system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why do only female mammals produce milk from their breasts to nurse young? Males have nipples too, wouldn't it be helpful energy-wise for a species if parents took turns?
gpt
Males have nipples because for roughly 2 weeks in the womb we all develop as female. After that sex begins to define itself and work on breasts and a vagina is indefinitely halted. 

Nipples are only the exit hole for milk, for males to produce milk we'd also need mammary glands. Not to mention milk production is tied to reproduction, too. So males would need to have periods of fertility, like ovulating females.