The ontogeny of sexual dimorphism in the skull of the springbok antelope (Antidorcas marsupialis) (918.7)

Abstract

Antelope (Artiodactyla: Bovidae) provide many classic examples of sexual dimorphism, but the ontogenetic pathway to adult sexual dimorphism has not been systematically studied. Moreover, it is unknown whether the development of obvious sexual differences in horn size correlate with the development of sexual differences in other horn traits, skull size, or skull shape. To better understand springbok skull development in general, and the development of sexual dimorphism in particular, I compared skulls from more than 50 springbok (Antidorcas marsupialis), which had been harvested, sexed and age‐estimated from a wild population in northern Namibia from 1992‐1994, and ranged from 7 months to >5 years old. Seven traits were examined: horn length, circumference, and number of horn rings, and skull size and shape in ventral and lateral views. Skulls were photographed in ventral and lateral views, landmarks were digitized, and 2D geometric morphometric analysis was performed using standard software and procedures. For each trait, male and female values were compared in yearlings and in adults to test the hypothesis that springbok sexual dimorphism increases after one year of age. By one year of age, males were significantly larger than females in horn length, horn circumference, and the sexes differed significantly in lateral skull shape. After one year of age, sexual differences increase in horn circumference and lateral skull size, and decrease and then reverse in the number of horn rings. Sexual differences were observed in nearly all trait studied. However, sexual differences arose at various times for the different traits. Whether these differences increased, were maintained, or reversed with age also depended on the trait. The developmental timing of sexual differences in size and shape indicates that sex‐specific changes in skull shape, and not allometry through male growth, are the primary mechanism for achieving sexual differences in skull shape.Grant Funding Source: Supported by an NSF‐GRF pre‐doctoral fellowship.