'Voyeurism' prolongs copulation in the dragon lizard Ctenophorus fordi

Risk of sperm competition is becoming increasingly appreciated as a determi nant of male reproductive behavior in evolutionary ecology. That is, a male is under selection to adjust his ejaculate investments into a female depen ding on the mating histories of both. I investigated such behavior in the A ustralian Mallee dragon lizard (Ctenophorus fordi). Females in this species mate repeatedly and apparently indiscriminately with several partners in s uccession. Intuitively, a male mating subsequent to a rival should mate lon ger if doing so transfers more competing spermatozoa or seminal fluids that may act as paternity guards. I tested the prediction that males mate longe r with females known to have mated recently with other males in a simple ex periment using wild-caught lizards kept in outdoor enclosures. Males allowe d to observe a female's first copulation with rival males and then copulate with the same female remained in copula 60% longer (15.9 s on average) tha n the males in the first matings (9.9 s on average). In the reciprocal tria ls in which males in females' second copulations could not observe the firs t male's copulation, second males did not mate for longer than first males. Furthermore, copula duration was positively correlated with transferred ej aculate volume. Thus, the results suggest that males may prolong copulation to become more competitive when under increased risk of sperm competition.