Abstract
Sexual dimorphism in body size (sexual size dimorphism) is common in many species. The sources of selection that generate the independent evolution of adult male and female size have been investigated extensively by evolutionary biologists, but how and when females and males grow apart during ontogeny is poorly understood. Here we use the hawkmoth, Manduca sexta, to examine when sexual size dimorphism arises by measuring body mass every day during development. We further investigated whether environmental variables influence the ontogeny of sexual size dimorphism by raising moths on three different diet qualities (poor, medium and high). We found that size dimorphism arose during early larval development on the highest quality food treatment but it arose late in larval development when raised on the medium quality food. This female-biased dimorphism (females larger) increased substantially from the pupal-to-adult stage in both treatments, a pattern that appears to be common in Lepidopterans. Although dimorphism appeared in a few stages when individuals were raised on the poorest quality diet, it did not persist such that male and female adults were the same size. This demonstrates that the environmental conditions that insects are raised in can affect the growth trajectories of males and females differently and thus when dimorphism arises or disappears during development. We conclude that the development of sexual size dimorphism in M. sexta occurs during larval development and continues to accumulate during the pupal/adult stages, and that environmental variables such as diet quality can influence patterns of dimorphism in adults.
Highlights
Sexual differences in body size are common in organisms and have attracted considerable interest in evolutionary biology for over a century [1,2,3]
The development time of 5th instar females was considerably longer than in males (F1,483 = 41.0, P,0.0001; Fig. 2B), but this effect depended on the quality of diet the larvae were raised on; females had about a 5% longer development time than males when raised on the 100% and 80% diet treatments while females had about a 11% longer development time when raised on the 60% diet treatments (F2,483 = 6.60, P = 0.0015; Fig. 2B)
Body mass – Within treatments Sexual size dimorphism first appeared at 25% of the development time from hatching to pupation when individuals were raised on the 100% diet treatment; females were about 15% larger than males during this stage (F1,180 = 6.35, P = 0.01; Fig. 3A,4)
Summary
Sexual differences in body size (sexual size dimorphism: SSD) are common in organisms and have attracted considerable interest in evolutionary biology for over a century [1,2,3]. Numerous studies have focused on these evolutionary explanations for how selection can generate variation in dimorphism in adult body size, few studies have focused on how and when sexual size dimorphism arises during development [6,7,8,9,10]. Such studies are critical to understanding how size dimorphism evolves in adults because the proximate target of selection is the developmental process that determines growth and body size in immature stages [10]
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