Within- and between-group dynamics in an obligate cooperative breeder

1. Cooperative behaviour can have profound effects on demography. In many cooperative species, components of fitness (e.g. survival, reproductive success) are diminished in smaller social groups. These effects (termed group-level component Allee effects) may lead smaller groups to grow relatively slowly or fail to persist (termed group-level demographic Allee effects). 2. If these group-level effects were to propagate to the population level, small populations would grow slowly or decline to extinction (termed population-level demographic Allee effects). However, empirical studies have revealed little evidence of such population-level effects. 3. Theoretical studies suggest that dispersal behaviour could either cause or prevent the propagation of group-level Allee effects to the population level. We therefore characterised within-and between-pack dynamics in a population of African wild dogs (Lycaon pictus) to test these contrasting model predictions. 4. Larger wild dog packs produced more pups and their members experienced higher survival than those in smaller packs. Nevertheless, larger packs grew more slowly than smaller packs, because natal adults dispersed away from them. Most packs either died out in whole-pack death events, or broke up when their founders died, irrespective of pack size. Overall, packs showed negative density dependence rather than group-level demographic Allee effects. 5. Larger packs produced more, but not larger, dispersal groups, and hence generated more, but not larger, new packs. Larger packs thus contributed more than smaller packs to the number of packs in the population, but their large size did not propagate to their daughter packs. This pattern helps to explain the absence of population-level Allee effects in this species. 6. Dispersal behaviour, itself driven by natural selection on individual reproductive strategies, played a pivotal role in population dynamics, leading to the formation of new packs and limiting the size of established packs. Understanding dispersal processes is likely to be important to understanding the population dynamics of other cooperatively breeding species.

1. 合作行为可对种群统计学（demography）产生深远影响。在诸多合作繁育物种中，适合度（fitness）的组成部分（如存活率、繁殖成功率）在小型社会群体中会出现下降。这类效应被称为群体水平组分阿利效应（group-level component Allee effects），可能导致小型群体增长相对缓慢，或无法持续存续，即群体水平种群统计学阿利效应（group-level demographic Allee effects）。
2. 若此类群体水平效应蔓延至种群层面，小型种群将增长缓慢甚至衰退至灭绝，这类效应被称为种群水平种群统计学阿利效应（population-level demographic Allee effects）。然而，实证研究鲜有发现此类种群水平效应的证据。
3. 理论研究表明，扩散行为既可能促成也可能阻碍群体水平阿利效应向种群层面的蔓延。为此，我们对非洲野犬（African wild dogs, *Lycaon pictus*）种群的群体内部及群体间动态进行了表征，以检验这两种对立的模型预测。
4. 体型更大的非洲野犬群能产出更多幼崽，且群体成员的存活率高于小型群体。但由于本群成年个体（natal adults）会从大群中扩散离开，大群的增长速度反而慢于小群。多数群体要么在全群死亡事件中彻底消亡，要么在创始个体死亡后解体，且这一情况与群体规模无关。总体而言，野犬群表现出负密度依赖效应（negative density dependence），而非群体水平种群统计学阿利效应。
5. 更大的野犬群能产生更多而非体型更大的扩散群体，因此能够形成更多而非更大的新群体。由此，大群对种群中群体总数的贡献高于小群，但其自身的大规模并未传递至子代群体。这一模式有助于解释该物种为何未出现种群水平阿利效应。
6. 扩散行为本身由基于个体繁殖策略的自然选择所驱动，在种群动态中发挥了关键作用：它促成了新群体的形成，并限制了现存群体的规模。理解扩散过程，或有助于理解其他合作繁育物种的种群动态。