Monotremes are a small and distantly related group of mammals, including five species found only in Australia and New Guinea. They have a low metabolic rate, lay leathery eggs, and have a lower body temperature than other mammals. Their reproductive mode is a cross between egg-laying and live births, and they produce milk without nipples. Monotremes were once poorly understood but are now regarded as another adaptive group among mammals.
Monotremes (“a hole,” referring to their genitalia) are members of the Order Monotremata, the smallest of three mammalian groups (the others are marsupials and placentals) and the most distantly related to other living mammals. Monotremes split from other mammals about 150 million years ago. In contrast, marsupials and placental mammals split from each other about 90 million years ago.
Although the monotremes were once a larger group, today they consist of only five species: the platypus, the short-beaked echidna, the western long-beaked echidna, Sir David’s long-beaked echidna, and the eastern long beaked. They are all found only in Australia and New Guinea, but there is evidence that monotremes were probably of global extent at one point. Some taxonomists propose placing platypi and echnidnas in two separate orders.
Monotremes share many characteristics of early amniotes, like mammalian ancestors: They have a single hole to do all their business, a low metabolic rate by mammalian standards, and they lay leathery eggs, unlike any other mammal. For a long time, monotremes were very poorly understood and thought to be “inferior” mammals. In modern times, biologists take a less hierarchical view of the evolutionary process and regard monotremes as just another adaptive group among mammals.
The body temperature of monotremes is about 90°F (32°C), compared to about 95°F (35°C) for marsupials and 100°F (38°C) for placentals. A lower body temperature means monotremes benefit from a slightly lower food requirement, but also have less energy available to fuel their muscles.
The reproductive mode of monotremes is an interesting cross between reptilian-style egg-laying (although they are no more closely related to reptiles than other mammals) and conventional mammalian viviparous (live births). They lay eggs, but the egg is retained inside the mother’s body and receives nourishment for a time. Like other mammals, monotremes are covered in fur and produce milk for their young. But without nipples, monotreme females “sweat” milk for their young from a specialized patch on their bellies.
Monotremes are a fairly mysterious and small order of mammals. When the platypus genome is sequenced in the near future, it will shed more light on this group and its relationships to extant mammalian orders.
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