Sunning themselves on rocks or waddling awkwardly across the beach, it’s easy to think of these immobile mammals less as sea lions, and more as sea house cats. But don’t be fooled by their beachside behavior. Under the waves, sea lions are incredible endurance hunters. Hurtling around at speeds from 4 to 18 miles an hour and hunting for up to 30 hours at a time, these majestic mammals live up to their name. And thanks to a suite of physical adaptations, finely tuned over millions of years, they make for resourceful foragers.
看到牠們在石頭上曬太陽 或笨手笨腳搖晃地走過沙灘, 我們很容易把這些不太動的 哺乳動物視為海家貓, 而不是海獅。 但不要被牠們在海灘上的舉止欺騙。 在海裡,海獅是有驚人耐力的捕獵者。 以每小時 4 到 18 英里的速度衝馳, 每次獵捕可長達 30 小時, 這魁梧的哺乳動物真不負其名。 由於身體經過一系列的演化, 歷經數百萬年不斷微調, 牠們變成很機智的覓食者。
To find their favorite food, sea lions hunt much deeper than many of their semi-aquatic peers. With some species diving to depths of nearly 400 meters, they’re able to cope with the mounting pressure by collapsing their pliable rib cage, and compressing a pair of springy lungs. This pushes air up through the smaller airways, collapsing rings of cartilage as oxygen travels out from the lungs, to be held in the larger, upper airways. Upon surfacing, this air will be used to re-inflate the lungs, but for now their heart slows down to preserve oxygen. Blood flow is redirected towards only the most essential organs like the heart, lungs, and brain, which rely on reserve oxygen stored in blood and muscle.
為了尋找喜歡的食物, 海獅會潛入 比半水生同類動物更深處捕食。 有些種類會潛入約 400 米深, 牠們能對抗漸增的水壓── 藉由塌扁柔軟的胸腔 和壓縮兩個富彈性的肺。 空氣會被推過較小呼吸道, 當氧氣離開肺時,導致軟骨環塌扁, 如此空氣會停留在較大的上呼吸道。 浮到水面時, 這空氣會使肺重新膨脹, 但在水裡時, 心跳會減慢以保留氧氣。 血流只會流向最重要的器官, 如心臟、肺及腦, 這些器官仰賴 貯存於血和肌肉的氧氣。
Once they arrive at their hunting ground, sea lions depend on their superior vision to find their prey. Most mammal eyes have a structure called a lens– a transparent, convex structure whose shape refracts light to enable sight. In humans, this lens is curved to process light waves traveling through air. But sea lions need to see their best at hundreds of meters deep. To accommodate, their eyes have a much rounder lens to refract light underwater, as well as teardrop-shaped pupils which can expand to 25 times their original size. This lets in as much light as possible, helping them pinpoint their prey in even the dimmest conditions.
抵達獵場時, 海獅靠牠們的超級視力尋獲獵物。 大部份哺乳動物眼睛有個構造 稱為「水晶體」, 一種透明的凸形構造, 可折射光線成為可見影像。 人類水晶體是弧形結構, 以處理通過空氣的光波。 但海獅需在數百米深處仍能看清楚, 為了適應,牠們有 更圓的水晶體以反射水中的光, 並且其淚滴狀的瞳孔 可放大到原來的 25 倍。 這能儘可能讓光進入, 使牠們在極暗處 也可精確看清獵物所在。
But once they’ve closed in, they rely on something akin to a sixth sense to actually catch their meal. Their whiskers, or vibrissae, are composed of keratin and full of nerve fibers that run deep into the connective tissue of their face. Sea lions have full directional control over these whiskers, which can lie flat against their face, or stick out at a 90-degree angle. When properly tuned, these whiskers can sense the slim trails of moving water fish leave in their wake. And they’re precise enough to let blindfolded sea lions tell the difference between objects less than two centimeters different in size.
但當靠近獵物時, 牠們依賴一種 類似第六感的感覺以捕獲食物。 牠們的觸鬚──也稱為感覺毛, 是由角蛋白所組成, 並佈滿連到臉部 結締組織深處的神經纖維。 海獅能隨意控制這些觸鬚, 讓它們可以完全貼臉, 或以 90 度角伸出去。 經由適當的調整, 觸鬚能覺察到魚游過後的細長水跡。 它們準確力高到 即使矇住眼睛的海獅 也能分辨物體之間 小於兩公分的差異。
With these tools a healthy sea lion can catch generous helpings of fish such as anchovy, mackerel, and squid on every outing. And with their exceptional memories, they can remember multiple hunting grounds, including those they haven’t visited in decades. This memory also extends to breeding territories and birthing areas, as well as which neighbors are friend and foe. There’s even evidence that sea lions can remember how to perform tasks after 10 years with no practice in between, letting them navigate old stomping grounds with ease.
用這些能力,健康的海獅 每次狩獵都可捕獲大量的魚, 例如鯷魚、鯖魚及魷魚。 憑著極佳的記憶力, 牠們能記住許多獵場, 包括數十年未再造訪的場所。 這記憶力還涵蓋到繁殖地及生產地, 以及哪些鄰居是敵或友。 還有証據顯示 海獅能記得如何執行技能, 僅管十多年都沒再練習。 這讓牠們在舊時的出沒場所 可輕鬆遨遊。
Yet despite these incredible adaptations, there are changes unfolding in their habitats too rapidly for sea lions to handle. As climate change warms the oceans, certain toxic algae species thrive. This algae is harmless to the fish who eat it, but for the sea lions which ingest those fish, the algae’s domoic acid can trigger seizures and brain damage. Changing ocean conditions keep this algae blooming year round, causing more and more sea lions to wash up on beaches.
雖有這些驚人的演化, 但海獅棲息地的一些改變 發生得太快, 使牠們來不及適應。 隨著氣候改變暖化了海洋, 有些毒海藻繁殖茂盛。 這些海藻對吃它們的魚無害, 但對於吃這些魚的海獅, 海藻的軟骨藻酸 能誘發痙攣及腦損傷。 海洋環境的改變 使得海藻整年不斷地繁盛, 這使得愈來愈多的海獅屍體 被沖到海灘上。
This tragic discovery is just one of the many ways the health of aquatic animal communities can help us better understand Earth’s oceans. These red flags help us take action to protect ourselves and other maritime mammals. And the more we can learn about the changing ocean that sea lions inhabit, the better equipped we’ll be to help these clever creatures thrive.
這悲慘的發現僅是其中一個 水生生物群的健康 能協助我們 更理解地球的海洋的方式。 這些危險訊號促使我們採取行動 保護自己及其他海洋哺乳動物。 對海獅居住的海洋環境變化了解愈多, 我們愈能有充份準備, 幫助這些聰明的生物繁衍。