Plots defining the kinematic variables discussed in the text. Data from primate Cebus. The upper graph plots the open—close displacements of the lower jaw during a complete chewing sequence (from ingestion to final swallow) (black line). The grey line represents the second derivative of the black line. The enlarged portion of the chewing sequence graph (below) represents a single gape cycle, divided into the four gape cycle phases. The black square represents minimum gape. The black circles represent maximum gape. Total cycle duration (T_c) is the time between maximum gapes. The grey squares represent the fast-close slow-close transition: the position of the largest negative peak between maximum gape and minimum gape on the second derivative (grey line). The grey circle on the black line represents slow-open fast-open transition: the position of the largest negative peak on the second derivative between minimum gape and maximum gape in the following chew. These four variables define the boundaries of the four gape cycle phases fast-close (FC), slow-close (SC), slow-open (SO) and fast-open (FO).

Bivariate plots of ε₁ peak magnitude in microstrain (με) against strain rate (με s⁻¹) for Tupinambis 2 and 5, and against both strain rate and load time in Tupinambis 3. Tupinambis 2 and 5 only ate grasshoppers. ε₁ was not significantly correlated with load time in Tupinambis 2 and 5.

Box plot of average coefficients of variation (CVs) of total gape cycle duration (cycle CV) and the CVs of the phases of the gape cycle (SO, FO, FC, SC) in lepidosaurs and primates. Outliers were deleted. Single factor ANOVAs of CVs reveal significant effects of phase (five levels: SO, FO, FC, SC and TC) on CVs within both lepidosaurs and primates. Among lepidosaurs, CVs of slow open (CV_SO) and slow close (CV_SC) are not significantly different from each other but differ from CVs of fast close and total cycle (CV_FC and CV_TC; P<0.001); the CV for fast open (CV_FO) does not differ from any of the other CVs. Among primates, CV_FO and CV_SC are not significantly different from each other but differ significantly from CV_FC and CV_TC; CV_SO is only significantly different from CV_TC. Among primates CV_TC is different and lower than the CVs of all its constituent phases (P<0.01). CV_SO and CV_SC in primates are not significantly different from those in lizards; CV_FC and CV_FO in primates are significantly different and larger than those in lizards (CV_FC: t=−2.102, P=0.04; CV_FO: t=−3.639, P=0.001).

Bar plots of the coefficients of variation (CVs) of the four phases of the gape cycle for the species studied here. Note the higher CVs for FO duration in the four primate species to the left. On average, the CVs for FC are significantly different and higher in primates than in lepidosaurs. Note also that contrary to the predictions of Ross et al. (Ross et al., 2007a), SC duration is not less variable in primates than it is in lepidosaurs. See Fig. 3 for primate and lepidosaur means.

Top. Stacked bar plots of Johnson's relative weights, which quantify the proportionate contributions of variance in each phase to variance in total cycle duration. The primates are to the left of the plots; the lepidosaurs are to the right. The data for each individual are shown in the top plot (numbers identify individuals), species averages are presented in the bottom left, and the overall averages for primates and lepidosaurs are compared bottom right. In lepidosaurs variance in total cycle duration is more strongly influenced by variance in SO than is the case in primates; in primates variance in total cycle duration is more strongly influenced by variance in FO and FC than is the case in lepidosaurs; and in primates variance in total cycle duration is more evenly influenced by variance in all four gape cycle phases than is the case in most lepidosaurs. These results persist when Johnson's relative weights are calculated only from chewing cycles in which the primates or lepidosaurs ate fruit or vegetables: specifically, apple, tomato and endive.