Professor University of Manitoba, Manitoba, Canada
Background/Question/Methods
Male Cape ground squirrels (Xerus inauris) are a relatively long-lived species when compared to other species of similar size, and it appears they invest primarily in survival due to advantages in post-copulatory sperm competition. Little is known about their annual survival and longevity, other than evidence that they can live up to 13 years. We hypothesized that body condition and age would positively influence male survival and that environmental factors such as rainfall would also influence annual survival. We used long-term trapping data (2011-2019) and program MARK to estimate survival based on individual effects (age and body condition) and environmental effects (rainfall) for each male (n = 460). Males were grouped by age class at first capture, either adults (n=240), subadults (n=99) or juveniles (n=121). Juveniles represented individuals of a known age while subadults would be approximately 8 months to 1 year older and adults are >1 year or older.
Results/Conclusions
We found that individuals in better body condition had increased survival and that survival was dependent on age, supporting our main hypothesis. Juveniles (Mean 0.534 SE = 0.047, 95%CI = 0.442-0.624) had the lowest estimated survival in their first year after marking compared to both subadults (Mean 0.645, SE = 0.052, 95%CI = 0.538-0.738) and adults (Mean 0.598, SE = 0.035, 95%CI = 0.528-0.664). Males first caught as subadults and adults had similar estimated survival patterns at each year. Estimated survival increased at each year since first capture for all groups until 5 years after they were marked. After this period, yearly survival decreased to between 0.254 and 0.352 for the oldest individuals in the study (7 years since marking), dependent on age class at first capture. The previous year’s rainfall had no impact on estimated annual survival. Our results suggest that males who are in better body condition have an increased likelihood of survival and that males who survive past the first 8 months to 1 year after birth see an increase in survival probability. Additionally, the decrease in estimated survival after 5 years, suggests that there is a major life event or physiological cause for decreased survival.
