Male mammals, including humans, possess nipples, a feature typically associated with feeding offspring. This raises a curious question: why do males have these structures if they generally do not lactate?
Early Development and Evolutionary Roots
The fundamental reason male mammals have nipples stems from the earliest stages of embryonic development. All mammalian embryos, regardless of their eventual sex, begin with the same basic genetic blueprint. Around the fifth week of human development, mammary ridges form along the embryo’s body, which are the precursors to mammary glands and nipples.
Nipple formation occurs before sexual differentiation. In humans, this differentiation begins around the sixth or seventh week of gestation. Since nipples have already started developing by this point, they are present in both male and female embryos. This shared developmental pathway is an evolutionary legacy, reflecting common mammalian ancestry.
Diverse Manifestations Across Species
While the presence of nipples in male mammals is a general rule, there are interesting variations and exceptions across the animal kingdom. For instance, monotremes, such as the duck-billed platypus and echidna, are egg-laying mammals that do not possess nipples at all. Instead, they secrete milk onto patches of fur, which their young then lap up.
Other species also present deviations from the norm. Male mice, unlike most other male mammals, do not have nipples. This is because testosterone, produced early in male mouse embryonic development, actively inhibits nipple formation. Similarly, male horses and some marsupials also lack nipples or mammary glands, showcasing the diversity in mammalian anatomy.
Functional Roles and Vestigial Traits
For the vast majority of male mammals, nipples are considered vestigial structures. This means they are remnants of an ancestral form or developmental pathway that no longer serve a primary purpose in the male. Their presence does not confer any known disadvantage, so there has been no evolutionary pressure to eliminate them.
Despite their general non-functionality, rare instances of male lactation occur. The Dayak fruit bat (Dyacopterus spadiceus) is an example of a wild male mammal that produces milk, and this has also been observed in the Bismarck masked flying fox. While uncommon, these cases demonstrate that the physiological capacity for lactation exists in males, even if it is not typically expressed or needed for offspring survival.