Abstract
We examined the sprinting
and jumping capabilities of eight West Indian Anolis species during
three natural activities (escape from a predator, feeding, and undisturbed
activity), and compared these field data with maximal performance under
optimal laboratory conditions to answer three questions: (1) Has maximal
(i.e., laboratory) sprinting and jumping performance co-evolved with field
performance among species? (2) What proportion of their maximum capabilities
do anoles sprint and jump in different ecological contexts? (3) Does
a relationship exist between maximal sprinting and jumping ability and
the proportion of maximal performance used in these contexts? Among
species, maximal speed is tightly positively correlated with sprinting
performance during both feeding and escape in the field. Sprinting
speed during escape closely matches maximal sprinting ability (i.e., about
90% of maximum performance). By contrast, sprinting performance during
undisturbed activity is markedly lower (about 32% of maximum) than maximal
sprinting performance. Sprinting ability during feeding is intermediate
(about 71% of maximum) between field escape and field undisturbed activity.
In contrast to sprinting ability, jumping ability is always substantially
less than maximum (about 40% of maximum during feeding and undisturbed
activity). A negative relationship exists among species between maximal
speed and the proportion to which species sprint to their maximal abilities
during field escape.
Non-Technical Summary
In recent years, emphasis has been placed on the measurement of maximal performance capabilities as a means of understanding relationships between morphology and habitat use or fitness. As a result, over the past 15 years, a great number of studies have measured the maximal abilities of organisms to run, jump, swim, crawl, glide, hop, and do almost all kinds of other activities. An underlying assumption in these studies is that organisms actually take advantage of these capabilities in nature. However, very little data exist to evaluate this assumption.
One organism that has been greatly studied in this regard are lizards of the genus Anolis living on Caribbean islands. Anoles differ in morphological traits, such as limb length, that are strongly correlated with differences in how fast different species can run and how far they can jump. In turn, these differences are associated with differences in microhabitat use. The assumption, thus, has been that differences in performance ability are the intermediate link between morphology and habitat use. However, this linkage assumes that differences in performance capabilities translate into differences in what the animals actually do in nature.
To test this hypothesis, we measured the speed at which lizards ran and the distance they jumped in three contexts: feeding, escaping a simulated predator, and in undisturbed activity. We found that species tended to run at top speed when confronted with a predator. By contrast, although a correlation exists between maximal speed and speed in feeding trials, most species moved at substantially submaximal speeds during feeding. During undisturbed activity, as one might expect, lizards generally moved at much lower speeds than they were capable. By contrast, in all contexts, jumps in nature were substantially shorter than maximal capabilities.
We thus conclude that maximal sprinting speed has evolved primarily in the context of escaping predators. Differences in speed used during feeding may also be related to the evolution of sprint ability, but can't explain differences in maximal capabilities, because such capabilities are not utilized during feeding.
The situation for jumping is also clearcut: lizards do not utilize their maximal capabilities. Hence, the evolution of different jumping capabilities might seem problematic. One possibility is that we failed to investigate activities, such as social interactions, in which maximal capabilities are utilized. An alternative possibility, which we favor, is that the evolution of maximal jumping capability has occurred as a byproduct of selection for differences in maximal sprinting capability. At a mechanistic level, sprinting and jumping capability are both strongly affected by hindlimb length. Hence, selection for great (or little) sprinting capability would lead to changes in hindlimb length, which would have a great effect on maximal jumping capabilities.
One intriguing result
is that a negative relationship exists between maximal sprint capabilities
and the extent to which these maximal abilities are used in escaping predators.
In other words, the slower the species, the greater the percentage of its
maximal abilities it actually used in escape trials. This relationship
might result because slower species have no choice but to go all out, whereas
faster species, being faster, don't have to go top speed to escape a predator.
This hypothesis suggests that some cost exists to running at top speed,
such that faster species avoid doing so whenever possible. Further
research is needed to investigate this possibility.