Originally designed to live on land, marine mammals are a diverse, charismatic group of animals that include more than 120 species. The animals share key characteristics of land mammals. They have hair, breathe air, give birth to live young, which feed off mother's milk when young. They have warm bodies and usually thick blubber to keep their body temperatures high. The bottlenose dolphin is probably the most widely recognized marine mammal, easily spotted just offshore from beaches around the world. Small groups of 20 or less can live in close proximity to shorelines, but groups living more offshore can reach several hundred. Bottlenose dolphin calves stay with their mothers for up to six years, learning how to hunt and become good dolphin citizens. Full-grown dolphins reach eight to 12 feet in length and can weigh up to 1,430 pounds. The bottlenose dolphin is protected in U.S. waters.
What makes them "marine" depends on the animal. They either live mostly in the sea or, like polar bears, depend on the ocean for food. The largest in the group are whales -- including humpback whales. These massive animals reach up to 50 feet in length and weigh up to 79,000 pounds. To maintain their weight, the animals feed on tons of krill and fish. They neared extinction due to whaling, but have recovered somewhat since a 1966 moratorium on whaling was introduced.
While polar bears live mostly on land or ice, they are excellent swimmers and have been known to swim up to 45 miles a day. The massive animals, weighing up to 1,500 pounds, hunt mostly seals. In recent years, biologists have observed that the bears are swimming now more than ever as melting stretches the distances between Arctic ice flows. Because they depend on sea ice to hunt seals, the polar bear is considered threatened as global warming melts and thins ice in this region.
This member of the weasel family is also the smallest marine mammal, with females weighing about 60 pounds and males weighing up to 90 pounds. They may be small, but they're also clever. They're the only marine mammals known to use tools. They use stones to break open clams and store food they gather in the folds of their armpits! Another feature that sets them apart is their lack of blubber. These marine mammals depend mostly on their fur to stay warm. That feature makes them particularly vulnerable to oil spills, which can compromise their fur's insulating effect.
Immediately recognizable by its long tusks and whiskers, the sea walrus is a hefty, flippered member of the Odobenidae family and is, in fact, the last living member of this group. Since both the males and females have big tusks and not much for teeth, the animals feed by sucking up shellfish from the ocean floor. So, just what are those tusks for? The longer they are (they grow to be up to four feet long in males), the higher an animal is ranked in the group. Males attack each other with their tusks to establish dominance. The ivory appendages are also handy for poking holes in the winter ice and for helping the animals pull themselves out of the water.
Manatees, also known as sea cows, are gentle herbivores that live in marshy areas in tropical and subtropical waters. The average adult manatee can weigh up 1,200 pounds and is around 10 feet long. Because of their slow metabolism, these animals can only survive in warm waters. Due to the unusually long, cold winter this year in part of the southeastern United States, populations of manatees throughout Florida were devastated. During the day, manatees usually like to stay close to the surface. At night, manatees will often sleep about three to 10 feet below sea level. This is why these gentle animals are so often accidentally injured, maimed or killed by passing boats.
Found up and down the North American coastlines, these marine mammals spend half of their lives swimming. Although they can reach up to six feet in length and weigh around 180 pounds, when on land and in plain sight harbor seals may not be easy to spot. Their spotted brown or tan fur allows harbor seals to blend in with sand and rocks. Unlike their very vocal relatives -- sea lions and elephant seals -- harbor seals are quiet creatures that make little noise. They like to hang out on beaches, sand bars and rocks during low tide to bask in the sun and sleep, but they never go far from the water. At the slightest sign of danger, they will quickly slip back under the waves. These expert swimmers have been known to plunge to depths of more than 1,600 feet and stay underwater up to 28 minutes.
Elephant seals have surprisingly high levels of naturally produced carbon monoxide — a noxious gas that is deadly at high concentrations — in their blood, a new study finds. In fact, the amount of carbon monoxide found in the blood of these large mammals is roughly the same as that in people who smoke 40 or more cigarettes each day, researchers say.
Carbon monoxide is an odorless and colorless gas that is naturally produced in small quantities in humans and animals. The scientists are unsure why elephant seals have such unexpectedly high levels of the gas in their blood, but the researchers suggest it could protect the animals from injury when they dive to extreme depths in search of food.
In humans and animals, carbon monoxide is a byproduct of the breakdown of hemoglobin, which carries oxygen in red blood cells throughout the body, said study leader Michael Tift, a comparative physiologist at the Scripps Institution of Oceanography at the University of California, San Diego. (Gallery: California's Deep-Diving Elephant Seals)
Carbon monoxide binds to hemoglobin, effectively suffocating the protein and preventing it from transporting oxygen. In healthy adult humans, about 1 percent of hemoglobin is bound to carbon monoxide. But the amount of hemoglobin incapacitated by carbon monoxide can reach as high as 10 percent in elephant seals and chronic, heavy cigarette smokers (who are exposed to carbon monoxide from burning and inhaling tobacco), the researchers said.
"Elephant seals are known to have the highest blood volume of any mammal, so we knew there was the potential for producing a lot of carbon monoxide," Tift told Live Science. "When we looked into the levels of carbon monoxide in the blood, we suspected there could be a lot."
Yet, while elephant seals appear to have elevated levels of carbon monoxide in their bloodstreams, the concentration of the gas is not so high as to cause harm, the researchers said.
"The levels in elephant seals are not high enough to inhibit oxygen transport or lead to carbon monoxide poisoning," Tift said.
Carbon monoxide's colorless and odorless properties have earned it a reputation as a "silent killer." When the gas invades up to 20 percent of hemoglobin stores, humans typically begin to suffer the effects of carbon monoxide poisoning — lightheadedness, headaches and other flu-like symptoms. The gas typically becomes deadly when it incapacitates more than 50 percent of hemoglobin stores, Tift said.
The researcher and his colleagues sampled 24 elephant seals, ranging in age from pups to juveniles to adults, at the Año Nuevo State Reserve near Santa Cruz, California.
The researchers are not sure why elephant seals naturally produce higher levels of carbon monoxide, but suspect it may have something to do with the animal's prolific diving abilities. The mammals can dive more than 5,000 feet (1,500 meters) beneath the surface of the sea, holding their breath and conserving oxygen for impressively long periods of time.
"Elephant seals will shut off blood to specific organs and tissues as they are diving," Tift said. "Recently, we found that low levels of carbon monoxide can be therapeutic in treating certain conditions where blood has been shut off to muscles."
As such, carbon monoxide could protect elephant seals from reperfusion injuries, which occur when blood returns to tissue after sustained periods of oxygen deprivation.
"We can't say for sure that the carbon monoxide is therapeutic for elephant seals, but it definitely has the potential," Tift said. "If they didn't have this high level of carbon monoxide, there's a chance we would see injuries from reperfusion."
The researchers are testing this hypothesis by studying other diving and non-diving animals, including sea lions, penguins and terrestrial birds.
"We want to know, is high carbon monoxide found in all marine mammals? Is it found in deep divers, or both divers and non-divers?" Tift said.
The results of the new study were published online today (May 14) in the Journal of Experimental Biology.
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