A case of gender equality: absence of sex-related costs in a dioecious tropical forest tree species

In dioecious species, females usually have higher sex-related costs than males. Consequently, trade-offs involving survival, growth, and reproduction are expected to differ between sexes. Here, we investigate several aspects of sex-related costs to test whether they are higher for females than males of a widely distributed, locally abundant, dioecious tropical forest tree species. For this, every adult of Mollinedia schottiana (Spreng.) Perkins (Monimiaceae) had its stem diameter at soil height (DSH) and spatial location measured in two 1-ha plots located at the Atlantic Rainforest, SE Brazil. Flowering phenology was also recorded over 12 months. At a second population census, the surviving individuals from the first census had their DSH remeasured. In comparison to males, females did not flower less frequently, less intensely, or in a lower proportion over 12 months. They also did not grow less between censuses, have larger DSH, or show spatial segregation from males. However, sex ratio was male biased, which, together with floral biology, is likely a strategy of M. schottiana to pollination by thrips. This study shows that dioecious species do not necessarily have differential sex-related costs as expected by the higher investment in reproductive structures by females. Sex ratios, which are often interpreted as a result of sex-related costs, can be driven by the reproductive biology of plant species.