Costly conspicuousness reveals benefits of sexual dimorphism in brood parasitic diederik cuckoos.

The existence of adult sexual dimorphism is typically explained as a consequence of sexual selection, yet coevolutionary drivers of sexual dimorphism frequently remain untested. Here, I investigate the role of sexual dimorphism in host-parasite interactions of the brood parasitic diederik cuckoo, . Female diederik cuckoos are more cryptic in appearance and pose a threat to the clutch, while male diederik cuckoos are conspicuous and not a direct threat. Specifically, I examine whether sexual dimorphism in diederik cuckoos provokes threat-level sensitive responses in Southern red bishop, , hosts. I use experimentally simulated nest intrusions to test whether hosts have the capacity to differentially (i) detect, and/or (ii) discriminate between, male and female diederik cuckoos, relative to harmless controls. Overall, I found no evidence that diederik cuckoos differ in detectability, since both sexes are comparable to controls in the probability and speed of host detection. Furthermore, neither male nor female hosts discriminate between sexually dimorphic diederik cuckoos when engaging in frontline nest defences. However, hosts that witnessed a male diederik cuckoo during the trial were more likely to reject odd eggs. Moreover, experimental eggs were significantly more likely to be rejected when female bishops observed a male compared to a female diederik cuckoo. While the cryptic appearance of female diederik cuckoos does not reduce detection by hosts, it does provide the benefit of anonymity given the egg rejection costs of conspicuous male-like appearance in the nest vicinity. These findings have implications for the evolution and maintenance of sexual dimorphism across the Cuculidae, and highlight the value of testing assumptions about the ecological drivers of sexual dimorphism.