Spatial and social learning of Icelandic threespine stickleback (Gasterosteus aculeatus L. ): Contrasting habitats and population divergence

Resource polymorphism and phenotypic differentiation is common in vertebrates as they adapted to local environments. Selection acts on the morphology, behavior and/or physiology of organisms increasing the efficiency of local resource use. Different resources use may lead to population diversification which in turn may result in emergent of distinct morphs within species even to speciation. Many ecological factors could promote divergence such as level of complexity and size of a lake, food availability, inter- and intraspecific competition and predation pressure. Different age classes, sexes and individuals within a population can exploit different resources with resulting ecological specialization which may lead to sexual dimorphism or population divergence. A model species in investigating morphological and behavioral divergence is threespine stickleback as it has undergone multiple adaptive radiations since the end of the Pleistocene period (10.000- 15.000 years ago) showing complex morphology, behavior and life history, and inhabits a wide range of aquatic habitat (small ponds, larger lakes, rivers). Specifically, behavioral differences have been revealed in spatial and social learning, response to predators and anti-predator behavior, and social behavior.In Iceland two distinct stickleback morphs have been described in neovolcanic lakes, the lava and mud morph which show morphological, behavioral adaptations to the distinct mud and lava habitats. The lava sticklebacks live predominantly in spatially structured lava substrate and they feed mainly on benthic invertebrates that are hosted in the numerous holes and crevice that the complex lava substrate provides while the mud sticklebacks are better adapted to pelagic prey and they are equally likely to forage on benthic prey and plankton. Behavioral differences were also described between the two morphs in anti-predator behavior most likely affected by the very different structural complexity of their habitats. In this study behavioral and morphological differences were examined among Icelandic threespine stickleback populations inhabiting different types of habitat. Specifically in the first part of the thesis (Manuscript I) stickleback from lava and mud habitat were used to examine the role of habitat complexity in shaping social and learning behavior. Specifically I investigated differences in stickleback from lava and mud substrate in: (1) their exploitation of socially transmitted information, through naïve fish joining performance and gain of food source information from a demonstrator, (2) spatial learning, represented by the individuals speed and ability to consistently locate a stable food source and (3) shoal cohesiveness and social interaction of individuals. Consistent differences were found between lava and mud stickleback in the use of social and spatial information to locate a food source. Sticklebacks originating from mud habitats were more likely to join a demonstrator at the feeder, approach conspecifics and aggregate more closely than lava sticklebacks. On the other hand, the lava fish, were found to be better learners (lower error rate and faster) suggesting stronger selection for individual cognitive ability in the more complex habitat. Discrepancies in the parallel pattern of aggregation, boldness and social learning between mud and lava populations indicate that the aptitude for social learning while foraging has evolved independently of general social behavior (tendency to approach and shoal). In the second part (Manuscript II) I investigate morphological differences related to sex and ecological opportunity in Icelandic threespine stickleback. The ecological opportunity theory suggests that in heterogeneous environments with high intraspecific competition and low interspecific competition or predation pressure lead to phenotypic variation as individuals that switch to under-exploited resources undergo weaker competition. The present study investigates sexual dimorphism and morphological variation in relation to substrate type and lake size among populations and the sexes in threespine stickleback (Gasterosteus aculeatus). Threespine stickleback were sampled from 33 sites in 22 lakes in Iceland inhabiting three main substrate types: mud, vegetation and rock. Lake size, generally representing increased niche availability, was positively correlated with phenotypic variance within populations and sexual dimorphism, most likely representing increased ecological opportunity. Finally, the result do not support that increased sexual dimorphism restricts phenotypic variation at the population level