Effects of testosterone on aggressive behaviour after social mixing in male lambs

The aim of this article was to study the effects of testosterone administration on the aggressive behaviour of male prepubertal lambs after mixing unacquainted individuals. Forty male lambs from two different flocks (1 and 2), 20-22 kg body weight and 10-12 weeks old, were distributed into four groups: C1 and C2 (control groups), and T1 and T2 (testosterone-treated groups). Groups C1 and T1 came from flock 1, while groups C2 and T2 came from flock 2. Animals in groups T1 and T2 were given 250 mg of testosterone enantate intramuscularly on the first experimental day. Animals from the same flock were kept as a group during the first 15 days; on Day 16, control animals (C1 and C2) were moved into one pen and testosterone-treated animals (T1 and T2) into another one, where they were kept for 3 additional days. Aggressive interactions and hormone plasma levels were recorded both before and after social mixing. Before social mixing, testosterone did not modify the frequency of aggressive interactions, while it increased the probability of an animal to receive an aggression. After social mixing testosterone- treated animals initiated and received more aggressions than control animals. Moreover, after social mixing, testosterone-treated animals directed their aggressions mainly towards unfamiliar animals, whereas control animals directed theirs aggressions towards familiar and unfamiliar animals in the proportion expected by chance. It is suggested that the effects of testosterone on aggressive behaviour are context dependent, and that testosterone increases an animal's tendency to establish dominance relationships, rather than its aggressiveness per se.