Association between horn size anthropometrical parameters and scrotal circumference of gemsbok bulls (Oryx gazella)

Abstract

Commercial game breeding practices place immense pressure on the selection of antelope species with long horns and improved genetics in order to maximise their monetary values. Current hunting policies are aimed at the longest-horned individuals within a population and the longevity of such individuals is drastically reduced. Eliminating long-horned animals from natural populations will shift their dynamics toward young and inexperienced males carrying short horns. In order to either support or dissuade current gaming industry directives, the genuine influence of extra-long horns on the male antelope’s fertility and overall masculine appearance should be thoroughly investigated. The objective of this study was therefore to determine the correlation between horn size and scrotal circumference of gemsbok bulls. The gemsbok population sampled in this study was periodically subjected to culling. Bulls (n = 144) were harvested by professional hunters on a random seeing-equals-shooting basis. However, due to continuous growth and the onset of senescence, respectively, males younger than 3 years and older than 10 years were excluded, reducing n to 106 animals. Individually marked carcases were categorised into either age group 1 (3 years ≤ x ≤ 5 years) or age group 2 (5 years < x ≤ 7 years), weighed and the relevant body and horn parameters recorded. These parameters were determined using a measuring tape and comprised of the following: heart girth, body length, face length, neck circumference, cannon bone length, cannon bone circumference, scrotal circumference (SC) and shoulder circumference, as well as horn lengths, apex lengths (distance between last ring and tip of horn), horn ring number, long horn (long horn), total horn (sum of right and left horns), tip-totip, and horn base circumferences. The influence of the different age groups on individual body and horn parameters was analysed and the only non-significant difference in means observed for body measurements was for cannon bone circumference (CBC). This observation meant that bone circumference did not increase, irrespective of age. With regards to horn measurements only horn rings left (HRL) showed to be significantly different (P≤0.05) between the age groups, indicating higher ring numbers for older bulls. All the parameters were then correlated with SC to test the perceptions that male appearance and weaponry would be significantly impacted by changes in scrotal size. Heart girth, body length, face length, neck circumference, and shoulder circumference were found to be greatly influenced by SC (P≤0.001) in young bulls (age group 1), suggesting that any developmental change in one would significantly affect the other. These strong relationships also implied that SC gave a good indication as to a gemsbok bull’s growth and physical conformation up to the age of 5 years, and that bulls exhibiting large testes would have increased body weights and larger anthropometric measures. It appeared, however, that all relevant growth parameters reached their maximum points as they showed no further increase in bulls older than 6 years (age group 2). Looking at age group 2, cannon bone length was the only parameter significantly (P≤0.01) correlated with SC. This finding suggested that significant bone growth only occurred after a specific threshold for SC was achieved. Both apex lengths for age group 1 were highly significant (P≤0.001), but negative in association with SC, suggesting a reduction in apex lengths with scrotal enlargement. These correlation coefficients allowed for the calculation of relevant parameter regressions per age group. Body weight, heart girth, body length, and face length (age group 1) increased linearly with SC, as well as cannon bone length (age group 2). Left horn length, total horn, and long horn for age group 1 increased simultaneously in correlation with SC until a circumference of approximately 24 cm was reached. An increase in ring number was hinted at when both apex lengths decreased for age group 1. The opposite was observed for age group 2: ring numbers decreased significantly (P≤0.05) as SC increased which suggested a reduction in overall horn lengths for older bulls due to honing or breakage during aggressive displays. Therefore, bulls with the largest scrotums were often the most dominant and did not exhibit the longest horns. However, no significant negative impacts of larger-than-average weapons masculinity and SC was observed in young gemsbok bulls while, a negative effect was noted in males older than 6 years, so H1 was conditionally accepted in the present study area. Since the animals sampled in this study were a free-roaming population, they were not subjected to intensive breeding protocols for trophy horns. There is a need for more research on the effects of horn size on the fertility and masculinity of male antelope in populations with intensive selection pressure for horn size.