before dawn. Their subsequent behaviour was monitored by radio-tracking, and energy expenditure was measured using the doubly labelled water technique. 4. Robins with high overnight energy expenditures (= 'negative' body-state; chilled) were less likely to initiate vocal behaviour, and less likely to respond to the song or calls of conspecifics than those with low overnight expenditures (= 'positive' body-state; warmed). 5. Chilled birds moved around their territories at almost twice the rate of warmed birds, and had significantly higher daytime energy expenditures. 6. We concluded that behaviour, including vocalizations and activity levels, and energy expenditures of robins, were sensitive to manipulations of body-state. 7. These behaviours are interpreted as adaptive responses to restore condition, while minimizing the risk of mortality from predation and starvation. Our results were consistent with the Balanced Effort Hypothesis, entailing an increase in energy expenditure on foraging and a reduction in expenditure on non- maintenance activities. 8. We suggest that inter-individual differences in body-state may be a key cause of previously unexplained variation in daily energy expenditure