Annual variation in the body condition of a small, arid-zone tortoise, Homopus signatus signatus

Homopus signatus signatus inhabits an arid region with unpredictable winter rainfall. To help understand the species’ response to variation in rainfall, we measured spring body condition (BC) over 5 years and seasonal BC over 15 months. Total rainfall influenced BC but the timing and frequency of rain were important. In 2 successive years with low rainfall, BC was significantly lower in the year when most rain fell in late winter, than in the year with more frequent showers. Females tended to have a higher spring BC than males, perhaps because females nest in spring. The low spring BC of juveniles, relative to adults, may be ascribed to the juvenile shell that is less ossified than that of adults, giving juveniles a lower body mass to shell volume ratio. Seasonal fluctuations in BC were substantial; female body mass declined by 18% from summer to mid-winter but increased after the first rains to high values in spring. We compared two BC indices, the ratio of body mass to (1) carapace length and (2) shell volume. The latter index varied more among years and provides a ratio that closely approximates the body density of tortoises.