Temperature Regulation in the Incubation Mounds of the Australian Brush-Turkey

Abstract

The Australian Brush-turkey, Alectura lathami, constructs incubation mounds of decomposing forest litter in which many large eggs are incubated by microbial heat generation. On Kangaroo Island, the average mound is about 12.7 m3 and weighs about 6,800 kg. It maintains an incubation of 330C in an average ambient air of 180C. When eggs are in the mound, the rate of heat production is estimated to be about 100 Watts, a value more than 20 times the heat production of the resting adult. Thus, the mound can incubate many more eggs than would be possible in a normal nest. Core is stable due to mound size and biophysical homeothermy. Mounds tend to reach a stable temperature at which the rate of microbial heat production equals the rate of heat loss to the environment. The bird adjusts equilibrium by adding or removing litter as required. A numerical computer model, incorporating experimental data on mound size, ambient temperature, and the mound material's rate of heat production, water content, dry density, and thermal conductivity, predicts that as little as 1 cm of litter added to the mound will raise core about 1.50C. Experimental manipulation of artificial and natural mounds uphold the model and indicate that functional mounds require (1) a critical mass of fresh litter (ca. 3,000 kg), (2) sufficient water content (>0.2 ml/g dry material), and (3) occasional mixing of the litter. Once constructed and adjusted, natural mounds require little attention, and larger ones can stay warm for several weeks without the bird. The mound characteristics appear to minimize the work required for maintenance. The bird maintains water content of the mound at a level (9 = 0.3 ml/g) that minimizes thermal conductivity and microbial heat production. Therefore, heat is retained in the mound and decomposition occurs slowly, reducing the requirement to collect fresh litter. Kangaroo Island mounds are larger than those in sub-tropical rainforest, probably because rates of decomposition of mound material are lower, not because of differences in either thermal conductivity of the material or ambient temperature.