Experimental reversal of courtship roles in an insect

Abstract

STUDIES of sexual selection¹ commonly examine female mating preferences or intermale competition², but there have been few experimental tests of factors controlling why these, the 'typical courtship roles', exist in the first place. Although the distribution of mates can affect sexual selection, and thus the degree of sexual competition³, it is the typically lower investment in offspring by males relative to females that is thought to determine the usually greater potential mating frequency, and hence greater sexual competition, among males^4,5. Reversal in the typical courtship roles is, therefore, expected to occur where there is high male parental investment⁴. The courtship roles are generally considered to be fixed for any particular species because of the usually inflexible nature of adaptations for parental care. Certain katydids (Orthoptera: Tettigoniidae)^6–8, however, show intraspecific variation in these roles. This has been suggested to result from variation in the relative importance of male parental investment⁹ in the production of offspring. Male parental investment comprises the nutritional value of the spermotophore eaten by the female. When other food is scarce therefore, females are predicted to compete sexually for males and their offerings^7,8,10. Here we confirm this prediction and show that an increase in food from a low level results in a change from role-reversal to the typical roles; this supports the theory^4,5 that variation in relative male parental investment controls sexual differences.