Alec Sand, under the name before he changed it by deed poll, Zoond, determined the mechanism by which the chameleon’s tongue is projected at its prey. There has been an additional and very important twist in the story that has come only recently but the essence of where the force is applied and by which muscle was all down to Sand 85 years ago.
There had been several attempts to explain the working of the tongue before the 1930s. Sand began his paper:
The projection of the chameleon's tongue constitutes a unique problem in muscular organisation. It has frequently been recorded that a chameleon can project its tongue to a distance as great, or slightly greater than the length of the animal's body from nose to anus, and that projection takes place with instantaneous rapidity. Various theories have been advanced to explain this remarkable mechanism, yet still it remains imperfectly understood. The variety of the explanations offered is clearly due to the fact that investigation of this problem has been entirely anatomical, and that no attempt has hitherto been made to apply the test of experiment to it Such an experimental investigation forms the subject of this paper. The experiments here described deal with the tongue, the hyoid apparatus and the accessory muscles, and permit of the formulation of a complete account of this highly complicated and highly perfected response.
Sand was able to dismiss some of the earlier ideas from the 19th Century very easily, as had others before him. Inflation of the tubular tongue by air from the lungs was impossible because there was no pathway for air to reach the lumen of the tongue. The idea that a penile-like engorgement with blood was responsible, suggested in 1828, was shot down in flames by Duvernoy in 1836. Things happened too fast for it to be other than a highly specialised muscular mechanism.
In a series of experiments, Sand determined which muscle was responsible and how it worked. In essence the knob of the tongue throws itself off a long bony projection aimed at the prey. It does so by squeezing that projection (the entoglossal process). As the muscle tightens it spreads so that it starts to slip off the tapered end of the entoglossal process and, still contracting, close down the lumen of the tongue completely. That squeezing force then becomes a force acting against the end of the entoglossal process and the tongue is projected forwards, carrying the corrugated neck of the tongue behind. The sticky tongue plus prey is hauled back into the mouth by the weak muscles in the extended neck of the tongue.
There has sometimes been confusion as to how the basic mechanism works, not helped by inadequate or no explanation in textbooks. For example, Angus Bellairs in his book, Reptiles, from 1957, did not describe the mechanism in the text, despite it being characteristic of chameleons, but just showed a tiny diagram drawn from Sand’s description with a misleading arrow for the direction of force.
More recent research, as I said above, has added to that story because high-speed photography couple with physiological knowledge has demonstrated that the muscle, aptly called the accelerator muscle, just cannot do the amount of work needed in the time it takes for the tongue to reach its prey. There is an additional, elastic, process that accounts for the projection of the tongue. The accelerator muscle squeezing inwards act to compress layers of connective tissue, the intralingual sheaths, that lie between the muscle and the entoglossal process. The collagen fibres within those sheaths are arranged such that when compressed by the muscle, strain within the fibres increases. Thus the intralingual sheaths build up a store of energy that is released suddenly as the muscle and sheaths themselves slide off the end of the entoglossal process. It also seems that the thickened tip of the entoglossal process provides a passive block allowing the accelerator muscle to generate considerable pressure on the intralingual sheaths before the forward edges of the sheath and overlying muscle can move forward over the tip to initiate the sudden release of energy from the sheaths and the continued contraction of the muscle. Although other muscles help in the process, their role is minor compared to that of the accelerator muscle.
The presence of the intralingual sheaths, which are in layers and extend like a telescope, Star Wars Light Sabres or the tubes of a photographic tripod, were of course described by the 19th Century anatomists. Any function, though, except for providing lubrication or mechanical protection, was not considered.
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| Diagram modified from Martin & Wolfe (Chameleons, London: Blandford, 1992) to show the direction of contraction by the accelerator or ring muscle on the intralingual sheaths and the bony entoglossal process |
Elasticity to generate rapid motion is known from other cases in animals. There is still though nothing quite like seeing the speed of the ballistic mechanism of the chameleon’s tongue. It is not surprising that it has excited such interest in the world of biomechanics. Experiments have been done on it, mathematical models have been built of it and manipulators designed and constructed on the principles attributed to it. But while Sand may not have foreseen the importance of the elastic component, the whole process is driven, as he said, by that ‘massive sphincter-like ring muscle’ of the tongue knob.
