(Alex Wild) Acrobat tree ants in a Paraguay rainforest. (Photo courtesy of Alex Wild, Copyright 2003, used with permission.)
Why Ants Don't Die When Falling from High Distances
FEBRUARY 06, 2009April Holladay, HappyNews ColumnistQ: Why don't ants die when falling from a high distance?
Sharon, Moose Jaw, Canada
A: It’s a hot muggy day on the steep slopes of a Malaysian rainforest. About 60 feet (20 m) above the jungle floor, a small brownish-red acrobat ant clambers out of a nest hidden under the whitish-gray bark of a macaranga tree. Locking clawed back feet into the tree trunk, she follows an ant-laid chemical trail down the trunk to forage for insects. A monkey hurtles past, brushing her off. Falling, the ant wildly splays her legs like a parachute to slow her descent, and spirals downward, picking up speed. In less than a second, she reaches terminal velocity, about 4 mph (6.4 km/hour), and falls at that speed until she hits ground. She crawls away — unhurt — to search for a chemical path back home, high above.How do ants fall such vast distances and survive? Easy. Two factors save them:
- They have so little mass relative to their air resistance that they fall slowly and, therefore, have little energy to dissipate when they hit.
- Their bodies are tiny deformable tanks, well designed to absorb blows.Falling slowly
Ants, like all objects falling through the atmosphere, have a terminal velocity that depends on their shape, size, and mass. An ant picks up speed as she falls through the air. The air, in turn, resists her movement with a force proportional to the square of her speed. Eventually she reaches a speed at which the upward drag forces exactly balance her downward weight and she stops accelerating. That speed is her terminal velocity.
The terminal velocity of a small to medium ant is about 4 mph (6.4 km/hour), according to the physics department of the University of Illinois. An ant would fall faster, given a ball-like shape, but the ant's no dummy. She thrusts her legs out, presenting more surface to the air, to fall slower, like a flat sheet of paper instead of a balled-up sheet. Indeed, a man has a terminal velocity of about 125 mph (200 km/hour) with arms and legs fully extended to catch the wind like a parachute and about 200 mph (320 km/hour) when curled into a ball. An ant slows similarly.
But, it isn't falling that hurts, it's the sudden stop. Hitting ground, however, reaps the big benefits of falling slowly. When the ant hits, she must dissipate her falling (kinetic) energy in order to halt. That kinetic energy depends on the square of the velocity — not just velocity. So she must dissipate much less energy on impact, than say a man falling at a higher velocity. An ant goes 4 mph when she hits --- about 1/30th times slower than a falling man on impact. She absorbs only 1/26,000,000th (1/26 millionth) times the energy of the man (assuming an ant weighs 1/10th of an ounce (0.3 g) and a man 180 pounds (82 kg)). No wonder the man probably dies and the ant walks away, unhurt.
"Sufficiently small animals cannot be hurt in a fall from any height: A monkey is too big, a squirrel is on the edge, but a mouse is completely safe," says biologist Michael C. LaBarbera of the University of Chicago.
But an ant has even more advantages to survive falls. She isn't built like a human.
An insect's skeleton surrounds its body like armor. But, unlike a tank's steel plating, an insect's armor is deformable to absorb and dissipate blows. It is made up of several layers; the outer layer is made of a tough substance called chitin (which is similar to the keratin that makes up our fingernails).
Even if impact with the ground rips a hole in the ant's exoskeleton, the ant is unlikely to bleed much, because insect blood has excellent clotting characteristics, says LaBarbera.
The nervous system is distributed throughout an ant's body, so the head can take blows, unharmed, that would probably knock a vertebrate unconscious or kill him.
Concrete ants and others may well impact concrete, but as Meyers points out, not so tree ants: "An ant falling 60 feet from a tree in the Malaysian rain forest is not likely to hit its head on a concrete sidewalk. The forest floor of vegetation and decaying leaves would make for a pretty soft landing!" emails entomologist John Meyer, professor at North Carolina State University.
The circulatory system is likewise distributed. An ant has a long tube "heart" that runs the length of her abdomen. The exoskeleton protects the fragile tube from impact.
Finally, some ants glide!
Glider ants of the Amazon have adapted an ant's slow fall into a glide to better survive the return trip.
When a glider ant falls from 100 feet (30 m), she doesn't fall to the forest floor. Instead, she falls a little bit, slows down, twists for a backward approach, swoops to the tree trunk (like a kid on a rope swing), hits on the back of her armor-plated abdomen, and plunks her clawed feet on the trunk — only about 30 feet (10 m) from where she started.
Had she fallen to the ground, how could she possibly find an ant-laid chemical trail leading home among the litter? Worse: predators abound. She'd be unhurt from the fall, but would die, nevertheless.
Almost certain death from predators is the major evolutionary driving mechanism behind the gliding behavior, says insect ecologist Stephen P. Yanoviak of the University of Texas Medical Branch in Galveston, the discoverer of such ants.
The biology of B_movie monsters by Michael C. LaBarbera, University of Chicago, Organismal Biology & Anatomy, Geophysical Sciences, the Committee on Evolutionary BiologyAnt behavior by Alex Wild, Myrmecos.netThe exoskeleton by John R. Meyer, North Carolina State UniversityCirculatory system of insects by John R. Meyer, North Carolina State UniversityAn ant dropped off the Empire State Building, physics department of the University of IllinoisGliding ants by Stephen P. Yanoviak, University of Texas Medical Branch at GalvestonAmazing ants "fly" when they fall by Robin Lloyd, Live Science
(Answered Jan. 12, 2009)