| Placental matrotrophy has evolved independently in many lineages, which is indicative of strong selection. Several ad-hoc hypotheses and one mathematical model have been proposed to explain its evolution, but little experimental data exists to support these claims. Recent studies have attempted to find correlates between placental matrotrophy and other life history traits, but the only common correlate found was a reduction in reproductive allocation (RA). Here I report on three studies. The first two focus on the Trexler-DeAngelis model for the evolution of matrotrophy. In chapter one, I use closely related placental and non-placental species from the northern clade of Poeciliopsis to test an assumption imperative to the model, that placental species abort a subset of developing offspring in low food conditions. The results show no evidence of abortion due to food level. Instead, placental species appear to be tethered to a brood once initiated, and sacrifice body condition to maintain reproduction when resources are restricted. However, an alternative explanation for these results is that the pattern of resource allocation is a function other life history traits, rather than placentation alone. Chapter two distinguishes between these alternatives, by performing a similar experiment on the southern clade of Poeciliopsis, which has the opposite relationship between life history traits and placentation seen in the northern clade. The results mirror those from the northern clade, indicating reproductive mode, rather than life history, dictates the pattern of resource allocation. This further adds to the difficulties of explaining the evolution of the placenta within the constraints of the Trexler-DeAngelis model. The third study characterizes locomotor consequences related to differences in reproductive allocation using the Trinidadian Guppy (Poecilia reticulata), because placentation is correlated with a reduction in reproductive allocation. Females with the highest RA were less streamlined, had lower escape velocities, and had to work harder during prolonged swimming. In low predation natural habitat, these same females were restricted to habitat with slower moving water, while females with lower RAs were able to use faster flows, which are thought to be preferred feeding grounds. All high predation fish used slow moving water, irrespective of RA. |
