What unique characteristic is generally found in mammals?

The unique characteristic generally found in mammals is live birth. While not all mammals give birth to live young—since monotremes like the platypus and echidna lay eggs—most mammals do reproduce through live births, which is a defining feature of the group. This reproductive strategy allows for more advanced development of the young inside the mother's body, enhancing their survival.

Mammals are also characterized by other traits such as having mammary glands that produce milk for feeding their young, fur or hair covering their bodies, and specific adaptations in their skeletal and respiratory systems. This live birth process is a significant evolutionary advantage, fostering a closer maternal bond and providing better protection and nourishment for the developing offspring compared to egg-laying strategies seen in other animal groups.