Adapted to trickery - traits favouring brood parasitism

Obligatory brood parasitic species do not care for their offspring themselves, but burden the costly parental tasks upon other species. This trickery has a very negative impact on the reproductive success of the host species in most cases, leading to the rapid evolution of countermeasures by the hosts. Enhanced host defences, in turn, force the brood parasites to evolve superior parasitic skills, resulting in evolutionary arms-races between parasites and hosts. All vertebrate obligate brood parasites except one are birds, and brood parasitic strategies evolved from an ancestral state of parental care in the parasitic bird species. Whether the transition from parental care to interspecific obligate brood parasitism included an intermediate step of intraspecific facultative parasitism or not, is still not entirely resolved. It is also unclear to what extend ecological factors may have facilitated this transition. So far, all studies on the evolution of obligate brood parasitism in vertebrates have been conducted in birds, and an alternative view on the topic will help to fully understand what promotes the development of brood parasitism. This study uses the only obligate vertebrate brood parasite outside the avian clade, the Lake Tanganyika cuckoo catfish Synodontis multipunctatus, to shed light on the role that ecological factors can play in the evolution of brood parasitism on a more general basis. Further, as parental care is not an ancestral state in the cuckoo catfish, the study will investigate what special adaptations are needed for a species to reproduce as a brood parasite when parasitism did not evolve out of parental care. The cuckoo catfish exploits the mouthbrooding behaviour of several cichlid species in Lake Tanganyika. The project will include field work at Lake Tanganyika in Zambia, where different Synodontis species will be caught to find out their preferred habitats, trophic positions, and diets. Experimental setups will be used to further test the species-specific food preferences. By reproducing particular species in the laboratory, the study will be able to depict reproductive traits, like for example egg characteristics and spawning frequencies. Experimental settings will be used to test the importance of those traits for the success of brood parasitism. A quantification of the carotenoid concentrations in eggs of different Synodontis species, and a further test for the Total Antioxidative Capacity of egg contents will expose potential adaptive effects of egg colouration on embryo development. Finally, A modelling approach within a Bayesian framework will be used to model the importance of specific traits and their combinations to equip S. multipunctatus for a life as a successful brood parasite. With this approach, the project will show what factors in the natural environment of these fishes, what traits of their reproductive biology, or what specific combination of these factors promote the evolution of brood parasitism.

Obligatory brood parasitic species do not care for their offspring themselves, but burden the costly parental tasks upon other species. This study used the only known obligate vertebrate brood parasite outside the avian clade, the cuckoo catfish Synodontis multipunctatus to investigate important ecological and life-history traits for the evolution and maintenance of brood parasitic reproduction aside avian model systems. Results from this project indicate that special combinations of ecological and life-history traits are needed to allow brood parasitism to evolve in a species without parental care in its evolutionary ancestry. It further provides valuable insight into the radiation of Synodontis in Lake Tanganyika (LT), Africa. The cuckoo catfish exploits the mouthbrooding behaviour of several cichlid species in the lake, and contrasts to the hitherto studied avian brood parasitic systems because it has evolved within a genus without any form of parental care. One theory predicts that the brood parasitic behaviour of cuckoo catfish may have evolved as consequence of a dietary specialisation on cichlid eggs. A comparison of diet and trophic position among LT Synodontis species indicated that S. multipunctatus is a generalist species while other species show more specialisation. A preference for cichlid eggs could not be confirmed and whether egg predation was critical for the onset of brood parasitism remained inconclusive. An analysis of the gonads showed that there are no significant differences in gonad structure among species. S. irsacae, was the only species showing strong seasonality, while all other species seem to reproduce continuously. Several reproductive traits (egg colouration, egg size, egg adhesiveness, frequency of gamete maturation) were tested among the species to investigate their importance for brood parasitism. Results showed that S. multipunctatus has larger and less sticky eggs compared to other LT Synodontis species. These traits are shared with S. granulosus (a species occurring in deeper waters below 40m), and represent potential preadaptations. The colour of eggs of cuckoo catfish are bright yellow compared to rather dull and colourless in other species. This indicated an adaptation to increase the acceptance of parasite eggs by hosts, which are attracted by carotenoid-based colours like yellow. The frequency of gamete maturation was high in S. multipunctatus, potentially as an adaptation to the spatially and temporally unpredictable occurring host spawnings. A behavioural study showed that the brood parasitic behaviour of cuckoo catfish is not innate but cuckoo catfish must learn how to parasitise their hosts effectively. A lab experiment simulating host incubation of multiply parasitised cichlid clutches indicated that cannibalism among predatory cuckoo catfish clutch-mates is no regular strategy to increase survival. A parentage analysis in parasitized host clutches collected in the natural habitat indicated group intrusions of wild cuckoo catfish to host spawnings and surprisingly divers mating patterns.