Masticatory muscular activation in elderly individuals during chewing

Background: Old‐age is the last stage of human evolution and, unfortunately, the ageing of the oral cavity and masticatory system seems accelerated. As a consequence, there is a reduction in the amount of food ingested, which can lead to an imbalance in nutrition. Objective: The purpose of this study was to investigate the levels of muscular activation of elderly individuals, during chewing, and to compare with young individuals. Materials and Methods: An electromyographical analysis of the masticatory system in 10 individuals aged between 60 and 75 years (elderly group) and a similar number between 23–30 years old (young group ‐ control) was carried out. The analysis was performed using a MyoSystem‐Br1 electromyographer with differential active electrodes. The test was recorded during functional conditions, and the muscles assessed were the temporalis and masseter. Data were normalised by maximum voluntary contraction (MVC), and the results were analysed using an independent t‐test for comparison between the groups. Results: The normalised electromyographic data obtained showed significant differences in both groups. Comparing the normalised values obtained for MVC, the mean values for the masseter and temporalis muscles of elderly group were statistically lower (p ≤ 0.05) than control group for harder foods, but there were no significant differences for food with the lowest consistency. Conclusion: It can be concluded that elderly individuals show slight hypoactivity of their masticatory musculature during chewing when compared to young individuals.     

Associations between self-assessed masticatory ability and some general health factors in a Swedish population

Objectives : The aims of this study were 1) to describe changes in self-assessed masticatory ability over a 14 year period (1975–89); and 2) to describe associations between self-assessed masticatory ability and age, dental state and some other background factors in a sample of the Swedish population in 1988/89. Design: The Swedish National Central Bureau of Statistics investigates annually the living conditions of the Swedish population by means of interviews by trained persons. The data were analysed by means of stepwise logistic regression and calculation of adjusted relative risks. Subjects: In the investigation in 1988/89, 12901 people above 16 years of age participated and the response rate was 80%. Results: In comparison between the investigations, the prevalence of reported impairment was lower in 1988/89 than in 1975 and 1980/81. Prevalence of reported impairment of chewing ability increased with ageing from 2% in young adults (16–34 years old) to 44% in older elderly (>85 years old). In most age groups, edentulous people reported the highest prevalence, and dentate people the lowest. Relative risks for impaired masticatory ability, independent of age, gender and dental state, were higher for people in rural areas, with low income and living single, as well as for those with skeletal, gastrointestinal, psychiatric and tumour diseases. In the elderly, results from the logistic regression showed that some disability and psychosocial factors were also associated with masticatory ability. Conclusions : The results indicated that there was a group of elderly people who reported several functional and health problems including impaired masticatory ability.     

Electromyography and mechanics of mastication in the albino rat.

The masticatory apparatus in the albino rat was studied by means of electromyography and subsequent estimation of muscular forces. The activity patterns of the trigeminal and suprahyoid musculature and the mandibular movements were recorded simultaneously during feeding. The relative forces of the individual muscles in the different stages of chewing cycles and biting were estimated on the basis of their physiological cross sections and their activity levels, as measured from integrated electromyograms. Workinglines and moment arms of these muscles were determined for different jaw positions. In the anteriorly directed masticatory grinding stroke the resultants of the muscle forces at each side are identical; they direct anteriorly, dorsally and slightly lingually and pass along the lateral side of the second molar. Almost the entire muscular resultant force is transmitted to the molars while the temporo-mandibular joint remains unloaded. A small transverse force, produced by the tense symphyseal cruciate ligaments balances the couple of muscle resultant and molar reaction force in the transverse plane. After each grinding stroke the mandible is repositioned for the next stroke by the overlapping actions of three muscle groups: the pterygoids and suprahyoids produce depression and forward shift, the suprahyoids and temporal backward shift and elevation of the mandible while the subsequent co-operation of the temporal and masseter causes final closure of the mouth and starting of the forward grinding movement. All muscles act in a bilaterally symmetrical fashion. The pterygoids contract more strongly, the masseter more weakly during biting than during chewing. The wide gape shifts the resultant of the muscle forces more vertically and moreposteriorly. The joint then becomes strongly loaded because the reaction forces are applied far anteriorly on the incisors. The charateristic angle between the almost horizontal biting force and the surface of the food pellet indicates that the lower incisors produce a chisel-like action. Tooth structure reflects chewing and biting forces. The transverse molar lamellae lie about parallel to the chewing forces whereas perpendicular loading of the occlusal surfaces is achieved by their inclination in the transverse plane. The incisors are loaded approximately parallel to their longitudinal axis, placement that avoids bending forces during biting. It is suggested that a predominantly protrusive musculature favors the effective force transmission to the lower incisors, required for gnawing. By grinding food across transversely oriented molar ridges the protrusive components of the muscles would be utilized best. From the relative weights of the masticatory muscles in their topographical relations with joints, molars and incisors it may be concluded that the masticatory apparatus is a construction adapted to optimal transmission of force from muscles to teeth.     

