The social system of white-breasted mesites (Mesitornis variegata)

Abstract

Animals show great diversity in their social systems, ranging from species living solitarily to species living in highly complex social groups. This variation in social systems provides the perfect setting to investigate evolutionary transitions among social traits. A particularly useful approach consists in comparing closely related species that exhibit different levels of sociality. Social systems have been described differently among distant taxonomic groups. In birds they are generally defined according to the breeding system or social mating system, and most species are socially monogamous, with more complex groups generally characterized by natal philopatry of juveniles and cooperative breeding. Several hypotheses related to the species demography, ecology, life-history and kinship relationships have been proposed to explain the transitions from pair-living to cooperative breeding systems in birds. White-breasted mesites (Mesitornis variegata) are medium-sized ground-dwelling birds that belong to the family Mesitornithidae. This family of birds is endemic to Madagascar and is composed of two additional allopatric species that show quite different levels of sociality: the brown mesite (Mesitornis unicolor), a pair breeder found in the eastern rainy forests, and the subdesert mesite (Monias benschi), which lives in groups and breed cooperatively in the southern spiny forests. White-breasted mesites are usually found in pairs or small groups, that are thought to be family groups, in the dry deciduous forests of western Madagascar, but there is little information on their breeding system and no study has yet examined the genetic relatedness of their small social units. The general aim of this thesis is first to describe several components of the social system of white-breasted mesites, a species that was previously suggested to breed cooperatively but has precocial chicks which in principle do not require much parental care, and then investigate proximate and ultimate mechanisms that may have shaped this system. To do so I combine genetic, behavioural, morphologic and spatial data collected on 10 to 15 social units of M. variegata in Kirindy forest, Western Madagascar, during five field seasons (October 2009 to April 2012). The results of this study indicate that white-breasted mesites live in very cohesive stable pairs or small families formed by delayed dispersal of juveniles and that care is provided only by parents. Additionally, contrary to previous suggestions, the food provisioning period for chicks in M. variegata is intense for 2 months, but can extend to up to 12 months, although at much lower rates. This long parental care period could be related to the juveniles’ slow development of foraging skills I observed in this species. The high cohesion of mesite social units, with inter-individual distances rarely exceeding 3 m, seems to be associated to predator avoidance and not to mate defence. Still, breeding partners’ high cohesion may indirectly explain their strictly monogamous mating system. Analyses of juvenile dispersal patterns revealed that male juveniles stay longer in families than females, matching with the slower adult male population turnover and the presence of some families containing a “stepmother”. Additionally, by comparing adults associated with juveniles with adults living in pairs I show that family-living can be costly for parents in terms of foraging efficiency and investment in future reproduction, which could underlie the parental intolerance I observed towards older juveniles. Overall, limitations for independent breeding and solitarily ranging in combination with slow development of foraging skills and parental costs seem to be important factors explaining juvenile dispersal patterns, family formation and stability in this species. Additionally, predation risk selecting for strong intra-group and pair cohesion may affect the mating system in white-breasted mesites and suggests a link between genetic monogamy and predation risk in a socially monogamous species. Based on comparisons with the other mesite species the breeding system of white-breasted mesites could be considered along the transition between a bi-parental and a cooperative breeding system, with some juveniles able to provide help but prevented from doing so by their parents.