Game theory predicts that at least some of the behaviour patterns displayed during aggressive encounters are used to assess asymmetries in variables that indicate fighting ability and resource value. Game theoretical models such as the sequential assessment game see assessment as the major activity during a fight. However, while these models acknowledge the existence of physical and motivational assessment parameters, there are only a few examples where a mechanism for the assessment of fighting readiness has been shown. In staged encounters between male Mediterranean field crickets, Gryllus bimaculatus, fighting behaviour follows a stereotyped escalation cascade with ritualized displays in the beginning and physical combat towards the end. Despite their larger size, heavier animals lost 30% of the encounters even if weight asymmetry was large. To examine whether the contestants provide assessment cues that might explain this surprising result, we analysed two stereotyped displays in detail (antennal fencing and mandible spreading). The duration of antennal fencing, which is necessary to initiate a fight, was independent of experience and weight asymmetry between the contestants, but was prolonged after shortening the antennae by almost 90%. Fights escalated only when antennal movement frequencies were high in both contestants. In blinded crickets few contests were settled by another ritualized display, mandible spreading, and fights that escalated beyond this stage were significantly shorter than in untreated crickets. We suggest that antennal fencing may be used to assess fighting readiness of the opponent, while mandible spreading may indicate fighting ability. We conclude that high willingness to fight may help crickets to overcome inferior fighting ability.
Many animal species show flexible behavioral responses to environmental and social changes. Such responses typically require changes in the neural substrate responsible for particular behavioral states. We have shown previously in the African cichlid fish, Haplochromis burtoni, that changes in social status, including events such as losing or winning a territorial encounter, result in changes in somatic growth rate. Here we demonstrate for the first time that changes in social status cause changes in the size of neurons involved in the control of growth. Specifically, somatostatin-containing neurons in the hypothalamus of H. burtoni increase up to threefold in volume in dominant and socially descending animals compared with cell sizes in subordinate and socially ascending fish. Because somatostatin is known to be an inhibitor of growth hormone release, the differences in cell size suggest a possible mechanism to account for the more rapid growth rates of subordinate and socially ascending animals compared with those of dominant or socially descending fish. These results reveal possible mechanisms responsible for socially induced physiological plasticity that allow animals to shift resources from reproduction to growth or vice versa depending on the social context.
The life-history strategies of organisms are sculpted over evolutionary time by the relative prospects of present and future reproductive success. As a consequence, animals of many species show flexible behavioral responses to environmental and social change. Here we show that disruption of the habitat of a colony of African cichlid fish, Haplochromis burtoni (Günther) caused males to switch social status more frequently than animals kept in a stable environment. H. burtoni males can be either reproductively active, guarding a territory, or reproductively inactive (nonterritorial). Although on average 25-50% of the males are territorial in both the stable and unstable environments, during the 20-week study, nearly two-thirds of the animals became territorial for at least 1 week. Moreover, many fish changed social status several times. Surprisingly, the induced changes in social status caused changes in somatic growth. Nonterritorial males and animals ascending in social rank showed an increased growth rate whereas territorial males and animals descending in social rank slowed their growth rate or even shrank. Similar behavioral and physiological changes are caused by social change in animals kept in stable environmental conditions, although at a lower rate. This suggests that differential growth, in interaction with environmental conditions, is a central mechanism underlying the changes in social status. Such reversible phenotypic plasticity in a crucial life-history trait may have evolved to enable animals to shift resources from reproduction to growth or vice versa, depending on present and future reproductive prospects.