The Proximate Causes of Sexual Size Dimorphism in Phrynocephalus przewalskii

Wei Zhao,Nai-Fa Liu,Michael Sears

doi:10.1371/journal.pone.0085963

Abstract

Sexual size dimorphism (SSD) is a common phenomenon and is a central topic in evolutionary biology. Recently, the importance of pursuing an ontogenetic perspective of SSD has been emphasized, to elucidate the proximate physiological mechanisms leading to its evolution. However, such research has seldom focused on the critical periods when males and females diverge. Using mark-recapture data, we investigated the development of SSD, sex-specific survivorship, and growth rates in Phrynocephalus przewalskii (Agamidae). We demonstrated that both male and female lizards are reproductively mature at age 10–11 months (including 5 months hibernation). Male-biased SSD in snout-vent length (SVL) was only found in adults and was fully expressed at age 11 months (June of the first full season of activity), just after sexual maturation. However, male-biased SSD in tail length (TL), hind-limb length (LL), and head width (HW) were fully expressed at age 9–10 months, just before sexual maturation. Analysis of age-specific linear growth rates identified sexually dimorphic growth during the fifth growth month (age 10–11 months) as the proximate cause of SSD in SVL. The males experienced higher mortality than females in the first 2 years and only survived better than females after SSD was well developed. This suggests that the critical period of divergence in the sizes of male and female P. przewalskii occurs between 10 and 11 months of age (May to June during the first full season of activity), and that the sexual difference in growth during this period is the proximate cause. However, the sexual difference in survivorship cannot explain the male-biased SSD in SVL. Our results indicate that performance-related characteristics, such as TL, HW, and LL diverged earlier than SVL. The physiological mechanisms underlying the different growth patterns of males and females may reflect different energy allocations associated with their different reproductive statuses.

Highlights

Sexual dimorphism is a common and long-discussed feature of the animal world that is reflected in a variety of biological characteristics
The importance of an ontogenetic perspective, in which sexual size dimorphism (SSD) is viewed as a developmental process of ‘growing apart’, has been emphasized [3] and the proximate causes of SSD has been determined in several lizards species under this framework [4,5,6,7]
It is clear that sexspecific growth plays an important role in SSD development, we still know little about when the divergence take place and whether the differences persist throughout life

Summary

Introduction

Sexual dimorphism is a common and long-discussed feature of the animal world that is reflected in a variety of biological characteristics. Sexual size dimorphism (SSD) is a widespread phenomenon observed in many taxa that is believed to have evolved through several non-exclusive ultimate selection pressures, including sexual selection, fecundity selection, and intersexual niche divergence [1]. These adaptive hypotheses explain relatively small proportions of the interspecific variance in lizard SSD [2] and relatively little is known about the proximate developmental mechanisms of SSD. There is little information concerning the proximate physiological mechanisms involved in this differentiation and the factors that influence the process To resolve these questions, we must first determine the critical periods when male and female sizes diverge

Methods

Results

Conclusion