Digest: A replication of sexual dimorphisms in size and longevity in a simulated baboon population *

Sexual selection, which arises when individuals of one sex compete with each other to secure mates and produce offspring, has the power to create sex differences in not only physical characteristics, but also complex life-history traits such as longevity. In species with sexual selection, males tend to have shorter life spans than females (Clutton-Brock and Isvaran 2007). A short life span means less time for reproductive opportunities, suggesting that a short life span would only be selected for if it is accompanied by a fitness benefit—or if there are fitness costs associated with long life spans. The disposable soma theory states that aging is the cause of accumulated damage to the body’s many systems (Kirkwood 2008). An individual can repair this damage, but doing so expends energy that might be better used in behaviors that directly enhance fitness, such as competing for mates. Each species has found its own optimal trade-off between body repair and investment in other functions. In this issue, King et al. (2017) quantitatively test whether sexual selection may underlie differences in energy allocation and therefore differences in life span between male and female savannah baboons. Baboons have sexual dimorphisms that suggest an energetic trade-off is at work: males are significantly larger; females live significantly longer. Females provide nearly all offspring care, whereas males compete for access to female mates. Males’ reproductive success depends on their ability to monopolize access to estrous females, and size confers a huge