豪有袋類ノーザンクオール、執拗な性欲が雄の寿命短縮か

【パリＡＦＰ＝時事】オーストラリア北部に生息する愛らしい有袋類、ノーザンクオール（ヒメフクロネコの一種）の雄は、一生に１回しかない繁殖期が終わると死に至る。これまでなぜ死ぬのかは謎だったが、雌との交尾を執拗（しつよう）に追い求めるあまり休息が不足し、自らの命を縮めている可能性を指摘する研究結果が１日、英学術誌「ロイヤルソサエティー・オープンサイエンス」に発表された。（写真はオーストラリア北部に固有の有袋類、ノーザンクオール）
　研究チームは、北部特別地域（準州）沖のグルートアイランド島で雄７匹、雌６匹のノーザンクオールの追跡調査を行った。
　繁殖期を含む４２日間のデータを分析した結果、最長４回の繁殖期を生き延びる雌よりも、雄の方がはるかに活発なことが判明した。調査期間中、休んだり寝たりしていた時間の割合は雌が２４％近かったのに対し、雄はわずか７％だった。
　論文の主著者を務めた豪サンシャインコースト大学のジョシュア・ガシュク氏はＡＦＰに対し、ノーザンクオールの雄の睡眠時間はまったく足りていないように見えると指摘。繁殖後に雄が死亡する原因について「初めて決定的な証拠をつかんだかもしれない」と語った。

■激情の繁殖期

　ノーザンクオールの生存はすでに、オーストラリア大陸固有の生物ではないオオヒキガエルやネコ、キツネなどが持ち込まれたことで脅かされている。
　ガシュク氏は雄が毎年死ぬということは種そのものが絶滅しかねないと危惧する。ただ、「自殺的生殖」と呼ばれるこの極端な繁殖戦略を「何千年も続けて来た」からには、何らかのメリットがあるはずだとガシュク氏は言う。
　有袋類に詳しい豪クイーンズランド大学のエイドリアン・ブラッドリー 氏は、ノーザンクオールよりもさらに小さいアンテキヌスのような小型有袋類の「交尾衝動」が、「雌の排せつ腔（こう）腺から放出されるフェロモンへの抗し難い反応」であることは確かだと述べた。そうしたフクロネコ科の小型種は、繁殖期にかなり体重が減るという。
　ただし、より体の大きいノーザンクオールについてはさらなる研究が必要だと同氏は忠告している。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2023/02/02-13:45）

For male northern quolls, sex is a death sentence.
The cute marsupials native to northern Australia are the world's largest semelparous mammal, which means that the males drop dead after their first breeding season.
But what exactly causes them to die has remained a mystery.
Research published on Wednesday suggests that the males are depriving themselves of rest in their relentless pursuit to mate with females, potentially threatening the survival of their already endangered species.
Hoping to shed light on this sex-driven death frenzy, a team of researchers strapped tiny backpacks carrying tracking devices to seven male and six female northern quolls on Groote Eylandt, an island off the coast of Australia's Northern Territory.
The data, collected over 42 days that included breeding season, was entered into a machine learning algorithm which analysed different quoll behaviours.
The males were found to be far more active than females, who live for up to four breeding seasons.
And while females rested or laid around nearly 24 percent of the time, the proportion for males was just seven percent, according to a study published in the journal Royal Society Open Science.
The study's lead author, Joshua Gaschk of the University of the Sunshine Coast, told AFP that the males didn't seem to be sleeping anywhere near as much as they should.
For the first time, we might have a smoking gun for what is causing the males to die after breeding, he said.
Smaller relatives of the northern quoll, such as the antechinus, are also semelparous.
But research has found that their males die from internal haemorrhaging and infection due to escalating stress hormones during mating season -- which is not what is killing the northern quolls.
- 'Mating frenzy' -
Gaschk said the yearly male die-off could threaten the survival of the northern quoll, which has been badly affected by the introduction of non-native cane toads, cats and foxes to Australia.
But the carnivorous marsupials have been using this extreme breeding strategy, also known as suicidal reproduction, for thousands of years -- there's got to be a benefit to it, Gaschk said.
Indeed on Groote Eylandt, an island with no cane toads and few cats, the northern quolls are not just surviving, but doing really well, he said.
Adrian Bradley of the University of Queensland, who was not involved in the study, called the new research significant.
Bradley said he was quite certain that the mating frenzy of smaller semelparous marsupials like the antechinus is stimulated as an irresistible response to the release of perfume-like pheromones from the cloacal glands of females.
The amount of weight lost during these frenzies likely explains why only smaller members of the dasyuridae family are semelparous, he said.
But for the larger northern quoll, Bradley warned it was not yet possible to conclusively say why some males generally do not survive the breeding season, calling for further research.