Eastern Garter Snakes (Thamnophis sirtalis) with proportionally larger heads are in better condition

Abstract

We investigated sexual size dimorphism and trophic morphology dimorphism in Eastern Garter Snakes (Thamnophis sirtalis) to test predictions derived from the reproductive role hypothesis. Our first objective was to examine whether structures involved in feeding (head and jaws) are more divergent than body size. Female T. sirtalis were larger than males and had proportionally larger heads. Our second objective was to look for an advantage of large head size by examining the relationship between body condition and head size. Body condition was positively correlated with relative head size in T. sirtalis for both sexes, in agreement with the reproductive role hypothesis.

Sexual dimorphism and skull size and shape in the highly specialized snake species,Aipysurus eydouxii(Elapidae: Hydrophiinae)

“…It is thought to be one means of compensating for greater energy expenditures (maintaining larger bodies and, in the case of females, providing nutritional components for developing embryos) (e.g. Elgee & Blouin-Demers, 2011;Borczyk, 2015). Females grow larger in approximately two thirds of snake species studied so far and usually females possesses larger heads (Shine, 1993).…”

“…For example, Arafura File Snakes (Acrochordus arafurae) reaching extremes with females being up to 10 times heavier than males (Shine, 1991). Sexual size dimorphism is followed by differentiated energy requirements that can manifest as food intake and type or size of prey (Elgee & Blouin-Demers, 2011;Borczyk, 2015). The latter aspect is of specific interest since, in many instances, it requires adjustment of not only ecological properties (like habitats, preferences towards appropriate food resources, or feeding niche divergence) but also of morphology (Camilleri & Shine, 1990).…”

“…Rossman, 1980;Young, 1989;Hampton & Kalmus, 2014;Andjelković, Tomović & Ivanović, 2016;Hampton & Moon, 2013). Under such circumstances, intraspecific food-niche partitioning followed by morphological adjustments are easily predictable, namely the larger sex, in response to elevated food requirements, increases the size of its skull to ingest larger prey (Elgee & Blouin-Demers, 2011). It is more difficult to explain a scenario where strong feeding specialisation is equalised between males and females and the size of ingested food particles does not change.…”

Allometry of head size and shape dimorphism in the grass snake (Natrix natrix L.)

“…However, because of the relatively low number of published studies on the sexual differences in head growth pattern in snakes, it is hard to make generalizations. In some snake species, males and females follow the same allometric pattern and the differences between them are constant, regardless of the presence and direction of sexual size dimorphism (Forsman, 1991;Vincent et al, 2004aVincent et al, , 2004bElgee and Blouin-Demers, 2011), but in other cases there are some differences (Luiselli et al, 2002). More results regarding species with both male-and female-biased sexual dimorphism covering a wide taxonomical range are needed to make more general conclusions possible and to test the ideas about parallel or independent evolution of such dimorphism.…”

Body condition and head size in snakes

“…The adaptive significance of snake head size, both absolute and relative, has been the focus of numerous publica-Animal Ecology Research Group, Hungarian Academy of Science, Hungarian Natural History Museum, Budapest, Hungary Department of Animal Ecology, Ecology Building, Lund University, S-22362 Lund, Sweden School of Biological Sciences, University of Wollongong, NSW 2522, Australia e-mail: madsen@uow.edu.au tions (e.g. Shine, 1991;Arnold, 1993;Forsman, 1996;Elgee and Blouin-Demers, 2011).…”