Decoupling of reproductive rates and parental expenditure in a polyandrous butterfly

Current theory postulates that the operational sex ratio (OSR) determines the relative degree of mating competition in the two sexes and is in turn influenced by a sexual difference in the potential reproductive rate (PRR) defined as 1/time out, where time out is the time an individual must spend recovering from a bout of mating activity and/or caring for offspring. In bushcricket mating systems where males provide females with a nuptial gift, relative energy expenditure in offspring influences the PRR of males and females and underlies a diet-mediated shift in the OSR. Here we investigated if there is a similar positive relationship between relative parental nutrient expenditure in offspring and PRR in the polyandrous butterfly Pieris napi, where female fecundity is strongly dependent on male nuptial gifts at mating. By varying the amount of nutrients females receive at mating and relating this to number of offspring produced, we show that male P.! napi have, on average, a nutrient expenditure in offspring equaling that of females. In spite of this, the male reproductive rate is 8-13 times higher than that of females. Hence the relative degree of parental expenditure in offspring is largely decoupled from the degree of mating competition in P. napi. Two alternative explanations are advanced to account for the difference between the butterfly and the bushcricket mating systems.