reading dialy 6

Shh! Quiet Bison Get More Sex

UC Davis geography graduate student Megan Wyman measures the amplitude of a bison's bellow in Nebraska's Fort Niobrara National Wildlife Refuge. The quieter bulls mate more. Credit: Paul Haverkamp/UC Davis

Photo is of a bison and a calf in Yellowstone National Park. Credit: Julie Larsen Maher/Wildlife Conservation Society

American bison block a lane of traffic in Yellowstone National Park. Credit: AP Photo/Robert F. Bukaty.

American bison graze in Wind Cave National Park in South Dakota. Credit: Kathryn Hoppe
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Bison with the loudest bellows lose out in the mating game, while the quietest bulls score the most mates and sire the most offspring.
"We were expecting to find that the bigger, stronger guys — the high-quality males — would have the loudest bellows, because they can handle the costs of it," said Megan Wyman, a graduate student in geography at University of California, Davis and the lead author of the study. "But instead, we found the opposite.
Wyman and colleagues also found that the volume of a bison bull's bellow was not related to its weight or age.
American bison are not buffalo. But these icons of America are prone to homosexual behavior. More than 55 percent of mounting in young males is with the same gender. Meanwhile, they eventually do get
the job of procreation done during the annual rut.
Wyman and Michael S. Mooring of Point Loma Nazarene University and a number of student interns spent two summers monitoring the mountings and other activity of 325 wild bison in Fort Niobrara National Wildlife Refuge in the Sandhills region of north-central Nebraska.
Quiet dominance
The reason a soft touch works is not clear, but it could be the bulls want to keep it down so other males don't horn in on their activity.
Bison bellows are loud, low-frequency vocalizations performed by bulls during the rut. They are most commonly used when one male challenges another, typically when the two are within 45 to 90 feet of one another. Yet sometimes a bellow will attract bulls from further away, and this may be one reason that a herd's dominant bulls keep their voices down, Wyman speculates.
"It could be that bulls provide information about their high quality through other signals — for example, the frequency or the duration of their bellows. So they don't have to be louder, they just have to be heard," she said. "If you bellow too loudly, it could bring in too many other bison to check you out."
Wyman wonders why lower-quality males don't turn down the volume of their bellows to emulate their more successful rivals.
"That's a lot harder to explain," she said. "It could be that if you use a quieter volume, other bulls have to approach even closer to check you out, and any time you bring someone that close, there's a higher risk of attack. And that's the type of cost that these low-ranking bulls may not be able to bear."
Watching the rut
Bison once numbered in the tens of millions across the continent, but they were wiped out by commercial hunting and habitat loss. By 1889, fewer than 1,100 individuals remained. Some 500,000 exist today, but only about 20,000 are wild; one conservation group says they could be poised for a big comeback.
Observing the herd for 14 hours each day during the two-month rut of July and August, the researchers recorded each copulation and to detail the tangled web of connections between males and females as bulls lost and gained cows during their intense competitions. To assess where each bull ranked in the herd's hierarchy of dominance, Wyman tallied outcomes of challenges between rivals, including threats that ended with an animal backing down in the face of combat, as well as full-blown, head-to-head fights.
When calves were born the following spring, DNA samples were taken to determine parentage.
For measurements of amplitude, Wyman used a hand-held sound-level meter from the safety of her vehicle. With each reading, she also recorded specific behaviors of the bull, his female and any challenging rivals, as well as noting the factors that could affect the level of the reading such as the bull's head orientation, its distance from the meter and wind conditions.
Her analysis showed that, on average, the least successful bulls — those with the lowest number of copulations and offspring — bellowed at least 50 percent louder than their more successful rivals, corresponding to decibel readings averaging from 109 per bull down to 103. This drop in volume correlated with a rise in the number of times a bull copulated from none to five, and the number of calves it sired from none to nine.

＊アドレス＊
http://www.livescience.com/animals/081216-bison-mating.html

＊要約＊
一般的にどの生き物も大きく強く自分をアピールしたほうが子孫を残しやすいと考えられていますが、アメリカバイソンの群れの行動を研究した結果、鳴き声が大きいオスほど喧嘩に弱くメスにもモテないことが判明した。
研究はカリフォルニア大学デービス校ミーガン・ワイマンさんによって行われた。ミーガンさんはまずアメリカバイソンの発情期の2ヶ月間群れを追跡しオスの鳴き声を手持ちマイクで採集、そのオスが何匹のメスと交尾したか、何匹の子どもが生まれたか、メスの取り合いの勝敗はどうだったのかを記録し、鳴き声の大きさと交尾の回数の関係を調べた。その結果、交尾の回数が多かったオスの鳴き声は最も少なかったオスに比べて3分の2しかなく、体重や年齢と鳴き声の大きさにも関係が見られなかった。おそらくオスは自分の強さを声の大きさではなく周波数や鳴く時間の長短で伝えていると考えられるため、大きな声を出す必要がなく小さい声で十分らしい。また、小さな声のほうが他のオスに交尾の邪魔をされることが少なくなって有利になる。

＊感想＊
人間もうるさいのはうざいので、みんなにこれを広めて、あまりはしゃぎすぎないように気をつけてもらえたらいいと思いました。

＊単語＊
bellows……………ほえる
dominant…………主要な
bulls………………雄牛
tangled……………もつれる
assess……………評価する
threats……………脅迫・きざし
calves……………子供を生む
measurements…測定・寸法
amplitude………十分