Manatees, also known as sea cows, are aquatic mammals that belong to a group of animals called Sirenia. This group also contains dugongs. Dugongs and manatees look quite alike – they are similar in size, colour and shape, and both have flexible flippers for forelimbs. However, the manatee has a broad, rounded tail, whereas the dugong's is fluked, like that of a whale. There are three species of manatees: the West Indian manatee (Trichechus manatus), the African manatee (Trichechus senegalensis) and the Amazonian manatee (Trichechus inunguis).

海牛，又称海牛类动物，是属于“海牛目”这一动物类群的水生哺乳动物。该类群还包括儒艮。海牛和儒艮在外形上颇为相似——它们的大小、颜色和形状都相近，而且两者都有用于前肢的灵活鳍状肢。不过，海牛的尾巴宽阔且呈圆形，而儒艮的尾巴则是带有波浪状的，类似于鲸鱼的尾巴。海牛有三种种类：西印度海牛（Trichechus manatus）、非洲海牛（Trichechus senegalensis）和亚马逊海牛（Trichechus inunguis）。

Unlike most mammals, manatees have only six bones in their neck – most others, including humans and giraffes, have seven. This short neck allows a manatee to move its head up and down, but not side to side. To see something on its left or its right, a manatee must turn its entire body, steering with its flippers. Manatees have pectoral flippers but no back limbs, only a tail for propulsion. They do have pelvic bones, however – a leftover from their evolution from a four-legged to a fully aquatic animal. Manatees share some visual similarities to elephants. Like elephants, manatees have thick, wrinkled skin. They also have some hairs covering their bodies which help them sense vibrations in the water around them.

与大多数哺乳动物不同，海牛的颈部只有六块骨头——而包括人类和长颈鹿在内的其他大多数动物都有七块。这种较短的颈部使海牛能够上下移动头部，但不能左右移动。要看到自己左边或右边的东西，海牛必须转动整个身体，依靠鳍肢来引导方向。海牛有胸鳍但没有后肢，只有尾巴用于推进。不过，它们确实有骨盆骨——这是它们从四足动物进化为完全水生动物过程中遗留下来的特征。海牛与大象在视觉上有一些相似之处。和大象一样，海牛有着厚厚的、布满褶皱的皮肤。它们的身上还覆盖着一些毛发，有助于它们感知周围水中的振动。

Seagrasses and other marine plants make up most of a manatee's diet. Manatees spend about eight hours each day grazing and uprooting plants. They eat up to 15% of their weight in food each day. African manatees are omnivorous – studies have shown that molluscs and fish make up a small part of their diets. West Indian and Amazonian manatees are both herbivores.

海草和其他海洋植物是海牛的主要食物来源。海牛每天大约会花八个小时来吃草和拔草。它们每天会摄入相当于自身体重 15%的食物。非洲海牛是杂食动物——研究表明，软体动物和鱼类只是它们饮食中的一小部分。西印度海牛和亚马逊海牛都是食草动物。

Manatees' teeth are all molars – flat, rounded teeth for grinding food. Due to manatees' abrasive aquatic plant diet, these teeth get worn down and they eventually fall out, so they continually grow new teeth that get pushed forward to replace the ones they lose. Instead of having incisors to grasp their food, manatees have lips which function like a pair of hands to help tear food away from the seafloor.

海牛的牙齿都是磨牙——扁平且圆润的牙齿，用于咀嚼食物。由于海牛以具有磨蚀性的水生植物为食，这些牙齿会逐渐磨损，最终脱落，因此它们会不断长出新的牙齿，这些新牙齿会向前移动以取代脱落的牙齿。海牛没有门齿来抓取食物，而是拥有嘴唇，其功能类似于一双手，帮助将食物从海底撕扯出来。

Manatees are fully aquatic, but as mammals, they need to come up to the surface to breathe. When awake, they typically surface every two to four minutes, but they can hold their breath for much longer. Adult manatees sleep underwater for 10–12 hours a day, but they come up for air every 15–20 minutes. Active manatees need to breathe more frequently. It's thought that manatees use their muscular diaphragm and breathing to adjust their buoyancy. They may use diaphragm contractions to compress and store gas in folds in their large intestine to help them float.

