Allometry and sexual selection of male weaponry in Wellington tree weta, Hemideina crassidens

Both male and female Wellington tree weta, Hemideina crassidens, use cavities in trees as diurnal shelters. That these galleries are often limiting in nature offers males the opportunity to increase their reproductive success by monopolizing galleries and the females residing in them. Male H. crassidens, can mature at either the 8th, 9th, or 10th instar, whereas females mature at the 10th instar only, and male head (and mandible) size positively covaries with ultimate instar number. It has been suggested that males fight for control of galleries by using their enlarged mandibles as weapons, and males with larger mandibles control galleries with more females. In the present study, I present a statistical examination of sexual dimorphism, showing that traits related to head size are on average significantly larger in males, whereas traits related to body size are on average significantly larger in females. I tested three predictions addressing the hypothesis that sexual selection is driving megacephaly in male H. crassidens. First, as predicted, traits related to head size show a positive allometric relationship with body size in males but not in females. Second, adapting a novel statistical technique based on maximum likelihood and bootstrapping revealed that males, but not females, exhibit a multimodal distribution in head and body size traits. This is likely a consequence of males maturing at one of three instars, which results in positive covariance between the ultimate instar number and morphological traits. Third, as predicted, single adult males with larger heads reside in galleries housing larger groups of adult females. Copyright 2005.