[livius]

Actually, reptiles and amphibians and even many fish are known for being able to do this when the “demographic winter” falls on them. That is, there’s no critter of the opposite sex around, and their bodies obviously trigger responses that probably allowed them to survive after the great extinction.

[sparklite2]

Some of ‘em even change sex.
They’re called transfish.

[rellimpank]

--in flocks of hens (chickens) one will also at least start to develop male characteristics if no rooster available--

-happened on my folks farm---