It is complicated: how sex, family, and season affect growth in a sexually size-dimorphic spider

Female-biased sexual size dimorphism (SSD), where females are larger than males, is widespread. While ultimate explanations for SSD are well established, the proximate developmental mechanisms remain poorly understood. Studying sex-specific growth trajectories to identify common vs. sex-specific growth periods is therefore key to uncovering when and how SSD emerges. Theory predicts that female-biased SSD may arise if females hatch larger, grow more rapidly, grow longer, or combine these pathways. We studied sex-specific growth trajectories in the African hermit spider, Nephilingis cruentata, where adult females are on average 75-times heavier and take 2.3-times longer until adulthood than males. We tracked growth trajectories of carapace width for 916 individuals. After initial growth trajectories common to both sexes, trajectories diverged after [~]80 days, coinciding with the onset of the male subadult stage. At this point, male growth decelerated and terminated at sexual maturity, whereas female growth accelerated until their subadult stage at [~]200 days. Thereafter, female growth rate also decelerated and eventually terminated at sexual maturity. Thus, females attain their larger size through both an extended growth period and a more rapid growth rate, which initiates in mid-development. Whereas we detected seasonal effects on growth that were similar in both sexes, family effects showed strong sex-specific signatures. Our results pinpoint a key developmental window in which male and female growth trajectories begin to diverge, providing a clear target for detailed investigation of the mechanisms underlying SSD. At the same time, the complexity of these patterns will likely hinder mechanistic studies of SSD development.