Thyroid Development in Precocial and Altricial Avian Embryos

Abstract

-Histological criteria indicate that thyroid glands are organized into follicles at about the same time of incubation in Japanese Quail (precocial) and Ringed Turtle Dove (altricial) embryos, but dove thyroids develop more slowly, are much smaller and are much less active than quail thyroids by the time of hatching. These results indicate thyroid readiness for participation in early endothermic responses of precocial hatchlings, and relative thyroid inactivity in young ectothermic altricial nestlings.-Department of Biology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061. Accepted 12 April 1976. THYROID hormones are known to be important determinants of metabolic rate and heat production in adult birds. In addition, the time course of developing thyroid function closely parallels developing thermoregulatory ability in precocial young that hatch at a relatively advanced stage of development (see Freeman 1971, for the data on domestic chickens; McNabb et al. 1972 and Spiers et al. 1974 for the data on Japanese Quail [Coturnix coturnix japonica]). These precocial hatchlings exhibit some thermoregulatory ability shortly after hatching although they do not become truly homeothermic for several weeks; the actual time course of homeothermic development depends on the species. In contrast, altricial birds hatch at a less developed stage and are dependent on parental care and feeding for some time. Altricial hatchlings are ectothermic for the first few days after hatching, but acquire thermoregulatory ability during the nestling period, and are homeothermic by the time of fledging (usually at about 2-3 weeks of age, Dawson and Hudson 1970). Using the single criterion of epithelial cell height, Dawson and Allen (1960) concluded that thyroid function in nestling Vesper Sparrows (Pooecetes gramineus) matches or exceeds that of adults and, therefore, anticipates the development of homeothermy. Rol'nik (1968) stated that the thyroid of altricial birds differentiates into follicles and becomes physiologically active relatively later than in precocial birds, but this comparison is based on birds that differ markedly in body size and length of their incubation periods. The objective of this paper is to compare thyroid gland development during the embryonic period in a precocial species (Japanese Quail), and in an altricial species (Ringed Turtle Doves, Streptopelia risoria). Although these species both have an incubation period of about 16+ days duration, quail exhibit endothermic responses to cold shortly after hatching (Spiers et al. 1974) while doves are presumably ectothermic for at least the first few days after hatching (Breitenbach and Baskett 1967 have shown that nestling Mourning Doves are ectothermic for at least the first 3 days). Our choice of these species was determined by the equal length of their incubation periods plus patterns of development typical of those considered precocial and altricial.