OLSON, Christopher R.*; VLECK, Carol M.; VLECK, David; Iowa State University, Ames: Growth costs increase with periodic cooling in house wren eggs

Avian embryos of many species grow rapidly, even in the face of periodic drops in egg temperature when the adult leaves the nest to forage. Consequences of obligatory bouts of cooling for embryonic growth are poorly understood. We periodically cooled house wren ( Troglodytes aedon ) eggs to 20°C fifteen times a day and compared their growth to eggs incubated at a constant 37.5°C. We measured oxygen consumption of all eggs at 37.5°C at two day intervals. We then sacrificed the eggs and measured dry embryo mass and remaining dry yolk mass after 12 days of development. Mean embryo mass of eggs that were cooled averaged 57% of embryo mass from eggs of the same age incubated at constant temperatures, and were about 2-days delayed in growth. Residual yolk mass, however, did not differ between the constant temperature and cooled eggs. The metabolism of cooled eggs was 84% that of same-aged eggs held at constant temperature. Compared to younger embryos of the same mass that developed at constant temperature, these periodically cooled eggs had metabolic rates that averaged 20% higher. When development was defined in terms of thermal dose (degree·days), embryos from eggs periodically cooled for 12 days were 74% the size of embryos incubated at constant 37.5°C for the same thermal dose. We used independent measurements of the effect of temperature on metabolic rate of developing embryos to incorporate non-linear effects of temperature and make empirical estimates of overall metabolic costs. Mass-specific growth cost (kJ/g dry tissue) of periodically cooled eggs was 17% greater than that of eggs held at constant temperature. Periodic cooling imposes significant costs to the embryo that include increased energy demand and lower efficiency of growth, in addition to the familiar extended development time.