More Evolutionary Studies on Lizards

A number of people have written on the recent news about the speedy evolution of some lizards. There is another study, recently published in the Journal of Experimental Biology that has some interesting evolutionary implications.

The study examined the relationship between biomechanics and foraging mode in 18 species of lizard spread across nine families. Like any good piece of research, the paper was concerned with testing a hypothesis based on the effects of sit and wait foraging versus wide foraging on the biomechanics of the lizards. The authors even make some specific predictions based on the model:

Based on these patterns we predict that the speed demands of foraging mode should be related to biomechanics and gait. Sit-and-wait species rely on rapid locomotion during prey capture, which should involve running mechanics and trotting gaits. On the other hand, WF lizards predominantly use slower locomotion to locate prey, which should involve walking mechanics and a shift toward single-foot gaits (while retaining faster locomotion with running mechanics and trotting gaits for predator evasion and social interactions).

By way of explanation, sit and wait foragers are primarily visually oriented ambush predators that have large tongues and short, broad snouts. Wide foragers, primarily, have forked tongues that they use to sense the prey and have long narrow jaws.
The researchers used a special track with built in force plates, and high speed cameras to analyze the lizard biomechanics. Current evidence indicates that wide foraging evolved from the sit and wait technique – which also has some interesting implications. For example, sit and wait foragers don’t walk, they use running gaits even at lower speeds. Wide foragers do walk and this raises the question as to how and why the walking gait evolved. Incidentally, those wide foragers that reverted to a sit and wait style also lost the ability to walk. Here is the problem. It seems intuitively obvious that if you have to cover a large amount of territory it would be energetically less expensive to walk than it would be to run. Unfortunately this is not the case:

First, lizards have body masses that are too small to realize relevant metabolic savings from mechanical energy savings during walking because their actual metabolic costs are two orders of magnitude greater than their total mechanical energy costs (Reilly et al., 2007). Therefore, no matter how much percent recovery of external mechanical energy the lizards attain, it is insignificant in relation to the actual metabolic cost of locomotion. Second, the cost of locomotion during walking has been shown to be greater than running, both on a per stride basis and on an absolute basis, because WF actually spend the majority of their activity budget walking slowly (Anderson and Karasov, 1981; Reilly et al., 2007).

So how does one explain walking in wide foraging species?

Wide foraging lizards have evolved entire suites of characters related to their shift to derived chemosensory systems (Cooper, 1994; McBrayer and Corbin, 2007; Reilly et al., 2007; Schwenk, 1993). From the brain to olfactory receptors to forked air sampling tongues, WF lizards exhibit a number of characters that enhance their ability to slowly search for food [Reilly et al. (Reilly et al., 2007) and references therein]. While foraging, slower locomotor movements may enhance wide foraging by allowing the chemosensory apparatus to meticulously sample a complex heterogeneous habitat for prey chemicals (Anderson, 2007; Cooper, 1994). The prey items that WF chemically search for reward them with a higher energy pay-off (Gasnier et al., 1994). Fast locomotor movements, while foraging, would preclude WF from being able to sample chemicals thoroughly and follow them in the environment. Thus, simply moving slower while foraging is of adaptive value to WF lizards because it affords them the ability to effectively locate and discriminate energy-rich prey. Our findings suggest that the convergent evolution of slower foraging locomotion in WF lizards is an important correlate of effective predatory chemosensory behavior.

Basically, what the research is saying is that locomotor mechanics, foraging mode, and gait coevolve.
McElroy, E.J., Hickey, K.L., Reilly, S.M. (2008). The correlated evolution of biomechanics, gait and foraging mode in lizards. Journal of Experimental Biology. , 211(7), 1029-1040.