White blood cell profiles of breeding american toads (Bufo americanus) relative to sex and body size

Immune traits may trade off against one another, or against other life history traits such as growth, development, and reproduction. Breeding introduces additional constraints on investment in immunity that may differ for each sex. During contests for access to females, males may be subjected to injuries that could result in infections. Thus, breeding males should show greater investment in first-line defenses against infection, as compared to other defenses, or as compared to investments by conspecific females. We tested these predictions of the immunoredistribution hypothesis by comparing white blood cell profiles of breeding male and female American toads. In this species, smaller males often occupy terrestrial positions as isolated satellites, while larger males are more likely to engage in attempted displacement of amplexed rivals, making the latter more likely to be injured and vulnerable to infections. Because heterophils are important first-line defenses against bacterial infections, we predicted that larger males would have higher proportions of heterophils in their leukocyte profiles than smaller males; this prediction was supported. However, contrary to expectation, females and larger males had similar proportions of heterophils, possibly because females were equally susceptible to injury during attempts to dislodge amplexed partners. Future work on white blood cell profiles of breeding amphibians is warranted, particularly for species where the sexes differ in likelihood of injury.