Males make a series of head-bobs that let neighboring males know their territorial claim—typically two or three trees that are also home to several females, ensuring the male has mating success.
Since territorial head-bobs are also energy-intensive, previous research has suggested they contain information about the fitness of the displayer, Ord noted.
"If you can produce a lot of these displays then other males are going to know you are in good physical condition" and thus unlikely to mount a challenge, he said.
As Ord was studying this behavior, he noticed that sometimes, but not all the time, the lizards would do the push-ups before starting the head-bobs.
The push-ups, he thought, seemed like an alert display.
To find out, he built realistic, robotic lizards that perform the push-ups, head-bobs, and an unusual display for this species called a dewlap extension.
A dewlap is loose skin that hangs under the lizard's throat.
Ord put these robots in the Puerto Rican forest and observed how the real lizards responded to the various gestures.
Based on an analysis of more than 300 responses, he concluded the lizards use alerts when their neighbors are far away or otherwise visually distracted.
The push-ups and dewlap extensions also elicited a response, confirming the hypothesis that the alerts are just alerts—they don't contain additional information.
In the wild, anole lizards communicate with the push-ups.
"You can imagine doing push-ups takes quite a bit of energy," Ord said. "So they don't always want to [do them] if they don't need to."
In addition, the push-ups might attract predators such as the Puerto Rican lizard-eating cuckoo and feral mongoose.
Leo Fleishman is a biologist at Union College in Schenectady, New York, who uses lizards to study the evolution of communication signals.
He said the study is a "nice confirmation" of earlier studies that indicate lizards initiate communication with highly visible physical displays when the viewer is inattentive and far away.
The new study proves that such motions do elicit the attention of potential viewers, he said.
"And the demonstration that [the lizards] are more likely to add it under a variety of what they call noisy conditions is also novel. The examples before only involved distance," he said.
According to Ord, the lizards may learn the signals via experimentation.
"Over time they'll get an idea that in some conditions they'll get responses from the lizards they're trying to communicate with when they add a particular component to their display," he said.
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