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Horses run on toenails, magnets zap data, kill mosquitoes while they’re young

Q: I am in a 4-H horse program and need to know how and why human and horse feet differ. —Marina

Wild horse on the barrier island, Assateague, between Virginia and Maryland [National Park Service]

A: Horses feet differ from humans in that a horse has only one functional toe on each foot and its toenail is thick—a hoof.

"The horse’s foot contacts the ground only with the hoof of the third digit [like our middle toe] in an almost vertical position [like a ballet dancer, en pointe]," says Gheorghe M. Constantinescu, professor of veterinary anatomy, University of Missouri-Columbia and author of the book Clinical Dissection Guide for Large Animals.

Moreover, the so-called cannon bone is much longer proportionately than ours. What do I mean by "cannon bone"? Look at your foot. The five bones going from your ankle to your toes correspond to the one bone in the horse’s foot: the cannon bone. But, it’s huge — running half way up the horse’s leg to what looks like its knee but is actually its ankle (called the hock). Almost all trace of its other toes has vanished. Only splint bones alongside the cannon bone remain of its second and fourth toes. Short, strong ligaments tie the splint bones to the cannon bone.

Only splint bones alongside the cannon bone remain of its second and fourth toes. [P.D. Garrett, DVD, MS]

To understand why horses and humans are so different, let's examine how horse evolved.

Horses started (about 70 million years ago) more like us, with five toes, but not really horses, rather rat-sized mammals (called Protungulatum) with five-toed clawed feet — the first mammal herbivore. These creatures split into different groups over the next few million years and became the forebears of whales, anteaters, and horses.

Gradually, (60 million years ago) Protungulatum changed into the first horse (Phenacodus, once known by the lovely name "Eohippus", meaning "dawn horse"). This dog-sized, doggish-looking equid ate fruit and soft foliage. Its legs were flexible and rotatable — like ours. Already, though, its feet were changing. Phenacodus ran through swampy undergrowth on long, splayed padded toes with small hoofs on each toe instead of claws. It had four-toed fore feet and three-toed hind feet. The hind feet had two additional tiny bone nubs: vestiges of the first and fifth toes.

The North American climate changed and with it the land and its animals. Grasses evolved; the vast forests shrank. "Dawn horses" grew larger with longer legs and fewer toes to better escape predators in the open plains. In 15 million years, they changed to three-toed creatures (Mesohippus celer).

By 18 million years ago, horse changed into a specialized runner. It got still bigger and longer legged. Its toes gradually reduced from five to three and only the middle toe touched the ground, says Constantinescu. Bones and muscles specialized for efficient forward-and-back strides. Finally, to get greatest speed, they stood on tiptoe. Springy ligaments that ran to the big toe supported their weight.

In another million years, horse stood permanently on tiptoe. The side toes were still there but smaller and to each side, probably only touching ground in wet patches. The big central toe developed a large hoof.

Finally, modern horse (Equus, 4 million years ago) made the last change. It runs on its toenail—the hoof—and has only one functional toe. It lost the other two about 15 million years ago to gain speed. The trace toes exist as side splints with ligaments binding the main supporting bone (the cannon bone) to act as a single bone and prevent the big toe from twisting while running.

So, a horse’s foot is different from ours to run superbly fast over hard ground. It runs about 40 mph [64 kph] (compared with the fastest human’s speed of 23 mph [37 kph] for 10 seconds) to sprint away from plains predators.

Update:  DNA Study Sheds New Light on Horse Evolution

Further Surfing:

Robert C. McClure, College of Veterinary Medicine, University of Missouri-Columbia: Functional anatomy of the horse foot

Kathleen Hunt, TalkOrigins.org: Horse evolution

Martin Jehle: Archaic hoofed mammals

Magnets zap data

Q: Why does placing magnets near computer disks damage them? — Saima, Islamabad, Pakistan

She floats in space.  No magnets near this hard disk [NASA]

A: Putting a powerful magnet near a hard disk doesn’t damage the disk. It’s the information on the disk that’s gone forever. Erased.

A computer disk is a glass, ceramic, or aluminum platter that is coated with a magnetic material, probably an iron compound. Iron, like nickel and cobalt, is riddled with tiny magnets — an intrinsic part of the metal — that normally are pointing every which way. Just a random jumble of north and south poles.

The computer disk stores information by sending a signal through a tiny electromagnet (a wire coiled around an iron core), which creates a changing magnetic field. The resulting magnetic lines of force magnetize the iron compound coating the hard disk according to a pattern defined by the changing current. The jillions of magnets that form the coating line up into a corresponding on/off, 0/1 bit pattern that represents the data.

What happens if you bring a big magnet close? You guessed it. Ziip! All the tiny magnets line up with the north/south poles of the big magnet and lose their carefully wrought magnetic pattern. The data’s gone.

Further Surfing:

Wikipedia: Hard disk

Maxtor Corporation: personal computing hard drives

How stuff works: How hard disks work

Kill mosquitoes while they’re young

Q: I have two small ponds and now have mosquito larvae in them. I would like to know if there is something I can put into the ponds to kill the larvae? — Sharon

Mosquitoes — kill ‘em young [Scott Camazine © used by permission]

A: Yes, there are plenty of larvae killers. Take your pick: fish, bacteria, poison, oil.

Mosquitofish (small minnows: Gambusia affinis) voraciously gobble mosquito larvae, reproduce fast, and survive cold. Many cities and counties provide them free. Contact your local environmental health department.

Another good bet is Mosquito Dunk (contains bacteria called Bacillus thuringiensis). It’s available at most hardware stores. Be careful and follow the instructions because this product can cause skin and eye irritation but is otherwise harmless to humans.

You’re wise to attack the larvae. Get them when they’re concentrated and immobile. Adult mosquitoes are hard to kill and can carry West Nile virus. Normally the virus infects birds but mosquito carriers can infect humans (as well as birds) and the disease can kill us.

Further Surfing:

Environmental Protection Agency: Lavicides for mosquito control

Albuquerque Bio-Disease Management: Mosquito control

(Answered Oct. 3, 2003)