Proximate determinants of bite force capacity in the mouse lemur C. Chazeau, J. Marchal, R. Hackert, M. Perret & A. Herrel Département d’Ecologie et de Gestion de la Biodiversité, UMR 7179 C.N.R.S/M.N.H.N., Paris Cedex 5, France Keywords head size; testosterone; muscle; age; force. Correspondence Anthony Herrel, UMR 7179 C.N.R.S/M.N.H.N., Département d’Ecologie et de Gestion de la Biodiversité, 57 rue Cuvier, Case postale 55, 75231, Paris Cedex 5, France. Tel: +33 140798120; Fax: +33 140793773 Email: anthony.herrel@mnhn.fr Editor: Andrew Kitchener Received 20 September 2012; revised 3 November 2012; accepted 20 November 2012 doi:10.1111/jzo.12011 Abstract Both mating system and diet are thought to drive inter-individual variation in bite force. Although previously published data suggest that bite force variation may be driven by variation in morphology (e.g. head morphology, body size, muscle size), age and physiology (e.g. fluctuating plasma testosterone levels) in some verte- brates, this remains untested in primates. Here, we explore the proximal determi- nants of bite force capacity in the grey mouse lemur Microcebus murinus. Our results show that in male grey mouse lemurs, bite force measurements are repeat- able across a 1-month period. Yet, bite forces were independent of fluctuation plasma testosterone levels. Head dimensions and body mass were all positively correlated with bite force. Among these, head width was the best predictor of bite force as has been observed for other vertebrates. Unexpectedly, age was highly significantly and positively correlated with bite force. Whereas older animals generally bit harder, the oldest age group (5.5 years) showed a decline in bite force capacity. These results suggest that bite force in the grey mouse lemur is mostly determined by morphology and age, yet is independent of variation in testoster- one. Future studies including a broader age range and animals of different sexes would be of interest to better understand the variation in bite force in this small lemur. Introduction Body size and dietary specialization have been suggested as the primary drivers of the evolution of cranial form and function in primates (e.g. Hylander, 1979; Ravosa, 1991; Daegling, 1992; Perry, Hartstone-Rose & Wall, 2011a). Whereas many studies have estimated bite forces in primates using anatomical estimates (e.g. Spencer, 1998; Perry, Hartstone-Rose & Logan, 2011b; Shi et al., 2012), muscle stimulation (Dechow & Carlson, 1983, 1990) or indirectly through measures of the forces need to break food items (Lucas, Peters & Arrandale, 1994), surprisingly few studies have provided direct measures of bite force (but see Ross et al., 2007). Yet, direct bite force measurements are of inter- est as they are non-invasive and can be used to test inter- individual correlations with morphology (Herrel et al., 2008), behavior (Herrel et al., 2009) and diet (Herrel et al., 2006). For a given gape angle, bite force is determined by the force generation capacity of the jaw muscles and the lever system of the jaw. The force-generating capacity of the muscles is in turn determined by muscle cross-sectional area and fiber type (e.g. Herrel et al., 2008). As individuals with bigger heads can harbor larger muscles, correlations between head dimensions and bite force generation capacity are often observed (Herrel et al., 1999, 2005a,b), and head size is often used as a proxy for male fighting ability in species that bite during territory defense or aggressive behaviors in general (Molina Borja, Padron Fumero & Alfonso-Martin, 1998). However, physiological parameter such as fluctuating plasma testosterone levels may also affect bite force capacity. Indeed, testosterone is known to affect anatomical and physi- ological traits that contribute to variation in physical perform- ance (Luine et al., 1980; Tobin & Joubert, 1991; Girgenrath & Marsh, 2003; but see O’Connor et al., 2011). However, evi- dence for testosterone affecting performance traits such as bite force is equivocal. Whereas in the lizard Anolis carolinensis, plasma testosterone levels are correlated with bite force capac- ity (Husak et al., 2007), in another species of lizard Gallotia galloti, experimental elevation of testosterone levels increased the size of the jaw adductor muscles, but not bite force (Huyghe et al., 2010). How general these results are, and whether other vertebrates also show correlations between fluc- tuating testosterone levels and bite force, remains, however, unknown. Finally, bite force is known to be strongly influenced by growth and development (Herrel & Gibb, 2006). As animals grow, their heads and jaw muscles increase in size as well, thus affecting bite force (Herrel & O’Reilly, 2006). Moreover, these patterns are often allometric (i.e. different components growing at different rates), and larger animals often have disproportionately large bite forces for their size (Herrel & Gibb, 2006). Yet, most of these studies are based on Journal of Zoology Journal of Zoology. Print ISSN 0952-8369 Journal of Zoology •• (2012) ••–•• © 2012 The Zoological Society of London 1