Influence of age and dental status on chewing behaviour studied by EMG recordings during consumption of various food samples

Objective: To evaluate the effects of age and dental status on chewing performance in humans. Design: Electromyography recordings (EMG) were made during chewing of six foods (rice, beef, cheese, crispy bread, apple, and peanuts) to compare the masticatory patterns of four subject groups with different ages and dental status. Subjects: Nineteen elders (mean age 67.2 years) classified into three categories according to their number of opposing post‐canine teeth pairs (i.e. functional units) and a control group of 10 young adults (mean age 26.5 years) with a high number of functional units. Main outcome measures: Number of chewing cycles, chewing time, total muscle activity and muscle activity per chew, burst and inter‐burst durations, maximum and mean voltages from EMG recordings. Results: Time‐related EMG parameters increased from young subjects to elderly subjects with high, middle and low dental status. Parameters related to EMG voltages per chew decreased in the same order among the different groups of subjects. These tendencies were observed for all the studied products. Subjects with weak muscle contraction may compensate for their poor chewing performance by lengthening both chewing cycle and sequence duration. Additional alterations in the chewing patterns were observed when age effect was associated with a dental status degradation in terms of number of functional units. Conclusion: Impairment in mastication for the elderly is due to both ageing and decreasing number of functional pairs of post‐canine teeth.     

Chewing number is related to incremental increases in body weight from 20 years of age in Japanese middle‐aged adults

doi: 10.1111/j.1741‐2358.2012.00666.x Chewing number is related to incremental increases in body weight from 20 years of age in Japanese middle‐aged adults Background: Eating habits are associated with both current obesity and incremental increases in body weight from young adulthood, but no study has focused on chewing number during meals among community residents. Objective: This study focused on the relationship between chewing number and incremental increases in body weight from 20 years of age. Methods: A total of 93 persons aged 35–61 years participated. The subjects were asked to set the device and record their chewing number during each meal on a particular day. They were also asked whether their body weight had increased by 10 kg or more since they were 20 years old. Results: The body weight of 28 subjects (30%) had increased more than 10 kg since the age of 20 years. Total chewing number showed a relationship with such body weight increases. The odds ratio of weight increments of more than 10 kg for the lowest tertile group was 4.6 [95% confidence interval (CI), 1.3–16.2] relative to the highest tertile group (Model 1). The odds ratio of weight increments for the lowest tertile group increased to 6.3 (95% CI, 1.6–25.4) in Model 2 and to 9.1 (95% CI, 1.7–49.8) in Model 3. Conclusion: Although this study was limited because it did not consider all risk factors, categorical chewing number was related independently to body weight increments of more than 10 kg from 20 years of age.     

