Cold is normally avoided by ectothermic animals, which rely on environmental temperature to keep their body temperature high. When exposed to cold, metabolic rate slows down, muscles stiffen, and escaping from predators or catching a meal becomes increasingly difficult. This is why the distribution of chameleon species across a wide range of thermal habitats, from deserts to alpine zones, is so intriguing. In 1982, Stephen Reilly reported that some species of chameleons can even feed at body temperatures as low as 3.5°C, allowing them to take advantage of thermal niches other lizards can't exploit. Although these observations were made almost 30 years ago, the answer to how chameleons are able to catch prey at very low temperatures had remained elusive until now.

Christopher Anderson and Stephen Deban, from the University of South Florida, USA, believe the secret is in their tongues. Chameleons are sit-and-wait predators; they ambush their prey by rapidly projecting their sticky tongue once their prey is within reach. This means they don't need fast leg muscles to catch their prey, they need a fast tongue instead. Interestingly, chameleons possess a unique mechanism of tongue projection that relies on rapid elastic recoil of collagen tissue, which works somewhat like a ‘bow and arrow’. Anderson and Deban set out to determine whether this elastic-recoil mechanism confers thermal independence to chameleon tongue projection.

Using a high speed camera, the team recorded tongue projection events of veiled chameleons (Chamaeleo caliptratus) catching crickets that had been placed at different pre-set distances. They then measured peak acceleration, peak velocity and peak mass-specific power of the tongue projections, as well as the tongue retractions, which do not use elastic recoil but instead are driven by contraction of the hyoglossus muscle.

The chameleons were able to project their tongues and capture the crickets over the same range of distances whether they were at 15, 25 or 35°C. Although the performance of tongue projection decreased by 10–19% from 25 to 15°C, even at 15°C performance was maintained at an extremely high level; the average peak velocity was 3.4 m s–1, average peak acceleration was 357 m s–2 and average peak mass-specific power was 1892 W kg–1. The drop in performance of tongue retraction, on the other hand, was much more pronounced, exhibiting decreases in peak velocity, acceleration and mass-specific power of between 42 and 63% over a 10°C range. These results confirm a high degree of temperature independence of tongue projection and a large temperature dependence in the performance of the tongue retraction.

The large differences in thermal dependence between tongue projection and retraction support the hypothesis that the elastic-recoil mechanism is responsible for the high degree of temperature independence observed during chameleon ballistic tongue projection. During this process, temperature-dependent muscle contraction ‘loads’ the tongue before launch. The relatively temperature-independent elastic-recoil mechanism then powers the tongue's projection, which launches it towards the prey with impressive speed and acceleration. The temperature independence of this process allows chameleons to feed early in the morning when it is too cold for other lizards to hunt and to inhabit thermal habitats other lizards can't exploit, which gives them a leading edge when competing for food. Now, you can't tell me that's not a cool tongue.

C. V.
S. M.
Ballistic tongue projection in chameleons maintains high performance at low temperature
Proc. Natl. Acad. Sci. USA