3 years ago

Field evidence challenges the often-presumed relationship between early male maturation and female-biased sexual size dimorphism

Field evidence challenges the often-presumed relationship between early male maturation and female-biased sexual size dimorphism
Marie-Claire Chelini, Eileen Hebets
Female-biased sexual size dimorphism (SSD) is often considered an epiphenomenon of selection for the increased mating opportunities provided by early male maturation (i.e., protandry). Empirical evidence of the adaptive significance of protandry remains nonetheless fairly scarce. We use field data collected throughout the reproductive season of an SSD crab spider, Mecaphesa celer, to test two hypotheses: Protandry provides fitness benefits to males, leading to female-biased SSD, or protandry is an indirect consequence of selection for small male size/large female size. Using field-collected data, we modeled the probability of mating success for females and males according to their timing of maturation. We found that males matured earlier than females and the proportion of virgin females decreased abruptly early in the season, but unexpectedly increased afterward. Timing of female maturation was not related to clutch size, but large females tended to have more offspring than small females. Timing of female and male maturation was inversely related to size at adulthood, as early-maturing individuals were larger than late-maturing ones, suggesting that both sexes exhibit some plasticity in their developmental trajectories. Such plasticity indicates that protandry could co-occur with any degree and direction of SSD. Our calculation of the probability of mating success along the season shows multiple male maturation time points with similar predicted mating success. This suggests that males follow multiple strategies with equal success, trading-off access to virgin females with intensity of male–male competition. Our results challenge classic hypotheses linking protandry and female-biased SSD, and emphasize the importance of directly testing the often-assumed relationships between co-occurring animal traits. Early male maturation (i.e., protandry), and female-biased sexual size dimorphism (SSD) are linked in innumerous taxa, and are often assumed to co-evolve. We merge field-collected data with modeled probabilities of success for males and females to test four specific predictions regarding the adaptive benefits provided by protandry and the relationship between female and male growth trajectories and size at adulthood. We show that protandry is not simply a consequence of selection for female-biased SSD, as male size does not trade-off with timing of maturation, and we provide only mixed support for the hypothesis that protandry leads to larger fitness benefits for males.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/ece3.3450

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