BEYOND STRUCTURE. “Although we usually focus on ‘useless’ (apparently non-functional) structural features, other aspects of an organism’s biology might be considered vestigial. These range from biochemical reactions or physiological properties, such as the mammalian dive reflex (present in humans but in much-reduced form relative to other mammals.”
https://evolution-outreach.springeropen.com/articles/10.1186/s12052-014-0012-5