Scaling of buccal mass growth and muscle activation determine the duration of feeding behaviors in the marine mollusc Aplysia californica.

The mechanical forces experienced during movement and the time constants of muscle activation are important determinants of the durations of behaviors, which may both be affected by size-dependent scaling. The mechanics of slow movements in small animals are dominated by elastic forces and are thus quasistatic (i.e., always near mechanical equilibrium). Muscular forces producing movement and elastic forces resisting movement should both scale identically (proportional to mass⅔), leaving the scaling of the time constant of muscle activation to play a critical role in determining behavioral duration. We tested this hypothesis by measuring the duration of feeding behaviors in the marine mollusc Aplysia californica whose body sizes spanned three orders of magnitude. The duration of muscle activation was determined by measuring the time it took for muscles to produce maximum force as Aplysia attempted to feed on tethered inedible seaweed, which provided an in vivo approximation of an isometric contraction. The timing of muscle activation scaled with mass0.3. The total duration of biting behaviors scaled identically, with mass0.3, indicating a lack of additional mechanical effects. The duration of swallowing behavior, however, exhibited a shallower scaling of mass0.17. We suggest that this was due to the allometric growth of the anterior retractor muscle during development, as measured by micro computed tomography scans (microCT) of buccal masses. Consequently, larger Aplysia did not need to activate their muscles as fully to produce equivalent forces. These results indicate that muscle activation may be an important determinant of the scaling of behavioral durations in quasistatic systems.