Unfavourable Environment Limits Social Conflict in Yuhina Brunneiceps
Total Page：16
File Type：pdf, Size：1020Kb
ARTICLE Received 20 Dec 2011 | Accepted 8 May 2012 | Published 6 Jun 2012 DOI: 10.1038/ncomms1894 Unfavourable environment limits social conflict in Yuhina brunneiceps Sheng-Feng Shen1,2, Sandra L. Vehrencamp2, Rufus A. Johnstone3, Hsiang-Ching Chen4, Shih-Fan Chan1, Wen-Yi Liao1,4, Kai-Yin Lin1,4 & Hsiao-Wei Yuan4 Identifying the factors that modulate cooperative and competitive behaviours is the key to understanding social evolution. However, how ecological factors affect social conflict and their fitness consequences remain relatively unexplored. Here, using both a game-theoretical model and empirical data, we show that Taiwan yuhinas (Yuhina brunneiceps)—a joint-nesting species in which group members are unrelated—employ more cooperative strategies in unfavourable environmental conditions. Fighting duration was lower, fewer total eggs were laid and incubation was more likely to start after all females completed egg laying (which causes more synchronous egg hatching). Surprisingly, as a consequence, there were more surviving offspring in unfavourable conditions because the cooperative strategies resulted in fewer dead nestlings. To our knowledge, this study is the first theoretical analysis and empirical study demonstrating that an unfavourable environment reduces social conflict and results in better fitness consequences in social vertebrates. 1 Biodiversity Research Center, Academia Sinica, Taipei 115, Taiwan. 2 Department of Neurobiology and Behavior and Lab of Ornithology, Cornell University, Ithaca, New York 14853, USA. 3 Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK. 4 School of Forestry and Resource Conservation, National Taiwan University, Taipei 106, Taiwan. Correspondence and requests for materials should be addressed to S.-F.S. (email: [email protected]) or to H.-W.Y. (email: [email protected]). NATURE COMMUNICATIONS | 3:885 | DOI: 10.1038/ncomms1894 | www.nature.com/naturecommunications © 2012 Macmillan Publishers Limited. All rights reserved. ARTICLE NatUre cOMMUNicatiONS | DOI: 10.1038/ncomms1894 ooperation and conflict are inevitable and opposing forces within social groups1–4. Much attention has focused on Chow ecological factors affect the benefits of cooperation5. For example, lower predation risk, greater foraging efficiency and better access to critical resources have been considered the three general classes of social benefits favouring group living6. It is also well accepted that ecological factors have a critical role in the evolu- tion of social groups, such as ecological constraints7 and fluctuat- ing environments8,9 that favour group formation and cooperative breeding. However, social conflict and its resolution within a sta- ble social group have mostly been understood by studying intrinsic individual properties, such as genetic relatedness between inter- acting individuals10, resource holding power11, and behavioural strategies such as tit-for-tat responding12, punishment13 and polic- ing14,15. Although the cost c and benefit b terms in Hamilton’s rule (rb-c > 0) inherently involve ecology, the precise role of ecology in influencing within-group conflict remains poorly understood16,17, presumably because the relative importance of social and ecologi- Figure 1 | A group of Taiwan yuhinas huddled together (© I.-D. Lin). cal factors are often difficult to disentangle in most social animals forming groups with kin. In plural breeding species, where two or more females breed in fitness, s(f, x), declines linearly with total brood size, f + (n − 1)f′, a social group and provide parental care to their offspring, conflict where a focal individual produces f offspring and f ′ represents over the relative share of reproduction is often intense. Extreme the population average number of a non-focal individual’s offspring expressions of social conflict, such as egg destruction and infanti- production. We also assume that parents’ competitive effort, x, cide, commonly occur in many plural breeding birds, mammals and also influences the offspring’s share of group resources and thus social hymenoptera, for example refs 18–20. Plural breeding spe- offspring fitness. For example, adult females’ competitive effort for cies are therefore good subjects for studying how group members early egg laying and early onset of nocturnal incubation resulted resolve breeding conflicts. However, little is known about how eco- in asynchronous hatching, and early hatchlings had a competitive logical factors affect the intensity and the fitness consequences of advantage in joint-nesting yuhinas (see text for details). An these social conflicts. individual young’s fitness is We develop a game-theoretical model to explore the theoreti- cal consequences of how ecological factors affect social conflict in x the context of cooperatively breeding species. We make four gen- s[, f f′′,, x x](=1 − f +()n − 1 f ′)/a , (1) x+() n −1 x′ eral assumptions to depict the tragedy of the common scenario in the joint-nesting system of yuhinas: first, we assume