Ecological consequences of scaling of chew cycle duration and daily feeding time in primates

Intraspecific Variation in Maximum Ingested Food Size and Body Mass in Varecia rubra and Propithecus coquereli

In a recent study, we quantified the scaling of ingested food size (V_b)—the maximum size at which an animal consistently ingests food whole—and found that V_b scaled isometrically between species of captive strepsirrhines. The current study examines the relationship between V_b and body size within species with a focus on the frugivorous Varecia rubra and the folivorous Propithecus coquereli. We found no overlap in V_b between the species (all V. rubra ingested larger pieces of food relative to those eaten by P. coquereli), and least-squares regression of V_b and three different measures of body mass showed no scaling relationship within each species. We believe that this lack of relationship results from the relatively narrow intraspecific body size variation and seemingly patternless individual variation in V_b within species and take this study as further evidence that general scaling questions are best examined interspecifically rather than intraspecifically.

The influence of experimental manipulations on chewing speed during in vivo laboratory research in tufted capuchins (Cebus apella)

Even though in vivo studies of mastication in living primates are often used to test functional and adaptive hypotheses explaining primate masticatory behavior, we currently have little data addressing how experimental procedures performed in the laboratory influence mastication. The obvious logistical issue in assessing how animal manipulation impacts feeding physiology reflects the difficulty in quantifying mechanical parameters without handling the animal. In this study, we measured chewing cycle duration as a mechanical variable that can be collected remotely to: 1) assess how experimental manipulations affect chewing speed in Cebus apella, 2) compare captive chewing cycle durations to that of wild conspecifics, and 3) document sources of variation (beyond experimental manipulation) impacting captive chewing cycle durations. We find that experimental manipulations do increase chewing cycle durations in C. apella by as much as 152 milliseconds (ms) on average. These slower chewing speeds are mainly an effect of anesthesia (and/or restraint), rather than electrode implantation or more invasive surgical procedures. Comparison of captive and wild C. apella suggest there is no novel effect of captivity on chewing speed, although this cannot unequivocally demonstrate that masticatory mechanics are similar in captive and wild individuals. Furthermore, we document significant differences in cycle durations due to inter-individual variation and food type, although duration did not always significantly correlate with mechanical properties of foods. We advocate that the significant reduction in chewing speed be considered as an appropriate qualification when applying the results of laboratory-based feeding studies to adaptive explanations of primate feeding behaviors.

Intraspecific scaling of chewing cycle duration in three species of domestic ungulates

In mammals, chewing cycle duration (CCD) increases with various measures of size, scaling with body mass0.13–0.28 and jaw length0.55. Proposed explanations for these scaling relationships include the allometry of body size, basal metabolic rate and tooth size, on the one hand, and pendular mechanics treating the jaw as a gravity-driven pendulum, on the other. Little is known, however, about the relationship between CCD and size within species. Recent research in dogs demonstrates altogether different scaling exponents and weaker correlations. This research suggests that breed-specific growth rates influence the maturation of the neural networks generating chewing rhythm, which may be altered because of changes in jaw mass during early postnatal growth. Here, we explored the intraspecific scaling of CCD within a sample of adult horses ranging from miniatures to draft breeds and an ontogenetic sample of goats and alpacas from infants to adults. In horses, CCD scales with body mass0.19 and jaw length0.57, although in neither case is the correlation significant. In the ontogenetic samples of goats and alpacas, CCD is significantly correlated with body mass, scaling as CCD∝body mass0.37 in both species. In goats, but not alpacas, CCD is also significantly correlated with jaw length, scaling as jaw length1.032. As in dogs, the scaling of CCD in horses may reflect the influence of selective breeding on growth trajectories of different breeds, resulting in reduced body and jaw size differences among infants, when CCD is established, compared with adults. However, the allometric scaling of tooth size in horses of different breeds may be a potential influence on the scaling of CCD. The scaling of CCD with body and jaw size in goats, and to a lesser extent in alpacas, also suggests that the development of peripheral masticatory structures such as the teeth and occlusal relations may play a role in changes in CCD during the earliest stages of postnatal ontogeny.

Maximum ingested food size in captive strepsirrhine primates: scaling and the effects of diet

Little is known about ingested food size (V(b)) in primates, even though this variable has potentially important effects on food intake and processing. This study provides the first data on V(b) in strepsirrhine primates using a captive sample of 17 species. These data can be used for generating and testing models of feeding energetics. Strepsirrhines are of interest because they are hypometabolic and chewing rate and daily feeding time do not show a significant scaling relationship with body size. Using melon, carrot, and sweet potato we found that maximum V(b) scales isometrically with body mass and mandible length. Low dietary quality in larger strepsirrhines might explain why V(b) increases with body size at a greater rate than does resting metabolic rate. Relative to body size, V(b) is large in frugivores but small in folivores; furthermore scaling slopes are higher in frugivores than in folivores. A gross estimate of dietary quality explains much of the variation in V(b) that is not explained by body size. Gape adaptations might favor habitually large bites for frugivores and small ones for folivores. More data are required for several feeding variables and for wild populations.

