Data from: Scented nectar and the challenge of measuring honest signals in pollination

1. Nectar scents are thought to function as honest signals of reward used
by pollinators, but this hypothesis has rarely been tested. 2. Using
Penstemon digitalis, we examined honest signalling of the nectar volatile
(S)-(+)-linalool and pollinator responses to linalool in both field and
laboratory settings. Because our previous work showed that linalool
emission was associated with higher female fitness and that nectar is
scented with linalool, we hypothesized that linalool was an honest signal
of nectar reward. To assess honesty, we measured linalool-nectar
associations including nectar volume, sugar amount, concentration, and
production rate for inflorescences and flowers in several populations. We
also assessed whether Bombus impatiens, the main pollinator of P.
digitalis at our sites, can use linalool as a foraging signal. We
supplemented real or artificial flowers in the field and lab with varying
linalool-nectar combinations to measure pollinator behavioural responses.
3. We found that an inflorescence’s linalool emissions could be used to
predict nectar rewards in P. digitalis, but this was driven by indirect
associations with display size rather than directly advertising more
profitable flowers. For flowers within inflorescences there was also no
evidence for an association between signal and reward. Field tests of
bumblebee behaviour were inconclusive. However, in laboratory assays
bumblebees generally used variation in linalool emissions to choose more
profitable flowers, demonstrating they can detect differences in linalool
emitted by P. digitalis and associate them with reward profitability.
These results suggest experiments that decouple display size, scent and
reward are necessary to assess whether (and when) bees prefer higher
linalool emissions. Bees preferred nectars with lower linalool
concentrations when linalool flavoured the nectar solution, suggesting
potential for conflicting pressures on scent emission in the field. 4.
Synthesis: Our results highlight the challenges of assessing function for
traits important to fitness and suggest that the perception of floral
signalling honesty may depend on whether pollinators use inflorescences or
flowers within inflorescences when making foraging decisions. We conclude
that future research on honest signalling in flowering plants, as well as
its connection to phenotypic selection, should explicitly consider among-
and within-inflorescence honesty, in theoretical and experimental
contexts.