OLSON, Christopher R.*; VLECK, Carol M.; VLECK, David; Iowa State University, Ames: Temperature and metabolic rate: embryonic birds depart from an Arrhenius relationship

Temperature has a direct effect on metabolism and growth. This effect is of particular importance to organisms that naturally experience a range of body temperatures induced by environmental or behavioral changes. We report the instantaneous metabolic response of house wren ( Troglodytes aedon) embryos to episodes of periodic cooling. House wren eggs regularly cool when the incubating adult leaves the nest to forage. Eggs at various stages of development were collected from wren nests and brought to the laboratory. A miniature thermocouple was inserted through the allantoic membrane and placed next to the embryo. Eggs were then cooled and rewarmed over a range of 37.5-15°C while we simultaneously measured instantaneous metabolism and egg temperature. Metabolic rate increased with embryo size and with embryo temperature. Under the Arrhenius model, the natural logarithm of metabolism plotted against the inverse of absolute temperature should yield a linear relationship whose slope is determined, in part, by the underlying physiology and biochemistry of the organism. For house wren embryos, metabolic rates show non-Arrhenius behavior over a biologically relevant range of temperatures. As eggs cool, embryo metabolic rate drops faster than the Arrhenius model predicts. Departure from an Arrhenius relationship across a range of biologically relevant temperatures may occur when different biochemical reaction rates and/or transport functions become uncoordinated. Such metabolic imbalances may account for the observations that wren embryos grow more efficiently at constant temperature than at variable temperatures. A differential growth efficiency should impose selection on incubating adults to maintain incubation temperatures within a narrow range.