SEXUALLY DIMORPHIC HEAD SIZES AND REPRODUCTIVE SUCCESS IN THE SLEEPY LIZARD TILIQUA-RUGOSA

In 1993, 458 males and 346 females of the large Australian skink, Tili qua rugosa, were captured in a study area near Mt. Mary, South Austral ia. Females were significantly longer than males, although there was b road overlap in snout-vent length measures. Males had significantly lo nger and broader heads than females of equivalent snout-vent length. I n the spring some, but not all, lizards formed monogamous pairs. Pairi ng was used as an indirect indicator of reproductive success. When all adult males were considered there was no significant difference in he ad size between those found paired or unpaired. However, among small a dults, paired males had significantly broader heads than unpaired male s. This supports the hypothesis that head size is under sexual selecti on. Individuals with wider heads could be more successful in male-male combat where jaws are the major offensive weapon. Younger, smaller ma les with a wide head could gain mates at an earlier age. Females showe d a different pattern. In all females, and most strongly amongst large r size classes, paired females had significantly larger heads than unp aired females. An explanation is that larger heads somehow reduce the chance that a female will skip a year of reproduction, although the me chanism is not clear.