USA Biology Olympiad (USABO) Practice Exam

Which of the following is NOT a characteristic of all mammalian species?

1. Hair or fur

2. Live birth

3. Warm-blooded

4. Placenta

The correct answer is: Placenta

The characteristic that is not applicable to all mammalian species is the presence of a placenta. While many mammals, specifically eutherians (or placental mammals), do give birth to live young that have developed within a placenta, this is not a universal trait among all mammalian groups. For instance, monotremes, such as the platypus and echidna, are a subgroup of mammals that lay eggs rather than giving birth to live young. Additionally, marsupials, such as kangaroos and opossums, give birth to relatively undeveloped young that continue to grow in pouches rather than having a prolonged gestation period supported by a placenta. In contrast, hair or fur, being a defining characteristic of mammals, is found across all species within this class. Warm-bloodedness, referring to the ability to maintain a constant body temperature independent of environmental conditions, is also a universal trait among mammals. Live birth is common to most mammals, with the notable exceptions being those that lay eggs or have unique reproductive strategies within their respective groups. Therefore, the presence of a placenta is specific to certain groups and does not apply to all mammals, making it the correct choice for this question.