An experimental test of density-dependent selection on temperament traits of activity, boldness and sociability.

Temperament traits are seen in many animal species, and recent evolutionary models predict that they could be maintained by heterogeneous selection. We tested this prediction by examining density-dependent selection in juvenile common lizards Zootoca vivipara scored for activity, boldness and sociability at birth and at the age of 1 year. We measured three key life-history traits (juvenile survival, body growth rate and reproduction) and quantified selection in experimental populations at five density levels ranging from low to high values. We observed consistent individual differences for all behaviours on the short term, but only for activity and one boldness measure across the first year of life. At low density, growth selection favoured more sociable lizards, whereas viability selection favoured less active individuals. A significant negative correlational selection on activity and boldness existed for body growth rate irrespective of density. Thus, behavioural traits were characterized by limited ontogenic consistency, and natural selection was heterogeneous between density treatments and fitness traits. This confirms that density-dependent selection plays an important role in the maintenance of individual differences in exploration-activity and sociability.

Immediate and delayed life history effects caused by food deprivation early in life in a short-lived lizard.

Detailed studies of the mechanisms driving life history effects of food availability are of prime importance to understand the evolution of phenotypic plasticity and the capacity of organisms to produce better adapted phenotypes. Food availability may influence life history trajectories through three nonexclusive mechanisms: (i) immediate and long-lasting effects on individual quality, and indirect delayed effects on (ii) intracohort and (iii) intercohort interactions. Using the common lizard (Zootoca vivipara), we tested whether a food deprivation during the two-first months of life influence life history (growth, survival, reproduction) and performance traits (immunocompetence, locomotor performances) until adulthood. We investigated the underlying mechanisms and their possible interactions by manipulating jointly food availability in a birth cohort and in cohorts of older conspecifics. Food deprivation had direct immediate negative effects on growth but positive long-lasting effects on immunocompetence. Food deprivation had also indirect delayed effects on growth, body size, early survival and reproduction mediated by an interaction between its direct effects on individual quality and its delayed effects on the intensity of intercohort social interactions combined with density dependence on body size. These results demonstrate that interactions between direct and socially mediated effects of past environments influence life history evolution in size-structured and stage-structured populations.