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

## TL;DR

This study explores how sexual dimorphism in diederik cuckoos influences host responses during brood parasitism, finding that cryptic female cuckoos avoid egg rejection costs unlike conspicuous males.

## Contribution

The study provides novel empirical evidence on the ecological drivers of sexual dimorphism in brood parasites, specifically in relation to host egg rejection behavior.

## Key findings

- Hosts did not detect or discriminate between male and female diederik cuckoos during nest intrusions.
- Hosts that saw male cuckoos were more likely to reject odd eggs compared to those that saw females.
- Cryptic female cuckoos avoid egg rejection costs associated with conspicuous male-like intruders.

## Abstract

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, Chrysococcyx caprius. 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, Euplectes orix, 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.

Sexual dimorphism is typically explained by sexual selection, yet other potential drivers frequently remain untested. I examine whether Southern red bishop, Euplectes orix, hosts of the brood parasitic diederik cuckoo, Chrysococcyx caprius, differentially detect, and/or discriminate between, the male and female forms of their brood parasite. Using experimental intrusions at hosts’ nests, I found no evidence that diederik cuckoos differ in detectability, and hosts were also indiscriminately aggressive toward nest intruders. However, hosts that witnessed a male diederik cuckoo model during the trial were more likely to reject odd eggs. Together, these findings suggest that the more cryptic appearance of female diederik cuckoos is beneficial given the egg rejection costs associated with conspicuous male‐like intruders at the nest. The female diederik cuckoo (left) exhibits less conspicuous plumage and facial colouration than the male (right; photography by Dominic Cram and Jenny York).