Crocodiles, Alligators May Lure Prey with Sticks
December 02, 2013
Egrets and other birds building nests near ponds that harbor crocodiles or alligators should be suspicious of sticks they see floating in the water. That twig or branch may be bait for a trap set by the reptiles, as new research suggests they may have joined the few species that use tools to lure prey.
According to Valdimir Dinets, a zoologist known for his studies of crocodilian (crocodiles and alligators) behavior, two species of the large reptiles have been observed using twigs and sticks as bird lures.
“At least one of them uses this method predominantly during the nest-building season of its prey,” he writes in a paper published in the journal Ethology Ecology & Evolution. “This is the first known case of a predator not just using objects as lures, but also taking into account the seasonality of prey behavior. It provides a surprising insight into previously unrecognized complexity of archosaurian behavior.”
According to the study, the use of objects as hunting lures is very rare among animals, only seen in captive capuchin monkeys, a few bird species and one insect.
Writing in the study, Dinets states that “it is common for some bird species to preferentially nest in trees growing in ponds with large numbers of crocodiles or alligators, apparently using the crocodilians as protection against tree-climbing nest predators such as snakes, monkeys and raccoons.”
But, he says, the birds have to “pay” for the protection because their chicks can sometimes fall into the water where they are usually devoured by crocodilians.
“But the protection seems to be worth the cost,” Dinets writes. “Almost any crocodile farm or alligator park with appropriate trees will sooner or later become the location of an egret rookery.”
Dinets says he repeatedly saw crocodilians laying in shallow water with small sticks or twigs across their snouts around rookeries.
“The crocodiles remained perfectly still for hours, and if they did move to change position, they did it in such a way that the sticks remained balanced on their snouts,” he writes.
Dinets says the predators get the sticks onto their snouts by submerging under them.
“Then it takes some balancing act to keep the sticks in place,” he said in an email.
Observing alligators at two egret nesting sites for a year, Dinets saw stick displaying mostly during the bird’s breeding season, between March and June and more frequently during nest building, from late March and April.
Dinets noted that the increase in the behavior could be explained by higher amounts of sticks in the water, either because of nest building or trees shedding. He thinks, however that that explanation “seems unlikely.”
“Virtually no freely floating sticks or twigs were seen by the observer at that time, and none are visible in photographs of rookery ponds made at that time,” writes Dinets. “Any available sticks were probably quickly picked up by birds looking for nest material.”
Also, he notes that the most common trees around the waters don’t often shed branches.
Dinets says it’s “unknown” what factors lead to the stick-displaying behavior at particular locations and at certain times of the year.
“The predators might be reacting to the presence of large numbers of wading birds flying low over water, to the sounds made by courting birds, or to some other environmental clue,” he writes.
He also doesn’t know if the behavior is a learned behavior or an evolved instinct.
While Dinets says crocodilians have historically been viewed as “lethargic, stupid and boring,” the new research adds to the complex behavior already known such as signaling, advanced parental care and “highly coordinated group hunting tactics.”
“These discoveries are interesting not just because they show how easy it is to underestimate the intelligence of even relatively familiar animals, but also because crocodilians are a sister taxon of dinosaurs and flying reptiles,” he concludes.
harbor (v) 庇護
suspicious (a) 猜疑的,疑心的XXXXX*後常接 “to”：be suspicious of something 表示「對. . .起疑」
prey (n) (肉食性動物的)獵物XXXXX*本字多當不可數名詞用
築巢在有鱷魚的水塘邊的大白鷺 (egret) 和其它鳥類得小心水面漂浮的樹枝了。研究發現，水面上的小樹枝很可能是這個爬蟲類用來設陷阱的誘餌 (bait)，這表示鱷魚也屬於那些少數會用工具來誘捕獵物 (prey) 的動物了。(編按：crocodile 跟 alligator 都是鱷魚，但可以由嘴部形狀和棲息地做區別。crocodile 的嘴部較尖，可出沒於淡水和鹹水域；alligator 的嘴部較寬扁，因此另有「短吻鱷」之名，出沒於淡水。文中還出有一個字crocodilian，是"鱷魚類"的意思，也可當形容詞用。)
predominantly (adv) 顯著地
predator (n) 肉食動物
captive (a) 被俘的；受監禁的
一位有名的、研究鱷魚行為的動物學家 (zoologist) 發表文章：「鱷魚很明顯 (predominantly) 會在獵物的築巢季使用這個方法。這是肉食性動物在使用誘餌捕獵時，還會考量 (take . . . into account) 獵物行為季節性的首次發現 (the first known case of . . .)，這令人驚喜的瞭解到之前我們所不知道的主龍類 (archosaurian) 行為的複雜度。」(編按：archosaurian 包含了鱷魚和鳥類，可看 wikipedia說明)
根據這個研究的說法，使用物品來當誘餌捕獵的行為是很罕見的，過去只在被圈養 (captive) 的捲尾猴 (capuchin monkey)，一些鳥類和一種昆蟲上發現過。
devour (v) 大口的吃
rookery (n) (鳥類) 群棲地;繁殖處
snout (n) (動物的)口鼻部
submerge (v) 把. . .浸入水中，淹沒
研究人員表示：「有一些鳥類偏愛 (preferentially) 在有大量鱷魚出沒的水域旁的樹上築巢，明顯是把鱷魚 (crocodilian) 當成護身符，用來防護像是蛇、猴子、浣熊 (raccoon ) 這些會爬樹的鳥巢掠食者 (nest predator)。」但這些鳥類也須為此付出代價，因為雛鳥 (chick) 有可能會掉倒水裡，被鱷魚吞食 (devour)。「但這似乎是值得付出的代價 (worth the cost)，幾乎任何鱷魚場旁的樹，遲早都會成為白鷺的棲息與繁殖地 (rookery)。」
wade (v) 涉水而行
court (v) 向. . .求愛
lethargic (a) 笨重的，瞌睡的
研究人員不斷看到鱷魚把小樹枝橫架在口鼻上 (snout)，在有鳥巢聚集的淺水區躺著。但鱷魚在特定的位置和時間做出這樣行為的因素還不明確，也許是因為出現了大量於水面掠過的水鳥 (wading birds) 的足跡，或是求偶中 (courting) 鳥類所發出的聲音，也或許是其它的環境線索讓鱷魚做出如此反應。研究人員也還不知道這是學習來的行為，還是演化來的本能 (evolved instinct)。
雖然鱷魚向來被視為是笨重 (lethargic) 、無趣的動物，但其實鱷魚有使用訊號的能力，有高等的親代照顧 (parental care) 表現，也展現具高度協調性的 (coordinated) 群體補獵術 (hunting tactic)，這項最新研究更增添了我們對其行為的複雜度的了解。
Check your vocabulary!
Fill in the blanks with a word or phrase from the list above. Make necessary changes. After you finish, select the text below to reveal the hidden answers.
1. The submarine submerged when enemy planes were sighted.
2. After the tennis match the boys devoured the sandwiches in seconds.
3. His son had been taken captive during the raid.
4. She was always suspicious of strangers.
5. The river was full but we managed to wade across.
6. Police believe someone must be harboring the killer.