Thursday, February 22, 2007
Lizards 'Shout' Against a Noisy Background Discovery
Lizards that signal to rivals with a visual display "shout" to get their point across, UC Davis researchers have found.
Male anole lizards signal ownership of their territory by sitting up on a tree trunk, bobbing their heads up and down and extending a colorful throat pouch (dewlap). They can spot a rival lizard up to 25 meters away, said Terry Ord, a postdoctoral researcher at UC Davis who is working with Judy Stamps, professor of evolution and ecology.
The lizards' signals need to be strong enough for a rival to see, but not vivid enough to say "eat me" to a passing predator. But their forest home can be a visually noisy environment, with branches and leaves waving in the breeze and casting patterns of light and shade.
"They have to have a strategy to get their message across," Ord said.
Image: Postdoctoral researcher Terry Ord says anole lizards, such as this one, create a strategy to get their message across to rivals. (Courtesy photo UCD)
Ord videotaped two species of anole lizards, Anolis cristatellus and Anolis gundlachi, in the Caribbean National Forest in Puerto Rico. He found that the more "visual noise" in the background, the faster and more exaggerated the movements of the lizards.
Anole lizards are interesting to evolutionary biologists because different species are found on different islands all over the Caribbean. The lizards are not particularly closely related - they are separated by 30 million years of evolution - but they live in similar environments with the same obstacles to communication. So Ord is using them as a model to investigate the evolution of such signals.
The other authors on the paper, which is published online in Proceedings of the Royal Society part B, are Richard A. Peters, Australian National University, Canberra; and Barbara Clucas, a graduate student in animal behavior at UC Davis. The work was supported by grants from the National Geographic Society, the National Science Foundation and the Australian Research Council.
Source: University of California Davis PR February 21, 2007
Lizards speed up visual displays in noisy motion habitats
Terry J. Ord, Richard A. Peters, Barbara Clucas, Judy A. Stamps
Proceedings of the Royal Society B: Biological Sciences
ISSN: 0962-8452 (Paper) 1471-2954 (Online)
Issue: FirstCite Early Online Publishing
Extensive research over the last few decades has revealed that many acoustically communicating animals compensate for the masking effect of background noise by changing the structure of their signals. Familiar examples include birds using acoustic properties that enhance the transmission of vocalizations in noisy habitats. Here, we show that the effects of background noise on communication signals are not limited to the acoustic modality, and that visual noise from windblown vegetation has an equally important influence on the production of dynamic visual displays. We found that two species of Puerto Rican lizard, Anolis cristatellus and A. gundlachi, increase the speed of body movements used in territorial signalling to apparently improve communication in visually 'noisy' environments of rapidly moving vegetation. This is the first evidence that animals change how they produce dynamic visual signals when communicating in noisy motion habitats. Taken together with previous work on acoustic communication, our results show that animals with very different sensory ecologies can face similar environmental constraints and adopt remarkably similar strategies to overcome these constraints.
The above paper references:
Evolution of Anoline Lizard Display Behavior
Thomas A Jennsen
American Zoologist* 1977 17(1):203-215; doi:10.1093/icb/17.1.203
Based on my conceptual framework of anoline display behavior, I am suggesting the following evolutionary trends. Lateral presentation during display was probably promoted by monocular vision. Along with lateral presentation, postures evolved to increase lateral outline. These postures which magnified body size were probably of selective advantage within aggressive social contexts since larger animals tend to dominate smaller ones through bluff. Body movement evolved along with lateral orientation and size-enhancing postures. These movements would be most effective if they complemented lateral orientation. Effectors available for such movements were primarily pre-adapted for vertical motion. The patterns of movement generated were probably simple oscillatory bobbing movements by the head which were weakly stereotyped, interspecifically similar, appearing in many contexts, and having a weakly defined information content. Events having selective advantage for species recognition promoted stereotypy of bobbing behavior into species-unique displays; each species had its unique signature display which served in a manifold communicatory capacity. The signature display appeared in assertion, courtship, and challenge contexts. Its information content varied depending upon context and recipient of the display (e.g., male or female). Besides the stereotyped aspects of the display, certain features remained variable with potential information significance. Core variability (see text) promotes individual recognition and may be the origin of new unique display patterns as sibling species emerge. Display modifiers (see text) are variable display features shared by members of a population (many being shared interspecifically) that provide a graded appearance to display performance; modifiers can indicate level of arousal and facilitate interspecific communication. For some species display repertoire size seems to have evolved from a single display (signature display) to repertoires of multiple displays; these subsequent displays are generally restricted to aggressive interactions.
*American Zoologist is now Integrative and Comparative Biology
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