Cognitive systems of obligate brood parasites

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The way an animal thinks (cognition) or behaves can influence its survival and reproduction (fitness). However, only recently have clear links been shown between behaviour, cognition and the brain in non-model organisms. Spatial memory is one of the strongest cognitive systems to demonstrate these links because of the clear role of (i) spatial memory in animal movement in relation to resources and (ii) the hippocampus in spatial memory, and ultimately, fitness. Obligate brood parasites, species that rely solely on other species to raise their young, are an ideal system to study specializations in the brain and cognition because females have more complex space use than males, which contrasts with other systems in which we see a sex difference favouring males (e.g. polygynous mammals). In some brood parasites, like brown-headed cowbirds (Molothrus ater), only females search for and parasitize up to 50 host nests per season. I showed that female cowbirds outperformed males on a spatial memory task that resembled host nest visits in the wild. In addition, differences in behaviour and cognition were linked to differences in the brain; female cowbirds had higher levels of hippocampal neurogenesis (new neurons recruited into the hippocampus) than males, with no sex difference in a non-parasitic relative. Strong tests of the role of sex in spatial memory in wild animals requires a system with both clear sex-specific demands on spatial memory and sex role reversal of those demands (to avoid the many confounding sex-specific factors), and obligate brood parasitism is one such system. Here, I propose to examine cognitive and neural specializations in several species of brood parasites using phylogenetic, developmental, mechanistic, and functional approaches. I aim to determine whether (i) female-biased sex differences in hippocampal neurogenesis persist in brood parasites across taxa, in addition to potential developmental specializations, (ii) hippocampal attributes such as neurogenesis affect ecologically-relevant spatial memory, and (iii) hippocampal neurogenesis affects fitness. I hypothesize that brood parasites have evolved adaptations in hippocampal attributes that maximize their ability to remember the location of host nests to increase their lifetime reproductive success. To test this hypothesis, I will combine cognitive testing in the lab, field work, and fine-scale measures of the hippocampus using immuhistochemistry and cellular compartment analysis of temporal activity by fluorescence in situ hybridization (catFISH). Examining brood parasites offers a unique opportunity within the field of cognitive neuroscience to study a system in which females, instead of males, have more complex space use, thus enabling us to show whether spatial demands, rather than sex itself, can shape these parameters. Using this system, I will provide a holistic perspective on the link between form and function in spatial memory in wild animals.