Do all mammals have fur at some point in their lives?

Yes, all mammals have fur at some point in their lives. Even marine mammals, such as whales and dolphins, are considered to be furry mammals because they evolved from ancestors that had fur. While whales and dolphins lack the dense fur coats of land mammals, they still have vestigial hair follicles that are covered in a fine layer of modified hair called "guard hairs." These guard hairs serve a sensory function and help these marine mammals navigate their environment. Embryos or fetal stages of all mammal species also possess fur at some point during their development, even if they lose it before or shortly after birth. Mammals are further categorised based on features in addition to mammalian traits like mammary glands and fur, for further understanding evolutionary connections, adaptations, and diversification of groups in the diverse class termed "Mammalia."