Horn Measurements of Sable Antelope (Hippotragus niger) and Their Value in Age Determination

Abstract

The objective of this study was to describe the macroscopically observed horn growth of sable antelope and to evaluate several horn measurements in terms of its suitable use for accurate, individual age determination in this species. One hundred and eighty-eight male and one hundred and fifty-five female sable antelope served as subject animals for this cross-sectional observational study. The animals are farmed in the Northern Cape Province, South Africa. Horn measurement data was recorded for sable antelope of known age whilst they were routinely chemically immobilized for management purposes. The data sets were individually identified by recording existing ear tag numbers and/or microchip numbers of the subject animals. The dates of data collections were recorded in order to calculate the age of the subject animal in days on the day of data collection. The following measurements were taken and recorded on a field list: - Smooth tip length (unridged section of initial horn growth) - Total horn length - Number of horn ridges - Horn base circumference - Secondary/Postmature horn growth section length Horn measurements excluding growth check counts were recorded for 343 sable antelope, of which 188 were male and 155 were female. Measurements were recorded for left and right horns of the same individual. Multiple observations were recorded at different ages for the same individuals, resulting in a total number of observations as follows: Tip length, female (n = 288), male (n= 341); total horn length, female (n = 367), male (n = 457); number of ridges, female (n = 293), male (n = 349) and horn base circumference, female (n = 289), male (n = 377). The age range for female sable antelope was 127 days to 6242 days and for males 301 days to 2669 days. There were 48 recorded lengths of the secondary/postmature growth sections available as well as 38 counts of growth checks, all for mature female sable antelope. Measurements were done in inches as per the game industry standard and then converted to centimetres using a factor of 2,54 for purposes of statistical analysis. Any abnormalities were recorded on the form used to record all field data. Data was split into a male and female group. Horn growth check counts were done photographically for mature female sable antelope. The number of growth checks was recorded together with the ear tag number read from the photograph, and the photograph date was used to determine the age of the individual at the time that the photograph was taken. A sufficient number of suitable photographs of mature animals was only available for female sable antelope, as males are either sold for breeding or hunting purposes once maturity is reached. Subjective incisor evaluation also formed part of the routine clinical examination, although it did not form part of this study. In many instances, this included taking a photographic record of the incisor teeth, particularly in mature animals. Results There was a strong correlation between total horn length and age in both male and female sable antelope (female (r2 = 0.949) and male (r2 = 0.911)). There was also a strong correlation between the number of horn ridges and age for both male and female sable antelope (female (r2 = 0.949) and male (r2 = 0.911). Exponential regression models were developed for male and female sable antelopes to predict an individual animal’s age based on horn length. Because the number of horn ridges exhibited multicollinearity with the horn lengths, it was not included as an explanatory variable in these models. Although the fitted regression models explained 96% of the variation in the males’ ages and 85% of the variation in the females’ ages, large prediction errors of more than 1 000 days in the ages for some sable antelopes indicated that accurate individual age prediction with these models is not possible. In mature female sable antelope, growth checks were present in 84% of individuals for which suitable photographs were on record (n = 38). Out of the 32 individuals with clear growth checks, 31 could be accurately aged to within a year or less of actual age based on the number of growth checks present. Conclusion The total horn length of sable antelope can be useful to determine the average age within the population for any specific horn length, but cannot be used to accurately determine the age of the individual animal. The previously published dental eruption pattern remains the most accurate practical method to determine age up to the point of complete dental eruption. Growth checks counts in the secondary/postmature growth section were found to be useful for age determination in mature female sable antelope. A limitation on this finding was that clearly distinguishable growth checks were not always present in all mature female sable antelope.