Strategic ejaculate allocation in the mostly monandrous moth Lobesia botrana: female size, male size, and their interaction effects

Experimental research and theoretical models question the Darwin-Bateman paradigm that male reproductive success is just limited by the number of mates. Males have evolved tactics to tailor ejaculates among their mates to maximise fitness. Males are expected to be choosy and to bias ejaculate investment depending on the perceived quality of females (cryptic male choice) and/or the reproductive social context. We investigated ejaculate allocation strategy in Lobesia botrana, a mostly monandrous moth, depending on male size, female size, and their interaction, the latter effect being usually overlooked in most studies. The explained variables were absolute ejaculate size (spermatophore volume) and relative ejaculate size (ejaculate size to lifetime ejaculate expenditure ratio). Absolute ejaculate allocation analysis showed that: (1) ejaculate expenditure increases with male size; (2) males tailor greater ejaculates to larger than smaller females; and (3) there was no interaction between male size and female size, so small and large males tailored equivalent increases in ejaculate volume per female weight unit. Quite differently, relative ejaculate allocation analysis showed that: (1) small males invest comparatively more than large males in females of similar size; and (2) there was an interaction between female size and male size, so small males invest comparatively more than large males when mated with large females. Results are discussed in the framework of strategic ejaculate allocation from an evolutionary perspective, as ecological variables, including the current climate change scenario, may promote coexistence in the field of quite different-sized adults in this moth species. Significance statement Available evidence supports that insects and other animals, including humans, exhibit strategic ejaculate allocation, which is shaped by an array of factors related to mate quality and reproductive social context. Among them, female size is especially important, as it positively correlates with fecundity and thereby largely regulates male fitness returns. We studied the ejaculate allocation pattern in the European grapevine moth, a usually monandrous species. We show that, in addition to female size, male size and male-by-female size interaction are critical drivers of ejaculate allocation. However, the interpretation of the results must be done carefully, as they may differ substantially depending on whether male investment is measured in absolute or relative terms.