Masticatory function in patients with xerostomia

The effects of reduced salivary output in patients suffering from xerostomia on masticatory function has not been previously studied. This study compares masticatory performance and kinematic activity of patients suffering from xerostomia with age-, sex-, and number of occluding pairs-matched healthy controls. Masticatory function was evaluated by assessment of chewing motion and muscle activity during chewing an artificial food (CutterSil®), chewing gum and swallowing a bolus of almond. Chewing motion was recorded with the Optotrak® computer system. Bilateral muscle activity of both masseter and anterior temporalis was recorded using surface electrodes. Results of this study revealed significant differences between patients and controls in their ability to process food and masticatory muscle activity. The majority of patients could not break down the artificial food, others had a larger median particle size than the controls. A significant difference was also observed in the number of chewing cycles required to swallow almonds, the patients required more than twice as many chews as the controls, P<0.001. The right masseter muscle displayed significantly less activity for the patient than the controls. These findings suggest that patients with xerostomia exhibit reduced ability to process food. The observed decline in masticatory performance is probably due to reduced activity of the muscles of mastication.     

Effects of aging on EMG variables during fatiguing isometric contractions

The purpose of this study was to evaluate the neuromuscular adaptation that occurred with aging, by comparing young and aged subjects with respect to changes in surface EMG from the tibialis anterior muscle during fatiguing contractions. EMG variables such as the averaged rectified value (ARV), median frequency (MDF), and muscle fiber conduction velocity (MFCV) were calculated during maximal (MVC, 3 sec) and submaximal (60% MVC, 60 sec) isometric contractions. Muscular force, ARV, MDF, and MFCV during MVC were significantly greater in the young than in the elderly (p < 0.05). EMG amplitude increased and the waveform slowed in all subjects during submaximal contractions, indicating the development of local muscle fatigue. As fatigue progressed, the ARV increased and the MDF and MFCV decreased significantly (p < 0.01). The fatigue-induced changes in the MDF and MFCV were significantly smaller in aged than in young subjects (p < 0.05), a trend also seen in the ARV change, which means that the elderly cannot be fatigued as much as the young with contractions of the same relative intensity. These results as a whole suggest that the aged subjects hold an adaptive motor strategy to cope with age-related neuromuscular deteriorations, due to the decline of motor unit activation and selective atrophy of fast twitch muscle fibers.     

Feeding mechanics and dietary implications in the fossil sloth Neocnus (Mammalia: Xenarthra: Megalonychidae) from haiti

Haitian species of the extinct ground sloth genus Neocnus (Mammalia: Pilosa: Megalonychidae) have previously been hypothesized to have a much reduced jugal bone and a correspondingly reduced masseter musculature but a paucity of specimens has prevented further investigation of this hypothesis. Recent discovery of jugal bones belonging to Haitian specimens of Neocnus within the University of Florida Museum collections enables the element to be more accurately described. The discovery also makes it possible to explore mastication in these sloths. Osteological characters related to feeding were examined, along with comparative estimations of bite force with the extant tree sloths, Bradypus and Choloepus, and their known dietary habits as a means to infer aspects of the paleodiet of Neocnus. There is a significant difference in moment arm calculations for m. masseter between predicted and actual jugals, but the overall significance for bite force is lost and hampered by small sample size. Neocnus demonstrates a variety of characters that are similar to those of Bradypus and not to Choloepus, which is a close phylogenetic relative. The masticatory musculature of Neocnus enabled a chewing cycle emphasizing a grinding combination of mesiodistal and linguobuccal movements of the molariform dentition. The orientations of m. masseter and m. temporalis are estimated to produce relatively high bite force ratios that imply a masticatory system with stronger versus faster components. Because of the similarity of bite forces and jaw mechanics to those of Bradypus, in addition to a number of osteological adaptations indicative of herbivorous grazers (elevated mandibular condyle, large and complex masseter, and robust angular process), the Haitian forms of Neocnus are considered to have been selective feeders with a folivorous diet. J. Morphol. 2011. © 2011 Wiley‐Liss, Inc.     

Skull Shape, Masticatory Apparatus, and Diet of Vassallia and Holmesina (Mammalia: Xenarthra: Pampatheriidae): When Anatomy Constrains Destiny

The form and function of the masticatory apparatus of the fossil genera Vassallia and Holmesina are analyzed so that the possible dietary behaviors of these pampathere xenarthrans might be inferred. Analysis is based on comparisons of dental morphology and skeletal features (through RFTRA) associated with the masticatory musculature among the pampatheres, the extant dasypodids Euphractus and Dasypus, and the glyptodont Propalaeohoplophorus. A method is proposed for generating a moment arm of the massetericus independently of the muscle's line of action, which allows direct comparison among extant and fossil mammals. The masticatory apparatus of the pampatheres strongly resembles that of Euphractus among extant forms, but the development of muscular attachment sites indicates a more powerful musculature, particularly the massetericus; the taxa differ most markedly in dental morphology. Long moment arms about the jaw joint and large ratios of muscle to bite moments indicate forceful rather than quick movements. The various skeletal and dental features analyzed suggest that the masticatory apparatus of the pampatheres was more powerful and efficient in transverse chewing than in dasypodids and that they were primarily grazers consuming mainly coarse vegetation. These features, some shared with herbivorous ungulates, include wide, relatively flat mandibular condyles; condyles well dorsal to muscular insertion sites; expanded angular processes; unfused symphysis; a posteriorly extended tooth row; open-rooted teeth; mesial teeth that bear mainly transverse striations; distal teeth that are mesiodistally elongated, bear basined occlusal surfaces, and in Vassallia possess a central island of resistant dentine that acted as a functional analogue of an ectoloph; and teeth with a stepwise arrangement. The results of this study indicate that detailed analysis and comparison of morphology lead to useful predictions of behavior.     

Reconstruction of the cranial musculature and masticatory function of the Pleistocene panamerican ground sloth Eremotherium laurillardi (Mammalia,Xenarthra, Megatheriidae)

Cranial musculature, dental function and mandibular movement patterns in Eremotherium laurillardi were reconstructed from the examination of crania and dentitions. Size, shape and pattern of muscle divisions were reconstructed from the examination of bony rugosities indicating muscle attachments. Details of masticatory muscle structure and function were based on dissections of the tree sloths Bradypus and Choloepus. Among sloths, masticatory muscles in E. laurillardi demonstrate a different synergist–antagonist pattern, reflecting greater emphasis on mediolateral mandibular movements. Eight cranial character complexes (anterior facial, zygomatic arch, superficial masseter, deep masseter–zygomaticomandibularis, pterygoid, temporal, occipital and occlusal) determined by interrelated contributions of each component made to group functions were identified. An elongate anterior face and predental spout in E. laurillardi allowed protrusion of a long narrow tongue at small degrees of gape, reflecting a probably ancestral xenarthran condition. Gape minimisation, in conjunction with the mediolaterally directed masticatory stroke in E. laurillardi, was a unique solution to increase masticatory efficiency by permitting molariform tooth shearing surfaces to remain in or near occlusion for a greater percentage of each chewing cycle.     

Shear Modulus Estimation on Vastus Intermedius of Elderly and Young Females over the Entire Range of Isometric Contraction

