Patterns of growth and heterochrony in moundbuilders (Megapodiidae) and fowl (Phasianidae)

Abstract

Megapode hatchlings are “superprecocial” compared to hatchlings of other Galliformes. In this paper, we examine patterns of growth and inquire about evolutionary processes that led to different degrees of precocity at hatching in Galliformes. The postnatal development of body mass and that of seven morphometric parameters of wing and leg of Brush Turkey Alectura lathami, Peacock Pavo cristatus, Pheasant Phasianus colchicus and Japanese Quail Coturnix japonica was characterized by the ratio of exponential growth rate (EGR) to relative size (ln (W/A); W=size at a given age; A=asymptote) at a given age. From that relationship, we derived developmental trajectories as the slopes of a least squares regression of EGR on relative size. The slopes of the developmental trajectories are indicative of the degree of precocity of development. We found that Brush Turkey, Peacock, and Pheasant follow the same developmental trajectory. However, the Brush Turkey hatches heterochronically delayed on that trajectory and consequently, its hatchlings are superprecocial. The Quail follows a significantly steeper developmental trajectory and hatches with a high degree of embryonic function (EGR). From a comparative phylogenetic analysis we reconstruct the ancestral galliform developmental trajectory and infer that hypermorphosis of Brush Turkey embryos led to the superprecocial hatchlings. The steeper developmental trajectory of Quail is evolutionary derived from ancestral patterns of growth by somatic paedomorphosis. Using a multivariate approach we show that development of different parts of the body is coherent, i.e., that patterns of development of wing and leg do not vary independently of each other. Consequently, selection for changed development of any part of the wing or the leg will cause the other parts to change too.