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:Comparison of testes size in lemurs and the sister taxon, the lorises, however, revealed no grade shift between the two groups of strepsirrhines in a previous study (Kappeler 1997a), making this explanation unlikely.
Category: