Why are zombie processes invisible when doing ps -f on mac?

Question

Given this C program:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(void) {
    pid_t pid;
    if ((pid = fork()) >= 0) {
        if (pid == 0) {
            exit(0);
        } else {
            sleep(999);
        }
    }
    return 0;
}

after running:

$ gcc -o zombie zombie.c
$ ./zombie &
$ ps -f 

only the parent process is showing:

501 77474 74651   0  2:36PM ttys000    0:00.00 ./zombie

On Linux the same program shows this:

osboxes   6900  6835  0 13:37 pts/1    00:00:00 ./zombie
osboxes   6903  6900  0 13:37 pts/1    00:00:00 [zombie] <defunct>

Now, strangely, on OSX when starting more than one zombie programs..

$ ./zombie &
$ ./zombie &
$ ps -f

.. the zombie processes (in brackets) show up:

501 77653 74651   0  2:39PM ttys000    0:00.00 ./zombie
501 77664 74651   0  2:39PM ttys000    0:00.00 ./zombie
501 77667 77664   0  2:39PM ttys000    0:00.00 (zombie)

What's the reason for this?