In 2007 and 2008, I conducted a large-scale meta-study of great ape cognitive research by reading and categorizing the abstracts of every great ape paper in the PsycINFO database published between 1992 and 2006.

In addition to significant growth of the field, I found an increasing emphasis on socially-oriented research. Though studies concerning physical cognition (such as problem solving) maintained a plurality at 16.6% of the total number of papers, research into social cognition (14.9%) increased steadily over my time period until it overtook physical cognition in the 2004-2006 sample.

The significant majority of research (57.7%) was conducted in the lab, while 28% was conducted in the wild, 2.8% in both the lab and the wild, and 5.3% in neither. Chimpanzees remained the dominant species under study, appearing in more than 65% of the published papers. Chimps were followed by gorillas (12.6%), bonobos (6.1%) and orangutans (4.9%).10.6% of studies concerned multiple species; of these, the vast majority compared another species with chimpanzees.

It is lamentable that a fast-growing interest in the field occurred alongside an alarming decrease in wild great ape population. Accurate censuses are nearly impossible to take, but even the highest estimates place the total number of great apes in the wild at just over 400,000, worldwide, and declining for all subspecies for which conclusions can be drawn. All subspecies are at least listed as endangered; the western gorilla and Sumatran orangutan, critically so.

As far as future research is concerned, loss and deterioration of habitat is the most relevant and dangerous cause of decline. As wooded passages are destroyed, the increasing confinement and isolation of great ape populations causes dangerous homogeneity and increased vulnerability to disease. Since diseases are easily spread between humans and other apes, researchers who need to work in close contact with wild apes must take strenuous precautions and even question the ethics of their research: as necessary and useful as these field studies are, the smaller and more splintered the population gets, the more dangerous they become. These problems, combined with the fact that each ape group under consideration must be habituated to the presence of the human researchers, limits the majority of wild ape studies to a relatively small number of locations within Africa and Asia. Given these factors, it is not surprising that cognitive studies on captive apes greatly outnumber studies performed in the wild, in spite of the enormous expense of keeping apes in captivity.

You can read the study in its entirety here: http://www.wesleyan.edu/psyc/mindmatters/volume03/article02.pdf