When males stop having sex: adaptive insect mating tactics during parental care

Abstract

The theory of prudent sperm allocation predicts that males should reduce their investment in sperm transfer when their mate's propensity to produce offspring is low. Furthermore, if mating is costly for females, they would benefit from signalling this propensity to males to keep them from attempting to mate. Here we document a remarkable flexibility in insect mating tactic during the period of parental care. Extensive video observations in the biparental burying beetle Nicrophorus vespilloides revealed that pairs mate frequently before and during oviposition, temporarily discontinue their sexual activity while young larvae are present, and resume mating when males are close to leaving the brood. An experiment, in which we switched males at different time intervals and determined their P2 value (i.e. the proportion of progeny sired by the second male), confirmed the lack of mating activity during the first few days of larval care and its resumption later in parental care. Females cease laying eggs when caring for larvae, but females will resume egg laying if none of the larvae from their first clutch reach the carcass. We manipulated females into producing such replacement clutches by withholding their larvae, and we found that in this situation males did not cease to mate, but continued to engage in frequent copulations. Hence, males have the ability to assess a female's propensity for oviposition during the period of parental care. Further experiments demonstrated that males do not use the presence or absence of larvae as a cue to adjust copulation rate, but instead use female-produced cues of reproductive state. Therefore, our study reveals that female beetles express cues that display their reproductive condition and males have evolved the ability to detect these cues and respond with prudent sperm allocation. Our findings further contribute to the developing picture of the sophisticated mating tactics that insects employ.