Varied responses of functionally different vertebrate pollinators to floral scent traits in <i>Banksia</i> (Proteaceae)

In many plants, pollinators are attracted to the flowers by scent. Among plants, Banksia (Proteaceae) is exceptional in being pollinated by several functional groups of vertebrates, including honeyeater birds, members of two marsupial orders, and rodents. Here, we test the hypothesis that in Banksia interspecific variation in the frequency of floral visitation by functionally different vertebrate pollinators can be explained by floral scent traits. We used GC-MS to analyse floral volatiles of nine co-occurring Banksia species and tested for correlations between floral visitation by vertebrate pollinators quantified through camera trapping and (i) the abundance in floral headspace of three candidate mammal-attracting compounds, and (ii) overall scent richness (number of volatiles detected). Visitation by non-flying mammals, but not birds, was positively correlated with scent richness, aligning with general trends for other functional groups of pollinators that use olfaction when foraging for nectar. Visitation by native and introduced mice, but not by Honey possums, was correlated with median abundance of 3-methyl-1-nitrobutane and 2-methyl-1-nitrobutane - structural isomers of 1-nitropentane, a known semiochemical in Black rat that is attractive to both sexes. Further, the known rodent semiochemicals, sulcatone (6-methyl-5-hepten-2-one) and 2-methylbutanoic acid were detected in species frequently visited by mice. Our results demonstrate that within mixed-vertebrate pollination systems, the frequency of visitation by functionally different vertebrate pollinators can be partially explained by interspecific variation in floral scent chemistry. While behavioural tests are needed to confirm attractiveness of specific compounds, our results provide the first evidence for chemical adaptation to pollination by rodents in Australian flora.