Does gliding when pregnant select for larger females? H. B. Fokidis 1,2 & T. S. Risch 1,2 1 Department of Biological Sciences, Arkansas State University, State University, AR, USA 2 Savannah River Ecology Laboratory, Aiken, SC, USA Keywords body size; flying squirrel; gliding; sexual size dimorphism; wing loading. Correspondence H. Bobby Fokidis. Current address: School of Life Sciences, Arizona State University, PO Box 874601, Tempe, AR 85287-4601, USA. Email: Bobby.Fokidis@asu.edu Editor: Virginia Hayssen Received 4 October 2007; revised 4 February 2008; accepted 18 February 2008 doi:10.1111/j.1469-7998.2008.00433.x Abstract For terrestrial vertebrates, gliding imposes unique constraints on the interaction of body mass and structural size, particularly with reference to minimizing wing loading. Females of gliding animals experience increases in wing loading during pregnancy or gravidity, and selection may favour increased structural size to compensate for the added mass. We tested whether pregnant southern flying squirrels Glaucomys volans had similar wing loading as males, and whether females with lower wing loading bore heavier litters, than those with greater wing loading. Males had greater wing loading than females, regardless of the latter’s reproduc- tive state (males: 38.4  3.62 N m 2 , pregnant females: 30.7  4.21 N m 2 and non- pregnant females: 26.8  5.13 N m 2 ). The slope of the linear relationship between planar surface area and body mass was similar between pregnant females and males, however (F = 0.383, P = 0.322). Thus female flying squirrels may optimize their litter mass to minimize wing loading during pregnancy. Contrary to our prediction, females with greater wing loading had heavier litters than those with lower wing loading, which suggests reproductive output may be influenced by other ecological factors. Introduction Sexual size dimorphism (SSD) is attributed to differential selection acting separately, but concurrently on each sex (Andersson, 1994). Intersexual divergence in body size may be associated with female fecundity (Darwin, 1874; Howard et al., 1998; Reeve & Fairburn, 1999), ecological niche separation (Slatkin, 1984; Shine, 1989; Temeles et al., 2000), sexual selection (Darwin, 1874; Webster, 1997) or differences in survival (Yoccoz & Mesnager, 1998; Schulte- Hostedde, Millar & Gibbs, 2002). Male-biased SSD is usually explained in the context of sexual selection acting on male body size during mate competition; however, selection could act to decrease female body size (Loison et al., 1999; Karubian & Swaddle, 2001). Hypotheses concerning the evolution of female-biased SSD vary and include: selection for smaller males (Ralls, 1976), female dominance (Kruuk, 1972), increased female fecundity (Howard et al., 1998; Schulte-Hostedde, Millar & Gibbs, 2004), intersexual differences in survival (Yoccoz & Mesnager, 1998) and enhanced mobility of pregnant or gravid females (Myers, 1978; Shine, 1988; Hayssen & Kunz, 1996). The latter hypothesis may be particularly relevant for organisms where body size are major determinants of locomotor efficiency, such as in flying, gliding and swimming species (McGuire & Dudley, 2005). Increases in body mass with reproduction can potentially interfere with selection for lighter or more aerodynamic body forms, which generally increases efficiency of locomotion (Shine, 1988). In numerous studies, potential constraints on reproduc- tion may have resulted from shifts towards novel locomo- tory modes (Hayssen & Kunz, 1996; Davenport, 2003; McGuire & Dudley, 2005; Fokidis & Risch, 2008). Evidence for a reproductive constraint come from studies that have assessed locomotor impairment during gravidity in snakes (Shine, 1988), lizards (Olsson, Shine & Bak-Olsson, 2000; Shine, 2003), raptors (Mueller & Meyer, 1985) and bats (Hayssen & Kunz, 1996). In flying species, increased body mass means an individual must generate more lift to over- come gravity (Hayssen & Kunz, 1996; Guillemette & Ouel- let, 2005), and thus added mass during reproduction may be particularly important in these species. Indeed, this ‘wing loading hypothesis’ may explain adaptations in birds, such as sequential ovulation and oviparity that can result from the need to minimize mass associated with reproduction (Blackburn & Evans, 1986). Most mammals exhibit male-biased SSD, however Ralls (1976) compiled data on mammals exhibiting female-biased SSD and suggested that, although less common, female- biased SSD is widespread in mammals (documented in c. 178 species from 13 orders). Ralls (1976) reasoned that female-biased SSD likely resulted from a variety of ecologi- cal factors that are not necessarily mutually exclusive, as opposed to direct sexual selection favouring large females. In mammals, the wing loading hypothesis has been applied to some bat species, where litter size and mass increase with the degree of female-biased SSD in forearm length and hence wing area (Myers, 1978). The mass gained during pregnancy increases wing loading. Thus, when females are Journal of Zoology Journal of Zoology 275 (2008) 237–244 c  2008 The Authors. Journal compilation c  2008 The Zoological Society of London 237 Journal of Zoology. Print ISSN 0952-8369