Sexual dimorphism in the baiji, Lipotes vexillifer

Abstract

Dimorphism was detected in body length and in skull dimensions of the baiji, Lipotes vexillifer. The age of a sample comprising 22 females and 14 males was estimated. A single-phase Laird growth model was fitted to the body length versus age data for females. Two curves were required for the males, one for those with 4 dentinal growth layer groups (GLGs) and another for those above that age, i.e., with more than 4 GLGs. Males and females have about the same growth rate until they have about 4 GLGs, after which males grow more slowly than females. Sexual dimorphism was also found in 9 external measurements, based on t-tests. Analysis of covariance showed that once the effect of body length was removed, only 1 of the 9 measurements, from the tip of the upper jaw to the genital aperture, differed significantly between males and females. Differences in skull measurements resulted mainly from an early slowdown in the rate of longitudinal growth of the male, before the increase in body length slowed. Females and males could be correctly sexed by discriminant analysis using the body length, condylobasal length, and zygomatic width, and also the condylobasal length, width of the rostrum at the base, and the zygomatic width, except for one young female. Eight skull length measurements were greater in adult females than in adult males after the effect of body length was removed, and five skull width measurements were greater in males when the effect of condylobasal length was removed. Asymmetry in the position of the blowhole and in skew of the skull is pronounced and is independent of body length and sex.