It is well documented that a wide variety of animal species modulate their auditory signals to enhance the transmission of their vocalizations when faced with adverse environmental conditions. For example, concave-eared torrent frogs, found in China’s Huangshang hot springs, produce ultrasonic calls when their audible vocalizations are masked by fast-flowing streams; common marmosets increase both the volume and length of their calls when white noise is played to them; and urban great tits sing faster and at a higher minimum frequency than their rural counterparts.
Many lizard species produce vocalizations, and some, such as the anole lizards, which are native to Florida, Jamaica and Puerto Rico, also use visual signals. Anole lizards begin to produce these visual signals soon after hatching, and continue to do so throughout their lives. Both male and female anoles are territorial, and produce dynamic visual displays to communicate their territoriality to each other. These signals consist of vertical head movements – a lizard signals its presence to others by bobbing its head. Male anoles also have an expandable flap of skin, called a dewlap, which hangs beneath the chin; this can also be used for signalling other males. These displays are performed on an elevated perch site, and can be seen by other lizards from up to 25 metres away.
Anoles live in or around forests, and their visual signals can, therefore, be masked by vegetation blown across their habitat. This reduces the lizards’ ability to detect the visual signals produced by conspecifics. Terry J. Ord and his colleagues at the University of California, Davis, together with collaborators at the Australian National University’s Centre for Visual Sciences, now show that two species of lizard can alter their visual signals to compensate for the masking effect of visual noise in the habitat. The findings, which are published online in the Proceedings of the Royal Society: B, show for the first time that the visual signals produced by animals can, like auditory signals, be modulated to prevent masking.
Ord’s team investigated two species of Puerto Rican lizards, Anolis cristatellus and A. gundlachi. The former occupy habitats along roads or near the edges of forests, while the latter live in deep shade forests. The visual displays of both species are therefore susceptible to masking by windblown vegetation. The researchers compiled a video library of male lizards in their natural habitats, in the vicinity of the El Verde Field Station in the Caribbean National Forest. A computer program called AIM v. 1.2, which is based on motion detector algorithms, was used to quantify the lizards’ movement-based displays.
Upon analysis of the video footage, it was found that the lizards increase the speed of their visual displays when there is background noise in the form of windblown vegetation. There was a strong positive correlation between the speed of the visual displays and the speed of the vegetation blowing across the background of the footage. The results obtained by Ord’s team show that the speed of each display is adjusted in response to the speed of visual noise in the background. This minimizes interference from the visual noise, making the signal more conspicuous to other lizards in the neighborhood.
Ord, T. J., et al. (2007). Lizards speed up visual displays in noisy motion habitats. Proc. R. Soc. B. DOI: 10.1098.rspb.2006.0263. [Full text]