Allometry of masticatory loading parameters in mammals

Considerable research on the scaling of loading patterns in mammalian locomotor systems has not been accompanied by a similarly comprehensive analysis of the interspecific scaling of loading regimes in the mammalian masticatory complex. To address this deficiency, we analyzed mandibular corpus bone strain in 11 mammalian taxa varying in body size by over 2.5 orders of magnitude, including goats, horses, alpacas, pigs, and seven primate taxa. During alert chewing and biting of hard/tough foods, bone-strain data were collected with rosette gauges placed along the lateral aspect of the mandibular corpus below the molars or premolars. Bone-strain data were used to characterize relevant masticatory loading parameters: peak loading magnitudes, chewing cycle duration, chewing frequency, occlusal duty factor, loading rate, and loading time. Interspecific analyses indicate that much as observed in limb elements, corpus peak-strain magnitudes are similar across mammals of disparate body sizes. Chewing frequency is inversely correlated with body size, much as with locomotor stride frequency. Some of this allometric variation in chewing frequency appears to be due to a negative correlation with loading time, which increases with body size. Similar to the locomotor apparatus, occlusal duty factor, or the duration of the chewing cycle during which the corpus is loaded, does not vary with body size. Peak principal-strain magnitudes are most strongly positively correlated with loading rate and only secondarily with loading, with this complex relationship best described by a multiple regression equation with an interaction term between loading rate and loading time. In addition to informing interpretations of craniomandibular growth, form, function, and allometry, these comparisons provide a skeleton-wide perspective on the patterning of osteogenic stimuli across body sizes.

Rhythmic chewing with oral jaws in teleost fishes: a comparison with amniotes

Intra-oral prey processing (chewing) using the mandibular jaws occurs more extensively among teleost fishes than previously documented. The lack of muscle spindles, γ-motoneurons and periodontal afferents in fishes makes them useful for testing hypotheses regarding the relationship between these sensorimotor components and rhythmic chewing in vertebrates. Electromyography (EMG) data from the adductor mandibulae (AM) were used to quantify variation in chew cycle duration in the bowfin Amia, three osteoglossomorphs (bony-tongues), four salmonids and one esocid (pike). All species chewed prey using their oral jaw in repetitive trains of between 3 and 30 consecutive chews, a pattern that resembles cyclic chewing in amniote vertebrates. Variance in rhythmicity was compared within and between lineages using coefficients of variation and Levene's test for homogeneity of variance. These comparisons revealed that some teleosts exhibit degrees of rhythmicity that are comparable to mammalian mastication and higher than in lepidosaurs. Moreover, chew cycle durations in fishes, as in mammals, scale positively with mandible length. Chewing among basal teleosts may be rhythmic because it is stereotyped and inflexible, the result of patterned interactions between sensory feedback and a central pattern generator, because the lack of a fleshy tongue renders jaw–tongue coordination unnecessary and/or because stereotyped opening and closing movements are important for controlling fluid flow in the oral cavity.

Chewing variation in lepidosaurs and primates

Mammals chew more rhythmically than lepidosaurs. The research presented here evaluated possible reasons for this difference in relation to differences between lepidosaurs and mammals in sensorimotor systems. Variance in the absolute and relative durations of the phases of the gape cycle was calculated from kinematic data from four species of primates and eight species of lepidosaurs. The primates exhibit less variance in the duration of the gape cycle than in the durations of the four phases making up the gape cycle. This suggests that increases in the durations of some gape cycle phases are accompanied by decreases in others. Similar effects are much less pronounced in the lepidosaurs. In addition, the primates show isometric changes in gape cycle phase durations, i.e. the relative durations of the phases of the gape cycle change little with increasing cycle time. In contrast, in the lepidosaurs variance in total gape cycle duration is associated with increases in the proportion of the cycle made up by the slow open phase. We hypothesize that in mammals the central nervous system includes a representation of the optimal chew cycle duration maintained using afferent feedback about the ongoing state of the chew cycle. The differences between lepidosaurs and primates do not lie in the nature of the sensory information collected and its feedback to the feeding system, but rather the processing of that information by the CNS and its use feed-forward for modulating jaw movements and gape cycle phase durations during chewing.