忙しくても安心！10分でサクッと学べる長文読解（テーマ：生物）
以下に、「イルカの生態」をテーマにした、英検準1級レベルの長文を作成しました。7～8分かけてじっくり読める文量で、単語や文法のポイント、内容に関する問題と模範解答、最後に日本語訳を追加しています。
※この長文の単語数は 874語 です。
The Fascinating World of Dolphins
Dolphins are among the most intelligent and social animals in the world. They belong to the family of cetaceans, which also includes whales and porpoises. Known for their playful behavior and remarkable communication skills, dolphins have captivated the imagination of humans for centuries. These marine mammals are found in oceans and seas across the globe, as well as in some rivers. In this article, we will explore the complex behaviors, communication methods, and social structures of dolphins, along with their challenges in the modern world.
Physical Characteristics and Species Diversity
There are over 40 different species of dolphins, ranging in size from the small Maui's dolphin, which is about 1.5 meters long, to the much larger orca, or killer whale, which can grow up to 9 meters. Although orcas are commonly referred to as "whales," they are technically the largest species of dolphin.
Dolphins are highly adapted to life in the water. They have sleek, streamlined bodies that allow them to swim quickly and gracefully. They also possess a thick layer of blubber, which helps them stay warm in cold ocean waters. Dolphins breathe through a blowhole located on top of their heads, and they must come to the surface to breathe air. Despite living in water, they are mammals, which means they give birth to live young and nurse their offspring.
Dolphin Communication
Dolphins are known for their complex communication systems, which include a variety of clicks, whistles, and body movements. These sounds are used for a wide range of purposes, such as navigating the ocean, coordinating group movements, and signaling danger. One of the most fascinating aspects of dolphin communication is their use of echolocation. Dolphins emit high-frequency clicks that bounce off objects and return to them, allowing the dolphins to "see" their surroundings through sound. This ability is especially useful for hunting in dark or murky waters.
Researchers have found that dolphins also use a form of "signature whistle," which functions like a name. Each dolphin has its own unique whistle that it uses to identify itself and communicate with other dolphins. This personalized communication system highlights the intelligence and social complexity of these animals.
Social Structure and Behavior
Dolphins are highly social creatures and often live in groups called pods, which can range in size from just a few individuals to hundreds of dolphins. These pods are usually made up of related individuals, such as mothers and their offspring, but they can also include unrelated dolphins that join the group. The social bonds within these pods are strong, and dolphins engage in cooperative behaviors such as hunting, protecting one another from predators, and even caring for sick or injured pod members.
Dolphins are also known for their playful nature. They often leap out of the water, ride waves, and interact with boats and humans. While these behaviors may seem purely for fun, scientists believe that play may also serve important social and developmental functions, helping young dolphins practice skills and strengthen their bonds with other pod members.
Intelligence and Problem-Solving
Dolphins are widely regarded as one of the most intelligent animal species on Earth. Their brains are large and highly developed, with complex structures similar to those of humans. Dolphins have been observed using tools, such as sponges to protect their snouts while foraging on the seafloor, and they are capable of solving complex problems.
Dolphins are also capable of learning new behaviors, both from humans and from each other. In captivity, they have been trained to perform a wide variety of tasks, and in the wild, they have been observed teaching their offspring how to hunt and navigate their environment. Some studies even suggest that dolphins may possess self-awareness, as they are able to recognize themselves in mirrors, a trait shared by only a few animal species.
Threats to Dolphin Populations
Despite their intelligence and adaptability, dolphins face numerous threats in the modern world. One of the biggest challenges is bycatch, which occurs when dolphins are accidentally caught in fishing nets meant for other species. This unintentional capture can result in injury or death. Pollution, particularly from plastics and chemicals, also poses a significant risk to dolphins, as toxins can accumulate in their bodies and cause serious health problems.
Climate change is another threat to dolphin populations. As ocean temperatures rise, the distribution of prey species such as fish and squid may shift, forcing dolphins to travel further to find food. Additionally, noise pollution from ships, oil exploration, and underwater construction can interfere with dolphin communication and navigation, making it more difficult for them to survive.
Vocabulary and Grammar Points:
Echolocation (エコーロケーション) – The use of sound waves to locate objects in the environment by reflecting sound off them.
Example: "Dolphins use echolocation to hunt in dark or murky waters."
Blubber (脂肪) – A thick layer of fat under the skin of marine mammals that helps insulate them from cold water.
Example: "Blubber keeps dolphins warm in cold ocean waters."
Bycatch (混獲) – The capture of unintended species in fishing nets.
Example: "Bycatch is one of the biggest threats to dolphins."
Pod (群れ) – A group of dolphins or other marine mammals that live together.
Example: "Dolphins live in social groups called pods."
Self-awareness (自己認識) – The ability to recognize oneself as an individual, separate from others.
Example: "Dolphins may have self-awareness, as they can recognize themselves in mirrors."
Questions (問題):
What is echolocation, and how do dolphins use it?
How do dolphins communicate with each other?
What is a pod, and what social behaviors do dolphins show within a pod?
