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www.thevillagenews.co.za
18 November 2020
MY ENVIRONMENT
Those magnificent chameleons and their ballistic tongues By Dr Anina Lee
H
ave you ever given much thought to animals’ tongues? We all have them – dogs, cats and people. Frogs use their tongues to catch prey and butterflies have coiled mouthparts that they unfurl to suck nectar from flowers. But probably the most amazing tongue belongs to the chameleon. As readers probably know by now, we at Whale Coast Conservation are particularly partial to chameleons. Chameleons are ambush predators. Their weapon of choice is the tongue. When they spot a potential prey, they keep very still, and focus both eyes on the insect before launching the ballistic tongue. The attack is lightning fast – it lasts only a fraction of a second.
this horn-like structure. Think of pulling some socks over a short stick. When the chameleon is preparing to strike, the retractor (hypoglossus) muscle (4) is pulled over the horn (3) and bunches up against the hyoid at the back, like a concertina. In front of that sits a tube of collagen fibres (5) also wrapped around the horn. A tube of accelerator muscles (6) wraps over the collagen fibres, but does not cover the tip of the tongue. This leaves the tip of the tongue or tongue pad (7) at the front of the mouth. All these soft tissues are attached to each other and work together. Let’s look at how all this works:
BELOW: A chameleon's tongue in action. PHOTO: National Geographic Society Newsroom
The hollow tongue fits like a sheath over the long, tapering cartilaginous spike called the hyoid horn. The horn is attached to the hyoid bone. The tongue consists of three primary components: the sticky tip, the retractor muscles and the accelerator muscles. When at rest, the whole assembly sits at the bottom of the chameleon’s mouth, the base placed down in the throat behind the head.
With the advent of ultra-slow motion photography, it has been possible to see exactly how the chameleon does what it’s been doing successfully for millions of years.
Before striking, the lizard moves it toward the front of the mouth and muscles raise the hyoid bone above the lower jaw. This is when we can see the tongue appearing in the chameleon’s mouth. After aiming with the entire head, the chameleon is ready to fire.
A chameleon’s tongue is made up of many different parts. A look at the diagram may help us visualise the parts. We know about the very sticky end (7) that sticks to its prey and can rip it off its perch. The stickiness is due to very viscous saliva that is 100 times stickier than human saliva. In other words, it sticks like glue. To help hold on to the prey, there is also a small flap of skin at the sticky end (8) that ‘pouches’ the prey to make sure it doesn’t escape.
The accelerator muscles are essentially ‘sphincters’. These are rings of muscle that squeeze the collagen fibres against the hyoid horn. The squeezing deforms the elastic collagen and locks in potential energy. It’s like an archer pulling back the string of his bow. His muscle energy is transformed into the potential energy in the bent bow. When the archer releases the string, all that potential energy in the bow propels the arrow forward at great speed.
At the other end – the chameleon end – the tongue is attached to the hyoid apparatus (1 & 2) at the back of the chameleon’s throat. There is a long bony projection from the hyoid – called the entoglossal process or horn (3) – that lies inside the mouth, pointing forward. You can imagine that this bony horn is at the heart of the missile, aiming at the prey.
The chameleon’s tongue is somewhat similar. When the accelerator muscles contract, they deform the collagen layer, loading it with potential energy. When the muscle pressure is relaxed, the collagen springs back with super-speed to its previous lower energy state, propelling the rest of the tubular tongue forward and out. Muscle contraction on its own could never propel the tongue at such amazing speed.
The tongue is hollow and is sheathed around
ABOVE: Bony Hyoid; (2) Cartilage Hyoid; (3) Entoglossal process; (4) Hyoglossus (contractor) muscle; (5) Collagen fibres; (6) Lingual accelerator muscle; (7) Tongue pad; (8) Tongue ‘pouch’. IMAGE: Madcham
How does the chameleon retract its tongue again – hopefully with an insect glued to it? Remember that the retractor muscles were folded around the hyoid horn like a compressed concertina. When the tongue extends, the elastic retractors stretch with the rest of the tongue and glide smoothly off the tip of the horn. These muscles are connected to the horn with a ligament, preventing them from flying clean off. Then, after a hit (or a miss), the retractor muscles contract, bringing the rest of the tongue
with it. The tongue can pull back a prey item of about half the chameleon's own body weight. Because speed is less important for reeling in prey than catching it, the retractors don't need great speed. Rather they need strength. Most of the time the tongue collapses and recoils on the way back, like a strand of spaghetti being sucked in. Chameleons have been around for almost 100 million years, but now they need our help. Let our grandchildren not say that they disappeared on our watch.
