Does Emacs return memory to the system on Mac OS X or *BSD?

[George Plymale II]

Hi,

So, this question is spurred by a discussion found in this reddit
thread: 
https://www.reddit.com/r/emacs/comments/5rswpn/freeing_dead_heap/ddae477/
I'm curious as to whether Emacs returns memory to the operating system
when running on Mac OS X or *BSD systems. As is made clear in the
discussion, such things are system-dependent and there is no unified
resource for which systems support this behavior and which ones do
not. If no one here knows about this off the top of their head, which
specific source file(s) can I browse to learn about this? Thanks.