Rotics Shay, Clutton-Brock Tim
Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK.
Kalahari Research Centre, Kuruman River Reserve, Van Zylsrus 8467, Republic of South Africa.
Proc Biol Sci. 2021 Feb 24;288(1945):20202104. doi: 10.1098/rspb.2020.2104. Epub 2021 Feb 17.
In cooperatively breeding species where rearing effort is shared among multiple group members, increases in group size typically reduce average per capita contributions to offspring care by all group members (load-lightening) but it is not known how changes in group size affect the distribution of workload among group members. The socioeconomic collective action theory suggests that, in larger groups, the incentives for free riding are stronger, leading to greater inequalities in work division among group members. Here, we use the Gini index to measure inequality at the group level in the contributions of helpers to three different cooperative behaviours (babysitting, pup-provisioning and raised guarding) in groups of varying size in wild Kalahari meerkats (). In larger groups, inequality in helpers' contributions to cooperative activities and the frequency of free riding both increased. Elevated levels of inequality were generated partly as a result of increased differences in contributions to cooperative activities between helpers in different sex and age categories in larger groups. After controlling for the positive effect of group size on total provisioning, increasing levels of inequality in contributions were associated with reductions in total pup-provisioning conducted by the group. Reductions in total pup-provisioning were, in turn, associated with reductions in the growth and survival of pups (but pup growth and survival were not directly affected by inequality in provisioning). Our results support the prediction of collective action theory described above and show how the Gini index can be used to investigate the distribution of cooperative behaviour within the group.
在合作繁殖的物种中，养育工作由多个群体成员分担，群体规模的增加通常会降低所有群体成员对后代照顾的人均平均贡献（减轻负担），但尚不清楚群体规模的变化如何影响群体成员之间的工作量分配。社会经济集体行动理论表明，在较大的群体中，搭便车的动机更强，导致群体成员之间的分工不平等加剧。在这里，我们使用基尼指数来衡量野生喀拉哈里狐獴不同规模群体中助手对三种不同合作行为（照顾幼崽、提供食物和警戒）贡献的群体层面不平等。在较大的群体中，助手对合作活动贡献的不平等以及搭便车的频率都增加了。不平等程度的提高部分是由于较大群体中不同性别和年龄类别的助手对合作活动贡献的差异增加所致。在控制了群体规模对总食物供应的积极影响后，贡献不平等程度的增加与群体进行的总幼崽食物供应减少有关。总幼崽食物供应的减少反过来又与幼崽的生长和存活减少有关（但幼崽的生长和存活不受食物供应不平等的直接影响）。我们的结果支持上述集体行动理论的预测，并展示了基尼指数如何用于研究群体内合作行为的分布。
