Ecology and evolution of cooperation in the Espanola mockingbird, Nesomimus macdonaldi, Galapagos

When Charles Darwin visited the Galapagos Archipelago in 1835 he collected several specimens of Galapagos mockingbirds, among other organisms. It has now been argued the Galapagos mockingbirds provided him with the first insights into his theory of natural selection. Yet Darwin's seminal book on the origin of species also devoted a full chapter to acknowledge the difficulties of the theory for explaining the evolution of seemingly altruistic behaviors. It was difficult to explain how cooperation evolves when organisms incur costs to themselves as a result of their helping behavior. Interestingly, the Galapagos mockingbirds also display cooperative breeding behavior. Understanding the puzzle of cooperation remains a challenge to evolutionary biologists.;My dissertation research studied the paradox of cooperation in one of Darwin's unstudied mockingbird species: Nesomimus macdonaldi from Espanola Island, Galapagos. I conducted a 4-year study investigating the ecology and evolution of cooperation in the Espanola Mockingbird. The first chapter describes the development of species-specific molecular markers and characterizes the genetic mating system of the species. The Espanola Mockingbird is a polygynandrous breeder: groups range from monogamous pairs (30%) to singular (30%) to plural (40%) cooperative breeding families with up to 4 breeding pairs and a variable number of 'non-breeding' subordinates that may or may not help. Sixteen highly polymorphic tri-nucleotide markers were developed to estimate parentage of 89 young from 35 broods in 20 social groups. A high incidence of female con-specific parasitism (26%) and extra-pair paternity (27%) was evidenced. The extra-pair young were sired by within-group males in 13% of the cases and by extra-group males in 18% of the cases. In addition, females displayed egg-dumping behavior by laying eggs in other female's nests, yet they did not nest jointly. Con-specific parasitism was attributed to within-group females in 12% of the cases and to extra-group females in 16% of the cases. In the cooperative groups the rates of extra-pair parentage increased as breeding pairs increased, suggesting a greater opportunity for extra-pair copulations as group size increased.;The second chapter employed classic inclusive fitness theory to evaluate the costs and benefits of cooperative (and non-cooperative) behaviors. I quantified the direct and indirect fitness components of cooperative breeding relative to solitary breeding. Complementary to the benefits of philopatry, I estimated ecological constraints on dispersers by obtaining the probability of surviving dispersal and successfully acquiring reproductive vacancies from one year to the next. I found that birds in co-breeding groups had greater reproductive output (reproduction and survival) per capita, than solitary breeders. However, dominants in the group gained the highest benefits. Solitary (non-cooperating) breeders gained the second highest, after co-breeding dominants, while subordinates received the lowest fitness payoffs. The same trend was observed in both sexes. Why did subordinates stay in a cooperative breeding group if solitary breeding offered higher fitness payoffs? Estimates of net kin-selected benefits for group females suggested subordinates had an initial modest kin-selected incentive to stay in the group, however the benefit became negative if female survivorship was taken into account. For subordinate males, kin-selected benefits were also negative, and remained negative even after survivorship benefits were taken into account. Complementary to direct and indirect fitness measures of group living, dispersal costs suggested that subordinates remained in social groups in part due to the influence of high ecological constrains to leaving. It was prohibitively costly for subordinates to disperse. Subordinate male dispersers only had a 4% chance of successfully finding a reproductive vacancy. By contrast, subordinate females had a 21% chance of finding a reproductive vacancy if they dispersed.;Lastly, the third chapter evaluated current group dissolution models of the controversial reproductive skew theory as a tool to evaluate the maintenance of group living behavior. These models attempt to predict the evolutionary dynamics of group formation and maintenance, as influenced by the individuals' group dissolution tactics and the ecological, social, and genetic environment. I observed a despotic distribution of parentage that favored dominant males and females. This partitioning of reproduction was not explained by the null hypothesis of random variance because the observed binomial distribution (Nonac's B index) values were positive and deviated significantly from zero (0). The study species was expected to have forcible eviction tactics displayed by both group members (dominant and subordinate) and by both sexes. The observed reproductive share in males and females was not consistent with the predicted quantitative and qualitative predictions provided by forcible eviction tactic models. Behavioral observations suggested voluntary departures and provided further lack of support for the group dissolution models of transactional reproductive skew. I discuss the empirical findings in light of behavioral and genetic observations in this system and current views on the development of reproductive skew models.;Key words. cooperative breeding, cooperation, Galapagos mockingbirds, Espanola Mockingbird, Nesomimus macdonaldi, genetic mating system, social structure, kin selection, group dissolution tactics, reproductive skew.