Which is NOT a characteristic of all mammals?

Mammals are known for several key characteristics, and one of them is the presence of mammary glands that produce milk to nourish their young, which is true universally for all mammals. Additionally, mammals possess a four-chambered heart, which efficiently separates oxygenated and deoxygenated blood, enhancing their respiratory efficiency. They also generally have a diaphragm that aids in breathing by creating negative pressure in the thoracic cavity.

While the majority of mammals do give birth to live young, there are exceptions to this characteristic. For instance, monotremes, such as the platypus and echidna, lay eggs instead of giving birth to live offspring. This unique reproductive trait in some mammals is what makes the option about giving birth to live young not universally applicable to all mammals, distinguishing it from the other traits that all mammals share.