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“海牛目动物”，是属于儒艮科(又叫做Sirenia)的水生哺乳动物。该科还包括另一种生物--儒艮。儒艮与海牛外形极为相似:它们体型相近，肤色相仿，形态雷同，前肢都有灵活的鳍肢。然而，海牛尾部宽大圆钝，而儒艮尾部则是类似鲸类有的分又尾鳍。现存海牛共有三个物种:西印度海牛(Trichechus manatus)、非洲海牛(Trichechus senegalensis)以及亚马逊海牛(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，海牛颈部仅有六块骨骼--其他哺乳动物，包括人类和长颈鹿，通常有七块。这种短颈结构使海牛能上下摆动头部，却无法左右转动。当需要观察两侧时，它们必须用鳍肢掌控方向，转动整个身体。海牛拥有胸鳍但无后肢，仅靠尾鳍推进。不过它们确实保留着骨盆骨--这是从四足进化成完全水生动物留下的痕迹。海牛在外形上与大象有些相似之处。像大象一样，它们有厚重起皱的皮肤，以及覆盖全身的毛发，这些毛发能帮助它们感知周围水流的振动。

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.

海草与其他水生植物构成海牛的主要食物来源。它们每日花费约8小时啃食并连根拔起植物。它们每日的进食量可达体重的15%。非洲海牛为杂食性--研究表明，软体动物和鱼类占其饮食的一小部分。西印度海牛与亚马逊海牛均为严格食草性动物。

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.

海牛的牙齿均为臼齿(扁平圆钝，适于研磨食物)。因其植物性饮食的磨损性，这些牙齿会逐渐磨蚀并最终脱落，所以新齿会不断生长并前移替代旧齿。海牛没有门齿来抓取食物，它们有嘴唇，像双手一样从海床撕扯植物。

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.

海牛是完全水生，但作为哺乳动物仍需浮出水面呼吸。清醒时，它们通常每2-4分钟换气一次，但它们屏息能力远超于此。成年海牛每日在水下睡眠10-12小时，但每15-20分钟上浮呼吸一次;活跃个体则需更频繁换气。人们认为海牛可能通过膈肌和呼吸调节浮力。它们也许通过收缩膈肌将气体压缩储存于大肠褶皱中，以辅助漂浮。

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500公斤。它们可往返于淡水与咸水环境，栖息地包括红树林、珊瑚礁、河流及内环礁湖。其下分两个亚种:安的列斯海牛分布于巴哈马至巴西海域，而佛罗里达海牛主要栖息于美国水域，少数个体出曾出现在巴哈马。当冬季水温低于20°C时，佛罗里达海牛会聚集在发电厂的排水区等天然或人工暖水域避难。

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.

非洲海牛体长同样约为3.5米，栖息于非洲西海岸，从毛里塔尼亚至安哥拉的海域。它们也会进入河流活动，在马里、尼日尔等内陆国家也有目击记录。

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.

亚马逊海牛是体型最小的物种，但仍属大型动物。体长约2.5米，重达350公斤。它们偏好水温高于23°C的平静浅水区。该物种多分布于巴西亚马逊河流域以及哥伦比亚、厄瓜多尔和秘鲁的淡水环境中。

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.

非洲海牛体长同样约为3.5米，栖息于非洲西海岸，从毛里塔尼亚至安哥拉的海域。它们也会进入河流活动，在马里、尼日尔等内陆国家也有目击记录。亚马逊海牛是体型最小的物种，但仍属大型动物。体长约2.5米，重达350公斤。它们偏好水温高于23°C的平静浅水区。该物种多分布于巴西亚马逊河流域以及哥伦比亚、厄瓜多尔和秘鲁的淡水环境中。

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.

所有三种海牛目前均濒临灭绝或面临极高的生存风险。根据国际自然保护联盟(IUCN)的评估，非洲海牛和亚马逊海牛均被列为“易危“物种。据统计，1935至1954年间，约有14万头亚马逊海牛因肉、脂肪和毛皮需求遭捕杀，后者用来制造皮革。近年来，非洲海牛数量下降主要归因于渔网误捕和盗猎。如今，非洲海牛猎杀已在各国被立法禁止。