When both species are juveniles they look the same. However when the mantled howler monkeys grow older they show a grey or brownish mantle on their shoulders, most notable on the males. The Mexican black howler monkeys do not have this aspect.
@carlos55 I see, now I understand the differences between these two species of howler monkeys. (I compared photos of these two species of howler monkeys, but the only difference seemed to be the presence or absence of a mantle on their shoulders.)