海牛完全适应了水生生活，但作为哺乳动物，它们仍需要浮出水面来呼吸空气。清醒时，它们通常每隔两到四分钟就会浮出水面一次，但它们也可以长时间不呼吸。成年海牛每天在水下睡觉的时间约为 10 至 12 小时，但每隔 15 至 20 分钟就会浮出水面呼吸空气。活跃的海牛需要更频繁地呼吸。人们认为海牛利用其肌肉性膈肌和呼吸机制来调节自身的浮力。它们可能会通过膈肌收缩来压缩并储存气体于其庞大的肠道中的褶皱处，以帮助自己保持浮态。

The West Indian manatee reaches about 3.5 metres long and weighs on average around 500 kilogrammes. It moves between fresh water and salt water, taking advantage of coastal mangroves and coral reefs, rivers, lakes and inland lagoons. There are two subspecies of West Indian manatee: the Antillean manatee is found in waters from the Bahamas to Brazil, whereas the Florida manatee is found in US waters, although some individuals have been recorded in the Bahamas. In winter, the Florida manatee is typically restricted to Florida. When the ambient water temperature drops below 20ºC, it takes refuge in naturally and artificially warmed water, such as at the warm-water outfalls from powerplants.

西印度海牛体长约 3.5 米，平均体重约为 500 公斤。它在淡水和咸水之间穿梭移动，利用沿海的红树林和珊瑚礁、河流、湖泊以及内陆的泻湖生存。西印度海牛有两个亚种：安提利亚海牛分布在从巴哈马群岛到巴西的水域，而佛罗里达海牛则分布在美国水域，尽管也有个体被记录出现在巴哈马群岛。在冬季，佛罗里达海牛通常只活动于佛罗里达州。当周围水温降至 20 摄氏度以下时，它会躲入自然和人工加温的水域中，比如发电厂的热水排放口。

The African manatee is also about 3.5 metres long and found in the sea along the west coast of Africa, from Mauritania down to Angola. The species also makes use of rivers, with the mammals seen in landlocked countries such as Mali and Niger.

非洲海牛体长约 3.5 米，主要生活在非洲西海岸的海域中，从毛里塔尼亚一直延伸到安哥拉。该物种还会利用河流，因此在一些内陆国家如马里和尼日尔也能见到这种动物的身影。

The Amazonian manatee is the smallest species, though it is still a big animal. It grows to about 2.5 metres long and 350 kilogrammes. Amazonian manatees favour calm, shallow waters that are above 23ºC. This species is found in fresh water in the Amazon Basin in Brazil, as well as in Colombia, Ecuador and Peru.

亚马逊河海牛是所有种类中体型最小的，但依然是体型较大的动物。它能长到约 2.5 米长，体重约 350 公斤。亚马逊河海牛喜欢水流平缓、水深不超过 23 摄氏度的浅水区域。这种动物生活在巴西亚马逊盆地的淡水中，同时也在哥伦比亚、厄瓜多尔和秘鲁有分布。

All three manatee species are endangered or at a heightened risk of extinction. The African manatee and Amazonian manatee are both listed as Vulnerable by the International Union for Conservation of Nature (IUCN). It is estimated that 140,000. Amazonian manatees were killed between 1935 and 1954 for their meat, fat and skin, with the latter used to make leather. In more recent years, African manatee decline has been tied to incidental capture in fishing nets and hunting. Manatee hunting is now illegal in every country the African species is found in.

这三种海牛均处于濒危状态或面临灭绝的高风险。非洲海牛和亚马逊海牛都被国际自然保护联盟（IUCN）列为易危物种。据估计，在 1935 年至 1954 年期间，有 14 万只亚马逊海牛因其肉、脂肪和皮肤而被捕杀，其中皮肤被用于制作皮革。近年来，非洲海牛的数量减少与在渔网中意外被捕以及被猎杀有关。如今，在非洲海牛的分布国中，海牛捕猎行为在所有国家均已违法。

The two subspecies of West Indian manatee are listed as Endangered by the IUCN. Both are also expected to undergo a decline of 20% over the next 40 years. A review of almost 1,800 cases of entanglement in fishing nets and of plastic consumption among marine mammals in US waters from 2009 to 2020 found that at least 700 cases involved manatees. The chief cause of death in Florida manatees is boat strikes. However, laws in certain parts of Florida now limit boat speeds during winter, allowing slow-moving manatees more time to respond.

西印度海牛的两个亚种已被国际自然保护联盟列为濒危物种。预计它们在未来 40 年内都将减少 20%的数量。对 2009 年至 2020 年期间美国海域约 1800 例海洋哺乳动物被渔网缠绕以及塑料摄入情况的调查发现，其中至少有 700 例涉及海牛。佛罗里达海牛的主要死亡原因是船只撞击。然而，佛罗里达某些地区的法律现在规定冬季时船只速度要降低，这样行动缓慢的海牛就有更多时间做出反应。