Why Some Animals Evolved to Sacrifice Themselves動物為何舍身繁衍

賈森·比特爾/文　陳曉璐/譯

A male musk ox can weigh up to 800 pounds and charge at speeds over 30 miles per hour. During the breeding season， these shaggy1， Arctic Circle behemoths crash into each other face-first and then stab their opponents with their huge， sharp horns.

一頭雄性麝牛體重可達800磅，奔跑速度超過30英里/小時。在繁殖季節，這些體毛粗長的北極圈巨獸會臉對臉地猛撞在一起，然后用它們巨大而鋒利的角刺向對手。

Whats more， over the course of their 10- to 12-year life-spans， a single male musk ox might rack up2 something like 2，100 hits to the head.

更夸張的是，在它們10到12年的壽命中，一頭雄性麝牛的頭部可能會受到2100次撞擊。

“People have always just assumed that animals that headbutt3， like musk oxen and bighorn sheep， are somehow immune to head injuries，” says Nicole Ackermans， a neuroscientist at the Icahn School of Medicine at Mount Sinai in New York. “Like they have magical horns， or something.”

紐約芒特西奈伊坎醫學院的神經科學家妮科爾·阿克曼斯說：“人們總是想當然地認為，像麝牛和大角羊這樣用頭撞擊的動物，其頭部怎么也不容易受傷，就好像它們的角有魔力似的?！?/p>

Ackermans and her colleagues acquired the brains of musk oxen and bighorn sheep from a combination of field expeditions， donations from subsistence hunters， and captive research herds.

阿克曼斯和她的同事們通過實地考察、自給獵人捐贈和研究用圈養獸群，獲得了麝牛和大角羊的大腦。

“We found a specific pattern in all of our specimens that looked really similar to early chronic brain trauma in a human，” says Ackermans， who led a recent paper on the findings， published in the journal Acta Neuropathologica.

阿克曼斯說：“我們在所有樣本中發現了一種明確的規律，看起來與人類早期慢性腦外傷非常相似。”她最近在《神經病理學學報》上發表了一篇關于這些調研結果的論文。

Its evidence that evolution can lead a species down surprisingly self-destructive paths. And in this regard， the musk oxen are far from alone.

這證明了進化可以引導某類物種步入令人驚訝的自我毀滅之路。在這方面，麝牛絕不是唯一的案例。

‘Just dont die

“不死即可”

For their research， Ackermans and her colleagues stained the brains of three musk oxen and four bighorn sheep with biomarkers. These chemicals can illuminate patterns of traumatic brain injury commonly associated with human conditions， such as Alzheimers disease and chronic traumatic encephal-opathy， or CTE. In this case， the scientists were searching specifically for something known as the tau protein.

在研究中，阿克曼斯和她的同事們用生物標志物對三只麝牛和四只大角羊的大腦進行染色。這些化學物質可以標亮人類常發生的創傷性腦損傷模式，如阿爾茨海默病和慢性創傷性腦病。就這項研究而言，科學家們專門尋找一種被稱為微管相關蛋白的東西。

“When your neurons get damaged， whether it be through aging， genetic problems， or mechanical impact， they get ripped， and this protein breaks down and forms into clumps，” Ackermans explains. “And if you see them in specific patterns， thats how you can tell whether its just a normal brain， or aging， or Alzheimers， or potentially trauma.”

阿克曼斯解釋說：“無論是由于衰老、遺傳問題還是機械沖擊，只要你的神經元受損，它們都會被撕裂，然后這種微管相關蛋白就會分解并形成團塊。如果你看到這些蛋白以特定的模式出現，你就能判斷出一個大腦當前的情況，是正常的還是老化的、是患有阿爾茨海默病還是受到潛在創傷的。”

Unfortunately， the biomarker method didnt work as well on the sheep brains， though they did show signs of tau buildup.

不幸的是，盡管生物標志物確實顯示出微管相關蛋白積聚的跡象，但這一方法在大角羊的大腦中并不奏效。

“Each year， a musk ox will headbutt many times， but if they reproduce even once successfully， thats kind of all you need，” she says. “The thing thats encouraged， evolutionarily， is just dont die.”

她說：“一頭麝牛每年會撞頭多次，但它們只要成功繁殖一次，就足夠了。從進化的角度來看，它們信奉的是不死就行?！?/p>

It probably helps that male musk oxen live less than 15 years and females live 15 to 23 years， she says. So even if tau proteins are building up throughout the animals lives， they may never accumulate to a point where they can cause conditions such as Alzheimers or other dementias.

她說，雄性麝牛的壽命不到15年，而雌性麝牛的壽命為15至23年，這大概算是幸事。這樣一來，即使微管相關蛋白在它們的一生中不斷積累，其累積量也可能永遠不會多到導致阿爾茨海默病或其他癡呆癥的程度。

And even if they did develop those conditions， who would know？ “Theres no behavioral scale for musk ox. So we cant say that theyre not feeling a little forgetful，” she says.

