A test of the cost of reproduction hypothesis in environments with different extrinsic mortality rates

Background/Question/Methods The cost of reproduction hypothesis predicts that if residual reproductive value declines as a female ages, then young females should allocate less of available energy to current fecundity and more to future reproduction; whereas, older females should allocate more of available energy to current fecundity and less to future reproduction.  Although this prediction has a long history its’ generality across various environmental gradients has seldom been tested.  We tested for patterns consistent with the cost of reproduction hypothesis in environments that varied in their extrinsic mortality rates.  We compared reproductive allocation, fecundity, and offspring size between old and young females of the livebearer Brachyrhaphis rhabdophora (family Poeciliidae) found in high-predation and low-predation environments.  We used life history trajectory analysis as a framework to quantify these patterns.

Results/Conclusions Females in the low-predation environment followed the pattern predicted by the cost of reproduction hypothesis and exhibited increased reproductive allocation with age (i.e., allometry).  In contrast, females from high-predation environments exhibited no age-related change in reproductive allocation (i.e., isometry).  Increased allocation in older females from low-predation environments resulted mainly from increased fecundity rather than increased offspring size.