The Ontogeny of Sexual Size Dimorphism of a Moth: When Do Males and Females Grow Apart?

Citation: Stillwell RC, Daws A, Davidowitz G ( The Ontogeny of Sexual Size Dimorphism of a Moth: When Do Males and Females Grow Apart? R. Craig Stillwell 0 Andrew Daws 0 Goggy Davidowitz 0 Casper Breuker, Oxford Brookes University, United Kingdom 0 1 Department of Ecology and Evolution, University of Lausanne, Lausanne, Switzerland, 2 Department of Entomology, University of Arizona , Tucson, Arizona , United States of America 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. - Funding: This work was supported by NSF-USA (http://www.nsf.gov/) grant numbers IBN 9975168 and IOS-1053318 to Goggy Davidowitz and by a Postdoctoral Excellence in Research and Teaching fellowship to R. Craig Stillwell through NIH-USA (http://www.nih.gov/) Training Grant #1 K12 GM00708 to the Center for Insect Science, University of Arizona. This work was also supported (the data analysis and writing were done during R. Craig Stillwells time in this current postdoc) by an SNSF (http://www.snf.ch/) grant to Tadeusz Kawecki. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. 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]. Sexual size dimorphism varies considerably across all taxonomic levels; the degree and direction of dimorphism may vary substantially among populations within species, among species and among the major animal groups such as birds, mammals and insects [1,4]. For example, females are often the larger sex in insects, whereas males are often the larger sex in mammals [4,5]. This variation in size dimorphism is due to multiple sources of selection acting differentially on the sexes: fecundity selection for increased female size, sexual selection for increased male size and selection favoring small size in both sexes (through selection for short development time, [4]). Although 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]. There are three mutually non-exclusive ways for males and females to reach different sizes during growth and development: Males and females can differ in their size at hatching/birth, their growth rates and/or their development time [4]. Some studies have shown that the female-biased (females larger) dimorphism of insects is due to females growing faster than males, while other studies have shown that females prolong their growth and thus increase their size relative to males [10,11,12,13]. Only a few studies have examined sex differences in size at hatching, but the general impression is that there are no differences in hatching size between males and females in insects [12]. In general, little is known about when males and females start to diverge in body size during development [7,14,15]. The results of the few studies that have examined when dimorphism develops in invertebrates are not consistent; differences in size between males and females does not occur until late in development in some species [12,16,17,18,19,20,21] whereas studies with other species show that dimorphism is present early in development [14,20,22,23,24]. Most prior studies on the ontogeny of sexual size dimorphism are limited because only one or a few life stages were examined. Detailed studies that explore the ontogeny of size dimorphism from egg hatching to adult eclosion are needed to fully understand when sexual size dimorphism arises during growth and development. Ecological and environmental variables might influence the growth and development of the sexes differently and this could affect when males and females diverge in size during ontogeny. For example, female insects frequently exhibit greater phenotypic plasticity in body mass than males, creating variation in sexual size dimorphism within species [4]. Most of this sex difference in plasticity in size is in response to variation in diet quality and quantity, although some studies have shown that developmental temperature can also create sex-specific plasticity in size [22,25,26]. Because diet quality/quantity is known to be important in controlling insect growth and development and because it generates substantial sex-specific plasticity in body size, it is possible that diet quality/quantity will affect the ontogeny of sexual size dimorphism. However, no study has investigated whether diet quality/quantity affects when males and females diverge in size during development. In this study, we use the hawkmoth, Manduca sexta, as a model system to examine sexual size dimorphism in each stage of development to determine when sexual dimorphism starts to develop and to see whether the magnitude of this dimorphism changes over time. In addition, we investigate if diet quality influences the ontogeny of sexual size dimorphism. We raised moths from egg hatching to (...truncated)