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:…or species’means and treated analytically as measurement error (Careau et al. 2008), but researchers have since demonstrated that individual differences in baseline metabolic rate can persist over the lifetime of individuals (Nespolo and Franco 2007; Burton et al. 2011; White et al. 2013).
Category: