Evolution of sex-roles in black coucals Centropus grillii and white-browed coucals C. superciliosus

Abstract

Animals exhibit large variations in breeding systems. For the majority of animal species males compete more strongly amongst themselves to mate with females while females are choosy about a male to mate with. For species that provide post-zygotic parental care, usually females provide all or most care. Because it is so common, male-male competition for mates and predominant female care are sometimes referred to as ‘conventional’ or ‘typical’ sex-roles. However, in a few animal species the sex-roles are reversed, such that females compete more strongly than males for matings and males provide most or all parental care. ‘Sex-role reversal’ is often combined with polyandry and then termed ‘classical polyandry’ – that is a mating system in which a female pairs with more than one male either simultaneously or sequentially during the same breeding season. Despite a great interest to understand which factors are responsible in shaping the evolution of sex-role reversal, this phenomenon is still poorly understood. Knowing which factors are responsible in shaping reversed sex-roles is important for understanding sexual selection, mating systems and parental care. Therefore, despite their rarity, species with reversed sex-roles are of great importance to test theories which were formulated based mostly on the knowledge from animals with conventional sex-roles. Among birds, social monogamy with stronger male-male competition for matings and bi-parental care with larger female than male contribution to care is the norm. However, about 1% of all bird species exhibit a complete reversal of sex-roles: that is females are polyandrous and they compete more strongly for territories or mates and males take sole care of the young. Until 1971, such classical polyandrous species were known only among shorebirds (Order Charadriiformes). Because of this, previous studies and hypotheses about the evolution of polyandry and sex-role reversal focused mostly on shorebirds. The discovery of classical polyandry in black coucals, which is currently the only known bird species that combines classical polyandry with an altricial development of young, demonstrated that this behaviour is not restricted to shorebirds only. This called for further research on black coucals as well as on other coucal species to understand which factors favoured the evolution and maintenance of classical polyandry in this taxon. v Therefore, the goal of my doctoral research was to collect and analyse empirical data from coucals to understand proximate factors for the evolution and maintenance of sex-role reversal in black coucals. In this thesis, I present the results, interpretations and discussion of the analyses to test three hypotheses about proximate factors underlying the evolution and maintenance of classical polyandry in black coucals. Data were collected mostly from two sympatric species of free living coucals that differ fundamentally in sex-roles: the African black coucal (Centropus grillii), which is polyandrous and has male-only parental care, and the white-browed coucal (C. superciliosus) which is socially monogamous and both parents cooperate in caring for their young. These two coucal species were subjects of a long-term study in the Usangu Plain of Southwestern Tanzania from 2001 until to date. Also, a few data were obtained from a third coucal species, the socially monogamous and bi-parental coppery-tailed coucal (C. cupreicaudus), which also shares its breeding habitat with the other two coucal species. Because of insufficient data, this third coucal species did not form a major part of my analyses. However, I try to integrate the scant knowledge of the biology of this third coucal species as well as data from other coucal species studied elsewhere in discussing the findings.In Chapter 1 I provided background information relevant to understand the research problem, the rationale, general and specific objectives, and the significance of the research presented in this thesis. I also provide a list of existing hypotheses about factors shaping sex-roles of which some were tested and the findings presented in the data chapters of this thesis. In Chapter 2 I provided an empirical analysis of whether parental care in black coucals is less demanding than in bi-parental white-browed coucals, such that a single black coucal parent can successfully raise an entire brood but the cooperative effort of both parents is necessary to successfully raise a brood in white-browed coucals. The results indicated that parental care in black coucals is not less demanding than in white-browed coucals and therefore, a single whitebrowed coucal parent should also be sufficient to raise a clutch. We suggested that monogamy and bi-parental care is maintained in white-browed coucals not because both parents are required to successfully raise the brood, but rather because both sexes lack opportunities to become polygamous — due to a more or less balanced adult sex ratio.