金學堂英語的部落格

【聯合報／By NATALIE ANGIER／陳世欽譯】

The drive to exchange presents is ancient, transcultural and not limited to humans, though our generosity is on display during the Christmas season. Researchers have found striking examples of gift-giving across the phyletic landscape, in insects, spiders, mollusks, birds and mammals.

Many of these donations fall under the rubric of nuptial gifts, items or services offered up during the elaborate haggle of animal courtship to better the odds that one’s gametes will find purchase in the next generation.

Nuptial gifts allow researchers to precisely quantify a donor animal’s investment in mating and reproduction, and to track the subtleties of sexual competition and collusion by analyzing the chemical composition of a given bag of courtship swag.

“This is an incredibly cool and important topic in sexual selection that we’re just beginning to explore,” said Sara M. Lewis, a professor of biology at Tufts University outside of Boston who has written extensively about nuptial gifts. “The bright side of nuptial gifts is, here’s a way that males can contribute things that are essential to his mate and to his future offspring.

“On the other hand, the gifts can be a source of sexual conflict, a way of manipulating the female into doing what he wants,” she said.

Other researchers are studying how animals use gifts socially, to foster alliances or appease dominant members of the group. Grooming among primates is considered a form of gift-giving.

A gift is, by definition, something that is voluntarily given, Dr. Lewis said. “But that doesn’t mean it’s voluntarily received,” she added.

One example of a possibly unwelcome gift, she said, is the snail’s love dart. Land snails like the ones in a garden are hermaphroditic, meaning they produce both eggs and sperm, and they mate by swapping sperm with other snails . This is why, before copulation, a snail will try to pierce its partner with a harpoonlike structure made of calcium carbonate and produced in the snail’s genital region.

The dart may penetrate anywhere on the other snail’s body . And when it does, it delivers a “gift” of potent hormones, which help promote the retention of the donor’s sperm over that of competing snails. No mere victim, the punctured mollusk retorts with a flirty fléchette of its own, at which point the dueling Cupids will copulate.

In most cases, though, a male’s nuptial gift is something the female wants or needs. Martin Edvardsson, an evolutionary biologist at Australian National University, studies Callosobruchus maculatus, a small, spotted beetle that is a major pest on beans and grains. The beetles have little access to water, and the females get very thirsty while making eggs.

A male sequesters most of the liquid he encounters as a larva inside a bean and adds it to his ejaculate. Should a parched female solicit him, he’ll mount and pump in the stored water along with his sperm packet.

Gifts can be costly. The salivary mass that a male scorpionfly secretes to lure in a female is packed with so much protein and nutrients that a less-robust suitor may resort to offering a female a dead insect instead. Unlike a spitball, that has the benefit of being reclaimable, at least in part, after mating is through.

In the semiaquatic Zeus beetle, the female is about twice the size of the male. Not only does she allow a male to piggyback on top of her for weeks at a time, she also has a depression on her back seemingly designed to accommodate him while he feeds on rich wax that she secretes for his convenience. Scientists suspect this behavior evolved to prevent more onerous problems, like constant male harassment.

Jingzhi Tan and Brian Hare of Duke University in North Carolina showed that bonobos may be alone among apes in preferring to share food with strangers over friends and family. When wildborn bonobos were placed in a room and supplied with slices of bananas, and given the choice of opening a gate to admit either a familiar or an unknown bonobo, the provisioned bonobo would lift the latch of the stranger’s enclosure and then push food in that ape’s direction.

The researchers propose that bonobos use food gifts to expand their social network, which could enhance their own odds of survival and reproductive success.

But bonobo xenophilia has its limits. In experiments where the bonobos could allow strangers into a food room but could not interact with them physically, they didn’t bother opening the door. What good does it do me if you don’t know what a good friend I can be?

中譯

我們會在耶誕節期間展現慷慨，其實這種交換禮物的意願由來已久，不但跨文化，而且不限於人類。科學家在包括昆蟲、蜘蛛、軟體動物、鳥與哺乳類在內的動物界發現驚人的實例。

這種贈與有不少可稱為結婚禮物。它是動物為求偶而費力討價時提供的服務，用以提高自己傳宗接代的機率。

結婚禮物使科學家得以準確量化供體動物在交配與繁殖過程中的投資，同時分析求偶禮物袋的化學成分，以追蹤性競爭及共謀的微妙之處。

波士頓郊區塔虎脫大學生物學教授莎拉‧路易士曾經廣泛撰文解釋結婚禮物。她說：「這是我們剛開始探討的非常酷又非常重要的性選擇話題。結婚禮物的光明面是，雄性動物可透過某些東西達到求偶與繁衍後代的目的。」

她說：「另一方面，禮物可能也是性衝突的根源，是雄性指使雌性遂其所欲為的工具。」

另有科學家研究動物如何把禮物當成社交工具、建立聯盟關係或討好群體中的支配者。哺乳類的理毛行為被視為送禮。

路易士說，以定義言，禮物應該是一方甘願給，「對方卻未必甘願受」。

她說，禮物可能不受歡迎的實例之一是蝸牛的愛之箭。花園的陸上蝸牛雌雄同體，會產生精與卵，並以與其他蝸牛交換精子的方式交配。因此蝸牛在交配之前，會嘗試以在其生殖器區域製造的碳酸鈣魚叉式結構體刺入夥伴體內。

這支箭可能刺入對方任何部位，釋出有效荷爾蒙作為「禮物」，相對於其他競爭的蝸牛而言，有助於保留自己的精子。被刺入的蝸牛不甘淪為受害者，同樣釋出調情的小鋼矛。兩個射出愛之箭的邱比特此時開始交配。

然而多數情況下，雄性的禮物是雌性需要的。愛德華森是澳洲國立大學的演化生物學家，專門研究危害豆類與穀類作物的小型斑點甲蟲紋豆象。牠們很難取得水，雌蟲製造卵時會非常渴。

雄蟲像豆莢內的幼蟲般封存遇到的多數液體，同時把它加入射出的精子中。如果極渴的雌蟲央求，雄蟲會把預存的水與精子包一起射入對方體內。

禮物成本可能很高。雄蠍蛉為吸引異性所分泌的唾液團塊包覆許多蛋白質與營養物，可能導致另一隻不那麼強健的求偶者向雌蟲獻上死昆蟲。不同於紙團，交配結束後，這至少可以局部回收。

半水棲性的雌性宙斯甲蟲體型是雄蟲的兩倍。雌蟲不僅允許雄蟲一次伏在它背上數星期，背部還有一個凹處，似乎用於容納雄蟲，讓雄蟲食用雌蟲為它分泌的蠟時更方便。科學家認為，這種演化來的行為可防止雄蟲不斷騷擾之類更大的麻煩。

北卡羅來納州杜克大學的譯景志（譯音）與哈爾發現，倭黑猩猩可能是唯一樂意與陌生倭黑猩猩分享食物甚於親友的黑猩猩。他們把野生倭黑猩猩帶到一個房間，給牠們香蕉片，並使牠們可以開門讓認識或陌生的倭黑猩猩進入，結果發現，獲得香蕉片的倭黑猩猩會拉起陌生倭黑猩猩的圍籠門閂，把食物往對方方向推。

科學家認為，倭黑猩猩可能透過食物贈予擴大社交圈，進而提高生存與繁殖成功的機率。

然而倭黑猩猩對陌生事物的喜愛有其限度。在倭黑猩猩可讓陌生倭黑猩猩進入放有食物的房間，卻無法與對方實質互動的實驗中，牠們可懶得開門。如果你不知道我會是多好的一個朋友，這對我有何好處？