My proficiency at eating is not usually determined by temperature, unless it's one of those ridiculously hot summer days on which a little air conditioning can go a long way. The same cannot be said for ectothermic vertebrates, whose digestive processes and skeletal muscle contractions depend heavily on ambient temperature – warmer generally means more effective and colder the opposite. However, in the last few years, work out of Steve Deban's laboratory at the University of South Florida, USA, has highlighted how a number of ectotherms get around this temperature dependence of their feeding systems. By relying on stored elastic energy rather than muscle work directly, chameleon, toad and plethodontid salamander tongues can project rapidly and forcefully across a wide range of temperatures. Elastic energy release is not nearly as susceptible to temperature as a contracting muscle. Continuing along this intellectual thread, Deban and his student Paula Sandusky recently studied the effects of temperature on the feeding behavior of the frog Rana pipiens.
A total of 46 feeding events from five frogs were imaged at 6000 frames s–1. Feeding trials were performed at three temperatures (10, 15 and 25 C) with crickets as prey held at varying distances from the animal. Movements of the frog itself, its lower jaw ° and its tongue were characterized during each feeding, and the amplitude, speed and timing of these movements were compared statistically across temperatures. A temperature coefficient (Q10) was calculated for each of the various performance variables as an indicator of the degree to which they were affected by temperature.
During feeding, frogs extend their legs, lunge toward the prey and rapidly open their mouths. Rapid depression of the lower jaw (9–24 m s–1) propels the tongue out of the mouth and onto the prey, after which the tongue is retracted with the prey into the mouth. Movement amplitudes including lunge, gape and tongue protrusion distance were not especially sensitive to temperature. However, the durations and speeds of those movements were. For example, the mean velocity of mouth closing at 10°C was ∼0.1 m s–1 and at 25°C it was ∼0.3 m s–1. But not all durations and speeds were equally sensitive to temperature. Velocities and accelerations associated with mouth opening had Q10 values less than 1.25, indicating a relatively low sensitivity to temperature, while those associated with tongue projection were much higher. The higher Q10 values indicate the relative importance of muscle contraction in driving the movement because we know muscle contractile performance depends heavily on temperature. Thus, while mouth opening appears to be driven largely by elastic energy release (i.e. is not especially sensitive to temperature), tongue projection presumably relies more heavily on muscle contraction for its power. This is quite distinct from what has been found for tongue protrusion in toads, lungless salamanders and chameleons, where elastic energy is the main driver.
While Deban's work highlights the diverse ways in which ectothermic vertebrates power feeding movements, another very appealing aspect of his work is that it demonstrates how temperature can be used as a means of teasing out the importance of muscle versus elastic energy in all kinds of biomechanical systems.