或者即使這些動物真的患上了這些疾病，誰又會知道呢？她說：“我們沒有為麝牛制定行為量表，所以無法斷定它們沒有感到有些健忘。”

Mating to death

交配至死

Small and carnivorous， antechinuses are a genus4 native to mainland Australia and Tasmania. In recent years， theyve made headlines for the way the males practice semelparity—or a single burst of reproductive output， followed by programmed death. Antechinus females can live two to three years or more. But the males rarely last more than 11 months.

小型食肉動物袋鼩是澳大利亞大陸和塔斯馬尼亞的本土物種。近年來，雄性袋鼩的單次繁殖（即繁殖一次，然后按預設過程死亡）屢屢見諸報端。雌性袋鼩可以活兩到三年或更長時間，但雄性很少能活過11個月。

“They have a very frantic mating season，” says Fisher. Breeding bouts5 can last 12 to 14 hours， and afterward each male will try to mate with as many other females as he can—which leads to his demise.

費舍爾說：“它們會經歷一個非常瘋狂的交配季。”單次交配可持續12至14個小時，而且每只雄性都會設法與盡可能多的雌性交配——這導致了它的死亡。

Self-destructive insects

自我毀滅的昆蟲

In large， social insect colonies， a similar but slightly different dynamic plays out.

在大型的群居昆蟲群體中，類似但略有不同的模式也在上演。

When a European honeybee stings a soft-skinned attacker， such as a bear， it dies when its stinger6 lodges7 in the victims skin. An exploding ant may rip its abdomen in two while defending its nest from attackers. And in some termite species， elderly workers can turn themselves into suicide bombers.

歐洲蜜蜂蜇傷皮膚柔軟的攻擊者（比如熊）時，會因為它的刺卡入受害者的皮膚而死亡。爆炸螞蟻在保護自己的巢穴免遭攻擊時，可能會把自己的腹部裂成兩半。在某些白蟻種類中，老年工蟻會把自己變成自殺式炸彈。

But how does killing oneself make sense， evolutionarily speaking？

但從進化的角度來看，自殺有什么意義呢？

“Easy，” says Thomas Seeley， a biologist at Cornell University and author of The Lives of Bees， in an email. “Workers achieve genetic （evolutionary） success not by reproducing themselves， but helping their mother， the colonys queen， do so.”

康奈爾大學的生物學家、《蜜蜂的生活》一書的作者托馬斯·西利在一封電子郵件中說：“很簡單，工蜂獲得遺傳（進化）上的成功不是通過自己繁殖，而是幫助它們的母親，也就是蜂群的蟻后繁殖?！?/p>

“One form of this help is colony defense，” he explains.

“這類幫助的一種形式是群體防御?！彼忉屨f。

“Some researchers call this a ‘superorganism，” says Alice Laciny， an entomologist who works on exploding ants at the Natural History Museum Vienna， in an email. “So an ant colony or bee hive is more like one large animal， with the queen representing the reproductive organs. The minor workers are numerous and only need small amounts of resources to rear， so they are similar to the bodys cells in a way.”

在維也納自然歷史博物館研究爆炸螞蟻的昆蟲學家愛麗絲·拉齊尼在電子郵件中說：“一些研究人員稱之為‘超級有機體。就是說，蟻群或蜂巢更像是一個大型動物，蟻后代表生殖器官。小工蟻數量眾多，只需要少量的資源來飼養，所以它們在某種程度上類似于身體的細胞。”

A mothers ultimate sacrifice

一個母親的終極犧牲

Another form of sacrifice in the animal kingdom is the lengths to which some mothers will go to give their young a fighting chance.

動物王國的另一種犧牲形式是一些母親為了給孩子一個生存的機會而犧牲自我。

After theyre born， some species of legless amphibians known as caecilians literally eat their mothers top layer of skin as a first meal. And African social spiders take this one step further， with some females allowing their young to practice matriphagy—or the killing and eating of their own mother.

一些被稱為蚓螈的無腿兩棲動物會在出生后，把母親最外層的皮膚當作第一餐。而非洲的群居蜘蛛則更進一步，它們的雌性蜘蛛允許其后代噬母——即殺死并吃掉自己的母親。

Giant Pacific octopuses may be the ultimate self-sacrificing mothers. Females can hold vigil over their eggs for an incredible four years—during which time they dont even eat.

太平洋巨型章魚可能是最終極的自我犧牲型母親。雌性可以守護它們的卵長達難以置信的四年——在此期間它們甚至不吃任何東西。

“Inevitably， the females deplete their entire body reserves and die while guarding the eggs，” says Fisher.

“不可避免的是，雌性會耗盡它們的整個身體儲備，在守護卵的過程中死去。”費舍爾說。

“You do feel sorry for them， but thats how a lot of species get the best offspring success in the next generation.”

“它們確實讓人感到難過，但這就是許多物種繁衍后代取得最大成功的途徑。”

1 shaggy（毛發）粗長而蓬亂的。? 2 rack sth up累積；聚集。? 3 headbutt用頭撞（某人的頭或臉）。

4 genus（動植物的）屬。

5 bout（某活動的）一段，一陣，一場。? 6 stinger（昆蟲或動物的）刺，蜇刺，蜇針。? 7 lodge（被）固定；卡住。