Effects of sex and body size on ectoparasite loads in the northern flying squirrel (Glaucomys sabrinus)

Ectoparasites can have profoundly negative fitness consequences for host organisms. Sex differences in parasite load have been documented in many mammals, and have been attributed either to the allocation of energy to growth rather than the immune system in mammals exhibiting male-biased sexual size dimorphism or to the immunosuppressive qualities of testosterone. In addition, ectoparasites can have negative effects on body size and condition, as energy is allocated to the immune system rather than to growth and maintenance. Here, we used the northern flying squirrel (Glaucomys sabrinus (Shaw, 1801)) and its ectoparasites to test two predictions: (1) males are more heavily parasitized than females and (2) individuals with high ectoparasite loads will be in poorer condition and be smaller than individuals with low ectoparasite loads. Males were significantly more parasitized than females, and there was a nonsignificant trend for small males to be more parasitized than large males. Because the northern flying squirrel is not sexually dimorphic, the immunosuppressive qualities of testosterone may explain the sex differences in ectoparasite load. Ectoparasites may also influence skeletal growth rates, and males that are more susceptible to ectoparasites may simply be unable to allocate as much energy to growth and are thus structurally smaller.