Changes in lifetime immunocompetence in male and female Gryllus texensis (formerly G-integer):: trade-offs between immunity and reproduction

Determining whether males make energetic trade-offs between immunity and reproduction remains difficult, partly because of the complexity of the vertebrate immune system. We studied this question in an insect, which has a simpler immune system. We estimated immunocompetence in the cricket Gryllus texensis by measuring: (1) phenoloxidase (PO) activity, (2) haemocyte number and (3) susceptibility to the bacterium Serratia marcescens. From the last instar to prereproductive adulthood, immunocompetence in males was similar to that observed in females. When males reached the age at which they begin to display reproductive behaviour, PO activity and resistance to S. marcescens declined relative to females and younger males. In females, PO activity increased after the onset of sexual behaviour, relative to female nymphs and prereproductive adult females. Both males and females showed a decline in immunocompetence in old age. Neither weight nor body length correlated with resistance to S. marcescens in either sex. Haemocyte number did not differ between males and females at any age, did not change significantly with the onset of sexual behaviour, and did not correlate with resistance to S. marcescens, suggesting that it does not reflect immunocompetence. These results suggest that immunocompetence decreased in reproductively active males, supporting the hypothesis that males trade off immunity for reproduction. Our results also suggest that immunocompetence may increase in reproductively active females. (C) 2001 The Association for the Study of Animal Behaviour.