(much like humans appear
to be losing theirs).
Actually that's a general trend in mammals, and human
Y chromosomes are not that much reduced -
_Sminthopsis_ is the crassest case known (one-fifth
the mammal one's size, 4 functional genes), and
they're marsupials. I don't know if there's a
phylogenetic pattern in Y chromosome reduction, but
the only thing that's really crucial on that thing is
the SRY gene. And that can transpose - rarely, but it
has happened at least once and perhaps twice in
_Ellobius_ (a rodent).