Suzi Eszterhas / Red-shanked douc langur (Pygathrix nemaeus) infant sucking thumb while caretaker holds it. Animal was rescued from illegal wildlife trade, Endangered Primate Rescue Center, Cuc Phuong National Park, V...

Andy Rouse / Mountain gorilla (Gorilla beringei) baby held by mother, Rwanda

Anup Shah / Eastern Chimpanzee (Pan troglodytes schweinfurthii) female, forty years old, holding her two month old granddaughter, Gombe National Park, Tanzania

Cyril Ruoso / Barbary Macaque (Macaca sylvanus) infant, Morocco

Fiona Rogers / Bonobo (Pan paniscus) male baby aged 10 months with his mother 'Nioki', lying on his mother's foot 'sucking his thumb', Lola Ya Bonobo Sanctuary, Democratic Republic of Congo. October.

Ingo Arndt / Equatorial Saki (Pithecia aequatorialis) sucking its thumb, Amazon ecosystem, Peru

Suzi Eszterhas / Mountain Gorilla (Gorilla gorilla beringei) three month old infant sucking on thumb, endangered, Parc National Des Volcans, Rwanda

Mary McDonald / Young Olive baboon sucking thumb (Papio anubis) Nakuru NP, Kenya

Mary McDonald / Baby Olive baboon sucking thumb. (Papio anubis) Nakuru NP, Kenya

Owen Newman / Jo / Hoan bushman baby sucking his thumb, Bushmanland, Namibia. 1996

Suzi Eszterhas / Hoffmann's Two-toed Sloth (Choloepus hoffmanni) orphaned baby chewing on thumb of caretaker, Aviarios Sloth Sanctuary, Costa Rica

Flip Nicklin / Bearded Seal (Erignathus barbatus) pup sucks on researcher's finger for comfort, Norway