Are there any known species of cold-blooded mammals?
I thought being warm-blooded was an essential part of the definition of a mammal.
30 seconds with Google:
Whether it’s really as clear-cut as the article makes out, I don’t know, but it seems like a reputable place to start.
Interesting stuff over at Wikipedia:
“The mammals are the class of vertebrate animals characterized by the presence of mammary glands, which in females produce milk for the nourishment of young; the presence of hair or fur; and which have endothermic or “warm-blooded” bodies.”
Click on “warm-blooded” and they go onto saying that the terms warm-blooded and cold-blooded are being changed to fit the further understanding of physiology.
"Thanks to more thorough research in the field of animal physiology, scientists have come to realize that body temperature types do not easily fit a simple either/or scenario. Body temperature maintenance incorporates a wide range of different techniques that result in a body temperature spectrum, with the traditional ideals of warm blooded and cold blooded being at opposite extremes.
Because of the generalness of the terms, as well as an increased understanding in this field, both warm blooded and cold blooded have mostly fallen out of favour. They have since been replaced with one, or more, of their variants (see: #Breaking down Warm Bloodedness)."
-Tcat
All mammals become cold-blooded eventually. All you have to do is stop feeding them for a while.
See also the Staff Report by me and Doug: What makes some animals cold-blooded and others warm-blooded?
As others have said, “cold-blooded” is a simplistic term that does not reflect the large variety of thermal physiologies shown by animals. The naked mole-rat is an endotherm, that is, it generates most of its enegy internally. However, it is to some extent poikilothermic, that is, its body temperature fluctuates with the external environment. There are a number of other mammals, such as sloths, that have poor thermoregulation, and others, such as those that undergo torpor or hibernation, that will allow their body temperatures to drop in special circumstances. As far as I know, however, all mammals are endotherms, and so cannot be called “cold-blooded” in the usual sense of that term.
It’s important to note that an animal could still be a mammal if even it was “cold-blooded” (or hairless, or without mammaries). Animal classifications are not based solely on traits, but also on descent. Any animal descended from mammals is a mammal. This is essence of cladistics.
More basic defining characteristics for mammals are the presence of three ear ossicles, the lower jaw comprised of a single bone (dentary), and the jaw articulation formed by the dentary and squamosal bones. But as Pleonast says, even if a mammal lost these characteristics it would still be a mammal, since it was descended from animals that had these characteristics.
But aren’t mammals descended from reptiles and reptiles from fish. So is everything a fish?
Yes. All Homo sapiens are fish. (There’s got to be a Monty Python quote to go with that.) See the Tree of Life for more fun.
According to cladistics, the traditional category “Reptiles” is an artificial one. A better term for the group that includes mammals, turtles, snakes, lizards, crocodiles and birds is “amniotes,” animals that have an egg with an amnion (a kind of membrane). Although most mammals don’t lay shelled eggs, they still possess an amnion, and are amniotes.
Of the groups listed, the mammals split off from the others first. Some taxonomists today refer to the remaining groups, including birds, as the “Reptilia” in a cladistic sense. According to this view mammals are not reptiles, but birds definitely are.
And yes, mammals are “fish” in a cladistic sense. One problem with the traditional class Osteichthyes, the bony fishes, is that it is also an artificial group. The lungfish and lobefins are more closely related to the tetrapods (amphibians + amniotes) than they are to the other bony fishes. Therefore, according to cladistics, one must either abandon the entire class, or else include the tetrapods within the Osteichthyes. (The other groups of “fish”, the lampreys, hagfish, sharks, and rays, are more distantly related.)
From the Tree of Life (Pleonast’s link):
Mammals (along with other terrestrial vertebrates) are included within bony fishes (Osteichthyes)