Factors influencing testes size in the Cape ground squirrel

The impact of body condition, age, parasite abundance, food abundance, reproductive tactic, and testosterone on the testes size of the Cape ground squirrel was assessed. Body condition was obtained from the residuals of a log mass log spine length regression. Age is estimated in years using the time of scrotal descent in males. A normalised difference vegetation index (NDVI) was obtained from Landsat 8 satellite imagery. A mean NDVI value was obtained from each image reflecting the squirrel's primary foraging habitat at the time of capture. The reproductive tactic of each male (natal or band) was recorded from behavioural observations and trap location data. Parasites were counted during an inspection of the inner thigh and groin area, and males were combed down the left, right, and central planes of the back with a metal flea comb. Blood from the femoral vein was assayed to determine circulating testosterone concentration. Testes size was recorded in mass (g), calculated from linear dimensions using the ellipsoid volume formula. Testes mass reflects the combined mass of each testicle. Relationships between each variable and testes mass were assessed using linear mixed models with the squirrel's unique identification number as a random effect.