Hippopotamus (Hippopotamus amphibius) threat-yawning in the Khwai River under the rain, Khwai Concession, Okavango Delta, Botswana, Africa

Alligator by Turner River, Everglades, Florida, USA

Formosan or Taiwan macaque (Macaca cyclopis) adult male giving threat yawn while sitting on branch of banyan tree (Ficus microcarpus), Chaishan, Kaohsiung, Taiwan.

Hippopotamus (Hippopatamus amphibius), adult, in water, threatening, yawning, Saint Lucia Estuary, Isimangaliso Wetland Park, Kwazulu Natal, South Africa, Africa

Hippos (Hippopotamus amphibius), Tanzania, East Africa

Southern elephant seal (Mirounga leonina), male, after breeding period on the Falkland Islands. South America, Falkland Islands, January

Close-up of female cheetah (Acinonyx jubatus) yawning at camera, Serengeti, Tanzania

Growling bear (Ursus arctos); Fort Collins, Colorado, United States of America

Hippopotamus (Hippopotamus amphibius) splashing with open mouth in Kruger National park, South Africa

Hippopotamus, Hippopotamus amphibius, threat-yawning in the Khwai river in the Okavango Delta's Khwai concession. Botswana.

Hippopotamus (Hippopotamus amphibius) splashing with open mouth in Kruger National park, South Africa

Portrait of Hippo mouth open in water, Botswana

Hippopotamus in water, Khwai Okavango Delta Botswana