Discussion

The aim of this study was to investigate whether muscle mass-specific power output has evolved within a closely related group of organisms in response to variation in microhabitat use and behavior. To our knowledge, ours is the first study that quantitatively links the variation in muscle power output to variation in ecology using a comparative approach. Our data show that muscle mass-specific power output varies considerably among species (more than twofold) and is indeed linked to the ecology and behavior of the species investigated.

12.4.1 Ecological Correlates of Power Output

Our results show a significant correlation between maximal power output and ecology across 10 species of Anolis lizards. More specifically, the evolution toward a higher incidence of escaping upward has been parallelled by the evolution toward higher muscle mass-specific power output. Muscle mass-specific power output, however, does not seem to correlate with an arboreal lifestyle per se. Anolis lizards are typically "active" lizards that run and jump around their natural habitat to search for food and/or partners, to defend their territories, or to escape predators [21]. However, it is clear that, on top of the large ecological variation, such as the degree of arboreality, there is also a large behavioral variation, such as antipredator behavior. Moreover, perching off the ground does not necessarily mean that the animals move up and down a lot, i.e., in a vertical direction. While some species typically jump down to other trees or to the ground, others "squirrel" to the opposite side of the

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FIGURE 12.6 Plot of two-dimensional angles at footfall in running trials (left) and takeoff in jumping trials (right) for A. valencienni and A. carolinensis. Shown are means and standard error bars. For running, only the ankle angle differs between the two species (F = 5.76, p = 0.029), whereas all three angles differed significantly for jumping (all F > 11.84, all p < 0.003). Stick figures representing the mean configuration of the limb segments during running and jumping, per species are shown in the top panel. • = hip; ■ = knee; ▲ = ankle.

trunk, i.e., stay at the same height, and/or run up [38]. In our sample, for instance, A. lineatopus typically does not escape upward when confronted with a (human) predator while A. valencienni does, although both show similar degrees of arbore-ality. Moreover, maximal power output is greater in A. valencienni (see Table 12.1 for raw values). Thus, it seems that higher power outputs are selected for in arboreal microhabitats, specifically in those species that frequently need to move rapidly by running or jumping against gravity.

12.4.2 Interspecific Variation

When comparing all 10 species, we found no overall difference in muscle mass-specific power output between jumping and running. Moreover, both power outputs seem to have coevolved. This might be explained by the fact that the same muscles are used in running and jumping.

Still, it is striking that even within a very closely related group of lizards such as Anolis lizards, large variation in muscle mass-specific power output exists. In our sample of 10 species, muscle mass-specific power output varies between 374 and 849 W kg-1, i.e., more than a twofold difference (Table 12.1). In general, differences in maximal power output, when executing the same task, are attributed to differences in the physiological properties of the muscle. At the moment, we do not have data on the physiology, for example muscle fiber-type of the different muscles in the different species. However, it does seem likely that differences in muscle fiber-type composition are present because similar results have been found in other lizard species [11,39]. On the contrary, while (theoretically) also being of great importance to muscular power output, the configuration of the musculoskeletal system, such as the origin and insertion of the muscles, is generally considered to be more conservative, i.e., less variation is found among species. We have some evidence that this might not be the case among Anolis lizards. When comparing the hind limb musculature of two extreme Anolis species — a ground-trunk anole, A. sagrei, and a twig anole, A. valencienni — we found distinct differences in the insertion of the hip retractor and knee extensor muscles between these two species [40]. Thus, our limited data set on Anolis lizards at least suggests that muscular morphology might be more variable than previously thought.

Surprisingly, the values for muscle mass-specific power output we find for some Anolis species, such as jumping in A. valencienni, are within the range of so-called extremely high power outputs in jumping frogs [41,42] and flying or running birds [5,10,43]. In the past, it has been generally assumed that power amplifiers are necessary to produce such high powers at the whole animal level [18,41,42]. Recently, however, it has been shown that power outputs of around 1000 W kg-1 are possible at the muscular level [5,44]. Although likely, it remains to be tested whether in Anolis lizards high power outputs at the whole animal level reflect power output at the muscular level. However, our data do show that high power outputs, i.e., in the range of 500 to 800 W kg-1, might not be as exceptional as previously claimed [5,10,41-43].

12.4.3 Power Output during Running and Jumping: a Two-Species Comparison

The comparison between A. carolinensis and A. valencienni shows that while muscle mass-specific power output is similar during running, it differs significantly during jumping. We suggest that biomechanical differences lie at the basis of this discrepancy. While the configuration of the hind limb segments is similar in both species in running trials, it differs greatly in jumping events. In the species producing the highest muscle mass-specific power during jumping, i.e., A. valencienni, the femur is significantly more protracted, i.e., the hip angle is negative (Figure 12.6), while the knee and ankle are more flexed, i.e., the knee and ankle angles are smaller (Figure 12.6). Could these differences be responsible for the differences in muscle mass-specific power output between the two species? The different configuration of the hind limbs may indeed result in changes in the instantaneous moment arms of the hip retractors and knee and ankle extensors that power locomotion. For example, inspection of Figure 12.6 shows that in, e.g., A. valencienni, the femur is maximally protracted. In this configuration, the dominant femur retractor (m. caudofemoralis) has a negligible moment arm and initially cannot contribute to femur retraction. This opens up the possibility for elastic energy storage in the broad tendon of the M. caudofemoralis inserting on the femur. However, for this to be the case, the muscle has to be active prior to femur retraction such that energy can effectively be stored. In A. carolinensis, the femur is significantly less protracted prior to the onset of the jump and may thus have less potential for energy storage. Although these differences may explain the observed differences in power output (and also the magnitude of power output in these animals), this hypothesis needs to be investigated further by electromyographic and sonometric recordings of the M. caudofemoralis during jumping.

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