Lizards found to send secret signals
SOME LIZARDS DISPLAY BOLD and conspicuous signs of aggression, flaring their frills, dancing or standing on two legs to scare off competitors, but one species prefers more clandestine communication, new research shows.
To ward off rivals, male Augrabies lizards flash covert UV signals from a patch on their throats. Scientists from Macquarie University, led by behavioural ecologist Dr Martin Whiting, have confirmed that these South African lizards are extremely adept at detecting UV signals.
“We’ve demonstrated that the increase in the number of UV photoreceptors enhances the lizard’s ability to discriminate between male throat colours, allowing them to accurately gauge a rival’s fighting ability,” he says.
Previous studies have shown that throat patches of alpha male Augrabies (Platysaurus broadleyi) reflect UV more brightly than those of weaker males, thus acting as an advertisement, or warning, says Martin.
Secret signals
UV light is a form of shortwave radiation that lies beyond the visible
spectrum. Humans cannot detect UV light, but the ability has been
documented in many fish, insects and bird species, some of which use it
to communicate. Even some flowers use UV patterns to attract pollinating
insects.
Scientists have several theories as to why species use
UV light for communication. “One idea that hasn’t gained much support,”
says Martin, “is that it is a special channel of communication that
might preclude eavesdroppers, such as predators.” Alternatively, he
says, “UV may be effective at indicating the quality of a signaller,
which at this stage best explains the situation in the Augrabies flat
lizard.”
To ascertain how well the Augrabies flat lizard could see UV light, the researchers analysed electrical impulses in the lizards’ optic nerves when their retinas were triggered with lights of different colours. The results were compared with those of a similar species, the brown anole (Anolis sagrei), whose visual system and signalling behaviours are already well studied. The Augrabies lizard was found to be three times more sensitive to detecting UV light than the brown anoles.
Previously, research into UV sensitivity and signalling has been dominated by studies of fish, birds and insects. Ambon damselfish (Pomacentrus amboinensis), for example, emit unique UV patterns which they use to distinguish individuals of their own species from similar-looking outsiders.
Taking advantage of daylight
“It could be that UV signalling is more prevalent in birds and insects,”
Martin says; however, it could simply be that birds are the more
popular subjects “because they have parental care.” Such traits makes
these subjects easier for biologists to determine how sexually
successful they are and how effective their signalling is, he says.
Mammals are also known to detect UV light, but not for communication.
Professor John Endler, evolutionary biologist at Deakin University says
that rodents can see UV light, allowing them to find their way in dimly
lit environments. “It is a way around the limited illumination at dawn
and dusk,” he says.
John believes the phenomenon of UV communication will prove to be common among lizard species. The covert signalling, he says, “is likely to be widespread, especially in desert or rock-dwelling species and probably in desert or rock-dwelling snakes too.” These species tend to be active during the day, when UV is abundant in the environment, he says. “Previous lizards studied are either nocturnal or found in vegetation.”
Knowledge of these invisible mechanisms may have real applications in conservation management, says John. “There are now numerous cases of human-induced changes in the environment which disrupt signalling… [this] can impact breeding efficiency, resulting in a decline in population.”
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