Very interesting, never heard of this before. I gave this some thought, and I think I can come up with a simple reason for the observed behavior and also why it

Very interesting, never heard of this before. I gave this some thought, and I think I can come up with a simple reason for the observed behavior and also why it does not apply to humans. Female mice mate with dominant, territorial male mice who's role after conception is to defend the nest from the attack of rival males, who will kill/cannibalize the babies if given the chance. At some population density, there are way too many rival mice for a dominant male to defend, and this reproductive strategy breaks down. The imagined limiting conditions like food and housing were not the actual limiting condition of the colony - the killer rivals were. Human females don't form harems under the protection of dominant males, and thus this would not be a limiting factor for humans.