Chemosensory systems, particularly olfaction, are believed to have been ecologically important in primitive mammals (Rowe and Shepherd, 2016; Van Valkenburgh et al., 2014). Yet some mammalian clades (i.e. primates and cetaceans) have evolved a reduced olfactory system (Van Valkenburgh et al., 2014). The timing and causes of these reductions in the olfactory system among different clades of mammals are intriguing because they diverge from one of the fundamental aspects of what makes a mammal mammal-like. Primates have traditionally been viewed as having anatomically reduced olfactory systems (Cartmill, 2012; Le Gros Clark 1959; Smith and Bhatnagar, 2004; Smith et al., 2014; Van Valkenburgh et al., 2014) and enhanced visual systems (Barton et al., 1995; Cartmill, 1970; Kirk and Kay, 2004; Le Gros Clark, 1959; Sussman et al., 2013). This pattern led early investigators to hypothesise a trade-off between olfactory and visual systems (Fobes and King, 1982; Le Gros Clark, 1959). If metabolically expensive sensory systems no longer contribute to fitness, selection may be relaxed, resulting in reduced anatomy reflecting reduced functionality (Nummela et al., 2013). Following the initial proposal of the trade-off hypothesis, sensory system evolutionary studies have disproportionately focused on examining the visual system (Barton et al., 1995; Cartmill, 1970; Kirk and Kay, 2004; Ross and Kirk, 2007; Veilleux and Kirk, 2014), while the olfactory system has received relatively little attention.