科学家发现一些哺乳动物的行为令人感到不可思议：它们在水下居然可以利用的嗅觉来追踪食物。
??该项研究发表在十二月21日的《自然》科学杂志上。美国范德比尔特大学的生物学助理教授Kenneth Catania在观察到他所研究的鼹鼠游泳时发现了这个现象。Catania在今年年初获得了John D.和Catherine T. MacArthur基金会50万美元的“天才奖”，他表示：这很令人惊奇，因为通常都认为哺乳动物的嗅觉在水下根本不起作用。当哺乳动物逐渐适应水下生活之后，它们的嗅觉基本就退化了。
??因此他设计了一系列试验来验证星鼻鼹以及另外一种小型的半水生哺乳动物——水鼩是否真的能够在水下利用嗅觉。在试验中，利用一个高速的照相机他观察到：星鼻鼹在用鼻孔吹出水泡后接着又吸了回去，频率大约为每秒5到10次。而这正是与其相当的陆地哺乳动物呼吸的速率，例如老鼠等。
??Catania在一个玻璃缸的底部安装了一个高速摄像机，并在玻璃缸的底部粘了各种物体：蚯蚓、小鱼、昆虫皮和蜡滴、硅点等，然后观察鼹鼠的动作。结果他发现：当鼹鼠接近这些物体时，它们都会吹出气泡接触到这些物体，然后再将这些气泡吸回鼻孔。Catania表示：因为鼻子中的嗅觉神经都隐藏在鼻黏液的下面，而气味分子都是可溶于水的，所以，当这些气泡接触到物体的时候，气味分子会不可避免地与空气混和被鼹鼠吸回鼻孔中。
??但是因为星鼻鼹在碰到物体的时候也会伸出它那如同章鱼触须一样的鼻子去接触，所以为了验证它只是通过嗅觉也能判断物体的可食性，Catania又做了一些附加试验。其中一个就是在星鼻鼹和食物之间设置一个栅栏，使得它的触须鼻子无法伸出接触食物，而只能通过呼出和吸入气泡来判断。结果发现鼹鼠只用嗅觉也可以追踪到食物的气味。
??Catania同时还用水鼩做了试验，结果发现水鼩也同鼹鼠一样可以在水下利用它的嗅觉。
英文原文：
Two small, semi-aquatic mammals blow bubbles while swimming and then inhale them to smell submerged objects
Courtesy of Kenneth Catania
Kenneth Catania
For some time, Kenneth Catania had noticed that the star-nosed moles he studies blow a lot of bubbles as they swim around underwater. But it wasn't until recently that he really paid attention to this behavior and, when he did, he discovered that the moles were blowing bubbles in order to smell underwater objects.
"This came as a total surprise because the common wisdom is that mammals can't smell underwater,' says the assistant professor of biology. "When mammals adapt to living in water, their sense of smell usually degenerates. The primary example of this are the cetaceans — whales and dolphins — many of which have lost their sense of smell."
Catania, who earlier this year won a $500,000 "genius grant" from the John D. and Catherine T. MacArthur Foundation, devised a series of experiments to determine whether the star-nosed mole and another small, semi-aquatic mammal, the water shrew, can smell objects underwater and used a high-speed camera to discover how they do it. The results are reported in the Dec. 21, 2006 issue of the science journal Nature.
One of the first things the researcher noticed was that the moles were blowing bubbles out of their nostrils and then sucking them right back in. "They often loose part of the bubbles, but most of the air goes right back into their nose," he says.
Catania also determined that the moles were exhaling and inhaling these bubbles rapidly, between five and 10 times per second. That is about the same rate as the sniffing behavior of comparably sized land mammals, like rats and mice. "Rats and mice don't sniff the way we do," he says. "They push air 'out-in out-in' in a fashion strikingly similar to what the star-nosed mole is doing, except that it is doing it under water!"
The researcher mounted a high-speed video camera so that it pointed up through the bottom of a glass tank. Then he stuck various objects on the bottom of the tank — pieces of earthworm, small fish, insect cuticle and blobs of wax and silicon — and observed the moles' behavior. He saw that, when the moles approached one of these targets, they would blow bubbles that came into contact with the target's surface and then were sucked back into the nostrils.
"Because the olfactory nerves in the nose are covered with mucous, odorant molecules are all water soluble," says Catania. "So, when these bubbles come into contact with an object, it is almost inevitable that odorant molecules will mix with the air and be drawn into the nose when the bubble is inhaled."
Just because the moles are getting whiffs of interesting odors underwater doesn't necessarily mean that they are actually smelling them. So Catania devised some additional tests.
One of the complicating factors was the star-nosed mole's unusual nose, which is ringed by a star-shaped set of fleshy appendages. It uses its star like a super-sensitive set of fingers to identify objects it encounters while burrowing and swimming. So, at the same time it is sniffing at an object it is also fingering it with its star.
To determine if the mole can identify edible objects by sniffing alone, Catania created underwater scent trails leading to food and recorded how well the moles' could follow them. To keep the moles from using their tactile star, he put a grid-work between the animals and the scent trails. The openings in the grid were too small for the star appendages to squeeze through but large enough so the air bubbles can pass without difficulty.
These trials demonstrated that the moles could follow the scent trail by sniffing alone (without the tactile star). Five moles were tested on earthworm scent trails and followed the trail to its reward with accuracies ranging from 75 percent to 100 percent accuracy. Two moles were tested with fish scent trails and followed them with 85 percent and 100 percent accuracy.
When the grid was replaced with a screen with openings too small for the air bubbles to pass through, however, the moles' performance dropped down to the level of chance — the same as their performance with no-scent trails.
In order to see if this capability was limited to the star-nosed mole or if other small semi-aquatic mammals also have it, Catania captured some water shrews and began testing them. He found that they also exhibit this underwater sniffing behavior and can use it to follow underwater scent trails.
"Now, the question is, 'What other semi-aquatic mammals do this?'" Catania says. "Do animals like otters and seals do anything similar? Or is there a size limit and it only works for smaller mammals?"
He hopes that publication of his paper will encourage researchers who are studying all kinds of semi-aquatic animals to take a closer look at how they are using their noses underwater.
The research was funded by a Faculty Early Career Development (CAREER) award from the National Science Foundation.
