Ernie Janes / Black howler (Alouatta caraya) male and female, captive, occurs in Brazil and Paraguay. With digitally added leaf pattern.

Suzi Eszterhas / Colombian red howler monkey (Alouatta seniculus) mother and baby in tree. Northern Colombia.

Ernie Janes / Black howler (Alouatta caraya) captive, occurs in South America.

Eric Baccega / Pack of Grey wolves (Canis lupus) howling in the snow, captive. Bavarian Forest National Park, Germany, March. Captive.

Kevin Schafer / Guatemalan Black Howler Monkey (Alouatta pigra) calling, Community Baboon Sanctuary, Belize, Central America. Endangered species.

Kevin Schafer / Guatemalan Black Howler Monkey (Alouatta pigra) climbing, Community Baboon Sanctuary, Belize, Central America. Endangered species.

Juan Carlos Munoz / Black howler monkeys (Alouatta caraya) male and two females calling from tree, captive, Apenheul Park, Netherlands.

Kevin Schafer / Guatemalan black howler monkey (Alouatta pigra) sitting in tree howling, Community Baboon Sanctuary, Belize. Endangered species.

Eric Baccega / Two European grey wolves (Canis lupus) howling in winter landscape, captive. Bayerischerwald National Park, Germany.

Eric Baccega / European grey wolf (Canis lupus) howling in snow covered forest, captive. Bayerischerwald National Park, Germany.

Mark Bowler / Red howler monkey (Alouatta seniculus) howling, captive

Mark Bowler / Red howler monkey (Alouatta seniculus) howling, captive, digitally modified