THE OPPORTUNISTIC SERPENT: MALE GARTER SNAKES ADJUSTCOURTSHIP TACTICS TO MATING OPPORTUNITIES by

Reproductivemales encounter potential mates under a range of circumstances that in uence the costs, bene ts or feasibility of alternative courtship tactics. Thus, males may be under strong selection to exibly modify their behaviour.Red-sided garter snakes (Thamnophis sir-talis parietalis) in Manitoba overwinter in communal dens, and court and mate in large aggre-gations in early spring. The number of males within a courting group varies considerably, as do the body sizes of both males and females.We manipulated these factors to set up replicated courtship groups in outdoor arenas, and analysed videotapes of 82 courtship trials to quantify courting behaviours of male snakes. Larger and more heavy-bodiedmales courted more vig-orously than did their smaller, thinner-bodiedrivals, and large females attractedmore intense courtship.The major effect, however, involved the number of rivalmales competing for copu-lation.Males in large groups not only reduced their overall vigour of courtship, but also mod-i ed their tactics in such a way as to bene t from the courtship activities of rival males. That is, they devoted less energy to inducing female receptivity (which requires energy-expensive caudocephalicwaving) and more effort to behaviour (tail-searching)that enhanced their own 2) Corresponding author’