There is an ant and she is next to my foot, dragging a fly up the same hill that I have just stopped trying to drag myself up. All worker ants are females and this particular female has a black fly in her mandibles, but she’s having a hard time keeping it there because she is negotiating her way backwards through lichen, and the uncooperative (i.e., dead) fly keeps getting hung up.
When she finally makes it out of the lichen, she turns around and carries the fly with no problems at all up the vertical face of a rock, over the top of the rock and back into more lichen.
I get down on the ground to watch this creature that doesn’t seem to know I exist wrestle food for the colony uphill. She’s a carpenter ant on her way back to her colony, which is up above me somewhere in a rotting log with sawdust piles at the entrance—the detritus of the workers’ excavations. Carpenter ants can be found throughout the Rocky Mountains. The higher in elevation you go, the bigger they get—the individual ants I mean. Their colonies are not all that large as far as ant colonies go. Normally, they include up to several hundred members. Compare this to leaf-cutter ant colonies in South America that can have up to several million individuals living together. But where leaf-cutters are farmers, carpenter ants are hunter-gatherers. And hunting and gathering, as we humans know, is not a way of life that can support large populations. Carpenter ants primarily collect insects and honeydew, but they like fruit juices, grains of sugar and other sweets, too. My ant, not finding honeydew or sweets in great supply, I suppose has settled for an insect.
She is carrying her reward in her mandibles, which are to her what a fully stocked Home and Garden Center at Sears is to us. She uses her mandibles to defend herself and her colony, to dig and bore through wood and soil, to hold on and carry things, to groom herself, to care for her colony’s brood, and to communicate with other ants. The muscle that attaches her mandible to her head is the largest muscle in her body. It is what allows her to carry, on average, between one-half and six times her body weight back from a scavenging expedition.
Ant mandibles are the result of some impressive evolutionary adaptation. And as useful, necessary and impressive as they are, there are some ants in the world whose jaws put hers to shame. One of these is the strumigenys ant, which ranges from the southeastern United States to Argentina. This species has evolved a special mechanism called a trap jaw that allows it to close its mandibles in 2.5 millionths of a second—much faster than any muscle can contract. The trap jaw is used for hunting and is similar to the mechanism that enables locusts and fleas to jump so high so quickly.
Normally, things like speed of running, jumping and predatory strikes are directly correlated to the size of the animal performing the action. The bigger the animal, the faster the strike or the farther the jump. This is not good news if you are small—if you are an ant, a locust or a flea. These insects, instead, adapted in such ways that they no longer depend on the speed of muscular contraction for jumping or striking. They have evolved muscular “catapults” that store slowly generated energy as elastic strain energy, which they can nearly instantaneously convert to kinetic energy. This means their muscles can stretch out slowly and latch. When released to jump or strike, they snap back with almost incomprehensible speed—speeds literally as fast as thought. Those speeds generate large forces as well. A human jumping from a standing position exerts forces on the ground of two or three times his or her body weight. A flea generates a force of 100 times its body weight when it jumps.
Some ants have even gotten creative with their lightning-fast mandibles. There is a type of trap jaw ant in South America that uses its mandibles to launch itself into the air. It puts its head against something hard, snaps its jaws shut and flies off like a pea squirted from between your fingers. It uses its mandibles mostly for hunting, but when the colony is under attack it will aim its backside at the attacker, chomp its jaws shut, fly through the air and start biting the attacker upon landing.
The carpenter ant I am following does not have a trap jaw mandible attached to her head, but with her mandible muscle she is still able to forklift a fly that is twice her size safely back to the nest, where other ants are on their way out. They stop and rub their antennae over my ant, asking her who she is, where she is coming from, what she has got with her. She tells them and disappears into the decomposing tree. The others follow their empty jaws down the hill.
"Field Notes" is produced by the Montana Natural History Center.