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:However, this resistance to drought is much lower than observed for many Pulmonates which may survive several months or even more to the desiccation (Barbosa and Barbosa 1959; Cridland 1967; Véra et al. 1994).
Category: