Ecological constraints of fish breeding systems

The widespread occurrence of animals caring for unrelated young has challenged evolutionary biologists for decades. Cooperative breeding, in which subordinate, mature individuals delay their own reproduction to care for the offspring of others, is one of the most extreme examples of alloparental care. A major explanation of why helpers care for the offspring of others is the ecological constraints model which proposes that helping behavior is an adaption to shortages of resources such as breeding territories and mates. The ecological constraints approach has also been used to explain polygyny, another breeding strategy enigma in species where both sexes provide parental care. The question is why some females choose to share a males parental effort with other females when they could receive the full effort of one male. The ability of males to monopolize the resources needed to attract multiple females should be determined by the distribution of those resources. Although the ecological constraints approach has been useful in studying cooperative breeding and polygyny, we continue to lack a comprehensive framework for explaining these phenomena. In this project we propose to manipulate ecological factors and hormones to attempt to induce cooperative breeding and polygyny in a non-cooperative, monogamous fish. We will perform parallel experiments with a congeneric, sympatric species that is both cooperative and polygynous. Our experiments are designed to sequentially determine key factors that permit or constrain the two breeding systems. If we are able to induce cooperative breeding, our experiments may produce the strongest evidence yet of the influence of particular ecological constraints on breeding systems.

Summary: The aim of the project was to identify and manipulate the cognitive, social and physiological factors determining the breeding systems of two species that live next to each other, feed both on plankton and nest in rock crevices, and are threatened by the same predators: Neolamprologus caudopunctatus and Neolamprologus pulcher. Yet, their breeding systems are substantially different: while N. caudopunctatus is a socially and genetically monogamous biparental breeder, N. pulcher is a cooperative breeder, where older related and unrelated offspring help the dominant breeding pair to rise their young. First, we tested whether the competition for the larger and more preferred shelters prevents the bi-parental species from living in groups. We found that N. caudopunctatus successfully defended their preferred breeding shelter with the head start, however, their breeding success and pair formation suffered considerably when we released both species simultaneously and concluded that the two different breeding systems are most probably not determined by the availability of breeding shelters. Second, we assessed with a series of behavioural experiments in the laboratory that not pair bonding or nest construction, but only spawning elicits parental care and inhibits egg cannibalism, that the presence of eggs maintains the carer's behaviour and that parents care equally for their own and unrelated offspring. Third, we investigated the physiological mechanisms behind the two different arental care systems: while the transition from a non-reproductive submissive individual to a breeding care giver is associated with opposing changes in N. pulcher, in N. caudopunctatus only an increase of galanin and no change of prolactin was linked to the breeding phase. In conclusion, we could show in this project that rather the physiological, cognitive and social abilities than the ecological environment constrains the breeding systems of N. caudopunctatus and prevents them from developing a more complex social cooperative system. Although the two species' ecology, like body size, nutrition, habitat, or predation pressure, are remarkably similar, the interaction of specific environmental and genetic events might have determined the prerequisites to develop cooperative breeding in one species but not the other.