Which of the following is a characteristic of Mammalia?

Mammals are a distinct class of animals characterized by several unique features that differentiate them from other groups, one of the most significant being the presence of mammary glands. These glands allow female mammals to produce milk, which is a crucial source of nutrition for their young during early development. This nurturing aspect is vital for the survival and growth of offspring, ensuring they receive the necessary nutrients to thrive.

Mammary glands are a defining trait of mammals and underscore the evolutionary adaptations that have allowed them to care for their young more effectively compared to other animal classes. This characteristic is fundamental to their classification and is critical for understanding the reproductive and developmental behaviors of mammals within the larger context of animal biology.

The other features listed are characteristic of different animal groups. Feathers are exclusive to birds and are primarily used for insulation and flight. Scales are a hallmark of reptiles, providing protection and minimizing water loss in their habitats. Gills are found in aquatic animals like fish, enabling them to extract oxygen from water. These distinctions highlight the unique adaptations of mammals, with mammary glands being a core feature of their identity.