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寶貝，外面很熱！ 適應暖化 斑胸草雀唱「孵蛋歌」
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本報2016年8月24日綜合外電報導，姜唯編譯；蔡麗伶審校
澳洲一項驚人的研究發現，當氣溫升高，當地的斑胸草雀便會對著孵化中的蛋唱歌，而這樣的「提醒」，竟讓雛鳥發育出較小的身型，體重也較輕，顯示鳥兒因應氣候暖化，正在為後代作準備。
斑胸草雀。圖片來源：Jim Bendon（CC BY-SA 2.0）。
全球升溫 鳥兒還沒出生就收到「愛的提醒」
科學家蒐集斑胸草雀的蛋，在孵育期分組控制，播放已錄製好的「提醒」歌聲。結果顯示，相較於控制組，接收到提醒的幼雛，成長較緩慢並較能適應高溫的環境。研究發表於「科學」期刊。
「這只是在鳥兒目前經驗範圍內，並不代表依然能夠在極端氣候下孵育。」研究主要作者，澳洲迪肯大學的馬利耶特（Mylene Mariette）表示，「而這也是第一次顯示，唱歌給未出生的雛鳥聽，會產生如此長期的影響。」
「我們了解那會對認知與學習產生短期影響，但這是第一次顯示，這影響到成長和發育，並持續至成年。」馬利耶特解釋。
「這並不是一項因為天氣炎熱所產生的自發性的行為；而是發生在熱天，以及胚胎具聽力的時候。」馬利耶特說。
「寶貝！外面很熱」 斑胸草雀用歌聲增加後代生存勝算
為了解「提醒」的特殊用意，馬利耶特以及他的同事布坎南（Katherine Buchanan）讓斑胸草雀住在大學的戶外鳥社。
他們搜集鳥蛋，於孵化前最後五天，對其中一些蛋播放成鳥一般呼喚的歌聲，其他則播放準鳥爸媽特別的「提醒」聲。
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| 斑胸草雀的蛋。圖片來源：Mohammad Abdullah（CC BY-NC 2.0） | 斑胸草雀幼鳥。圖片來源：William Warby（CC BY 2.0） |
「我們發現，依據幼鳥是否聽見來自父母的提醒聲，會對高溫有不同的反應。」馬利耶特解釋。
「幼鳥會因溫度不同去調整其生長，以及跟父母尋求不同的食物。」尤其是「提醒」的影響，若鳥巢位於鳥舍中較熱的角落，似乎會讓雛鳥發育較慢，也較輕。
體重輕，對演化整體而言不是好消息。但研究人員追蹤長至成鳥的斑胸草雀，卻呈現相反的結果。在較熱鳥巢中成長的輕量鳥兒，明顯比那些較重的鳥兒生育出更多後代。
鳥爸媽「胎教」成功 傳遞未來環境訊息
此外，主要聽見「孵化提醒」的鳥蛋，未來會選擇較溫暖的地點築巢。
微妙的是，較小的身型在高溫中擁有更多好處，因為在高溫的成長環境下，可能導致較多的氧化壓力在組織上，對身體的耗費也較多。
「他們未必能思考，但身體擁有某些生理效應後會導致成長速度降低。」
總體而言，全球暖化造成野生鳥類數量漸減，對於相關品種的鳥類產生負面影響。英國雪菲爾大學研究鳥類繁衍的研究夥伴荷明斯（Nicola Hemmings）說，「鳥爸媽不但在雛鳥還未孵化時就教育他們，同時也給予他們未來孵化後的環境訊息。」
Zebra finch 'heat song' changes hatchling development
- 19 August 2016
- Science & Environment
CHRIS TZAROS
When the weather is hot, zebra finches in Australia sing to their eggs - and these "incubation calls" change the chicks' development, a study has found.
The surprising discovery suggests that the birds are preparing their offspring for warm conditions after they hatch.
Scientists collected eggs and incubated them in controlled conditions, playing recordings of the incubation song.
Compared to a control group, hatchlings that received these calls grew more slowly and coped better in the heat.
Writing in the journal Science, the researchers say this is the sort of adaptation that could help animals acclimatise to rising global temperatures.
"It doesn't mean that they will still be able to breed at extreme temperatures - this was within the range they currently experience," said the paper's lead author Mylene Mariette, from Deakin University in Geelong.
"But what's encouraging is that it's a strategy that the birds use to adjust the growth of their offspring to temperature, that we didn't know about."
It is also the first time that singing to unborn chicks has been shown to yield such long-term results.
Baby, it's hot outside
Previous studies have mostly concentrated on egg-bound embryos learning particular calls from their parents.
"We knew it could have some short-term effects on cognition and learning, but our study is the first to show that is has an effect on their growth and their development - and that those effects last until adulthood," Dr Mariette told BBC News.
"It means that the acoustic environment before birth has more impact than we thought."
ANDY TD BENNETT
She first noticed the rhythmic, high-pitched calls while making field recordings during her PhD.
"I was looking at how the parents talk to each other to coordinate parental care. That's when I noticed that sometimes when a parent was by itself in the nest, incubating the egg, it produced a quite different call.
"I wondered - hmm, are they talking to their eggs?"
When she started to study this squeaky serenade in detail, combing through Dr Mariette found that the birds were specifically singing it in the few days before eggs hatched - and only when the day's ambient temperature rose above about 26C.
"It's not that they do it spontaneously whenever it's hot; they do it when it's hot and when the embryos could potentially hear them," she said.
To figure out the specific effect of the calls, Dr Mariette and her colleague Katherine Buchanan used zebra finches living in the university's outdoor aviaries.
They collected eggs and, for the final five days of their incubation, played them either a recording of the birds' special egg-song, or a typical parent-to-parent call instead.
As soon as they hatched, the 175 chicks were returned to various nests and the team observed the youngsters' development in detail.
To begin with, the two groups were indistinguishable; the song hadn't changed the chicks' hatching weight.
MYLENE MARIETTE
Within a few days, however, that changed.
"We found that, depending on whether or not they had heard the 'hot call' from their parents, they reacted differently to heat," Dr Mariette explained.
"They adjusted their growth to temperature differently, and also solicited food from their parents differently."
Specifically, the "heat song" seemed to make the chicks develop slower - and remain lighter - if their nest was in a hot corner of the aviary.
Being a lightweight is generally bad news in evolutionary terms, but when the researchers followed the finches' fortunes into adulthood, it proved the opposite: those lighter birds that grew up in hot nests produced significantly more fledglings than their heavier childhood companions.
Information transfer
Furthermore, the chicks that were primed with a "heat song" in the egg went on to prefer warmer nesting spots.
"It's as if the parents are preparing the chicks for the temperature they will experience after hatching," said Dr Mariette.
It is a mystery, she added, how being smaller gives the birds an advantage in the heat. But it may be because growing in hot conditions is costly for the body, placing more oxidative stress on tissues - so staying small becomes a better strategy.
This could also help explain why, as global temperatures rise, wild bird populations in general are slowly shrinking - though mostly, that process has been seen to have negative consequences for the species involved.
CHRIS TZAROS
Whatever their evolutionary implications, the Deakin team's findings are an intriguing case of communication between parents and unborn offspring.
Nicola Hemmings, a research fellow studying bird reproduction at Sheffield University in the UK, said the experiments were elegantly designed and the results convincing.
"They've shown that not only can parents teach their offspring a call while they're still in the egg - but that they're giving them quite specific information about the environment they're about to hatch into," she told the BBC.
Just like the study's authors, Dr Hemmings said she was fascinated by what might be driving the changes in the chicks' development.
"They're not actually thinking, 'Mum says it's hot out there, I better take it slow when I get outside!' But it's having some kind of physiological effect on their body which is making their growth rate slow.
"Does it have a hormonal basis? Is it a stress response? That would be the interesting thing to look at next."
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