Elderly people often suffer from sarcopenia in their lower extremities, which gives rise to the increased susceptibility of fall. Comparing the mechanical properties of the knee extensor/flexors on elderly and young subjects is helpful in understanding the underlying mechanisms of the muscle aging process. However, although the stiffness of skeletal muscle has been proved to be positively correlated to its non-fatiguing contraction intensity by some existing methods, this conclusion has not been verified above 50% maximum voluntary contraction (MVC) due to the limitation of their measurement range. In this study, a vibro-ultrasound system was set up to achieve a considerably larger measurement range on muscle stiffness estimation. Its feasibility was verified on self-made silicone phantoms by comparing with the mechanical indentation method. The system was then used to assess the stiffness of vastus intermedius (VI), one of the knee extensors, on 10 healthy elderly female subjects (56.7±4.9 yr) and 10 healthy young female subjects (27.6±5.0 yr). The VI stiffness in its action direction was confirmed to be positively correlated to the % MVC level (R² = 0.999) over the entire range of isometric contraction, i.e. from 0% MVC (relaxed state) to 100% MVC. Furthermore, it was shown that there was no significant difference between the mean VI shear modulus of the elderly and young subjects in a relaxed state (p>0.1). However, when performing step isometric contraction, the VI stiffness of young female subjects was found to be larger than that of elderly participants (p<0.001), especially at the relatively higher contraction levels. The results expanded our knowledge on the mechanical property of the elderly’s skeletal muscle and its relationship with intensity of active contraction. Furthermore, the vibro-ultrasound system has a potential to become a powerful tool for investigating the elderly’s muscle diseases.     

Specific strength and voluntary muscle activation in young and elderly women and men.

The extents to which decreased muscle size or activation are responsible for the decrease in strength commonly observed with aging remain unclear. Our purpose was to compare muscle isometric strength [maximum voluntary contraction (MVC)], cross-sectional area (CSA), specific strength (MVC/CSA), and voluntary activation in the ankle dorsiflexor muscles of 24 young (32 +/- 1 yr) and 24 elderly (72 +/- 1 yr) healthy men and women of similar physical activity level. Three measures of voluntary muscle activation were used: the central activation ratio [MVC/(MVC + superimposed force)], the maximal rate of voluntary isometric force development, and foot tap speed. Men had higher MVC and CSA than did women. Young men had higher MVC compared with elderly men [262 +/- 19 (SE) vs. 197 +/- 22 N, respectively], whereas MVC was similar in young and elderly women (136 +/- 15 vs. 149 +/- 16 N, respectively). CSA was greater in young compared with elderly subjects. There was no age-related impairment of specific strength, central activation ratio, or the rate of voluntary force development. Foot tap speed was reduced in elderly (34 +/- 1 taps/10 s) compared with young subjects (47 +/- 1 taps/10 s). These results suggest that isometric specific strength and the ability to fully and rapidly activate the dorsiflexor muscles during a single isometric contraction were unimpaired by aging. However, there was an age-related deficit in the ability to perform rapid repetitive dynamic contractions.     

Mastication in the musk shrew,Suncus murinus (mammalia,Soricidae)

High-speed cinematography shows that Suncus murinus (Crocidurinae) masticates fast (mean 5.5, 5–10 masticatory cycles per sec). Their grasping behavior is not stereotyped. The unilateral mandibular movements combine vertical, anteroposterior, and lateral displacements; and any masticatory sequence may include crushing, repositioning, shearing, and grinding components. Size and consistency of food influence the duration of individual chewing cycles. As food is transferred to the new working side, the chewing direction reverses, either near maximum closure or near maximum opening. An unfused mandibular symphysis permits tilting movements of the two halves of the mandible. Food may be squeezed between the lower incisors. The working side tilts outward during closing; this may improve shearing or grinding action. The closing phase is posteriorly directed. Thus, the masticatory movements of these shrews differ from those that have been described in many other mammals.     

Functional evolution of the feeding system in rodents

The masticatory musculature of rodents has evolved to enable both gnawing at the incisors and chewing at the molars. In particular, the masseter muscle is highly specialised, having extended anteriorly to originate from the rostrum. All living rodents have achieved this masseteric expansion in one of three ways, known as the sciuromorph, hystricomorph and myomorph conditions. Here, we used finite element analysis (FEA) to investigate the biomechanical implications of these three morphologies, in a squirrel, guinea pig and rat. In particular, we wished to determine whether each of the three morphologies is better adapted for either gnawing or chewing. Results show that squirrels are more efficient at muscle-bite force transmission during incisor gnawing than guinea pigs, and that guinea pigs are more efficient at molar chewing than squirrels. This matches the known diet of nuts and seeds that squirrels gnaw, and of grasses that guinea pigs grind down with their molars. Surprisingly, results also indicate that rats are more efficient as well as more versatile feeders than both the squirrel and guinea pig. There seems to be no compromise in biting efficiency to accommodate the wider range of foodstuffs and the more general feeding behaviour adopted by rats. Our results show that the morphology of the skull and masticatory muscles have allowed squirrels to specialise as gnawers and guinea pigs as chewers, but that rats are high-performance generalists, which helps explain their overwhelming success as a group.     

