Background/Question/Methods The placenta, defined as a close apposition between maternal and fetal tissues for sustenance and physiological exchange, has evolved many times throughout the animal Kingdom. In fishes, initial steps in the evolution of placentation are believed to have involved a shift from (i) egg-laying to livebearing followed by (ii) a transition from lecithotrophy (a trophic mode in which females allocate all resources to the egg prior to fertilization) to placentotrophy (in which females continue to transfer resources to their developing embryos throughout the pregnancy via a placenta). The potential selective advantages driving this latter transition remain unclear. Trexler & DeAngelis (2003) proposed that placentotrophy allows a higher fecundity under high and predictable food conditions, but that embryo abortion and resorption of energy is a necessary preadaptation to the evolution of placentotrophy. We address several assumptions of the Trexler & DeAngelis model by comparing the effects of fluctuating resources on life history traits of two closely related Poeciliids: the lecithotrophic Poeciliopsis gracilis and the placentotrophic P. turneri. In a 12-week experiment females were assigned to either one of four food treatments: high food-high food, high–low, low-high and low-low. Our goal was to study the differences in the way these species allocate resources to reproduction. Results/Conclusions P. gracilis and P. turneri both show an increase in brood size under high food conditions related to an increase in female size (growth) during the experiment. Under low food conditions, P. gracilis shows a reduction in brood size (starting after 4 weeks), while P. turneri maintains a stable brood size throughout the experiment. Neither species abort embryos under low food. P. turneri females have a higher % body fat alluding to the fact that placental species rely more on postfertilization maternal provisioning. P. turneri females show a more severe reduction in fat reserves under low food compared to P. gracilis, due to continued embryo nourishment. Offspring size at birth decreases under prolonged low food conditions. The interbrood interval is not affected by food availability in either species, though preliminary analyses suggest that placental species may skip entire brood under low food conditions. Conclusion: High fat reserves, rather than embryo abortion, allow placental species to deal with short-term adverse conditions. However, both female condition and offspring quality decline under prolonged periods of low food. This supports the idea that placentotrophy evolved in stable, high resource environments.