科學(xué)家發(fā)現(xiàn)一些哺乳動物的行為令人感到不可思議：它們在水下居然可以利用的嗅覺來追蹤食物,。
??該項研究發(fā)表在十二月21日的《自然》科學(xué)雜志上,。美國范德比爾特大學(xué)的生物學(xué)助理教授Kenneth Catania在觀察到他所研究的鼴鼠游泳時發(fā)現(xiàn)了這個現(xiàn)象,。Catania在今年年初獲得了John D.和Catherine T. MacArthur基金會50萬美元的“天才獎”,，他表示：這很令人驚奇,，因為通常都認(rèn)為哺乳動物的嗅覺在水下根本不起作用,。當(dāng)哺乳動物逐漸適應(yīng)水下生活之后,，它們的嗅覺基本就退化了,。
??因此他設(shè)計了一系列試驗來驗證星鼻鼴以及另外一種小型的半水生哺乳動物——水鼩是否真的能夠在水下利用嗅覺,。在試驗中，利用一個高速的照相機他觀察到：星鼻鼴在用鼻孔吹出水泡后接著又吸了回去,，頻率大約為每秒5到10次,。而這正是與其相當(dāng)?shù)年懙夭溉閯游锖粑乃俾剩缋鲜蟮取?/p>
??Catania在一個玻璃缸的底部安裝了一個高速攝像機,，并在玻璃缸的底部粘了各種物體：蚯蚓,、小魚、昆蟲皮和蠟滴,、硅點等,，然后觀察鼴鼠的動作。結(jié)果他發(fā)現(xiàn)：當(dāng)鼴鼠接近這些物體時,，它們都會吹出氣泡接觸到這些物體,，然后再將這些氣泡吸回鼻孔。Catania表示：因為鼻子中的嗅覺神經(jīng)都隱藏在鼻黏液的下面,，而氣味分子都是可溶于水的,，所以，當(dāng)這些氣泡接觸到物體的時候,，氣味分子會不可避免地與空氣混和被鼴鼠吸回鼻孔中,。
??但是因為星鼻鼴在碰到物體的時候也會伸出它那如同章魚觸須一樣的鼻子去接觸，所以為了驗證它只是通過嗅覺也能判斷物體的可食性,，Catania又做了一些附加試驗,。其中一個就是在星鼻鼴和食物之間設(shè)置一個柵欄，使得它的觸須鼻子無法伸出接觸食物,，而只能通過呼出和吸入氣泡來判斷,。結(jié)果發(fā)現(xiàn)鼴鼠只用嗅覺也可以追蹤到食物的氣味。
??Catania同時還用水鼩做了試驗,，結(jié)果發(fā)現(xiàn)水鼩也同鼴鼠一樣可以在水下利用它的嗅覺,。
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.
