Traditionally, behavioural ecologists or ethologists studied certain behaviours under certain conditions, assuming that an individual can freely choose from the entire behavioural repertoire of the population in every moment and situation. Later, it has been repeatedly shown that different behaviours (like aggression and boldness) or the same behaviour in different contexts (like aggression towards competitors or offspring) can be correlated across a group of individuals. Such correlations are called behavioural syndromes or animal personalities, and invoke several interesting questions the main ones being: why and how can the correlations exist? The why is still mysterious but there are two main hypotheses about the how. The constraint hypothesis suspects genetic correlations between behaviours to be responsible, while the adaptive hypothesis assumes that natural selection had selected for the correlated behaviours. Irrespective of the possible correlations, different selective environments are expected to cause divergence in not only one but several behaviours in the same time.
We measured and compared the behaviour (drive to feed, aggression and boldness) of eight months old, individually reared ninespine sticklebacks (Pungitius pungitius) from four geographically and genetically isolated populations, representing two markedly different habitat types. In coastal marine environments (Baltic and White Seas), ninespine sticklebacks coexist with a number of predatory and competitor fish species, while in ponds (one from Sweden and one from Finland, separated by > 500km) they are the only fish species. We adopted a framework for studying complex behaviour based on Bell (2007. Proc Roy Soc B. 274:755-761) by comparing the presence/absence patterns of behavioural syndromes (correlated behaviours – a population trait) and the mean behavioural types (configuration of behaviours – an individual trait) between the populations. We found minimal evidence for the existence of behavioural syndromes: from the 12 possible correlations (four populations * three behaviours) only one was significant. However, evidence for habitat-dependent population divergence in behavioural types was profound. First, the ratio of individuals responding to the treatments (i.e. actually eating, attacking a conspecific or leaving the refuge) was habitat specific: pond fish responded in a much higher ratio than marine fish. Second, we found that pond fish were quicker feeders, more aggressive and bolder than marine fish, irrespective of population origin. Such pattern strongly implies natural selection to be the cause.
The lack of syndromes clearly rejects the constraint hypothesis about the existence of syndromes. However we could not support or reject the adaptive hypothesis. One can argue that it is possible that natural selection acted against behavioural correlations in all studied populations. Another possible line of argumentation is simply against the genetically based existence of behavioural syndromes. Most stickleback behavioural syndrome studies were based on wild-caught or group-reared fish, where the emergence of behavioural correlations can easily claimed to be a result of experience during ontogeny: either being more or less chased by predators and/or being dominant/subordinate in a social group might have the power to affect several behaviours (e.g. activity, boldness, aggression) parallel and thus to result in emerging behavioural syndromes. In fact, Bell & Sih (2007. Ecol Lett 10:828-834) have shown that exposure to predation resulted in the emergence of previously lacking behavioural syndromes in threespine sticklebacks (Gasterosteus aculeatus) as a common result of selection and plasticity.
Interestingly, the different behaviours were independent from each other within the populations (= lack of behavioural syndromes), but shifted together in response to presumed differences in the selection pressures (divergence in population mean behavioural types). Hence, we suggest that in our case drive to feed, aggression and boldness evolved together, but on an independent genetic basis as a response to the changes in predation pressure.
Herczeg G, Gonda A, Merilä J 2008. Predation mediated population divergence in complex behaviour of ninespine stickleback (Pungitius pungitius). Journal of Evolutionary Biology, in press.