Sophie's in the Congo for the summer, where she helps the bonobos at her mom's sanctuary. Bonobos are an endangered species, and mothers get killed in the wild so that the locals can sell the babies as pets. That is how Sophie meets Otto. The ape is in horrible shape, and he pulls at her heartstrings. She resolves to give him a better life. 

It is supposed to be back to Miami and her dad in time for school in September, but those plans get shattered when the Congolese president is assassinated. Anarchy ensues, and the rebels take over the town with their guns and machetes. They steal anything and everything they can... even bonobos, for food. Sophie and Otto's lives are at stake.

Survival demands their complete attention. Avoiding aggressive bonobos and humans, finding leaves, insects and manioc to eat, and finding safe drinking water fills their days. Sophie also wants to ensure that her family is safe, and she would like to return to a peaceful civilization. 

Otto and Sophie hold each other close, and it keeps them going.  

Endangered by Eliot Schrefer, 2012

I loved learning about bonobos and reading a story about a girl who clearly loves the apes. They say that bonobo society is peaceful because food is plentiful in their habitat. Chimps, by contrast, must compete for food. Supposedly, that is why they murder and rape each other.

• Eliot Schrefer's web site
• Jane Goodall's web site 
• Bonobo Conservation Initiative
• Books about bonobos