S9-2.2 Jan. 6 The evolution of modulation of amniote chewing ROSS, C.F.*; ECKHARDT, A.; HERREL, A.; METZGER, K.; SCHAERLAEKEN, V.; WASHINGTON, R.; WILLIAMS, H.; University of Chicago; University of Chicago; University of Antwerp; Brown University; University of Antwerp; University of Chicago; Ohio University firstname.lastname@example.org
Mammalian mastication is a highly coordinated, highly modulated behavior. Determining which modulatory systems were in place prior to the evolution of mammalian mastication and which may have been key innovations is vital for understanding how this complex functional system evolved. Mammals are distinguished from most other vertebrates in having sensory afferents from the periodontal ligament and gamma motor neurons to the muscle spindles, both of which are implicated in feed-forward control of mastication. We hypothesize that mammals are unique in their ability to use feed-forward control of bite force and muscle activity during mastication, and that this feed-forward control made possible rhythmic, regular chewing. The hypothesis that mammals exhibit more rhythmic, regular chewing behavior than lepidosaurs was evaluated using data on variation in cycle length from the literature and from our own in vivo studies of EMG, bone strain and/or kinematics. Comparisons between 44 species of mammals and 9 species of lepidosaurs from a range of body sizes and eating a variety of foods suggest that mammals chew with less variable cycle lengths than most lepidosaurs. Mammals (n=61 species) also exhibit size-related changes (i.e., scaling) of cycle length, and shorter cycle lengths than lizards (n=22 species) of equivalent mandible lengths. The origin of periodontal afferents and fusimotor control of muscle spindle sensitivity in stem mammals was a key innovation in the evolution of mammalian mastication. It facilitated feed-forward, or anticipatory, control of jaw movements, allowing rhythmic, regular chewing behavior. This is hypothesized to have increased the rate of intra-oral food processing, fueling the elevated metabolic rates characteristic of mammals.