What are some signs of dolphin intelligence?
What are the main threats facing dolphins today?
Sample Answers (模範解答):
Echolocation is the use of sound waves to locate objects. Dolphins emit clicks that bounce off objects, allowing them to "see" through sound.
Dolphins communicate using clicks, whistles, and body movements. They also have unique signature whistles to identify themselves.
A pod is a group of dolphins that live together. Within pods, dolphins hunt, protect each other, and care for sick members.
Dolphins show intelligence through tool use, problem-solving, and learning new behaviors. They may also have self-awareness.
The main threats to dolphins are bycatch, pollution, climate change, and noise pollution.
Japanese Translation (日本語訳):
イルカの魅力的な世界
イルカは、世界で最も知的で社交的な動物の一つです。彼らはクジラやネズミイルカと同じく鯨類に属し、遊び好きな性格や驚くべきコミュニケーション能力で知られています。イルカは、世界中の海洋や一部の川に生息しており、その生態や社会構造は長年にわたって人々の関心を集めてきました。この記事では、イルカの複雑な行動、コミュニケーション方法、社会構造について探り、現代の世界で直面している課題についても考察します。
身体的特徴と種の多様性
イルカには40以上の種が存在し、最小のマウイイルカは約1.5メートル、最大の種であるシャチ（オルカ）は9メートルにもなります。シャチは一般的に「クジラ」と呼ばれますが、実際にはイルカの中で最大の種です。
イルカは水中生活に非常に適応しており、流線型の体は迅速で優雅な泳ぎを可能にします。また、寒い海でも体温を保つために厚い脂肪層を持っています。イルカは頭の上にある噴気孔を使って呼吸し、水中から浮上して空気を吸います。彼らは哺乳類であるため、水中で暮らしながらも空気呼吸をし、子供を産み、母乳で育てます。
イルカのコミュニケーション
イルカは多様なクリック音、ホイッスル音、そして身体の動きを使った複雑なコミュニケーションシステムを持っています。これらの音は、海を航行したり、グループの動きを調整したり、危険を知らせたりするために使用されます。特に注目すべきなのは、イルカがエコーロケーションを使うことです。イルカは高周波のクリック音を発し、それが物体に反射して返ってくることで、音を使って周囲の状況を「見る」ことができます。この能力は、暗い水中や濁った水中での狩りに特に有効です。
研究者たちは、イルカが「シグネチャーホイッスル」という個別のホイッスル音を持っていることも発見しました。これはイルカにとっての「名前」のようなもので、他のイルカとコミュニケーションを取る際に使われます。この個別のコミュニケーションシステムは、イルカの知性と社会的複雑さを示すものです。
社会構造と行動
イルカは非常に社交的な動物で、通常はポッドと呼ばれるグループで生活しています。このポッドのサイズは数頭から数百頭までさまざまです。ポッドは通常、母親とその子供などの血縁者で構成されていますが、時には血縁関係のないイルカもグループに加わることがあります。ポッド内の社会的な絆は強く、イルカたちは一緒に狩りをしたり、捕食者から仲間を守ったり、病気やけがをした仲間の世話をしたりします。
イルカはまた、遊び好きな性格でも知られています。彼らはよく水から飛び出したり、波に乗ったり、船や人間と交流したりします。これらの行動は一見すると単なる遊びのように見えますが、科学者たちは、遊びが若いイルカのスキルを磨き、ポッド内での絆を強める重要な社会的および発達的な役割を果たしていると考えています。
知性と問題解決能力
イルカは、地球上で最も知的な動物の一つと広く認識されています。彼らの脳は大きく、複雑で、人間の脳と似た構造を持っています。イルカは道具を使うことが知られており、たとえば、海底で餌を探すときに鼻を保護するためにスポンジを使うことがあります。また、彼らは複雑な問題を解決する能力も持っています。
イルカは新しい行動を学ぶことができ、人間からも、仲間からも新しいスキルを習得します。飼育下では、さまざまなタスクを実行するように訓練されており、野生では、子どもたちに狩りや環境のナビゲーション方法を教える姿が観察されています。さらに、イルカは自己認識を持っている可能性があり、鏡に映った自分を認識できることが示されています。これは、ほんの少数の動物だけが持つ特性です。
イルカの脅威
その知性と適応能力にもかかわらず、イルカは現代の世界で多くの脅威に直面しています。最大の課題の一つは混獲です。これは、イルカが他の魚を捕まえるための漁網に誤って捕まってしまうことを指します。この偶発的な捕獲は、怪我や死亡につながることがあります。また、プラスチックや化学物質による汚染も大きなリスクをもたらしており、これらの毒素がイルカの体内に蓄積し、深刻な健康問題を引き起こすことがあります。
気候変動もイルカの生存にとって大きな脅威です。海水温が上昇すると、魚やイカなどの獲物の分布が変化し、イルカが食料を求めて長距離を移動しなければならなくなる可能性があります。さらに、船舶や石油探査、海底建設による騒音汚染がイルカのコミュニケーションやナビゲーションを妨げ、生存をより困難にしています。