Games among cannibals: competition to cannibalize and parent-offspring conflict lead to increased sibling cannibalism

Sibling cannibalism occurs in many species, yet understanding of sibling cannibalism as an adaptation currently lags behind understanding of other antagonistic interactions among siblings. Observed sibling cannibalism phenotypes likely reflect the interaction between competitive games among siblings and parent-offspring conflict. Using a game-theoretic approach, we derive optimal offspring cannibalism behaviour and parental modifiers that limit or facilitate cannibalism. The results are compared to contemporary frequency-independent analysis. With the addition of game interactions among siblings or parent-offspring co-evolution, our model predicts increased cannibalism (compared to the frequency-independent prediction), as offspring compete to eat siblings. When infertile eggs are present--strengthening competition--offspring risk eating viable siblings in order to gain access to infertile eggs, intensifying parent-offspring conflict. We use the results to make new predictions about the occurrence of sibling cannibalism. Additionally, we demonstrate the utility of trophic egg laying as a maternal mechanism to promote egg eating.     

Seed provisioning within holly fruits: test of the hierarchical model

Seed provisioning within holly Ilex aquifolium fruits was examined after a controlled pollination experiment to obtain full-sibs. The distribution of dry mass in stony endocarp and endosperm was determined according to the rank in the size hierarchy of the sibs within fruit. Resources were more equally distributed among sibs in the endocarps than in the endosperms, which might suggest sibling competition among endosperms and no evidences of competition in the tissue controlled by the maternal sporophyte. When the sibs are ranked by size, seed provisioning fit reasonably with the hierarchical model. Differences among consecutive sibs increased down the hierarchy, the second sib in the size hierarchy was less variable in resource acquisition than the others, and the disparity in the distribution of resources among sibs decreased with fruit size increases. These results seems to be in accordance with both kin selection model and hierarchical model, which indicates at the same time control of resource provisioning by the maternal plant and differences in vigour among sibs. This revised version was published online in August 2006 with corrections to the Cover Date.     

Competitive interactions between parasitoid larvae and the evolution of gregarious development

We report experiments using two closely related species of alysiine braconids directed at understanding how gregarious development evolved in one subfamily of parasitoid wasps. Theoretical models predict that once siblicide between parasitoid wasps has evolved, it can only be lost under stringent conditions, making the transition from solitary to gregarious development exiguous. Phylogenetic studies indicate, however, that gregariousness has independently arisen on numerous occasions. New theoretical models have demonstrated that if gregarious development involves reductions in larval mobility, rather than a lack of fighting ability (as in the older models), the evolution of gregariousness is much more likely. We tested the predictions of the older tolerance models (gregariousness based on non-fighting larval phenotypes) and the reduced mobility models (gregariousness based on non-searching larval phenotypes) by observing larval movement and the outcome of interspecific competition between Aphaereta genevensis (solitary) and A. pallipes (gregarious) under multiparasitism. Differences in larval mobility matched the prediction of the reduced mobility model of gregarious development, with the solitary A. genevensis having larvae that are much more mobile. The proportion of hosts producing the solitary species significantly declined after subsequent exposure to females of the gregarious species. This contradicts the prediction of the older models (fighting vs non-fighting phenotypes), under which any competitive interactions between solitary and gregarious larvae will result in a highly asymmetrical outcome, as the solitary species should be competitively superior. The observed outcome of interspecific competition offers evidence, with respect to this subfamily, in favour of the new models (searching vs non-searching phenotypes).     

