<p> Life-history research explores how environmental variation and physiological constraints shift the energy allocation decisions to competing functions of the body among individuals and populations. Variation in the energy available across the environmental landscape is of particular interest as it dictates the amount energy available for organisms to acquire and allocate to their life-history. Additionally, individuals vary in their ability to capture and assimilate energy from the environment. This dissertation seeks to understand how environmental variation in energy availability and physiological constraints of attaining this energy shape allocation decisions among individuals and populations. I examined diamond-backed watersnakes (Nerodia rhombifer) from five populations that differed in their access to energy resources of prey. I compared measures of reproduction and growth to detect the presence of differential allocation decisions. I tested whether differences in allocation among individuals and populations arose as a constraint of their ability to assimilate energy resources in the environment, or as a function of the variation in prey characteristics across the landscape. Lastly, I measured how variation in reproductive investment may limit energy acquisition among individuals.</p>