that offspring fitness increases as its share of group resources increases; second, where x′ represents the average population competitive effort, an offspring’s share of group resources decreases as joint-brood size offspring’s fitness is proportional to their parents’ competitive increases, and increases as its parent’s competitive effort increases; effort and a represents the rate of decline of offspring fitness with third, breeding adults sustain personal costs of producing young the increase in total brood size (a > n(f + (n − 1) f ′) so that offspring and competing for group resources in the form of reduced future fitness s will be greater than zero). Small values of a indicate that fitness; and finally, we assume that ecological conditions influence there is a larger negative impact on offspring fitness with larger the costs of investing in offspring production and engaging in social brood size. We also assume that producing offspring and investing conflict, and these costs are higher in poorer ecological conditions. in competitive effort entail personal costs, ef f and exx, respectively; We then studied how ecological conditions affect social conflict ef and ex increase as ecological conditions deteriorate. Furthermore, and fitness of plural breeding and joint-nesting Taiwan yuhinas we assume that investing in current reproduction and social conflict (Y. brunneiceps) to test the predictions of our models (Fig. 1). Most entails an accelerating future fitness cost26 to the mothers and thus 2 individuals (about 90%) live in social groups that are comprised of the cost function is equal to (eff + exx) . two to three unrelated monogamous pairs and sometimes one non- To determine the evolutionarily stable producing and competing breeding helper21. The joint-nesting behaviour of yuhinas, in which strategies, we adopt the ‘direct fitness’ approach27–29; this is equiva- multiple females contribute eggs to a group nest and provide paren- lent to the neighbour-modulated fitness of Hamilton12. The direct tal care, is a rare form of cooperative breeding22. We examined four fitness payoff of a group member is given by types of competitive strategies between co-breeding female yuhi- nas, namely, preventing other co-breeding females’ access to the w= fs[, f f′′,, x x](− e f + e x),2 (2) nest during egg laying; laying more eggs; early onset of nocturnal f x incubation to facilitate the earlier development of one’s own eggs; and egg tossing. The evolutionarily stable levels of competitive effort x′ and number of offspring produced f* adopted by group members must Results then satisfy Description of the cooperatively breeding model. We explore the theoretical consequences of how ecological factors affect ∂w ∂w ∂w ∂w social conflict in the context of cooperatively breeding species. + r f= f ′= f * = 0 , and + r f= f ′= f * = 0, (3a,b) ∂f ∂f ′ x= x′=x* ∂x ∂x′ x= x′=x* We explicitly consider that ecology can affect the costs of offspring production and cost of investing in social conflict in groups with n breeders. We assume that if ecological conditions are poor, the where r denotes the relatedness among group members. This equa- cost of producing offspring is higher. Specifically, building on the tion represents the condition that the evolutionarily stable levels of costly young model23–25, we assume that an individual young’s competitive effect and brood size should be the ones yielding the NatUre cOMMUNicatiONS | 3:885 | DOI: 10.1038/ncomms1894 | www.nature.com/naturecommunications © 2012 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1894 ARTICLE highest fitness in a population of individuals exhibiting those same where zij and c(zij, E) are competitive effort and the personal cost of strategies. Therefore, no alternative mutant strategy can invade the the competitive effort, respectively, of the jth individual of the ith population. The solutions, which correspond to the maximum of group. E indicates ecological condition (larger E represents better the individual fitness, are equal to ecological conditions). zi is the average level of competitive effort and g(zi, E) is the group productivity function in the ith group. 4 2 17,27,31 ((−2ef + an 1+() n −1 r)( H +ef an((1 − 3 n +n − 1)) r + 2H Frank and subsequent models have focused on specific group x* = , and 2e e an( 4e 2(((1+n − 1)( r +an 1 + n+() n − 1 r)2 productivity functions and no personal cost of social conflict. Here, f x f we explore a range of alternative possibilities, subject only to the ∂ ∂ ∂2 ∂ 2≤ 1 conditions that: (a) g/ z < 0 and g/ z 0, representing group = f * 2 (4a,b) productivity as a decreasing and linear or accelerating function of ef + an((1 + n +n − 1)) r + H competitive effort; and (b)∂ 2g/∂z∂E > 0 and ∂g/∂E≥0, indicating that group productivity is an increasing function of ecological condi- where H= e2((e 2 + 2 an n − 1 + r− nr). tion with diminishing returns but group productivity decreases less f f rapidly in good ecological conditions; (c) ∂c/∂z > 0, ∂2c/∂z2 > 0 and ∂2c/∂z∂E < 0, implying that proportional personal cost is an increas- Results of the cooperatively breeding model.