When Males Carry no Eggs: Species‐Specific Mate‐Choice Cues in Two Water Bug Species

ABSTRACT Male parental care is a prominent and sexually selected trait in giant water bugs (Belostomatidae), where females preferentially mate with egg‐carrying males. However, females frequently encounter males that are not carrying eggs, and the criteria underlying mate choice under such conditions remain poorly understood. Here, we examined female mate choice in two closely related species, Appasus major and A. japonicus , by experimentally restricting mate‐choice assays to males that were not carrying eggs. We quantified male courtship behaviour and analysed female acceptance using a common statistical framework in both species. Female mate choice differed strikingly between the species. In A. major , female acceptance was strongly associated with male courtship behaviour, whereas male body size had no detectable effect. In contrast, in A. japonicus , courtship behaviour did not predict female acceptance; instead, females were more likely to accept smaller males. We interpret these contrasting patterns in light of species‐specific reproductive ecology. In A. japonicus , multiple females sequentially contribute to communal egg masses, and male body size may provide females—particularly early ovipositing females—with a predictive cue for how quickly males transition from mating activity to dedicated egg care. In A. major , where eggs are larger and egg care is expected to be more energetically demanding, sustained courtship may instead function as an indicator of male endurance and parental capacity. Our results demonstrate that removing a dominant parental‐status signal reveals divergent mate‐choice rules even among closely related species. These findings highlight how differences in reproductive ecology can shape the informational value of male traits and generate contrasting selection pressures on morphology and behaviour in parental‐care systems.