1. Two often cited hypotheses explaining sexual head size dimorphism in lizards are: sexual selection acting on structures important in intrasexual competition, and reduction of intersexual competition through food niche separation. 2. In this study some implicit assumptions of the latter hypothesis were tested, namely that an increase in gape distance and bite force should accompany the observed increase in head size. These assumptions are tested by recording bite forces, in vivo, for lizards of the species Gallotia galloti. In this species, male lizards have significantly larger heads than female conspecifics of similar snout–vent length. 3. Additionally, the average force needed to crush several potential prey species was determined experimentally and compared with the bite force data. This comparison clearly illustrates that animals of both sexes can bite much harder than required for most insect food items, which does not support the niche divergence hypothesis. The apparent ‘excess’ bite force in both sexes might be related to the partially herbivorous diet of the animals. 4. To unravel the origin of differences between sexes in bite capacity, the crushing phase of biting was modelled. The results of this model show different strategies in allocation of muscle tissue between both sexes. The origin of this difference is discussed and a possible evolutionary pathway of the development of the sexual dimorphism in the species is provided.

In this study a ground-dwelling (Eublepharis macularius) and a highly specialised climbing (Gekko gecko) lizard were chosen as study objects. The fore-and hindlimbs of two individuals of each species were dissected, and muscle masses, mean fibre lengths, crosssectional areas and moment arms were determined. Special attention was paid to general muscle architecture (origin, insertion, fibre orientation, etc.) and pennation angles. Using these variables (cross sectional areas and moment arms), maximal moments exertable across the shoulder/hip, elbow/knee and wrist/ankle were calculated for both species. In accordance with the biomechanical predictions related to the preferred locomotor substrate of each species (i.e. level running for E. macularius and climbing for G. gecko), the results of this study indicate that climbers such as G. gecko generally possess powerful retractor muscles crossing the shoulder and hip joints. Additionally, the specialised climber is able to exert higher flexion moments across the elbow, which prevents the animals from falling backwards. However, G. gecko appears to be constrained in its ankle extension capabilities by the presence of the adhesive toe pads. The level-running species, on the other hand, shows a relatively stronger development of the extensor muscles in the lower limbs, allowing these lizards to run in an erect posture. In general, both species show large similarities on a gross morphological level as expected when considering their phylogenetic relatedness. Adaptations to their preferred locomotor substrate only become apparent when considering the functional properties (i.e. joint moments) of the appendicular musculature.& b d y :P. Aerts is a research director of the Fund for Scientific Research Flanders. A. Herrel is a postdoctoral research associate of the

The differences in angulation and length observed for the fibers of anatomical muscles may reflect two distinct mechanical requirements: arrangement for pinnation, reflecting an increase in physiological cross-section and arrangement for equivalent placement of sarcomeres, possibly associated with coordination. The observed differences in fiber angulation and length have different effects upon the responses of sarcomeres, specifically on their extent and rate of shortening and on the force they may generate. The basic mechanisms governing these effects and the various arrangements of muscles are reviewed. Fiber length and angulation in the complex M. adductor mandibulae externus 2 of a lizard were measured stereotactically; these values correlate well with the hypothesis that the muscle shows equivalence and demonstrate that angulation for pinnation is less constant. An outline for the study of muscle architecture and function, detailing the kinds of information require to estimate forces and evaluate muscle and fiber placements, is presented.

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Prey capture in Agama stellio was recorded by high-speed video in combination with the electrical activity of both jaw and hyolingual muscles. Quantification of kinematics and muscle activity patterns facilitated their correlation during kinematic phases. Changes in angular velocity of the gape let the strike be subdivided into four kinematic phases: slow open (SOI and SOII), fast open (FO), fast close (FC), and slow close-power stroke (SC/PS). The SOI phase is marked by initial activity in the tongue protractor, the hyoid protractor, and the ring muscle. These muscles project the tongue beyond the anterior margin of the jaw. During the SOII phase, a low level of activity in the jaw closers correlates with a decline of the jaw-opening velocity. Next, bilateral activity in the jaw openers defines the start of the FO phase. This activity ends at maximal gape. Simultaneously, the hyoid retractor and the hyoglossus become active, causing tongue retraction during the FO phase. At maximal gape, the jaw closers contract simultaneously, initiating the FC phase. After a short pause, they contract again and the prey is crushed during the SC/PS phase. Our results support the hypothesis of tongue projection in agamids by Smith ([1988] J. Morphol. 196:157-171), and show some striking similarities with muscle activity patterns during the strike in chameleons (Wainwright and Bennett [1992a] J. Exp. Biol. 168:1-21). Differences are in the activation pattern of the hyoglossus. The agamid tongue projection mechanism appears to be an ideal mechanical precursor for the ballistic tongue projection mechanism of chameleonids; the key derived feature in the chameleon tongue projection mechanism most likely lies in the changed motor pattern controlling the hyoglossus muscle. © 1995 Wiley-Liss, Inc.

The interaction of organismal design with ecology, and its evolutionary development are the subject of many functional and ecomorphological studies. Many studies have shown that the morphology and mechanics of the masticatory apparatus in mammals are adapted to diet. To investigate the relations between diet and the morphological and physiological properties of the lizard jaw system, a detailed analysis of the structure of the jaw apparatus was undertaken in the insectivorous lizard Plocederma stellio and in closely related herbivorous lizards of the genus Uromastix. The morphological and physiological properties of the jaw system in P. stellio and U. aegyptius were studied by means of dissections, light microscopy, histochemical characterisations, and in vivo stimulation experiments. The skull of Uromastix seems to be built for forceful biting (high, short snout). Additionally, the pterygoid muscle is modified in P. stellio, resulting in an additional force component during static biting. Stimulation experiments indicate that jaw muscles in both species are fast, which is supported by histochemical stainings. However, the oxidative capacity of the jaw muscles is larger in Uromastix. Contraction characteristics and performance of the feeding system (force output) are clearly thermally dependent. We conclude that several characteristics of the jaw system (presence of extra portion of the pterygoid muscle, large oxidative capacity of jaw muscles) in Uromastix may be attributed to its herbivorous diet. Jaw muscles, however, are still faster than expected. This is presumably the result of trade-offs between the thermal characteristics of the jaw adductors and the herbivorous lifestyle of these animals.

We reexamined the morphological and functional properties of the hyoid, the tongue pad, and hyolingual musculature in chameleons. Dissections and histological sections indicated the presence of five distinctly individualized pairs of intrinsic tongue muscles. An analysis of the histochemical properties of the system revealed only two fiber types in the hyolingual muscles: fast glycolytic and fast oxidative glycolytic fibers. In accordance with this observation, motor-endplate staining showed that all endplates are of the en-plaque type. All muscles show relatively short fibers and large numbers of motor endplates, indicating a large potential for fine muscular control. The connective tissue sheet surrounding the entoglossal process contains elastin fibers at its periphery, allowing for elastic recoil of the hyolingual system after prey capture. The connective tissue sheets surrounding the m. accelerator and m. hyoglossus were examined under polarized light. The collagen fibers in the accelerator epimysium are configured in a crossed helical array that will facilitate limited muscle elongation. The microstructure of the tongue pad as revealed by SEM showed decreased adhesive properties, indicating a change in the prey prehension mechanics in chameleons compared to agamid or iguanid lizards. These findings provide the basis for further experimental analysis of the hyolingual system.

This wide-ranging, omnivorous lizard of Australia has a very complex adductor muscle mass, with fibers differing in length by a factor of three and in insertion angle by 90 degrees. Stimulated muscles produce maximal moment with the mouth nearly fully open. The opening mechanism appears to involve only simple rotation and no translation of the mandible. EMGs indicate that the entire mass is activated equivalently in crushing and there are no temporal subdivisions, for instance, matching activity to angle of opening. During crushing of hard objects, the chin is brought into contact with the ground so that the subvertebral muscles may aid buccal closure. The lizards also activate the muscles in a pulsatile staircase effect leading to an unfused tetanus that generates forces several times the twitch level. Application in parallel of a maximum number of sarcomeres to the crushing bite appears to be the major design characteristic. Hence, this species offers an ideal case for analysis of the effects of different sarcomere placements on the simple movement generated. For the primary adductor muscles, the angles of fiber insertion relative to the lines connecting each insertion with the jaw joint are equivalent; this relation persists as the mouth opens. Also, fiber lenghts are proportional to the distance between jaw joint and site of insertion so that each sarcomere contributes equally to the movement generated. Complex tendons provide additional space for muscle placement. Some of these also extend beyond the bony attachment sites, producing tendinous "coronoid processes." The fibers of laterally and ventrally placed muscles are short relative to the length of the entire muscle, insert at relatively short moment arms, and undergo short excursion during opening; however, there are many such fibers. Also, muscles with a low incident angle are crossed; they apparently protect the jaw joint from horizontal (disarticulating) forces.