Data from: Adaptive shaping of the behavioural and neuroendocrine phenotype during adolescence

Environmental conditions during early life can adaptively shape the
phenotype for the prevailing environment. Recently, it has been suggested
that adolescence represents an additional temporal window for adaptive
developmental plasticity, though supporting evidence is scarce. Previous
work has shown that male guinea pigs living in large mixed-sex colonies
develop a low-aggressive phenotype as part of a queuing strategy that is
adaptive for integrating into large unfamiliar colonies. By contrast,
males living in pairs during adolescence become highly aggressive towards
strangers. Here, we tested whether the high-aggressive phenotype is
adaptive under conditions of low population density, namely when directly
competing with a single opponent for access to females. For that purpose,
we established groups of one pair-housed male (PM), one colony-housed male
(CM) and two females. PMs directed more aggression towards the male
competitor and more courtship and mating towards females than did CMs. In
consequence, PMs attained the dominant position in most cases and sired
significantly more offspring. Moreover, they showed distinctly higher
testosterone concentrations and elevated cortisol levels, which probably
promoted enhanced aggressiveness while mobilizing necessary energy. Taken
together, our results provide the clearest evidence to date for adaptive
shaping of the phenotype by environmental influences during adolescence.