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Are sharks and dolphins enemies; What mammals are poisonous; does soap break down bacteria?

Q: I've heard that dolphins and sharks are natural enemies. If this is so, why? (Adam, Bridgend, Llangeinor, Wales, UK)

A: Sharks and dolphins aren’t enemies. Instead they "show a natural toleration of each other," says Chicago Zoological Society biologist Randall Wells, who has been studying dolphins in Florida’s Sarasota Bay since 1970. They get along, especially if the shark is small and harmless (to the dolphin), which most sharks are.

Dolphin cavorting in the bow wave of a NOAA ship. [Grady Tuell, NOAA]

Sometimes sharks can be hostile. About 31% of the dolphins in Sarasota Bay show scars from shark bites (presumably big sharks). If possible, dolphins flee the big ones.

A dolphin researcher, Richard Connor, once witnessed such flight while studying dolphins in the Indian Ocean. An 8-foot great white shark cruised quietly into a school of resting dolphins. The dolphins panicked, leapt frantically out of the water in a wild attempt to escape, bolted away as fast as they could, and didn’t stop fleeing for ten minutes.

Popular legend may have prompted your question about sharks and dolphins being natural enemies. "People say" that dolphins gang up on a shark and attack it by ramming the shark with their heads. Certainly, if true, this behavior indicates dolphins don’t like sharks.

It is true that bottlenose dolphins in Scottish waters have killed harbor porpoises. They do gang up on a porpoise, batter it with their heads, and pound it to death with their tails — perhaps as a game. But porpoises are little harmless creatures that don’t threaten dolphins. "No biologist (first-hand account) has ever seen wild dolphins beat up sharks," says Connor.

Further Reading:

National Wildlife Federation: Machiavellians of the deep? By Roger Di Silvestro

Q: We know the platypus has venom. I'm wondering if there are any other poisonous mammals. Venom is routine in the insect world, but seems rare in the mammal world. (Dia, Washington DC)

A: Yes. Only four mammals besides the platypus are poisonous (but not lethal to humans). They are all insectivores: two types of shrews (the Blarina and Neomys species) and two types of the rare shrew-like solendons.

Southern short-tailed shrew has venom strong enough to kill mice but not lethal to people. [USGS]

The short-tailed shrew (Blarina species) — a small mousy-looking insectivore — paralyzes insects and other prey with a poison that is both a neurotoxin (damages nerves) and a hemotoxin (destroys blood cells). The poisoned insects stay alive and immobile for three to five days as fresh food. The short-tailed shrew is the only poisonous mammal in North America.

The water shrew (Neomys species) of Eurasia weakens its aquatic prey (snails, mollusks, and freshwater insects) with a similar saliva poison.

Both species (Solenodon paradoxus and Solenodon cubanus) of the solenodon of the West Indies also are poisonous. In the dark of the night, the rat-size slow-moving animals sniff with their long tubular snouts for ants, insects, grubs, and small reptiles that venture forth.

A solenodon or shrew kills by biting her victim. She stores the poison with her saliva in glands. When she bites a prey, the venomous saliva oozes down ducts to the base of her lower incisors, thence along side channels grooved in the teeth, and into the wound. Her teeth serve as a poison hypodermic — piercing the victim’s hide and injecting the venom.

Further Reading:

American Zoo: Solenodon paradoxus

Q: How does soap break down bacteria? (Sarah, Philadelphia, Pennsylvania)

A: Actually soap doesn’t break down bacteria directly. Instead, soap loosens and suspends oil-containing bacteria so water can flush the germs away.

Oil — maybe from your hands or from a greasy dish — contains germs and dirt. Simply washing your hands or the dish with water won’t get rid of the bacteria because water and oil don’t mix well — as anyone knows who’s tried to mix oil and vinegar (which is 95% water) to pour on a salad.

A soap molecule's fatty end grabs oil and its ionic end grabs water. 

Soap to the rescue! A soap molecule is built like a two-ended monster — grabbing oil with one end and water with the other. See diagram. The hydrocarbon (fatty) end attaches to an oil molecule and the electrically-charged ionic end attracts a water molecule.

The net result is that soap breaks up oil and water into tiny globs and scatters them together so that water can do its job of flushing oil away.

Furthermore, soap loosens the surface tension  of the water so that water lies flatter instead of balling up like rain droplets. The flat-lying water can seep into crevices better and, therefore, wash away the oil.

By the way, Babylonians made and used soap as early as 2800 BC. Archeologists found a soap-like material in cylinders at the ancient site of Babylon. Inscriptions on the cylinders describe how to make soap — boil fats with ashes.

Further Reading:

Wikipedia: Soap

Soap and Detergent Association: Soap history

(Answered May 6, 2005)