Animals Apart


By Dana M. Hawley /Julia C. Buck D. M. ホーリー /J. C. バック
English 日本語 日本語
On a shallow reef in the Florida Keys, a young Caribbean spiny lobster returns from a night of foraging for tasty mollusks and enters its narrow den. Lobsters usually share these rocky crevices, and tonight a new one has wandered in. Something about the newcomer is not right, though. Chemicals in its urine smell different. These substances are produced when a lobster is infected with a contagious virus called Panulirus argus virus 1, and the healthy returning lobster seems alarmed. As hard as it is to find a den like this one, protected from predators, the young animal backs out, into open waters and away from the deadly virus.  フロリダ半島南端沖,フロリダキーズ近くの浅い岩礁で,1匹の若いアメリカイセエビがおいしい貝を探しに出た夜の狩りを終え,岩の割れ目にある狭い巣に戻ってきた。イセエビはこうした巣を他の仲間と共有しているのが普通で,今夜は新顔が1匹入り込んでいた。
The lobster’s response to disease—seen in both field and laboratory experiments—is one we have become all too familiar with this year: social distancing. People’s close interactions with family and friends have been cut off to reduce the spread of COVID-19. It has been extremely hard. And many have questioned the necessity. Yet despite how unnatural it may feel to us, social distancing is very much a part of the natural world. In addition to lobsters, animals as diverse as monkeys, fishes, insects and birds detect and distance themselves from sick members of their species.  病気へのイセエビのこの反応(自然環境でも実験室でも確認されている)は,私たち全員が今年になって身をもって知った「ソーシャル・ディスタンシング」だ。新型コロナウイルス感染症COVID-19の拡大を抑えるため,家族や友人と間近で接触する行為が断たれた。これは非常につらく,多くの人はそんな必要があるのかと疑問に思った。だがソーシャル・ディスタンシングは,私たち人間にとっては実に不自然に感じられるものの,自然界では当然の行いだ。イセエビに加え,サルや魚,昆虫,鳥など様々な動物が病気の仲間を検知し,距離を置く。
This kind of behavior is common because it helps social animals survive. Although living in groups makes it easier for animals to capture prey, stay warm and avoid predators, it also leads to outbreaks of contagious diseases. (Just ask any human parent with a child in day care.) This heightened risk has favored the evolution of behaviors that help animals avoid infection. Animals that social distance during an outbreak are the ones most likely to stay alive. That, in turn, increases their chances to produce offspring that also practice social distancing when confronted with disease. These actions are what disease ecologists such as ourselves term “behavioral immunity.” Wild animals do not have vaccines, but they can prevent disease by how they live and act.  この種の行動が広く見られるのは,それが社会性動物の生き残りに役立つからだ。動物は集団で生活すれば獲物を捕らえやすく,暖かな状態が保たれ,捕食者を避けることも容易になるが,一方では感染症のアウトブレイクにもつながる(保育園に通う子供を持つ親なら先刻ご承知のはず)。そうした危険があるため,動物には感染症を避けるのに役立つ行動が進化した。アウトブレイクの最中に社会的距離を取った動物は生き延びる可能性が大きい。つまり子孫を残す機会が増え,病気に直面した際にソーシャル・ディスタンシングを実行する形質がそれらの子孫に引き継がれていく。
Immunity through behavior does come with costs, though. Social distancing from other members of your species, even temporarily, means missing out on the numerous benefits that favored social living in the first place. For this reason, researchers have learned that complete shunning is just one approach animals take. Some social species stay together when members are infected but change certain grooming interactions, for example, whereas others, such as ants, limit encounters between individuals that play particular roles in the colony, all to lower the risk of infection.  だが,行動を通じたこの防疫は代償を伴う。自分と同じ生物種のメンバーから社会的距離を置くと,それが一時的なものであっても,社会的生活をそもそも有利にしていた数多くの利点を失うことになる。