Size of male (blue) and female (red) spiders in cm at each molt date following initiation of treatment (X axis).
Developmental asynchrony: A potential cost of extreme sexual size dimorphism in seasonal environments
Abstract
Female-biased sexual size dimorphism reaches an extreme in the genus Trichonephila Dahl, 1911 (Araneae: Nephilidae). Large female size is accomplished through the addition of juvenile instars and is likely the result of fecundity selection. However, additional instars in only one sex could result in significant developmental asynchrony. To minimize asynchrony, males should grow more slowly and females more rapidly. To test this prediction, we reared spiders from six disjunct populations of Trichonephila clavipes (Linnaeus, 1767) on three different diets, inducing variation in growth rates. Males on all diets grew more slowly than females, but still matured significantly earlier. In the field, mature males cohabit with juvenile females, and in the laboratory, mean male age at maturity corresponded to the age at which females achieve the minimum size observed to host males. However, a significant fraction of males mature earlier than concurrently-emergent females reach that minimum size—and before any females reached sexual maturity. Thus, early-maturing males may perish before reproducing, especially in temperate environments. We propose that (1) sexual size dimorphism and early male maturation evolved in tropical environments characteristic for T. clavipes, where adult females are found all year round. Univoltinism is likely a secondary trait in highly seasonal environments. Seasonal populations are likely mixes of local individuals and recent tropical immigrants, which could mitigate the loss of early males and retain this apparently maladaptive trait outside the tropics. (2) Environmental variation generates asynchronous emergence at the beginning of the growing season, allowing later-emergent males to mate with early-emergent females.
Ingi Agnarsson, Charles Goodnight, Linden Higgins "Developmental asynchrony: A potential cost of extreme sexual size dimorphism in seasonal environments," The Journal of Arachnology, 52(1), 75-85, (21 May 2024) https://doi.org/10.1636/JoA-S-22-058
