Definition: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: Monotreme eggs have a leathery shell, like the eggs of reptiles. The eggs are retained inside the mother's body for at least a couple of weeks. During that time, the mother provides the eggs with nutrients. Platypus females lay their eggs in a burrow. Echidna females have a pouch in which they store their eggs. Female monotremes have mammary glands but lack nipples. Instead, they "sweat" milk from a patch on their belly. Answer: monotreme
Output:
Females of what mammalian group have mammary glands but lack nipples?