Knott, C. 
"Changes in orangutan diet, caloric intake and ketones in response to 
fluctuating fruit availability." 

Int J Primatol (1998).19: 1061-1079.
 
This article addresses ketosis in apes when seasonal fruit is not available.  
The researcher spent several years following Orangs catching their urine on keto 
sticks when they let loose from the treetops ( and you thought your job sucked 
).  

 
All the apes were in ketosis for about half the year with the heaviest levels 
found in nursing and pregnant females.