A great advantage to a ballistic mechanism (with the stored energy built up from muscle power) compared with one relying on muscle contraction acting in real time, is not only the velocity of the movement but its relative resistance to a lowering of temperature. Since first keeping chameleons in the early 1960s I have had an interest (plus an unsuccessful grant application in 1968 over which I sometimes occasionally rant) in what happens to reptiles as their temperature changes during the day. Yes, they can by behavioural thermoregulation achieve a preferred body temperature which is optimal for their metabolism (chameleons are often desperate to flatten their bodies in the first rays of the morning sun and absorb heat) but may be operating sub-optimally for much of the time. In that respect it is particular interest that ballistic tongue projection works at high performance over a 20°C range. By comparing, projection (elastic) with retraction (simply muscle-powered) of the tongued in the Veiled Chameleon (Chamaeleo calyptratus) at different environmental temperatures, Christopher Anderson and Stephen Deban found that peak velocity and power declined by 10-19% with a 10°C drop in temperature for projection of the tongue compared with a greater than 42% decline for retraction. That finding means that chameleons may be slower in the cold of early morning but that they can feed and take advantage of prey whose movements may be affected by the cold.
Chameleons come in different sizes, from Brookesia micra at 2.9 cm long to what is thought to be the largest, Parson’s Chameleon (Calumma parsonii) at 68 cm. By comparing the performance of the tongue of 20 species over a five-fold difference in length Christopher Anderson found that small species ‘project their tongues proportionately further than large species, achieving projection distances of 2.5 body lengths’. The small chameleons achieved the highest accelerations and outputs of power ever recorded in any movement of a reptile, bird or mammal. The whole arrangement of the jaws and tongue is such that small chameleons can capture eat relatively large prey items compared with large chameleons.
Finally, Sand’s observations on the chameleons in his laboratory discovered the trick that has been used to get chameleons in captivity to feed when the supply of live insects has dried up.
They were kept indoors on a small privet bush planted in a tub, and most of the original stock have now survived in this situation for over three months. Every day a batch of house-flies is released in the room, and the tree is sprayed with water, of which the chameleons require a regular supply, as Gadow has pointed out It is generally stated that the chameleons will only take live food. I have found that they have extraordinarily poor discrimination. Not only will they take dead, even mouldy flies, but also such miscellaneous objects as a dried up piece of chameleon embryo, raw or cooked liver, a small piece of twig, the head of a burnt match and a piece of putty have been repeatedly taken. The three latter were never swallowed, but were always rejected soon after being drawn into the mouth. But the object, whatever it be, must be kept in motion, which can be done by presenting it on the end of a dissecting needle. These observations are of interest in that they show that neither auditory nor olfactory stimuli play any part in attracting a chameleon to its food. The stimulus is purely visual, and apparently any oscillating object of appropriate size, indefinite colour, and irregular shape will induce a chameleon to shoot out its tongue and seize it.
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| Middle Dwarf Chameleon (Bradypodion thamnobates) Photographed in my office in the late 1980s when I had a breeding group. |
Below I have just shown a few of the key papers. Those interested can find references to other and earlier work there.
Anderson CV. 2016 Off like a shot: scaling of ballistic tongue projection reveals extremely high performance in small chameleons. Scientific Reports Reports 6, 18625 doi:10.1038/srep18625
Anderson CV, Deban SM. 2010 Ballistic tongue projection in chameleons maintains high performance at low temperature. Proceedings of the National Academy of Science of the USA 107, 5495–5499. doi:10.1073/pnas.0910778107
Anderson CV, Deban SM. 2012. Scaling of the ballistic tongue apparatus in chameleons. Journal of Morphology 273,1214–1226 doi: 10.1002/jmor.20053
Debray A. 2011 Manipulators inspired by the tongue of the chameleon. Bioinspiration & Biomimetics 6, 1–15. doi:10.1088/1748-3182/6/2/026002
de Groot JH, van Leeuwen JL. 2004 Evidence for an elastic projection mechanism in the chameleon tongue. Proceedings of the Royal Society B 271, 761–770. doi:10.1098/rspb.2003.2637
Moulton DE, Lessinnes T, O’Keeffe S, Dorfmann L, Goriely A. 2016 The elastic secrets of the chameleon tongue. Proceedings of the Royal Society A 472, 20160030. http://dx.doi.org/10.1098/rspa.2016.0030
Zoond, A. 1933. The mechanism of projection of the chameleon’s tongue. Journal of Experimental Biology 10, 174-185.
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