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First published online October 21, 2005
Journal of Experimental Biology 208, v (2005)
Copyright © 2005 The Company of Biologists Limited
doi: 10.1242/jeb.01888
Outside JEB |
FLYING LIZARDS FALL FAST
Mount Holyoke College
ggillis{at}mtholyoke.edu
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A `flying' lizard doesn't actually fly, it glides, which means that the power involved in remaining aloft comes from the animal's potential energy (how high it is, and how much it weighs). Sure, a little work is generally done by limb muscles to initiate a glide or control the shape and position of the aerodynamically active patagial membranes, or `wings', but essentially, gravity is the ingredient fueling a flying lizard's aerial activities. In Malaysia, Borneo and Sumatra, species of the flying lizard genus Draco can range in body mass over an order of magnitude, and some of the greatest instances of size disparity are seen among sympatric species whose microhabitats overlap. In a recently published paper, J. A. McGuire and R. Dudley test whether size impacts gliding performance in Draco lizards and explore how this might relate to the ecology of these fascinating animals.
To conduct gliding performance trials, McGuire and Dudley persuaded lizards from 11 different Draco species to glide 9 m between two poles, 4-6 m in height. They allowed the lizards to climb to the top of the takeoff pole and then used a long bamboo rod to encourage the animals to glide to the landing pole. McGuire and Dudley recorded the animals' glides on video at 60 fields s-1, digitized the videos and produced two-dimensional plots of each glide trajectory. They determined three measures of performance for each trial: (1) maximum velocity, (2) total height lost over the distance between poles and (3) total glide angle, which is the angle between the point of takeoff and point of landing (or projected intersection with the landing pole), relative to the horizontal. To explore possible correlations between the animals' body size and performance variables, the pair sacrificed the animals and took morphological measures.
McGuire and Dudley's morphological analyses revealed that larger animals have higher wing loading (more weight is supported by a given area of patagial membrane), which suggests that larger lizards should be less effective gliders than small ones. Indeed, the pair found a positive correlation between wing loading and total height lost during an animal's best glide; larger lizards lose more height, and thus have larger glide angles, than their smaller congeners. Finally, they noticed that performance variation decreases with increased wing loading. For example, large lizards consistently lose nearly 5.5 m in height over a 9 m horizontal glide distance, whereas small lizards lose anywhere from 2.6 to 5.5 m. Such results suggest that smaller lizards have the capacity to glide well - i.e. lose little height - yet have the flexibility to not have to do so. By contrast, large lizards always glide relatively poorly but are probably trying to maximize performance, nevertheless, because they have little margin for error.
What do the authors think this all means for flying lizards? Given their greater height losses during a glide, bigger animals are not as likely to utilize the lower parts of trees, implying a lower capacity for niche exploitation. In fact, 6 m would seem to be the minimum height from which a large lizard could successfully glide to a tree 9 m away, a typical distance in the forests these lizards inhabit. Clearly, bigger is not always better.
References
McGuire, J. A. and Dudley, R. (2005). The cost of living large: comparative gliding performance in flying lizards (Agamidae: Draco). Am. Nat. 166,93 -106.[CrossRef][Medline]
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