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Funder: National Science Foundation; Id: http://dx.doi.org/10.13039/100000001, Funder: Tanzanian Commission for Science and Technology (COSTECH), Funder: NERC, Funder: Max-Planck-Gesellschaft; Id: http://dx.doi.org/10.13039/501100004189, Funder: Ministry of Education; Id: http://dx.doi.org/10.13039/501100002701, Funder: Tanzania Wildlife Research Institute (TAWIRI), Funder: University of Cape Town; Id: http://dx.doi.org/10.13039/501100007112, Funder: German Academic Exchange Service (DAAD), Movement of the embryo is essential for musculoskeletal development in vertebrates, yet little is known about whether, and why, species vary. Avian brood parasites exhibit feats of strength in early life as adaptations to exploit the hosts that rear them. We hypothesized that an increase in embryonic movement could allow brood parasites to develop the required musculature for these demands. We measured embryo movement across incubation for multiple brood-parasitic and non-parasitic bird species. Using a phylogenetically controlled analysis, we found that brood parasites exhibited significantly increased muscular movement during incubation compared to non-parasites. This suggests that increased embryo movement may facilitate the development of the stronger musculoskeletal system required for the demanding tasks undertaken by young brood parasites.

Most studies on sexual size dimorphism address proximate and functional questions related to adults, but sexual size dimorphism usually develops during ontogeny and developmental trajectories of sexual size dimorphism are poorly understood. We studied three bird species with variation in adult sexual size dimorphism: black coucals (females 69% heavier than males), white‐browed coucals (females 13% heavier than males) and ruffs (males 70% heavier than females). Using a flexible Bayesian generalized additive model framework (GAMM), we examined when and how sexual size dimorphism developed in body mass, tarsus length and bill length from hatching until fledging. In ruffs, we additionally examined the development of intrasexual size variation among three morphs (Independents, Satellites and Faeders), which creates another level of variation in adult size of males and females. We found that 27–100% of the adult inter‐ and intrasexual size variation developed until fledging although none of the species completed growth during the observational period. In general, the larger sex/morph grew more quickly and reached its maximal absolute growth rate later than the smaller sex/morph. However, when the daily increase in body mass was modelled as a proportion, growth patterns were synchronized between and within sexes. Growth broadly followed sigmoidal asymptotic models, however only with the flexible GAMM approach, residual distributions were homogeneous over the entire observation periods. These results provide a platform for future studies to relate variation in growth to selective pressures and proximate mechanisms in these three species, and they highlight the advantage of using a flexible model approach for examining growth variation during ontogeny.

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. published

Background Certainty of paternity is considered an important factor in the evolution of paternal care. Several meta-analyses across birds support this idea, particularly for species with altricial young. However, the role of certainty of paternity in the evolution and maintenance of exclusive paternal care in the black coucal (Centropus grillii), which is the only known altricial bird species with male-only care, is not well understood. Here we investigated whether the differences in levels of paternal care in the black coucal and its sympatric congener, the bi-parental white-browed coucal (Centropus superciliosus), are shaped by extra-pair paternity. Results We found that male black coucals experienced a substantially higher loss of paternity than white-browed coucals. Further, unlike any previously reported bird species, extra-pair offspring in black coucals represented mainly the last hatchlings of the broods, and these last hatchlings were more likely to disappear during partial-brood loss. Conclusion The results suggest that exclusive paternal care in black coucals is not maintained by male certainty of parentage, and extra-pair fertilizations are unlikely to be a female strategy for seeking ‘good genes’. Extra-pair paternity in black coucals may reflect the inability of males to guard and copulate with the female after the onset of incubation, and a female strategy to demonstrate her commitment to other males of her social group. Electronic supplementary material The online version of this article (10.1186/s12862-018-1225-y) contains supplementary material, which is available to authorized users.

Providing parental care often reduces additional mating opportunities. Paternal care becomes easier to understand if trade-offs between mating and caring remain mild. The black coucal Centropus grillii combines male-only parental care with 50% of all broods containing young sired by another male. To understand how much caring for offspring reduces a male's chance to sire additional young in other males' nests, we matched the production of extra-pair young in each nest with the periods during which potential extra-pair sires were either caring for offspring themselves or when they had no own offspring to care for. We found that males which cared for a clutch were not fully excluded from the pool of competitors for siring young in other males' nests. Instead, the relative siring success showed a temporary dip. Males were approximately 17% less likely to sire young in other males' nests while they were incubating, about 48% less likely to do so while feeding nestlings, followed by 26% when feeding fledglings, compared to the success of males that currently did not care for offspring. These results suggest that real-life care situations by males may involve trade-off structures that differ from, and are less strict than those frequently employed in theoretical considerations of operational sex ratios, sex roles and parenting decisions.

Assessments of biodiversity patterns and threats among African reptiles have lagged behind those of other vertebrate groups and regions. We report the first systematic assessment of the distribution, threat status, and climate change vulnerability for the reptiles of Tanzania. A total of 321 reptile species (including 90 Tanzanian endemics) were assessed using the global standard IUCN Red List methodology and 274 species were also assessed using the IUCN guidelines for climate change vulnerability. Patterns of species richness and threat assessment confirm the conservation importance of the Eastern Arc Mountains, as previously demonstrated for birds, mammals and amphibians. Lowland forests and savannah-woodland habitats also support important reptile assemblages. Protected area gap analysis shows that 116 species have less than 20% of their distribution ranges protected, among which 12 are unprotected, eight species are threatened and 54 are vulnerable to climate change. Tanzania's northern margins and drier central corridor support high numbers of climate vulnerable reptile species, together with the eastern African coastal forests and the region between Lake Victoria and Rwanda. This paper fills a major gap in our understanding of the distribution and threats facing Tanzania's reptiles, and demonstrates more broadly that the explicit integration of climate change vulnerability in Red Listing criteria may revise spatial priorities for conservation.

The decision to provide parental care is often associated with trade-offs, because resources allocated to parental care typically cannot be invested in self-maintenance or mating. In most animals, females provide more parental care than males, but the reason for this pattern is still debated in evolutionary ecology. To better understand sex differences in parental care and its consequences, we need to study closely related species where the sexes differ in offspring care. We investigated parental care in relation to offspring growth in two closely related coucal species that fundamentally differ in sex roles and parental care, but live in the same food-rich habitat with a benign climate and have a similar breeding phenology. Incubation patterns differed and uniparental male black coucals fed their offspring two times more often than female and male white-browed coucals combined. Also, white-browed coucals had more ‘off-times’ than male black coucals, during which they perched and preened. However, these differences in parental care were not reflected in offspring growth, probably because white-browed coucals fed their nestlings a larger proportion of frogs than insects. A food-rich habitat with a benign climate may be a necessary, but—perhaps unsurprisingly—is not a sufficient factor for the evolution of uniparental care. In combination with previous results (Goymann et al . 2015 J. Evol. Biol . 28 , 1335–1353 ( doi:10.1111/jeb.12657 )), these data suggest that white-browed coucals may cooperate in parental care, because they lack opportunities to become polygamous rather than because both parents were needed to successfully raise all offspring. Our case study supports recent theory suggesting that permissive environmental conditions in combination with a particular life history may induce sexual selection in females. A positive feedback loop among sexual selection, body size and adult sex-ratio may then stabilize reversed sex roles in competition and parental care.