Biro PA, Abrahams MV, Post JR, Parkinson EA. (2006) Behavioural trade-offs between growth and mortality explains evolution in submaximal growth rates. Journal of Animal Ecology 75: 1165-1171. doi: 10.1111/j.1365-2656.2006.01137.x
Stamps JA. (2007) Growth-mortality tradeoffs and ‘personality traits’ in animals. Ecology Letters 10: 355-363. doi: 10.1111/j.1461-0248.2007.01034.x
Body size of animals is one of the most important traits of ecological and evolutionary interest. A large body of data shows that body size is strongly correlated to many physiological and fitness traits. Body size, or growth, often used as a proxy for fitness. In general, fecundity selection, male-male competition and female mate-choice –sexual selection in short – all favour larger body size. Hence, the fact that not all populations of all animal species are evolving into giants (being limited by only physiological and developmental constraints) suggests a serious cost of large body size and/or high growth rate. One such cost is expected to be predation. In order to have higher growth rate (or to maintain a larger body), besides some physiological shifts, an individual mainly needs to increase its energy input. Increased energy input might translate to higher foraging efforts and higher predation risk as a consequences in many cases.
Biro et al. (2006) tested the above idea by whole-lake introduction experiments. They introduced lab-reared rainbow trout (Oncorhynchus mykiss) to small experimental ponds that differed in predation. There were three predator-free ponds and four ponds with high predation. By introducing similarly sized young fish of two trout strains, one wild and one domestic (= fast growing), in similar amounts into the ponds, Biro et al. could test for risk taking, survival and growth of the two strains under different predation pressures. They found that genetically fast-growing individuals in the predatory ponds occurred in the risky but profitable habitats far more often, and also grew faster, but faced higher mortality than fish from the wild strain. Size difference between the strains was not affected by predation.
The former study suggests a complex behavioural switch linked to faster growth. Behavioural ecologist thinking had partly turned towards studying multiple behavioural traits and/or traits under different contexts together in the last years, where correlations between traits are interpreted as behavioural syndromes or animal personalities. Up to date, the idea that behavioural syndromes are emerging as a result of adaptation is supported, but the causes and mechanisms remain enigmatic. Stamps (2007) suggests that physiological constraints or needs can affect a suit of behavioural traits together and might result in behavioural syndromes. For instance, in a certain situation where high growth rate is favourable, many behavioural traits (activity, aggressiveness, boldness, risk taking, etc.) should change together to achieve it. This way of argumentation might be valid both at the levels of individual “decisions” (plasticity) and evolution, and can be useful in explaining both within and between population differences.
I think that the Stamps’s hypothesis is a very tempting one that could be tested straight on natural systems where populations differ in predatory risk and hence, where the cost-benefit ratio of high growth and large body size differs.
GH
