Which heart structure is present in monotremes, marsupials, and placental mammals?

• A. Three-chambered heart
• B. Four-chambered heart
• C. Five-chambered heart
• D. One-chambered heart

Answer: (B)

All three groups—monotremes, marsupials, and placental mammals—share a four-chambered heart, where two atria and two ventricles create a complete separation between oxygen-poor blood returning from the body and oxygen-rich blood coming from the lungs. This full division keeps the pulmonary and systemic circuits separate, allowing efficient oxygen delivery to support higher metabolic demands and endothermy found in mammals. Monotremes (like platypus and echidna) are basal mammals, but even they have this four-chambered arrangement, as do marsupials and placental mammals.

A three-chambered heart, by contrast, is typical of many reptiles (and some amphibians) where the ventricle isn’t fully divided, so mixing of blood can occur. A one-chambered heart is characteristic of fish, with a single chamber for both receiving and pumping blood. A five-chambered heart isn’t a standard structure for these vertebrate groups.