Dynamics of testes size compensates for variation in male body size

Lemurs demonstrate various mating systems but paradoxically lack sexual dimorphism. No convincing mechanism can explain the lack of large-bodied males. In mammals, testicle size correlates with body size; here, we examine how testicles vary relative to body weight within a population of Malagasy lemurs. We show that the significant positive relationship between body size and testicles in non-breeding seasons dissipates in breeding seasons because the testicles of lighter males grow more and match heavier males' testicles. Low and unpredictable plant productivity characterizes Madagascar's ecosystems, and natural selection may favour testicle-body size decoupling during challenging years, inhibiting the evolution of dimorphic males.