Relationship between Body Composition and Homeothermy in Neonates of Precocial and Semiprecocial Birds

We dissected carcasses of neonates belonging to ducks and geese (Anatidae; 8 species), shorebirds (Charadriidae and Scolopacidae; 12 species), gulls and terns (Laridae; 3 species), and nonanseriform water birds (Podicipedidae and Rallidae; 2 species) ranging in yolk-free lean wet body mass from 2.5 to 70 g. We have fitted allometric relationships between the lean wet mass of each component and the lean wet yolk-free neonatal body mass. The exponents of the relationships of the brain mass (0.73) or head mass (0.85) to neonatal body mass were significantly lower than 1. The exponents did not differ significantly from 1 for the heart, whole leg, leg bone, liver, intestines, pectoral muscles, skin, stomach, wings, feathers, yolk (wet and dry), and remainder of the body. The exponent for leg muscle mass (1.18) was significantly higher than 1. This suggests that larger chicks may have a higher potential for thermogenic heat production. At a given body mass, no differences could be detected with respect to the lean fresh leg muscle mass among ducklings, shorebirds, and the nonanseriform water birds. However, the high fractional lipid-free dry lean matter content of the leg muscles of ducklings (which might represent a high amount of contractile proteins in these muscles) could explain their observed high thermogenic heat production in response to cold stress. The exponents of feather mass and lean wet skin mass to body mass were significantly higher than 0.67 (i.e. surface-to-volume relationship of a sphere), in accordance with our previous finding that large neonates have a relatively lower minimal thermal conductance per unit surface area than smaller chicks.