Abstract
Individuals within species differ in their behavioural reactions to the environment. Consistent individual
differences in these behaviours (personality traits) are often correlated and known to be under natural
selection. These differences are frequently associated with variation in physiological traits, such as
endocrine profiles. For example, variation in circulating testosterone levels is associated with variation in
several personality traits and has been hypothesized to be a marker for personality in humans and
rodents. The importance of testosterone in controlling both behavioural strategies and individual
physiological differences suggests that direct selection on personality traits might cause pleiotropic
selection on the physiological mechanisms underlying these traits. To test this hypothesis, we quantified
levels of plasma testosterone levels and measured phytohaemagglutinin (PHA)-induced immune
responses of male great tits, Parus major, in lines artificially selected for diverging levels of avian
personality (‘fast’ and ‘slow’ exploratory behaviour). We found that testosterone levels were highly
repeatable within individuals and fluctuated predictably over the season. Contrary to our expectations,
‘slow’ explorers had consistently higher levels of baseline testosterone and higher immune responses
than ‘fast’ explorers. These results show that phenotypic selection for variation in personality traits
corresponds to consistent differences in hormone profile and immune function, but that higher
aggression levels do not need to be associated with higher baseline testosterone levels. Our results
confirm that personality traits have evolved as a result of selection on both the underlying controlling
physiological mechanisms and the phenotypic traits.
| Original language | English |
|---|---|
| Pages (from-to) | 1055-1061 |
| Journal | Animal Behaviour |
| Volume | 81 |
| Issue number | 5 |
| DOIs | |
| Publication status | Published - 2011 |