Maximum running speed limitations on terrestrial mammals: A theoretical approach

Loading effects on jump performance in green anole lizards, Anolis carolinensis

Locomotor performance is a crucial determinant of organismal fitness but is often impaired in certain circumstances, such as increased mass (loading) resulting from feeding or gravidity. Although the effects of loading have been studied extensively for striding locomotion, its effects on jumping are poorly understood. Jumping is a mode of locomotion that is widely used across animal taxa. It demands large amounts of power over a short time interval and, consequently, may be affected by loading to a greater extent than other modes of locomotion. We placed artificial loads equal to 30% body mass on individuals of the species Anolis carolinensis to simulate the mass gain following the consumption of a large meal. We investigated the effects of loading on jump performance (maximum jump distance and accuracy), kinematics and power output. Loading caused a significant 18% decline in maximum jump distance and a significant 10% decline in takeoff speed. In other words, the presence of the load caused the lizards to take shorter and slower jumps, whereas takeoff angle and takeoff duration were not affected. By contrast, jump accuracy was unaffected by loading, although accuracy declined when lizards jumped to farther perches. Finally, mass-specific power output did not increase significantly when lizards jumped with loads, suggesting that the ability to produce mechanical power may be a key limiting factor for maximum jump performance. Our results suggest that mass gain after a large meal can pose a significant locomotor challenge and also imply a tradeoff between fulfilling energy requirement and moving efficiently in the environment.

Causes and significance of variation in mammalian basal metabolism

Mammalian basal metabolic rates (BMR) increase with body mass, whichs explains approximately 95% of the variation in BMR. However, at a given mass, there remains a large amount of variation in BMR. While many researchers suggest that the overall scaling of BMR with body mass is due to physiological constraints, variation at a given body mass may provide clues as to how selection acts on BMR. Here, we examine this variation in BMR in a broad sample of mammals and we test the hypothesis that, across mammals, body composition explains differences in BMR at a given body mass. Variation in BMR is strongly correlated with variation in muscle mass, and both of these variables are correlated with latitude and ambient temperature. These results suggest that selection alters BMR in response to thermoregulatory pressures, and that selection uses muscle mass as a means to generate this variation.