A model for comparison of masticatory effectiveness in primates

A model is presented to express how effectively animals increase the exposed surface area of their food by chewing. It includes a coefficient of masticatory effectiveness (E) the value of which increases with effectiveness of exposing new food surface area with each chew. Humans and other species of primates differ significantly in their values of E; among the nonhuman primates studies, Lemur catta has a higher coefficient than Lemur fulvus, and both have higher coefficients than either Varecia variegatus or Galago crassicaudatus argentatus. The differences among the coefficients to these prosimians are correlated with variations in specific features of the molar morphology. Of six lower molar shearing crests considered, the relative length of the post-metacristid correlates most highly with the coefficient of masticatory effectiveness for the prosimian species. Also, among comparable-sized prosimians, E correlates significantly with the absolute postmetacristid length. Both these findings indicate that the relative size of molar shearing crests is related significantly to how effectively an animal chews its food. There are also implications for an adaptation to a high-fiber diet.     

Influence of occlusal stabilization splint on the asymmetric activity of masticatory muscles in patients with temporomandibular dysfunction

The aim of present study was to evaluate the symmetry of masticatory muscles' activity at various clenching levels in the intercuspal position in patients with functional disorders and in healthy subjects. The purpose was also to determine the effect of full-arch maxillary stabilization splint on the asymmetry of masticatory muscle activity in patients with temporomandibular dysfunction. In this study 6 TMD patients and 12 healthy subjects were investigated. Surface EMG recordings were obtained from left and right anterior temporal, left and right masseter and from the sub-mandibular group in the region of the anterior belly of the digastric muscle on the left and right side during clenching with the maximum 100% voluntary contraction (MVC) as well as during clenching at 50% and 25% of the maximum activity in the position of maximal intercuspation of teeth. In order to quantify asymmetrical masticatory muscle activity, the asymmetry index (AI) was calculated for each subject and for each muscle from the average anterior temporal, masseter and digastric potentials recorded during each test (100% MVC, 50% MVC and 25% MVC). In the group of patients EMG recordings were repeated during and after the splint therapy. The asymmetries of masticatory muscle activity was present in both groups, but in the group of TMD patients the asymmetry indices for anterior temporal muscle at 100% MVC (p = 0.049) and 50% MVC (p = 0.031) were significantly higher. Results have shown that the use of splint suppressed the asymmetry of all muscles, as during the splint therapy the asymmetry indices were lowered. After the therapy, the level of temporal muscle symmetry during submaximal clenching in the intercuspal position increased significantly (p = 0.046). This investigation points out that electromyography may be a valuable method of documenting that asymmetric activity of masticatory muscles improves after occlusal splint therapy in patients with TMD.     

Postactivation potentiation influences differently the nonlinear summation of contractions in young and elderly adults.

