Immunity and the expression of a secondary sexual trait in a horned beetle

Resource-based trade-offs are known to be important in determining investment in both sexual ornamentation and immunity in insects. Because of the strict resource limitation experienced during metamorphosis, we predict that if insects are trading off ornament size and immunity this should be most apparent immediately after eclosion and that the relationship between sexual ornament size and immunity should change with maturation feeding because of the changing patterns of resource availability during this process. We therefore present an investigation into the relationship between morphology, sex, and two measures of immune response (phenoloxidase [PO] activity and encapsulation ability) in the horned beetle Euoniticellus intermedius, immediately after eclosion and after maturation feeding. Both measures increased with maturation, with sex differences in PO activity becoming discernable after maturation feeding. PO activity was positively correlated with horn length in male beetles both on eclosion and after maturation feeding, and encapsulation ability was positively correlated with elytra length. We conclude that resource limitation in the pupating insect does have effects on immunity but that this resource limitation does not translate into a measurable trade-off between horn length and immunity. The correlation between horn length and PO activity may arise because both horn length and PO activity are correlated with the same elements of the animal's biology that allow some animals both to grow a long horn and to sustain a higher level of PO activity.