Which mammals are not social animals?

Is there any distinct features that you can give to differentiate between Mammals and other classes of animals?

• Mammals are supposed to be warmblooded but so are birds Mammals have hair or fur but then there are exceptions Mammals are viviparous but then we have egg laying platypus and also there are some species of snakes that are ovo-viviparous that is they lay eggs inside its body but the young ones come out like normal birth Is there any distinct features you can say for saying clear cut that so and so species is a mammal?

• Answer:

  <<Mammals have hair or fur but then there are exceptions>> Even the exceptions tend to have some hair at some stage of their lives especially, in the case of dolphins, around the nose. <<Is there any distinct features you can say for saying clear cut that so and so species is a mammal?>> All mammals have a jaw-skull joint called the dentary-squamosal. No extant non-mammal has one of those. (However, there's at least one exception -/Pachygenelus/ a non-mammalian cynodont from the fossil record of the Lower Jurassic.) All extant mammals (but not all fossil mammals) have three sound-processing ossicles in the middle ear. No non-mammals have more than one. Furthermore, the casing of the middle ear is composed of a single bone called the petrosal for mammals. The casing is multi-boned for non-mammals. All extant mammals (but not all fossil mammals) have a lower jaw composed of a single bone, the dentary. No non-mammals have a single boned lower jaw. All female mammals have mammary glands, and these produce milk to feed the young. Some non-mammals (eg. pigeons) also produce a form of milk for babies known as crop-milk, but they don't have mammary glands. For mammals with teeth, there is at most a single episode of replacement at any position. Non-mammals with teeth replace them more than once. However, there are such things as entirely toothless mammals, eg. echidnas. Update <<... and they have nipples>> Not quite. Female monotremes, such as the platypus, have mammary glands but no nipples.