Recently there was great excitement over the possibility that American alligators – Alligator mississippiensis – place sticks on their snouts to lure birds in search of nesting material, thus marking them out as the only reptiles known to use tools.
According to a study at a lake in Louisiana by Vladimir Dinets of the University of Tennessee in Knoxville, the alligators wait patiently for hours and then lunge at nesting snowy egrets that attempt to collect a stick. Dinets is convinced the alligators’ behaviour is deliberate because he only observed them displaying sticks when close to the egret rookeries, not elsewhere in the lake. Furthermore, the reptiles only exhibit such behaviour during the birds’ nesting season.
The researcher first hypothesised that alligators and crocodiles use hunting lures following a 2007 visit to India where he saw mugger or marsh crocodiles – Crocodylus palustris – lying in shallow water with sticks on their snouts.
As animal behaviourists collect more and more data, they are uncovering many examples of tool use across the animal kingdom. It is perhaps surprising to discover that tool use is much more common than we once thought: where previously we might have associated using tools with mammals and some birds, we now have examples of fish using tools and some invertebrates too – including insects and cephalopod molluscs.
For instance, octopuses are associated with using coconut shells as artificial shelters in areas where little other shelter exists. The coconut or veined octopus – Amphioctopus marginatus – has been filmed retrieving, manipulating, stacking and carrying shells up to 20 metres, and then reassembling them to use as a shelter. Under lab conditions, another species of octopus – Octopus mercatoris – has been observed using a plastic Lego brick to block its lair.
In the insect world, ants of the species Conomyrma bicolor pick up stones and other small objects with their mandibles and drop them down the vertical entrances of rival colonies – presumably to block the entrance – so enabling workers to forage for food without competition.
Hunting wasps of the genus Prionyx camouflage and seal the entrance of egg chambers that have been provisioned with live prey using a stone tool. They do this by vibrating their wing muscles while holding a weight such as a small pebble in their mandibles; the wasps apply the vibrating pebble to tamp down the area around the burrow, causing the sand to settle. Meanwhile, another hunting wasp – Ammophila – uses pebbles to close up burrow entrances.
Unsurprisingly, it’s not just humans who find shellfish hard to open. Both seagulls and fish are known to use tools to crack open bivalve molluscs – the birds by dropping them from the air onto a rock, while fish such as a wrasse may hold a shell in its mouth and then thrash it against a rock used as an anvil.
Mammals are also partial to shellfish and some use tools to crack the shells open. Sea otters – Enhydra lutris – are well known for using a personal anvil, which they store along with food collected during dives in a loose pouch of skin that extends across their chest. The otters use the rock to break open shellfish such as a clam; to open a particularly hard shell, an otter may pound its prey with both paws against the rock, which it places on its chest.
Sea otters may also use their stone to dislodge marine gastropods such as abalone from underwater rocks, repeatedly hammering them over the course of two to three dives. They have to use considerable force because an abalone can cling to a rock with a force equal to 4,000 times its own body weight.
And, of course, some primates are capable of using tools to open shellfish, although many more use tools to crack seeds and nuts. In Thailand and Myanmar (Burma), crab-eating macaques – Macaca fascicularis – do both, using stone tools to open nuts, oysters and other bivalves, as well as various types of sea snail.
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