Clutch-size adjustments and skew models: effects on reproductive partitioning and group stability

Reproductive skew theory seeks to integrate social and ecological factors thought to influence the division of reproduction among group-living animals. However, most reproductive skew models only examine interactions between individuals of the same sex. Here, we suggest that females can influence group stability and conflict among males by modifying their clutch size and may do so if they benefit from the presence of subordinate male helpers or from reduced conflict. We develop 3 models, based on concessions-based, restraint, and tug-of-war models, in which female clutch size is variable and ask when females will increase their clutch size above that which would be optimal in the absence of male-male conflict. In concessions-based and restraint models, females should increase clutch size above their optima if the benefits of staying for subordinate males are relatively low. Relatedness between males has no effect on clutch size. When females do increase clutch size, the division of reproduction between males is not influenced by relatedness and does not differ between restraint and concessions-based models. Both of these predictions are in sharp contrast to previous models. In tug-of-war models, clutch size is strongly influenced by relatedness between males, with the largest clutches, but the fewest surviving offspring, produced when males are unrelated. These 3 models demonstrate the importance of considering third-party interests in the decisions of group-living organisms.

Man-induced activities modify demographic parameters in a long-lived species: effects of poisoning and health policies

Recent changes in sanitary policies within the European Union (EU) concerning disposal of carcasses of domestic animals and the increase of non-natural mortality factors, such as illegal poisoning, are threatening European vultures. However, the effects of anthropogenic activities on demographic parameters are poorly studied. Using a long-term study (1994–2011) of the threatened Pyrenean Bearded Vulture Gypaetus barbatus population, we assess the variation in the proportion of breeding pairs, egg-laying dates, clutch size, breeding success, and survival following a sharp reduction in food availability in 2005 due to the application of restrictive sanitary policies decreasing livestock carcass availability. We found a delay in laying dates and a regressive trend in clutch size, breeding success, and survival following policy change. The maintenance of specific supplementary feeding stations for Bearded Vultures probably reduced the negative effects of illegal poisoning and food shortages, which mainly affected subadult survival. A drop in food availability may have produced changes in demographic parameters and an increase in mortality due to an increased exposure to contaminated food. As a result, supplementary feeding as a precautionary measure can be a useful tool to reduce illegal poisoning and declines in demographic parameters until previous food availability scenarios are achieved. This study shows how anthropogenic activities through human health regulations that affect habitat quality can suddenly modify demographic parameters in long-lived species, including those, such as survival, with high sensitivity to population growth rate.

Mothers adjust egg size to helper number in a cooperatively breeding cichlid

Mothers should adjust the size of propagules to the selective forces to which these offspring will be exposed. Usually, a larger propagule size is favored when young are exposed to high mortality risk or conspecific competition. Here we test 2 predictions on how egg size should vary with these selective agents. When offspring are cared for by parents and/or alloparents, protection may reduce the predation risk to young, which may allow mothers to invest less per single offspring. In the cooperatively breeding cichlid Neolamprologus pulcher, brood care helpers protect group offspring and reduce the latters' mortality rate. Therefore, females are expected to reduce their investment per egg when more helpers are present. In a first experiment, we tested this prediction by manipulating the helper number. In N. pulcher, helpers compete for dispersal opportunities with similar-sized individuals of neighboring groups. If the expected future competition pressure on young is high, females should increase their investment per offspring to give them a head start. In a second experiment, we tested whether females produce larger eggs when perceived neighbor density is high. Females indeed reduced egg size with increasing helper number. However, we did not detect an effect of local density on egg size, although females took longer to produce the next clutch when local density was high. We argue that females can use the energy saved by adjusting egg size to reduced predation risk to enhance future reproductive output. Adaptive adjustment of offspring size to helper number may be an important, as yet unrecognized, strategy of cooperative breeders.