Non-native species cause major ecological and economic damage in their introduced range; it is important to identify predictors of invasiveness to prevent future invasions. This information might also prove to be important is assessing the vulnerability of native species to environmental change. One of the most recent introduction events, the house sparrow (Passer domesticus) to Kenya, is still expanding beyond the site of initial introduction. Kenyan house sparrows do not have high genetic diversity, however still exhibit extensive behavioral and physiological variation; interestingly, the observed variation is correlated with the estimated age of the population. House sparrows closest to the edge of the range are more exploratory and release more stress hormones (glucocorticoids) in response to stressors than those closer to the site of initial introduction. However, what these trait differences mean in terms of fitness along the range expansion is unknown; further, how these differences arise given the lack of genetic diversity is also unknown. The goal of the present research is to determine A) if increased exploration and stress hormones grant a fitness advantage at the range edge, but a disadvantage in more familiar habitats; and B) whether these differences arise due to differences in maternal care during offspring development. To test these questions, observation of natural parental care as well as manipulattion of a mother's ability to provision during the nestling period in nestbox colonies will be carried out in Kenya. The PI predicts that increased exploration and stress hormones increase fitness at the edge of a range, but decrease it at the site of introduction. Further, it is also expected that food supplementation will generate offspring most like those naturally occurring at the site of introduction and a reduction of provisioning will produce offspring similar to those at the range edge. The proposed study will strengthen international collaborations and provide research opportunities for underrepresented minority STEM undergraduates.
Work in the Martin lab focuses on understanding what traits facilitate the range expansion of certain species generally; further, we are interested in how these traits are facilitated (e.g. genetically or through phenotypic plasticity). To conduct this work, we use a population of recently introduced house sparrows with sites ranging in time since colonization from 5 years to 60 years ago. In previous work, we showed individuals at the edge released more stress hormones in response to a short term stressor than those in older sites. As stress hormones can influence many behavioral traits, we wanted to see if certain behaviors also varied with range expansion and whether or not stress hormones played a direct role in that variation. We first determined how response to novelty changed throughout the range expansion. To do this, we exposed individual house sparrows from eight different locations throughout the range expansion to two novel foods and two novel objects to see i.) how quickly individuals responded to novelty, and ii.) how individuals differed in response depending on the novel item (i.e. if individuals responded similarly regardless of item or if individuals responded differently dependent on the item type). We predicted individuals closest to the range edge would respond fastest to all items. Second, we determined how memory formation changed throughout the range expansion (with the prediction that individuals at the range edge would create and recall memories more readily). Here, we trained house sparrows from three cities throughout the range to associate food with a particular color for four days, after which we measured the latency to correctly peck the color they had been trained to associate with food. Finally, we tested the role of stress hormones on memory (to determine if this was a mechanism through which individuals manipulated memory. To do this, we pharmacologically increased stress hormones in some individuals during the training period to compare memory formation to unmanipulated individuals. As hypothesized, individuals at the range edge approached and ate novel foods faster than individuals from older populations (although, interestingly, the same did not hold true when novel objects were used). However, individuals responded differently dependent on the item presented, indicating individuals have some degree of plasticity and are able to adjust their behavior dependent on environmetnal cues. Further, individuals from newer sites were better able to recall what color they had been trained to associate food with than indivdiuals from more established sites. However, experimentally manipulating stress hormones had no immediate effect on memory performance. Rapidly approaching potential novel food sources may be particularly important at a range edge where environments are relatively unfamiliar and therefore resources less known. Additionally, once those resources have been identified, it would also be important for enhanced memory formation to recall those resources. In older areas, where resources are well known, the time and energy required for identifying and remembering resources can be dedicated to other processes (e.g. reproduction). That stress hormone manipulation did not alter individual's capacity for memory formation suggests stress hormone differences are not the immediate mechanism by which memory changes throughout the range. However, stress hormones may still be important in the differential phenotypes we observed; for instance, increased exposure to stress hormones during development (or throughout life, rather than during the time of learning) may be more indicative of memory formation later in life. The behavioral differences we observed throughout the range expansion could have been induced due to genetic differences, developmental environment differences, or in response to an individual's current surroundings; more work needs to be done to elucidate which mechanim(s) are most likely. This research contributes to ongoing research to determine what traits facilitate rapid adaptation to changing environments. It would be interesting to identify the existance of these traits in other introduced species and/or species exposed to environmental (e.g. climate) change.
