A recent visitor to our house – a keen naturalist from southern Australia – was startled the first time he heard the sound of an Asian House gecko, and was even more surprised that a gecko was responsible for the call. It is unusual for a lizard to be so loud. I don’t hear the garden skinks shrieking, or the water dragons wailing. I’ve never heard a bluetongue ‘tut-tut’. But in the warmer months the geckos in our house call everyday: a perky and penetrating ‘chuk-chuk-chuk’. Why do they call so loudly, and why are the other lizards mostly silent?
Hearing is the youngest of the major senses – the most recent to evolve – and it has evolved differently among different groups of animals. There is much variation in ear anatomy and the ability to hear amongst reptiles, and even amongst different types of lizards. Little is known about the sense of hearing in lizards, and there are very few experimental studies of the reactions of lizards to sounds.
Even though most humans communicate verbally, we shouldn’t assume that other animals use sounds and hearing in the same way. The tuatara, an enigmatic lizard¹ from New Zealand, makes sounds that are beyond its ability to hear. Clearly these lizards are not talking to each other, or even to themselves. It seems that most lizards use their sense of hearing more to build an audio picture of the world around them, including detecting food and danger, than for communicating with their own kind. Many lizards use complex visual cues to signal to each other instead.
Most lizards can hear sounds in the middle range of frequencies heard by you and I, but not at the very low or very high ends of the frequency range. Some lizards like skinks have good hearing, but remain mostly silent. Geckos are among the most vocal of lizards. They include the Pygopodids, or ‘flap-footed lizards’ which are geckos that have lost their limbs and adopted an underground way of life.
Pygopodids are endemic to Australia and New Guinea, and seem to have a few tricks up their burrow when it comes to vocalizing. Some species of Delma squeak at a frequency that other Delmas can hear, but one not audible to animals that eat Delmas for dinner (other, larger Pygopodids and birds). The calls of some Delma species are also unique within each species. This is very unusual for a lizard. But what about the house gecko, which is much louder than its legless relatives?
The Asian House gecko Hemidactylus frenatus has been cohabiting with humans for a very long time in southern and south-east Asia, and is the subject of many superstitions in India and Bangladesh. It is now found throughout the tropics and is a recent immigrant to Australia. Although there was concern about the potential negative impact of this species on Australia’s native geckos, a recent study concluded that Asian House geckos like to live in houses and don’t stray far from human habitation. (A much greater threat is the spread of humans and their houses per se, and not the geckos they might contain).
The house gecko is a great traveler, no doubt assisted by its inclination to live with people, and the gecko-knack of clinging to smooth vertical surfaces and upside-down on horizontal ones. Unlike many other geckos it also has relatively desiccation-resistant eggs that it glues to surfaces (perfect for transport in shipping containers). It also guards feeding spots (generally around lights and lighted windows) that attract high numbers of insects. This territoriality is uncommon in geckos. Experiments have shown that male house geckos do most of the calling and males respond more frequently to the loud ‘chuk-chuk-chuk’ call of others (by moving away). The softer ‘churring’ call (which I often hear around my kitchen windows at night) is also mostly made by males when they are being antagonistic over territory, or a particularly tasty insect.
So the voice of the house gecko seems to be linked to the defense of a territory containing a high abundance of food. Perhaps a bit like adolescent hoodlums playing loud music around the local fast-food outlet. Since the house gecko is active at night, a call may be more effective than a visual cue to ward other geckos off its territory. The acoustics of buildings might also further amplify the house gecko’s call, increasing its effectiveness. House geckos were probably nocturnal callers before they moved into buildings, but centuries of cohabitation with people, and even veneration in some cultures, may have strengthened the usefulness of this call, and even selected for louder calls.
And the other lizards? Well, they gave me a little nod, but were silent on the matter.
1. Not actually a lizard, just looks a lot like one. But that’s another story.
Thanks to Eric Vanderduys for improving the veracity of this post. All remaining errors of fact are my own.
Csurhes and Markula 2009 Asian house gecko Hemidactylus frenatus Pest animal risk assessment. Queensland Primary Industries and Fisheries; Fay and Popper 2000 Evolution of hearing in vertebrates: the inner ears and processing. Hearing Research 149:1-10; Köppl 2009 Evolution of sound localisation in land vertebrates. Current Biology 19:R635-R639; Manley 2011 Lizard auditory papillae: An evolutionary kaleidoscope. Hearing Research 273:59-64; Manley et al. 2013 The remarkable ears of geckos and pygopods. p.111-131 In Köppl et al. (eds.) Insights from Comparative Hearing Research. Springer, New York.; Marcellini 1977 The function of a vocal display of the lizard Hemidactylus frenatus (Sauria: Gekkonidae) Anim. Behav. 25:414-417; Vanderduys and Kutt 2012 Is the Asian house gecko, Hemidactylus frenatus, really a threat to Australia’s biodiversity? Aust J Zool 60:361-367; Wikipedia http://en.m.wikipedia.org/wiki/Common_house_gecko ; Wilson 2011 Asian House Geckos Fact Sheet. Queensland Museum.; Young et al. 2013 Reptile auditory neuroethology: What do reptiles do with their hearing? p.323-346 In Köppl et al. (eds.) Insights from Comparative Hearing Research. Springer, New York.