Sex-dependent immune response in a semelparous spider.

Previous studies on arthropods showed that seasonality and parity in breeding considerably impact the direction of sex differences in immunocompetence, and it has been suggested that life span and the time window available for breeding play key roles in shaping sex-differences in immunity. One proposed mechanism behind this phenomenon is differential investment into life history traits in sexes. Here, we tested whether in a seasonally breeding semelparous arthropod sexes differ in their immunocompetence, predicting that females would show weaker immune response than males. We compared encapsulation efficiency (a well-established and widely used method for assessing immunocompetence) of freshly matured, virgin males and females of the lycosid spider Pardosa agrestis (Westring, 1861). On average, males mounted stronger immune response than females and the extent of encapsulation was positively associated with prosoma length in males, but not in females. Also, time until maturation was positively related to the extent of encapsulation in both sexes, but did not significantly affect adult prosoma length. We propose that sex-difference in encapsulation is likely shaped by combined effects of relatively higher costs of reproduction in females, narrow time window of reproductive activity, and the absence of trade-off between current and future reproduction.