Bystanders affect the outcome of mother–infant interactions in rhesus macaques

Animal communication involves the transfer of information between a sender and one or more receivers. However, such interactions do not happen in a social vacuum; third parties are typically present, who can potentially eavesdrop upon or intervene in the interaction. The importance of such bystanders in shaping the outcome of communicative interactions has been widely studied in humans, but has only recently received attention in other animal species. Here, we studied bouts of infant crying among rhesus macaques (Macaca mulatta) in order to investigate how the presence of bystanders may affect the outcome of this signalling interaction between infants and mothers. It was hypothesized that, as crying is acoustically aversive, bystanders may be aggressive to the mother or the infant in order to bring the crying bout to a close. Consequently, it was predicted that mothers should acquiesce more often to crying if in the presence of potentially aggressive animals. In line with this prediction, it was found that mothers gave infants access to the nipple significantly more often when crying occurred in the presence of animals that posed a high risk of aggression towards them. Both mothers and infants tended to receive more aggression from bystanders during crying bouts than outside of this time, although such aggression was extremely rare and was received by less than half of the mothers and infants in the study. Mothers were also found to be significantly more aggressive to their infants while the latter were crying than outside of crying bouts. These results provide new insight into the complex dynamics of mother–offspring conflict, and indicate that bystanders may play an important role in shaping the outcome of signalling interactions between infants and their mothers.     

Aggressive monopolization of mobile carers by young of a cooperative breeder

Competition between young of the same brood or litter is of particular interest in the fields of behavioural and evolutionary ecology, because the competing individuals are likely to be closely related, where evolutionary theory predicts a greater degree of cooperation. Studies of cooperative breeding species typically concentrate on who contributes care to rearing young, and assume a passive role of the young. Relatively, little attention has been devoted to considering how intralitter competition between young affects the distribution of care in cooperative breeders. In banded mongoose (Mungos mungo) groups, the majority of pups each form a stable exclusive one-to-one association with an adult group member (its 'escort') that is its principal care provider. This paper presents experimental evidence that each pup aggressively defends access to its escort, preventing other pups approaching, and therefore monopolizes the care provided by its escort. Each pup travels with the group and follows its escort, around which its exclusion zone is fixed: a form of mobile territoriality. This represents a novel system of care of young in a mammal species, but has general implications for the study of the distribution of care of young, particularly in cooperative breeding species. Parents and helpers may provide biased care to young, not due to preference but due to the competitive actions of the young within the brood or litter.     

Begging at high level simultaneously impairs growth and immune response in southern shrike (Lanius meridionalis) nestlings

Theoretical models suggest that begging should be costly in order to be evolutionarily stable. However, evidence for such a cost is contradictory (e.g. for growth costs) or scant (e.g. for immunological costs). Here, we experimentally test the existence of both costs in southern shrike (Lanius meridionalis) nestlings. Nestlings were paired by nest of origin and similar body mass. In each pair, a nestling was forced to beg for about 30 s h(-1) , whereas the other begged for only 2 s, both nestlings receiving the same quantity of food. At the same time, the nestling response to an antigen (phytohaemagglutinin) was measured. Nestlings forced to beg for longer showed a reduction in growth rate and in immunocompetence when compared to control chicks. The two costs occurred independently of each other and were negatively correlated to time begging. These results strongly support models of honest signalling as well as scramble competition, which predict that begging should be costly in order to be evolutionarily stable.     

Parent-offspring conflict and co-adaptation: behavioural ecology meets quantitative genetics

The evolution of the complex and dynamic behavioural interactions between caring parents and their dependent offspring is a major area of research in behavioural ecology and quantitative genetics. While behavioural ecologists examine the evolution of interactions between parents and offspring in the light of parent-offspring conflict and its resolution, quantitative geneticists explore the evolution of such interactions in the light of parent-offspring co-adaptation due to combined effects of parental and offspring behaviours on fitness. To date, there is little interaction or integration between these two fields. Here, we first review the merits and limitations of each of these two approaches and show that they provide important complementary insights into the evolution of strategies for offspring begging and parental resource provisioning. We then outline how central ideas from behavioural ecology and quantitative genetics can be combined within a framework based on the concept of behavioural reaction norms, which provides a common basis for behavioural ecologists and quantitative geneticists to study the evolution of parent-offspring interactions. Finally, we discuss how the behavioural reaction norm approach can be used to advance our understanding of parent-offspring conflict by combining information about the genetic basis of traits from quantitative genetics with key insights regarding the adaptive function and dynamic nature of parental and offspring behaviours from behavioural ecology.