Abstract
Differences in how males and females respond to foreign antigens are common across taxa. Such sexual differences in the immune system are predicted to be greater in species with high promiscuity and sociality as these factors increase the likelihood of disease transmission. Intense sperm competition is thought to further this sexual dichotomy as increased investment in spermatogenesis likely incurs additional immunological costs. Xerus inauris, a ground squirrel found throughout southern Africa, is extremely social and promiscuous with one of the highest male reproductive investments among rodents. These life-history attributes suggest males and females should demonstrate a large dichotomy in immunity. Contrary to our prediction, we found no difference in spleen mass between the sexes. However, we did find significant biases in leukocyte types and red blood cell counts, possibly reflecting responses to parasite types. Among males, we predicted greater investments in spermatogenesis would result in reduced immunological investments. We found a negative association between testes and spleen size and a positive relationship between testes and number of lice suggesting trade-offs in reproductive investment possibly due to the costs associated with spermatogenesis and immunity. We suggest when measuring sexual differences in immunity it is important to consider the effects of reproductive pressures, parasite types, and life history costs.
Highlights
Sexual selection imposes different selective pressures on males and females resulting in a dichotomy in fitness strategies [1]
We found the percentage of red blood cells to be significantly lower in males than females (t62 = 2.97, P = 0.004; Table 1)
Sex biases in parasitism often are attributed to sex-specific lifehistory strategies that affect parasite susceptibility and exposure such that differences in reproductive strategies should determine male and female immunocompetence [14]
Summary
Sexual selection imposes different selective pressures on males and females resulting in a dichotomy in fitness strategies [1]. In efforts to maximize their number of progeny, females typically invest more in processes affecting longevity [2,3]. Previous research across multiple taxa found tradeoffs between reproduction and immunity that can occur pre- and post-mating, there is little consistency in the patterns between these two processes when looking at natural populations [1,4]. This inconsistency may be attributed to the many behavioral and physiological processes an individual utilizes in an effort to counter parasitic infections [5]
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