造成生物滅絕的新原因：性擇
某些雄性用來吸引伴侶的「長處」短期來說是值得的。但根據史密森尼國家自然歷史博物館的新研究，科學家表示在繁殖方面過度投入資源也會付出代價。
由國家自然歷史博物館的古生物學家Gene
Hunt領導的科學家團隊，在分析了數以千計的古代甲殼類化石之後，發現投注大量能量在求偶競賽或許會損害物種應對環境變化時的彈性，而提高牠們滅亡的風險。Hunt和同所博物館的博士後研究員M.
João Fernandes Martins，以及威廉與瑪麗學院和南密西西比大學的合作研究人員，於4月11日將他們的發現發表在期刊《自然》(Nature)之中。
現今許多物種可以做為雌雄異型(sexual
dimorphism)的明顯例子――也就是雄性和雌性具有不同的特徵，通常是雄性用來吸引並保衛伴侶的工具。將基因傳遞給未來世代的需求是如此強大，以致於某些物種的雄性會挹注大量精力來產生這些專門用在性方面的外貌特徵，像是孔雀華麗的尾羽、象鼻海豹用來保衛領地的巨大身形。
Hunt表示過往有科學家提出動物為了發展這類特色而付出的能量，可能會限制牠們在存活方面所能取得的資源多寡，尤其是當牠們所處的環境出現某些變動的時候。結果可能造成雌雄異型相當明顯的物種也有更高的風險滅亡。但是要測量雌雄異型對物種存活能力的影響卻相當困難，因為在絕大部分的案例之中，科學家並無法分辨一具動物化石到底是雄性還是雌性。
介形蟲是一種例外。這類生物已經在地球上存活了將近5億年。牠們的外觀狀似蝦子，生活在比芥子還小的雙瓣殼之中。介形蟲的特色在於其雌雄分明的外型可以保存在化石紀錄當中。身體和外殼比較長的為雄性，而雌性通常較小，外型也比較矮胖。Hunt表示雄性延長的外殼是為了容納巨大的性器官。科學家推測該器官越大則產生的精子就越多，並且可以增加個體成功繁衍下一代的機會。
Hunt和他的同事利用國家自然歷史博物館、南密西西比大學和路易斯安那州立大學收藏的大量介形蟲化石，以及他們自己採集的其他化石，來探討雄性和雌性區別最為明顯的種類，是否比其他種類更容易受到環境變化影響。
團隊仔細審視數千具介形蟲樣品，根據採集到這些樣品的地層年代來標記出不同種類的生存時間，並且評定超過6000具個體的形狀和大小。他們最後的分析中包括了93種介形蟲，牠們生存的年代為8500萬年前至6500萬年前的白堊紀晚期。
某些種類在化石群集中一再地出現，團隊發現牠們幾乎活過了這整整2000萬年的時間，其他種類則只活了幾十萬年。而雄性的大小和形狀看起來對物種的壽命來說確實是一個重要因子。「我們發現當雄性比雌性更大且更長時，這些物種在化石紀錄中的出現時間也較不長久。牠們滅亡的風險更高。」
Hunt表示如果這也可以適用於其他的物種，那麼保育生物學者在評估某個物種有多容易被現今的環境危害影響時，或許可以試著把兩性異型納入考量當中。「如果過去的物種曾因為投入太多能量在繁殖方面，使得牠們難以適應變動的環境，也許今日我們在規劃物種的保育工作時也能用上同樣的原則。」
New Driver of Extinction：Sexual Selection
The lengths
that some males go to attract a mate can pay off in the short term. But
according to a new study from scientists at the Smithsonian’s National Museum
of Natural History, extravagant investments in reproduction also have their
costs.
By analyzing the fossils of thousands of ancient
crustaceans, a team of scientists led by National Museum of Natural History
paleontologist Gene Hunt has found that devoting a lot of energy to the
competition for mates may compromise species’ resilience to change and increase
their risk of extinction. Hunt, the museum’s postdoctoral fellow M. João
Fernandes Martins and collaborators at William & Mary and the University of
Southern Mississippi reported their findings April 11 in the journal Nature.
Many present-day species offer showy examples of
sexual dimorphism—characteristic differences between males and females that
usually help males attract and secure mates. The need to pass genes to future
generations is so critical that in some species, males put a lot of energy into
generating these sex-specific features—a peacock’s elaborate tail feathers or
an elephant seal’s massive, territory-defending body size, for example.
Hunt says some scientists have proposed that the
energy animals devote to developing these traits may limit the resources they
have available for survival, particularly when something in their environment
changes. That would put species with strong sexual dimorphism at greater risk
of extinction. But it has been difficult to determine how sexual dimorphism has
impacted species survival because in most cases, scientists cannot tell from a
fossil whether an animal was male or female.
Ostracods, tiny crustaceans that have been on the
planet for nearly 500 million years, are an exception. The shrimp-like
creatures, which live inside hinged shells smaller than a poppy seed, have
distinctive sex-specific shapes that are preserved in the fossil record.
Animals with elongated bodies and shells are males, whereas females are usually
smaller with a squatter shape. An elongated shell shape accommodates the male’s
large sex organs—with larger organs presumably producing more sperm and improving
individuals’ opportunities for reproductive success, Hunt said.
Hunt and his colleagues drew on large collections of
ostracod fossils from the National Museum of Natural History, the University of
Southern Mississippi and Louisiana State University, as well as additional
fossils they collected themselves, to investigate whether species in which this
male/female distinction was most prominent had been more vulnerable than others
to changes in their environments.
The team pored over thousands of the specimens,
noting when different species had lived based on the geological layer from
which they were collected and assessing the shapes and sizes of more than 6,000
individuals. Their final analysis included 93 different species of ostracods
that lived during the late Cretaceous period, between about 85 and 65 million
years ago.
Some species appeared over and over again in the
fossil collections, and the team found that some had lived throughout almost
the entire 20-million-year span. Others lasted just a few hundred thousand
years. And the size and shape of the males did appear to have been a
significant factor in species’ longevity. “We showed that when males are larger
and more elongated than the females, those species tend to not last as long in
the fossil record. They have a higher risk of extinction,” Hunt said.
If the same holds true for other species, Hunt said,
conservation biologists may want to take sexual dimorphism into account when
assessing species’ vulnerability to current environmental threats. “If devoting
so much energy to reproduction made it harder for species in the past to adapt
to changing circumstances, perhaps that same should apply to species we’re
concerned about conserving in the present day.”
原始論文：Maria João
Fernandes Martins, T. Markham Puckett, Rowan Lockwood, John P. Swaddle &
Gene Hunt. High male sexual investment as a driver of extinction in
fossil ostracods. Nature, 2018 DOI: 10.1038/s41586-018-0020-7
引用自：Smithsonian. "New driver of extinction:
Adaptations for sexual selection: Museum collections help scientists understand
the role of sexual selection in extinction."
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