The force enhancement of a twitch after a maximal conditioning muscle contraction [i.e., postactivation potentiation (PAP)] is reduced with aging, but its influence on the summation of force in response to repetitive stimulation at different frequencies is not known. The purpose of this work was to compare the electrically evoked mechanical responses of the tibialis anterior muscle between young and elderly adults after a 6-s maximal voluntary contraction (MVC). The results showed that, immediately after the conditioning MVC, twitch torque and its maximal rate of development and relaxation were significantly enhanced in both groups, but the magnitude of potentiation was greater in young (148.0 +/- 14.2, 123.7 +/- 16.5, and 185.4 +/- 36.5%, respectively) compared with elderly adults (87.4 +/- 15.2, 63.8 +/- 9.9, and 62.9 +/- 11.0%, respectively). This age-related difference in potentiation of the twitch disappeared completely 1 min after the conditioning MVC. The potentiation of torque and speed-related parameters in response to two- and three-pulse trains, delivered at a constant interval of 10 ms (100 Hz), was less than for a single pulse for both groups. In young adults, the magnitude of PAP on the successive individual mechanical contributions within a train of stimuli declined progressively such that the third contribution did not differ significantly from the same contribution before the conditioning MVC. In contrast, the second and third contributions did not potentiate (P > 0.05) in elderly adults. Although these contributions did potentiate significantly at a lower frequency of stimulation (20 Hz) in the two groups, the difference in PAP between young and elderly adults still persisted. This overall attenuation of potentiation with aging, however, appears to have a moderate influence on the decrement of the muscular performance.     

Recovery following exhaustive dynamic exercise in the human biceps muscle

The aim of the study was to investigate the recovery of the maximum voluntary contraction force (MVC), the endurance time and electromyographical (EMG) parameters following exhaustive dynamic exercise of the m. biceps brachii. EMG recordings were made in ten healthy subjects using bipolar surface electrodes placed over the common belly of the left arm biceps muscle. Up to 25 h post-exercise, the maximum contraction force and the EMG signal were recorded alternately at regular intervals. The EMG signal was recorded during 30-s contractions at 40% of the pre-fatigued MVC. Four hours and 25 h post-exercise, the endurance time of a 40% pre-fatigued MVC was recorded. Up to 25 h after the exercise the maximum contraction force, the endurance time and the EMG parameters were significantly different from the pre-exercise values. Nine out of ten subjects complained that muscle soreness had developed. Thus, long-lasting changes are found after exhaustive dynamic exercise, not only in the MVC and the muscle's endurance capacity, but also in the EMG signal.     

Ontogeny and phyletic size change in living and fossil lemurs

Lemurs are notable for encompassing the range of body‐size variation for all primates past and present—close to four orders of magnitude. Benefiting from the phylogenetic proximity of subfossil lemurs to smaller‐bodied living forms, we employ allometric data from the skull to probe the ontogenetic bases of size differentiation and morphological diversity across these clades. Building upon prior pairwise comparisons between sister taxa, we performed the first clade‐wide analyses of craniomandibular growth allometries in 359 specimens from 10 lemuroids and 176 specimens from 8 indrioids. Ontogenetic trajectories for extant forms were used as a criterion of subtraction to evaluate morphological variation, and putative adaptations among sister taxa. In other words, do species‐level differences in skull form result from the differential extension of common patterns of relative growth? In lemuroids, a pervasive pattern of ontogenetic scaling is observed for facial dimensions in all genera, with three genera also sharing relative growth trajectories for jaw proportions (Lemur, Eulemur, Varecia). Differences in masticatory growth and form characterizing Hapalemur and fossil Pachylemur likely reflect dietary factors. Pervasive ontogenetic scaling characterizes the facial skull in extant Indri, Avahi, and Propithecus, as well as their larger, extinct sister taxa Mesopropithecus and Babakotia. Significant interspecific differences are observed in the allometry of indrioid masticatory proportions, with variation in the mechanical advantage of the jaw adductors and stress‐resisting elements correlated with diet. As the growth series and adult data are largely coincidental in each clade, interspecific variation in facial form may result from selection for body‐size differentiation among sister taxa. Those cases where trajectories are discordant identify potential dietary adaptations linked to variation in masticatory forces during chewing and biting. Although such dissociations highlight selection to uncouple shared ancestral growth patterns, they occur largely via transpositions and retention of primitive size‐shape covariation patterns or relative growth coefficients. Am. J. Primatol. 72:161–172, 2010. © 2009 Wiley‐Liss, Inc.