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:Furthermore, it differs from the characteristics of apparent “active” urea transporters in the kidneys of amphibians (Schmidt-Nielsen and Shrauger 1963) and teleost fish (McDonald et al. 2000; McDonald et al. 2002), where thiourea is transported more effectively than acetamide.
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