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, Tiliqua rugosa, were captured in a study area near Mt. Mary, South Australia. Females were significantly longer than males, although there was broad overlap in snout-vent length measures. Males had significantly longer and broader heads than females of equivalent snout-vent length. In the spring some, but not all, lizards formed monogamous pairs. Pairing was used as an indirect indicator of reproductive success. When all adult males were considered there was no significant difference in head size between those found paired or unpaired. However, among small adults, paired males had significantly broader heads than unpaired males. This supports the hypothesis that head size is under sexual selection. Individuals with wider heads could be more successful in male-male combat where jaws are the major offensive weapon. Younger, smaller males with a wide head could gain mates at an earlier age. Females showed a different pattern. In all females, and most strongly amongst larger size classes, paired females had significantly larger heads than unpaired females. An explanation is that larger heads somehow reduce the chance that a female will skip a year of reproduction, although the mechanism is not clear.