Robbins Andrew M, Grueter Cyril C, Abavandimwe Didier, Stoinski Tara S, Robbins Martha M
Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany.
The Dian Fossey Gorilla Fund International, Atlanta, GA USA.
Behav Ecol Sociobiol. 2021;75(5):82. doi: 10.1007/s00265-021-03016-1. Epub 2021 Apr 22.
Two mechanisms have been proposed to explain why scramble competition can increase the travel requirements of individuals within larger groups. Firstly, individuals in larger groups may be more likely to encounter food sites where other group members have already eaten, leading to greater asynchronous "individual" travel to find fresh sites. Secondly, when food sites are aggregated into patches, larger groups may need to visit more patches to obtain the same amount of food per capita, leading to greater synchronous "group" travel between patches. If the first mechanism can be mitigated by increasing group spread, then we expect the second mechanism to be more sensitive to group size. Here, we examine the individual travel and group travel of the Virunga mountain gorillas, along with potential implications for the two mechanisms of scramble competition. Asynchronous individual travel accounted for 67% of the total travel time, and the remainder arose from group travel. Group spread increased significantly for larger groups, but not enough to prevent an increase in individual travel. Contrary to expectations, group travel decreased with size among most groups, and we found only limited evidence of patch depletion that would cause the second mechanism of scramble competition. Collectively, our results illustrate how the influence of group size can differ for individual travel versus group travel, just as it differs among species for overall travel. Studies that distinguish between the two mechanisms of scramble competition may enhance our understanding of ecological constraints upon group size, including potential differences between frugivores and folivores.
Feeding competition provides insight into how group size can influence the foraging patterns of social animals, but two key mechanisms are not typically compared. Firstly, larger groups may visit more patches to access the same amount of food per capita (group travel). Secondly, their individuals may also need to move past more spots where another member has already eaten (individual travel). Contrary to expectations, we found that group travel decreased with size for most groups of mountain gorillas, which may reflect extra travel by smaller groups to avoid larger groups. Individual travel increased with size in most groups, even though gorillas in larger groups compensated by spreading out over a broader area. The two mechanisms revealed patterns that were not apparent in our previous study of overall travel. Our approach may help to explain potential differences between folivores and frugivores.
已提出两种机制来解释为什么争夺竞争会增加较大群体中个体的移动需求。首先，较大群体中的个体可能更有可能遇到其他群体成员已经进食过的食物地点，从而导致更多异步的“个体”移动以寻找新的地点。其次，当食物地点聚集形成斑块时，较大的群体可能需要访问更多斑块才能获得相同人均食物量，从而导致斑块间更大的同步“群体”移动。如果第一种机制可以通过增加群体分散度来缓解，那么我们预计第二种机制对群体规模更敏感。在这里，我们研究了维龙加山地大猩猩的个体移动和群体移动，以及争夺竞争两种机制的潜在影响。异步个体移动占总移动时间的67%，其余部分来自群体移动。较大群体的群体分散度显著增加，但不足以阻止个体移动的增加。与预期相反，大多数群体中群体移动随规模减小，并且我们仅发现有限的斑块枯竭证据，而斑块枯竭会引发争夺竞争的第二种机制。总体而言，我们的结果表明群体规模对个体移动和群体移动的影响可能不同，就像其在不同物种的总体移动中存在差异一样。区分争夺竞争两种机制的研究可能会增进我们对群体规模生态限制的理解，包括食果动物和食叶动物之间的潜在差异。
觅食竞争有助于深入了解群体规模如何影响群居动物的觅食模式，但通常不会对两种关键机制进行比较。首先，较大的群体可能需要访问更多斑块以获取相同人均食物量（群体移动）。其次，其个体可能还需要经过更多其他成员已经进食过的地点（个体移动）。与预期相反，我们发现大多数山地大猩猩群体的群体移动随规模减小，这可能反映出较小群体为避开较大群体而进行的额外移动。大多数群体中个体移动随规模增加，尽管较大群体中的大猩猩通过在更广阔区域分散分布来进行补偿。这两种机制揭示了在我们之前对总体移动的研究中不明显的模式。我们的方法可能有助于解释食叶动物和食果动物之间的潜在差异。
