Turning tables: food availability shapes dynamic aggressive behaviour among asynchronously hatching siblings in red kites Milvus milvus

Aggression represents the backbone of dominance acquisition in several animal societies, where the decision to interact is dictated by its relative cost. Among siblings, such costs are weighted in the light of inclusive fitness, but how this translates to aggression patterns in response to changing external and internal conditions remains unclear. Using a null-model-based approach, we investigate how day-to-day changes in food provisioning affect aggression networks and food allocation in growing red kite (Milvus milvus) nestlings, whose dominance rank is largely dictated by age. We show that older siblings, irrespective of age, change from targeting only close-aged peers (close-competitor pattern) when food provisioning is low, to uniformly attacking all other peers (downward heuristic pattern) as food conditions improve. While food allocation was generally skewed towards the older siblings, the youngest sibling in the nest increased its probability of accessing food as more was provisioned and as downward heuristic patterns became more prominent, suggesting that different aggression patterns allow for catch-up growth after periods of low food. Our results indicate that dynamic aggression patterns within the nest modulate environmental effects on juvenile development by influencing the process of dominance acquisition and potentially impacting the fledging body condition, with far-reaching fitness consequences.

Seasonal variation in cuckoldry rates in the socially monogamous cichlid fish Variabilichromis moorii

Mating patterns in animal populations can respond to environmental conditions and consequently vary across time. To examine this variation in nature, studies must include temporal replicates from the same population. Here, we report temporal variation in genetic parentage in the socially monogamous cichlid Variabilichromis moorii from Lake Tanganyika, using samples of broods and their brood-tending parents that were collected across five field trips from the same study population. The sampled broods were either spawned during the dry season (three field trips) or during the rainy season (two trips). In all seasons, we detected substantial rates of extra-pair paternity, which were ascribed to cuckoldry by bachelor males. Paternity shares of brood-tending males were consistently higher, and the numbers of sires per brood were consistently lower, in broods that were spawned in the dry seasons compared to broods from the rainy seasons. In contrast, the strength of size-assortative pairing in our V. moorii population did not vary temporally. Seasonal fluctuations in environmental conditions, such as water turbidity, are proposed as a mechanism behind variable cuckolder pressure. Our data demonstrate the utility of long-term monitoring to improve our understanding of animal mating patterns.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10750-022-05042-0.

Monks relax sibling competition over parental resources in Tibetan populations

Why parents in some societies induce some of their sons to become religious celibates is an evolutionary puzzle. Some have speculated that this might be associated with brother competition for family resources. However, the behavioral ecology of monks and the possible links with competition between brothers remain unexplored. Here, we use demographic data from Amdo Tibetan agropastoralists in western China to evaluate what factors determine the probability of becoming a monk and explore the possible association between wealth and having a monk brother. We found that boys with at least one older brother are more likely to become celibate monks. Patrilocal heads of household, who inherit parental property, are more likely to be first-born sons, whereas men who marry uxorilocally, that is they move to their wife’s household, are generally second- or later-born sons. Moreover, we find that men with at least one monk brother are wealthier than men who only have non-celibate brothers. Together, these results suggest that sending a son to the monastery is a way for parents to decrease competition between brothers over family resources. Harsh and resource-limited environments, like the one we consider, can lead to the emergence of communal households, including polyandrous families, which used to be common in Tibetan areas. Directing one son to become a religious celibate offers a potentially effective solution to brother competition in our population.

The functional role of sibling aggression and “best of a bad job” strategies in cichlid juveniles

Siblings often compete for limited resources, such as food provided by their parents. However, although several functions of nonlethal sibling (nonsiblicidal) aggression have been proposed, there is currently little empirical evidence for these, apart from food monopolization. Here, we investigated the functions of nonlethal sibling aggression in the biparental-caring territorial herbivorous cichlid Varibilichromis moorii. We found that the juveniles of this species are highly aggressive and that larger juveniles are more aggressive toward their smaller siblings. Larger juveniles feed on algae more frequently than smaller siblings, thereby indicating a dominance hierarchy. Sibling aggression decreased when algae in the nest was experimentally removed. Furthermore, the removal of smaller juveniles decreased sibling aggression among the remaining larger juveniles, whereas the removal of larger juveniles increased aggression among smaller juveniles. The algal feeding rate of juveniles only increased when larger individuals were removed from the nest. Moreover, larger juveniles attained higher growth rates and remained in natal nests longer than smaller individuals. Our results indicate that sibling aggression may facilitate the monopolization of resources by larger juveniles and extend the parental care period. Interestingly, a small subset of juveniles was observed to migrate to other nests. These juveniles were larger than those of the host brood, and their growth rate increased within the new nests. We suggest that subordinate juveniles may disperse from natal nests and sneak into new nests to enhance their rank, which may represent a novel example of a “best of a bad job” strategy associated with sibling competition.