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:This might be because females of many species usually work close to their maximum capacity regardless of their work load (e.g., brood size), while males tend to increase their amount of care with increasing work load (Santos and Nakagawa 2012).
Category: