 Nesting Bears(穴籠りの熊) |
A black bear in a den.
Say “bear den” and for many, the cartoonish image of a cave comes to mind, the entrance littered with bones. Boyd Porter and his colleagues spent the past four years studying black bear dens on Prince of Wales Island in southern Southeast Alaska, and for him the word that comes to mind is “nest.” “They’re nesters, although people wouldn’t necessarily put them on a list of nesting animals in Alaska,” said Porter, a state wildlife biologist based in Ketchikan. “They bring in nesting materials as a buffer between them and the cold ground.”
“熊の巣穴”と言え、大半の人は、その入口に骨が散らばった洞穴のような漫画のイメージを心に思い浮かべる。Boyd Porterと同僚はアラスカ南東部のプリンス・オブ・ウェールズ島で黒クマの巣穴を４年間研究してきた。そして、彼の心に浮かんだ言葉は「巣穴」だ。人々はアラスカの巣穴動物のリストに熊を上げないが、彼らは巣穴動物なのだ、とKetchikanをベースに活動している州の生物学者Porterは言う。
Evergreen boughs (hemlock and cedar) are the preferred material, although salmonberry stalks and other vegetation is also used. Porter described bears gathering mouthfuls of material and making multiple trips to the den site in preparation for winter hibernation. These winter bedrooms are not year-round homes for bears, but they are used year after year and represent an important resource for the animals. “It looks like they add to the nesting material each year, and some of the structures have been around a long time and could be pretty ancient, there is obvious historic use of many dens,” Porter said.
常緑の大枝（ドクニンジン、シダ）は熊のお好みの食料であるが、ストロベリーの茎と他の草木も使われる。Porterは「冬の冬眠の準備で、熊は口いっぱい食料を集め、巣穴に何度も出入りする」と述べている。これらの冬の寝床は年中使う住処でないが、毎年使っていて、動物にとって重要な資源であることを示している。「毎年、巣の材料を加えているようで、その構造物は長くズーッとあり、かなり古い物もある。そして、たくさんの巣穴には明らかに古い使用跡がある」とPorterは言った。
Porter and his crew handled 65 black bears and documented 52 bear dens. All the dens were associated with trees and “woody material,” hollow tree trunks, root wads and cavities. Prince of Wales is famous for its limestone caves, but none of the 65 bears Porter tracked hibernated in natural caves. “There was also very little use of excavated dens,” he said. “That’s different than the majority of dens in the Interior and in South-central Alaska, where the majority (for both black and brown bears) are excavated and carved into side hills.” Although the Prince of Wales bears were not digging dens, there was evidence that bears modified the entrance of tree dens, to make it easier to move in and out. “It looked like there were multiple years of use at many sites,” he said. “Like for example a 400 or 500-year-old cedar tree, which could’ve been used for a bear den for 200 or 300 years.” Some of the most remarkable “nests” really were up high in trees. Some of these “elevated dens” had nesting material that represented multiple trips by the bear, probably climbing the tree with mouthfuls of vegetation to make the winter bed.
Porterと彼のチームは65頭の熊を扱い、52の巣穴について調査した。全ての巣穴は木と木材、空っぽの木の幹、木の根の詰め物、洞が関連していた。プリンス・オブ・ウェールズ島は石灰岩の洞穴で有名だが、Porterが追跡した65頭のどれも自然の洞穴に冬眠していなかった。「掘り起こした巣穴の使用例も非常に少ない」と彼は言った。この点は、内陸部と南中央部（黒熊と灰色熊が､主に斜面に巣穴を掘る）の巣穴と違っている。プリンス・オブ・ウェールズの熊は巣穴を掘らないが、入ったり出たりをし易くするために、熊が木の穴を修正した証拠はあった。たくさんの場所で長年使っていたようだ、と彼は言う。例えば、樹齢400-500年シダの木が、200-300年の間、使われていなかったように。もっとも顕著な巣穴は木の高いところにある。そのような高所の巣穴のいつかには穴籠り用の材料があり、熊の出入りを意味していて、たぶん冬籠りの寝床を作るために口一杯に草木を摘めて、木を上ったのだろう。
Bears were initially captured in summer using foot snares and equipped with radio tracking collars that enabled the researchers to find them at all times of year including in winter. In one case, Porter had a strong signal coming from inside a nearby tree, with no obvious way into the tree. The trunk where the signal was coming from was 40 inches in diameter and very clean on the outside. It became apparent the tree was hollow and the hidden entrance was at the very top where the last 20 feet of the tree had sheared off, maybe during a winter storm. “The bear had climbed up the tree and then down into the deep cavity and was resting peacefully just above our heads,” Porter said.
熊は、まず足罠で夏に捕獲される。無線追跡の首輪が装着され、冬を含む年中､研究者が彼らを見つけることができるようになった。ある例では、Porterは近くの木の内部から強い信号を捕らえた。木の中への明らかな入口は見えなかった。信号が来る方向の幹は直径４０インチあり､外側は大変綺麗だった。木は空洞で、最後の２０フィートはもぎ取られていて（たぶん、冬の嵐の間に）、隠れた入り口は高いところにあった。熊は木に登り深い空洞に降りて、我々の頭上で静かに休んでいた、とPorterは言った。
A bear den at the base of a tree.
But how would a bear ever find such a den?
“Bears are good at finding those sorts of things,” Porter said. “It makes you wonder if during their daily routines and summer explorations they make a mental note of potential sites and come back later. They probably identify multiple sites, as we had some bears that moved from one den to another in the same winter.”
A wet or flooded den is a likely scenario for a bear relocating during mid-winter, but there may also be other factors that influence a mid-winter move.
Another time a strong collar signal indicated a bear was in a jumble of windfall trees, all lying horizontal in a big pile. “We were climbing under these windfall trees, thinking it was in some cavity under the brush pile,” Porter said. Standing on the horizontal tree trunk with the collar signal blasting, they realized the bear was directly beneath them, inside the hollow tree under their feet. “She was 20-feet straight in the tree and it was very tight quarters inside. We didn’t (re)capture that particular bear because of safety concerns for the bear and for the researchers. It was just too tight to wiggle inside and even if we were able to tranquilize the bear there was no way to handle her or remove her collar.”
しかし、熊はどのようにしてそのような巣穴をみつけるのだろうか？
「熊はそのようなものを見つけるのが得意だ」とPorterは言う。毎日の行動や夏の探検の間、彼らはその場所を覚えていて、あとで戻ってくるか否かは疑わしい。濡れた、または水浸しになった巣穴は熊が真冬に場所を変えることはありそうだが、真冬の引っ越しに影響させる他の要素もある。他の時期に、熊の存在を示す強い信号は、水平になった大きな木の山である、風倒木の寄せ集めであった。「我々はその倒木の集まりの下を登り、草わらの下に洞穴があると考えている」とPorterは言った。雷のような強いシグナルがある水平な木の幹(trunk)に立ってみて、足元直下の木の洞穴内に、熊が居ると分かった。「木の中の20フィートまっすぐな部分に熊が居て、かなり狭かった。熊および研究者の安全のために、その熊を捕獲できなかった。内部でビックと動くにはあまりにも狭かったし、もし熊を落ち着かせることができたとしても、熊を捕まえ、発信器を外すことは出来なかった」
Because this portion of the study focused on den site descriptions, researchers did not spend a lot time “processing” bears, as is often done in bear research, where samples of blood, hair, and tissue are taken, a small premolar tooth is pulled to determine the bear’s age, and where bears are often tagged and marked. This work had already been accomplished during the summer capture. Researchers conducted a quick health assessment of the animals, noted if any cubs had been born and counted them, and swapped out the bear’s radio collar for one with fresh batteries. The collar also had valuable location data stored on board the small computer showing everywhere the animal had been with a location point recorded every six hours.
この記事は熊の巣穴に絞って述べているので、研究者は熊の研究でたびたびなされるような熊の解体に時間を使っていない。そこでは血液、体毛、細胞組織のサンプルが採取される。熊の年齢を調べるために、小さな臼歯が抜かれ、タグと印が付けられる。この作業は夏の間にすでに行われていた。研究者は迅速に動物の健康調査、小熊が生まれいるか、その頭数を調べ、発信器のバッテリー交換を行う。発信器は、６ヶ月間動物が移動した位置を記録している。
An elevated den.
At the 52 dens located during 2010-2013, the following details were measured and recorded: entrance and cavity size, elevation, slope and aspect, distance above or below ground level, nest material type, tree diameter at breast height (DBH), tree species, surrounding timber stand type, and den structure (e.g., standing, downed, dead, logged, roots). Trail cameras were also placed at den sites to document when the bears emerged in the spring, and how many cubs might be present.
One interesting detail is that males denned at higher elevation than females: the average den elevation was 313 meters for males and 193 meters for females. Bears preferred den sites in timbered habitat on steeper slopes.
2010-2013年の間に見つけた52の巣穴では、次の点が詳細に調べられ記録されている。入口と空洞のサイズ、標高、傾斜と外観、地面からの高さ、巣穴の材料、胸の高さの直径、木の種類、周辺の灌木のタイプ、巣穴の構造(立っている、枯れ木、木、根など)。トレイルのカメラもまた巣穴に設置され、春に熊が現れたとき、何匹小熊がいるかが調査された。１つの興味は「メスよりオス熊は高い位置に巣ごもる」と言うことだ。平均的高さはオスが標高313m、メスが193mだった。急斜面の木の茂った生息地の巣穴が好まれた。
Crawling into a bear den mid-winter to examine a hibernating bear required precautions to insure the safety of the researchers and the bears. Bears were sedated using a CO2 powered dart gun that delivered a sedative, or a “jab stick” style hypodermic needle.
“We soon realized, after seeing how tight these cavities were, we didn’t want to pull bears out of the den during capture,” Porter said. “We did most of the work in the den so we didn’t have to push and jam them back in – like pushing a 200-pound sack of beans back into a small space. We were concerned for the bears’ safety, and our safety, we talked about a lot of different scenarios with the drugs and bears in the planning process, and took a lot of latitude so that everyone was safe. We also had a carefully written animal care plan that insured the bears were treated well and both adults and cubs were safe.”
冬眠(hibernation)している熊を調査するために真冬に熊の巣穴に腹ばいで入ることは、研究者と熊の安全を確保するために安全対策が必要である。熊は鎮静剤(sedative)を含んだCO2発射の吹矢か、皮下注射(hypodermic needle)を用いて麻酔された。巣穴がいかに狭いかを見て、巣穴から熊を引っ張り出したくないかった、と直ぐに分かった、とPorterは言う。小さな空間に200ポンド(約90Kg)の豆袋を押すように、熊を押したり､押し戻す必要がないように、巣穴の中で大半の作業を行った。熊と我々の安全に関心があった。我々は、計画段階で薬と熊の様々なシナリオについて話し合った。そして､みんなが安全になるようにかなりの許容度を取った。また、熊をうまく扱い、親と小熊が安全になる動物のケア計画を注意深く書いた。
Part of the issue is that black bears are not true hibernators, like marmots or ground squirrels, which can reduce their body temperature to near freezing and their heartrate and respiration to just a few beats and breaths per minute. Bears are in a torpor - they are metabolically processing stored fat, although they don’t urinate or defecate. Instead they reabsorb the urine and feces in the form of proteins. This is an amazing evolutionary process that allows bears to live on stored fat reserves and maintain themselves, even giving birth to young during a time when most bear food is not available. They don’t hibernate because it is cold outside but rather because there is no food during winter months. They reduce their body temperature by 7 or 8 degrees and heartrate is reduced somewhat, but they can wake up fast. They still burn about 4,000 calories per day while in hibernation, which is why they need to put on so much fat during summer and fall. Bears can lose 25-40 percent of their body weight during hibernation burning fat for fuel.
この議論は、黒クマは体温を０度近くまで下げ、心拍と呼吸を数回／分に減少させるマーモットやジリスのように、本当の冬眠はしないことだ。熊は
冬休眠(torpor)していて、生態的には蓄えた脂肪を消費しているが、排尿(urinate)や排便(defecate)していない。かわりに、タンパク質の形で排尿・排便を再吸収している。これは熊のほとんどの食料が利用できないようなときに子どもを産むときでさえ、熊が蓄えた脂肪で自足で生きることができるという驚異的な進化プロセスを示している。体温を7～8度下げ、心拍を少し下げるが、すぐに起きることが出来る。彼らは、外は寒いが、むしろ冬の数ヶ月は食料がないので、冬眠しない。彼らは冬眠中に4kcal／日消費している。これが、夏と秋の間、何故脂肪を付ける必要があるかの理由だ。熊は燃料としての脂肪を燃やしながら、冬眠の間に体重を25-40%減らす。
A biologist tracking a bear in its winter den.
Some days as the researchers approached a den, the tracking signal indicated the bear was up, awake and aware of their presence from a long distance away. “Most bears met us with eyes wide open and fully alert at the den entrance,” said Porter.
Other times they left the den and moved away before the researchers arrived.
“We tried to go in as stealthy as we could, but on some of the cold, crunchy, icy days the noise of our approach was just too much for them,” he said. “That might be a strategy they use for predators such as wolves, not sitting tight in a den if a pack of wolves is moving in. The majority of the dens were not fortified or protected, and several had multiple entrances - not good if you were going to be dealing with a pack of wolves that could kill you and or your small cubs.”
That could make the elevated dens, or standing hollow tree dens, pretty appealing to a black bear. Porter said a bear could be very safe and more relaxed in a really good elevated den. So if bears spend the summer identifying potential den sites, who gets the best ones?
“Who knows, maybe it’s first come first served for available den sites. If a pregnant sow or a sow with last year’s cubs comes to a den and somebody is already there, does she go to the second choice – right on down the list of identified dens that she knows of, or maybe the one she was born in with her mother? A longer term study would tell you, but this project was concluded after four years.”
研究者が巣穴に近づいたある日、熊からの発信器の追跡シグナルが高く、遠くからでも彼らの存在がわかった。「大半の熊は目を大きく見開いた状態で我々に遭遇し、巣穴の入口をかなり警戒している」とPorterは言った。ある場合は、研究者がそこに着く前に、彼らは巣穴を去り､移動していた。
できるだけ、気づかれないように行くことを試みた。寒くて、バリバリ音のするような、極寒の日に、我々が熊に接近するときの音が彼らを大きすぎた、と彼は言った。狼の群れのような捕食者達に対して使われる戦略だろう。もし狼が入ってきたときのために、巣穴にしっかり座っていない。巣穴の多くは無防備で､防護されていなかった。そのいくつかには複数の入口があった。もし、あなたが、あなたや小熊を殺すかもしれない狼の群れを利用するつもりなら、それは良い方法でない。
それらが高い場所に巣穴や、立った空洞の木の巣穴を作らせることになる。それは黒クマには大変アピールするものだ。熊は高い場所にあるいい巣穴では安全で安心している、とPorterは言う。tsもし熊がそのような可能性のある巣穴を探して夏を過ごしたなら、どの熊が最高の巣穴を得るのか？
たぶん、早い者勝ち(first come first served)で巣穴を利用しただろう。妊娠した雌熊(sow)か、子連れの雌熊が巣穴にきて、既に使われていたとき、次の巣穴を選択するだろうか？彼女が知っている巣穴のリスト１つ下げる。それは彼女が埋めれたか、母熊と過ごした穴か？より長い研究で分かるだろうが、このプロジェクトは４年間後に終了した。
In nearby British Columbia during a similar black bear study radio-collared bears reused dens seven out of 25 potential occasions. During this long term study in Coastal British Columbia they found 71 percent of the identified dens were reused at least once over a ten year period.
As a wildlife biologist managing a population of black bears, Porter said the take home lesson is that existing dens are a valuable resource that should be protected if possible. That’s especially important in areas that are logged. Not only are the dens removed (the trees that create the dens) in the logging process, but the other old trees that could potentially become den sites over time are also removed.
He paraphrased a statement from a research project in British Columbia, where bears were using root wads and other woody structure as dens, year after year. “If you’re interested in managing black bears responsibly, you should be making special allocation for woody structures and insure there are plenty of potential sites available into the future,” he said.
近くのＢＣでの同じような黒クマの研究の間、発信器付きの熊は7/25の割合で巣穴を再利用した。CBの海岸地帯での長い研究の間、少なくとも、見つかった巣穴の71(%)は10年間再利用されていることが分かった。黒クマの個体数を管理する野生生物学者として、Porterは今後の課題(the take home lesson)は既存の巣穴は可能な限り保護すべき貴重な資源であるということだ、と言う。木材が伐採されている地域では特に重要だ。伐採の過程で巣穴が除去されるだけでなく、そのうちに巣穴になるかもしれない古木までも除去されてしまう。彼は、熊が巣穴として木の根の塊や木の構造物を使っているCBの研究プロジェクトの文面を言い換えた。もし、黒クマをしっかり管理することに興味があるなら、木製の構造物に特別な配置をして、多久さんの将来使える巣穴を保証すべきだ、と彼は言う。
A trail camera captured this photo of a sow nursing her cubs shortly after emerging from the den.
“This could help us in terms of comments on timber sale designs and locations,” Porter said. “We went to the Forest Service with 50-some recent den locations, and suggested they provide some sort of individual tree protection, or a small buffer around these known bear den sites. We discussed den sites with the Forest Service several times and were encouraged by some positive comments from Forest Service staff initially. However, in the end they did not incorporate our den site protection suggestions for some upcoming large timber sales in the same area. There’s nothing in the Forest Plan that says they have to make any allocations for black bear dens.”
木材の売出しの設計と場所に関するコメントに関して、これは助けになる、とPorterは言う。我々は最近の50の巣穴の場所のデータを持って、森林サービス局へ行き、それは熊の巣穴周辺の各木の保護、または小さな緩衝になるべきであると提案した。我々はたびたび森林サービスと巣穴場所について議論し、最初は職員からの肯定的コメントに励まされた。しかし、結局、同じエリアにおける近日中の大規模な木材セールに対する巣穴保護の提案に協力しなかった。黒クマの巣穴に対してどのように配置するか、森林サービスの計画は無かった。
Porter and his colleagues are still working with the data they gathered and plan to compare their findings to other work done across North America. Collars provided telemetry information (GPS) that can be used to create maps of the bears’ movements over the course of the year. Researchers will be looking at average times bears entered dens in fall and den emergence times in spring, habitat selection (and whether that changes over time), food resource preferences at different times of the year, and the bears’ activities and movement patterns.
Portetと同僚はこのデータを基になお調査していて、彼らが見つけた巣穴と北米全域でなされた他の研究を比較している。首輪は熊の年間の移動地図を作るために使われるGPS情報を発信する。研究者たちは秋に熊が巣穴に入る平均回数と春の巣穴の出現回数、生息地の選定、別の時期の餌の好み、熊の行動と移動パターンを見ている。
Dave Gregovich, a research analyst working with Porter, said he’s lucky to have such a wealth of data to work with. He participated in initial radio-collaring efforts and he visited dens with Porter and others in winter, and was struck by the cleanliness of the dens. He speculated that may simply be healthier for the inhabitant. A messy den could breed parasites, but a bear nesting in a pile of fresh cedar boughs could be in pretty good shape for the long dry and warm winter.
“The dens that I went to didn’t smell at all, they’re as clean as a whistle,” he said. “For a bear being in a small space all winter, it was cleaner than my apartment.”
Dave Gregovich(Porterとの共同の分析者)は「そのように豊富なデータを得ることは幸運だ」と言った。彼は初期に発信器付き首輪の仕事に、冬にPorterや他と一緒に従事し、巣穴の清潔さに感銘を受けた。生息動物にはより健康的になると、推測している。散らかった巣穴は寄生虫を繁殖させることになり、持ち運ばれた新鮮なシダの束での巣作りは長く乾いていて、冬暖かく、非常に良い形になる。「私が行った巣穴はまったく臭いせず、汚れ１つなく綺麗だった(as clean as a whistle)」と彼は言う。「冬中、小さな空間にずーっといる熊にとって、私のアパートより綺麗だった」
For more on Prince of Wales black bears and bear research, see “Bear research on Prince of Wales Island" from AFWN, 2012
A series of trail camera images of a black bear investigating a bucket snare (and evading capture).
Riley Woodford is the editor of Alaska Fish and Wildlife News and participated in bear captures on POW in 2012.
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