Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Biology

Supervisor

Dr. David Sherry and Dr. Scott MacDougall-Shackleton

Abstract

The hypothesis underlying all of neuroecology proposes that natural selection can modify cognition and its neural mechanisms if these modifications enhance fitness. I tested for sex and seasonal differences in cognition and the brain of brood-parasitic brown-headed cowbirds (Molothrus ater) and the closely related non-brood-parasitic red-winged blackbird (Agelaius phoeniceus) to determine whether cognitive and neural patterns were associated with space use and singing in the wild. Cowbirds show a reversal of sex-typical space use often seen in mammals with only female cowbirds parasitizing, searching for, and revisiting host nests. Cowbird and blackbird males sing more than females, especially in breeding condition, but female blackbirds sing, whereas female cowbirds do not sing at all. I tested cowbirds on a foraging task that required them to move through the testing environment in Chapter 2 and stationary spatial and colour memory touchscreen tasks in Chapter 3. I then examined sex and seasonal differences in the brain regions involved in spatial memory, the hippocampus, (Chapter 4) and singing behaviour, the HVC (proper name) and the robust nucleus of the arcopallium (RA), (Chapter 5) of cowbirds and blackbirds. Female cowbirds outperformed males on the foraging task and female cowbirds and blackbirds had a larger hippocampus relative to the telencephalon than male cowbirds and blackbirds, regardless of breeding condition. Female cowbirds had higher doublecortin immunoreactivity (DCX+), a measure of neurogenesis, in the hippocampus than male cowbirds, but no sex difference existed in blackbirds. However, male cowbirds outperformed female cowbirds on the spatial touchscreen task, demonstrating that females have enhanced spatial memory only on tasks resembling their behaviour in the wild. Male and female cowbirds performed better on the spatial touchscreen task than on the colour touchscreen task, suggesting that cowbirds may have enhanced spatial memory relative to other forms of memory. Indeed, cowbirds had a larger hippocampus with higher DCX+ than blackbirds. Finally, the size of HVC and RA were positively associated with singing and DCX+ in HVC was negatively associated with singing. In conclusion, my results support the central tenet of neuroecology, namely that the brain and cognition are specialized for an organism’s ecology.


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