Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:In addition to the three study species, yellow baboons (Papio cynocephalus) and Sanje mangabeys (Cercocebus sanjei) are also present but, being predominantly terrestrial, are rarely sighted from line transects (Rovero et al. 2006, 2012), and the former occur only along the lowland forest edge.
Category: