Science and technology

Marine biology

Scents and sensibility

Turtles love a smell that humans loathe. For them, it means dinner

JELLYFISH may not be most people's idea of a tasty snack, but if you are a loggerhead turtle, they are top of the menu.

Though jellyfish can swim, they are not exactly the greyhounds of the ocean, so they are easily caught.

And since munching a large jellyfish can keep a turtle going for days, loggerheads love them when they can find them.

Yum！

The best place to do so is in an upwelling zone.

This is an area where the wind's action draws cold, nutrient-rich water to the surface.

That encourages the growth of planktonic algae, and thus of everything that feeds on such plankton, or feeds on what feeds on them—including jellyfish, and also various molluscs and crustacea that loggerheads enjoy as an appetiser.

And upwelling zones do, indeed, attract turtles.

But nobody knows how, for such zones are scattered, local and often temporary phenomena, and locating them in the wide expanse of the sea is hard.

Courtney Endres, a biologist at the University of North Carolina at Chapel Hill, however, thought the reptiles might be smelling such zones from afar.

She knew from an experiment she conducted a few years ago that loggerheads have a good sense of smell.

She also knew, because passing sailors frequently comment on the fact, that upwelling zones tend to stink.

Specifically, they stink of cabbages.

That is because many planktonic algae, when crunched up during the process of being eaten, release a substance called dimethylsulphoniopropionate.

This quickly degrades into another chemical, dimethyl sulphide, which generations of those subjected to unimaginative school lunches will instantly recognise.

But to loggerheads it is the smell of a banquet.

Ms Endres and her colleague Kenneth Lohmann showed this by collecting 11 loggerhead hatchlings from local beaches and testing them to see which smells most interested them.

The young turtles were each put in a tank of seawater that had various scents blown across its surface.

These included cinnamon, jasmine, lemon—and dimethyl sulphide.

When a turtle came up for air , the researchers recorded how long it sniffed around.

They found, as they report in Experimental Biology, that the animals spent an average of five seconds breathing air scented with lemon,jasmine or cinnamon, all pleasant odours from a human point of view.

That was no different from the amount of time they spent at the surface when no scent was used at all.

When the air was scented with dimethyl sulphide, however, they spent an average of ten seconds breathing in the stinky scent of cabbages—or upwellings.

That does not prove that loggerheads smell their way to the table, of course. But it makes the hypothesis plausible.

For turtles, it seems, the perfumes of Araby hold little charm.

The scent of cabbages, by contrast, is as attractive as the odour of roasting chicken is to a hungry human.

科学技术

海洋生物学

气味与识别

海龟喜欢一种人类讨厌的味道，对它们来说，这表示大餐

水母或许不符合大多数人对于美味小食的定义，但如果你变成一只红海龟，水母便成了顶级美食。

虽然水母可以游动，但它们可算不上海洋中的快艇，所以要逮它们很容易。

而吃上一只大水母能让一只海龟维持好几天，所以红海龟一旦发现水母便会大快朵颐。

味道好极了！

上升流区是海龟大快朵颐的最佳地点。

在这种区域里，水流会在海风的作用下将富含营养物质的冷水带到海洋表面，

这有利于浮游藻类生长，因此也有利于以这些浮游生物为食的动物，或以这些动物为食的动物生长，包括水母，还有各种各样的软体动物和甲壳动物，这些都是红海龟的最爱。

所以上升流区确实会吸引海龟，

但没人知道海龟是怎样找到上升流区的，这些区域都是局部性的，非常分散，且常常只是暂时现象，要想在宽广的海洋中找到它们绝非易事。

然而考特尼恩德斯认为两栖动物离很远便能通过气味察觉到上升流区，她是美国北卡罗来纳州立大学的一名生物学家。

她是从几年前进行的试验中发现红海龟拥有良好的嗅觉。

她还了解到上升流区往往会发出异味，因为经过上升流区的水手们经常会提到这一事实。

明确的说，是卷心菜的异味。

这是由于浮游藻类在被吃掉的过程中被咬碎时，会释放出一种叫做硫代甜菜碱的物质，

该物质很快会降解为另一种化学物质-二甲基硫化物，闻到这种物质产生的气味会立刻让人想起千篇一律的学校午餐的气味。

但这对红海龟来说是盛宴的气息。

恩德斯及其同事肯尼斯罗曼通过实验表明了这点。她们在当地海滩上抓了11只红海龟的幼龟，并测试它们对哪种气味最感兴趣。

每只幼龟被分别放入一个装有海水的水缸中，并将各种各样的气味吹过水面，

包括肉桂，茉莉，柠檬，以及二甲基硫化物。

当海龟到水面上换气时，研究人员记录了它们露出水面的时间。

他们发现，从人类的角度上看，柠檬，茉莉，或肉桂都是很悦人的气味，而海龟在呼吸含有以上气味的空气时，平均只在水面停留了五秒钟，

与在其呼吸没有气味的空气时的停留时间没什么不同。

然而在空气中含有二甲基硫的气味时，海龟在这种散发着卷心菜或者说上升流异味的空气中平均停留了十秒钟。该报道刊登在《实验生物学》上。

当然，这并不能证明红海龟能顺着气味找到它们的餐桌，但是这让以上假设似乎可以说得通。

看来阿拉伯香水对海龟也不会有什么吸引力，

相比之下，卷心菜的气味之于海龟的吸引力就像烤鸡之于饿汉。