Abstract
ABSTRACTIn leghorn chickens (Gallus gallus domesticus) of standard breed (large) and bantam (small) varieties, artificial selection has led to females being permanently gravid and sexual selection has led to male-biased size dimorphism. Using respirometry, videography and morphological measurements, sex and variety differences in metabolic cost of locomotion, gait utilisation and maximum sustainable speed (Umax) were investigated during treadmill locomotion. Males were capable of greater Umax than females and used a grounded running gait at high speeds, which was only observed in a few bantam females and no standard breed females. Body mass accounted for variation in the incremental increase in metabolic power with speed between the varieties, but not the sexes. For the first time in an avian species, a greater mass-specific incremental cost of locomotion, and minimum measured cost of transport (CoTmin) were found in males than in females. Furthermore, in both varieties, the female CoTmin was lower than predicted from interspecific allometry. Even when compared at equivalent speeds (using Froude number), CoT decreased more rapidly in females than in males. These trends were common to both varieties despite a more upright limb in females than in males in the standard breed, and a lack of dimorphism in posture in the bantam variety. Females may possess compensatory adaptations for metabolic efficiency during gravidity (e.g. in muscle specialization/posture/kinematics). Furthermore, the elevated power at faster speeds in males may be linked to their muscle properties being suited to inter-male aggressive combat.
Highlights
Many avian species exhibit sexual dimorphism in morphology, physiology and behaviour, linked to differential specialization of the sexes towards mate competition, reproduction and parental care (Dunn et al, 2001)
A significant interaction between variety and sex for hhip was found because hhip was 33.04 mm taller in B♂ compared to B♀, whereas in the standard breed, hhip was 21.60 mm taller L♀ compared to L♂ (Table 1)
It is possible that they avoided higher U and grounded running gaits in order to reduce peak forces on their bones and avoid fracture as their bones may be weakened by the provision of medullary calcium towards eggshell formation (Bloom et al, 1941; Whitehead, 2004). This may be pertinent in white leghorns, which are prone to osteoporosis during eggshell construction (Dacke et al, 1993). The sexes of both standard breed and bantam varieties of leghorn chicken differed in all measured aspects of terrestrial locomotion
Summary
Many avian species exhibit sexual dimorphism in morphology, physiology and behaviour, linked to differential specialization of the sexes towards mate competition, reproduction and parental care (Dunn et al, 2001). With the few exceptions of reverse sexual size dimorphism, where females are the larger sex (Reynolds, 1972; Hakkarainen et al, 1996; Pande and Dahanukar, 2012), males are often larger than females and these size differences are more pronounced in cursorial species (Hoglund, 1989). Previous studies investigating physiological differences between the sexes in birds have focused on maximum performance and aerobic limits and/or scopes (Chappell et al, 1996, 2011; Hammond et al, 2000). Despite well documented influences of body size and shape on the mechanics and energetics of locomotion (Taylor et al, 1982; Alexander and Jayes, 1983), the influence of sexual dimorphism on locomotor performance in birds has been given little attention (Brackenbury and Elsayed, 1985; Lees et al, 2012; Rose et al, 2014)
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