Understanding how much energy an organism allocates when breeding to maximize reproduction over their lifetime, or the cost of reproduction, is essential in exploring reproductive patterns in living organisms. One commonly observed pattern is that species which become sexually mature at an earlier age tend to die earlier and vice versa. As a result, species must find a balance between survival and reproduction. We have proposed two models that demonstrate this trade-off. First, species may follow a “step” model where reproductive cost is additive for each reproductive attempt. Second, species may follow a “constant cost” model where the cost to reproduce is constant and independent from the number of reproductive attempts. However, which model is more correct and how reproductive trade-offs are influenced by age, resource quality, and number of prior reproductive experience is unclear.
Burying beetles provide the perfect experimental conditions that will allow us to independently look at age and reproductive experience on the cost of reproduction. Burying beetles are unique because they breed in discrete cycles that allows for precise evaluation of reproductive costs. With this advantage, we created a factorial design of “age x resource quality x number of prior reproductive experience” to evaluate the different types of interactions that can influence the cost of reproduction.
Results/Conclusions
In our experiment, we found that 1) There was a significant difference (P < .0001) between the average lifespan of the beetles in the early, middle, and late trials. By delaying reproduction from early to middle, or middle to late, beetles gained about 10 days of additional lifespan; however, the reproductive delay between early to middle, and middle to late reveals a large diminishing return—22 days. 2) The average lifespan of the beetles in the reproduce once and reproduce twice trials did not differ (P < .8020). Individuals in both treatments lived about 63 days.
Our results suggest that the cost of reproduction is mainly dependent on the age at which a beetle reproduces, not how many times it reproduces. Once a beetle starts paying the cost of reproduction, they will continue to pay it for the rest of their lives independent of how many times they reproduce.