It's Complicated: How Sex, Family, and Season Affect Growth of 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 versus 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 sex-specific carapace-width growth for 916 individuals from 153 families, which allowed us to test and account for the importance of family effects on sex-specific growth. After initial growth trajectories common to both sexes, trajectories diverged after ∼80-90 days, coinciding with the male subadult stage onset. At this point, male growth decelerated and terminated at sexual maturity, whereas female growth accelerated until their subadult stage at ∼200 days. Thereafter, the female growth rate also decelerated and 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 female mid-development. Whereas we detected seasonal effects on growth that were similar in both sexes, among-family variation showed sex-specific signatures. Our results pinpoint a key developmental window of growth trajectory divergence between sexes, providing a target for investigation of the mechanisms underlying SSD and emphasizing the importance of family effects.