"Livebearing is a reproductive strategy that confers both costs and benefits to females. One important cost is a reduction in locomotory performance of the mother during her pregnancy. This can detrimentally influence her feeding efficacy and her ability to avoid predation. Some livebearing organisms have evolved reproductive life-history adaptations that help minimize these locomotory costs. For example, in livebearing snakes the transition to an arboreal life style is accompanied by a repositioning of the paired ovaries along the length of the body, to reduce overlap; this reduces bodily distension and enhances climbing ability of the females during gestation.
I recently proposed that the placenta is a life-history adaptation that evolved in livebearing fishes to minimize adverse effects on locomotory performance during gestation (Pollux et al. 2009). I propose to test this novel hypothesis in a biomechanical study by comparing the consequences of pregnancy for locomotory performance between closely related livebearing species with and without placentation. The fish family Poeciliidae offers a unique opportunity to test this hypothesis because the placenta evolved several times in this family (Pollux et al. 2009). In this project we take advantage of this unique feature by selecting two species-pairs that each represent an independent evolutionary origin of placentation within the family. Specifically, I will test (1) whether placental species have a lower reproductive burden and a smaller abdominal distension during pregnancy than non-placental species; (2) whether these differences decline over the course of gestation; (3) whether pregnancy influences the composition and volume of the trunk muscles, and how this differs between placental and non-placental species; and (4) whether placental species have an improved swimming performance compared to closely-related, non-placental livebearing species."
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