Social rank, hormones and reproductive behaviour of male brushtail possums (Trichosurus vulpecula) : implications for biocontrol

Abstract

The relationship between social rank and plasma testosterone and cortisol levels was investigated in brushtail possums (Trichosurus vulpecula) in captive triads held at the Landcare Research Manaaki Whenua animal facility, Rangiora, New Zealand. Potential biocontrol methods that inhibit possum reproduction may target immunosterilisation or sex steroid function. However their success relies on behaviour remaining unchanged. Assumptions about behaviour are also important when the effects of immunocontraception are being modelled. I assessed firstly whether changes in testosterone concentrations cause changes in male possums' social and reproductive behaviour. Fourteen triads of two male and one female possum were observed in outdoor pens and their interactions scored to ascertain their dominance relationships during periods of hierarchy formation and hierarchy stability. Possums were blood sampled weekly to measure plasma testosterone and cortisol levels. During the period of hierarchy formation, dominant males had higher plasma testosterone than subordinate males, but there was no correlation between social rank and cortisol concentrations. Once hierarchies had stabilised, there was no difference in testosterone and cortisol levels among dominant and subordinate males. Seven dominant males were castrated during the nonbreeding season and observed to determine whether there were any changes in social rank in the triad. Hierarchy stability was unaffected by castration of these males, and there was no change in intensity or frequency of interactions among the members of each triad. However during the breeding season, dominant males had higher plasma testosterone levels than subordinates. When the dominant male was removed from triads, the testosterone concentrations of subordinates did not increase. Male reproductive behaviour was also observed during the breeding season. Of three matings seen, all were by dominant males. Although three attempted matings were by subordinate males were observed (14% of attempts), all of these were repelled by the female. Although sample sizes were small, it seems likely that both male rank and 'female choice' are important in reproductive behaviour. Both disruption of sex steroid production in male possums and immunocontraception appear to be promising avenues for biocontrol. However further information on reproductive behaviour in the wild and the effect of castration on behaviour during unstable periods would be useful to ensure that social behaviour is not altered by these control methods, and that modelling assumptions are met.