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individual flowers have evolved traits of higher attractiveness to pollinators. We
showed in dual choice experiments in the wind-tunnel that individual male flow-
ers are indeed more attractive to the males of the main pollinator moth. Male
flowers, despite being smaller than female flowers, produced significantly higher
amounts of odor, both in the Swiss and Spanish population. The significant inter-
action between sex and population indicates that the sex differences in the amount
of scent produced is different among the two populations. Indeed, in the Spanish
population, the sex difference was less pronounced, and these plants emitted an
overall lower amount of scent. Population specific differences in sex specific traits
may be related to differences in pollinator composition, or differences in Swiss
and Spanish Hadena moths, however, data are not available.
Interestingly, we found that in both populations more of the compounds in-
volved in pollinator attraction, so-called active compounds, were significantly dif-
ferent among sexes, suggesting that selection for higher attractiveness is mediated
by the sensory ecology of the pollinator. Overall, our results strongly suggest that
in S. latifolia, scent is more important for the attractiveness of individual flowers
than size, as we used flowers of similar size in our behavioral assays. Other studies
on S. latifolia floral scent emission failed to detect a statistically significant higher
odor production by males [20], but similar trends were found [31]. Both studies,
however, were not designed to investigate the effect of gender on floral fragrance
and analyzed fewer plants than our study; given the usually high variation in floral
scent, large sample size is an important factor in detecting significant effects. Sex
differences in amounts of scent were also detected in other plant species, with
male plants releasing more attractive volatiles or higher amounts than conspecific
females [32,33]. Ashmann et al. (2005) showed that in the gynodioecious wild
strawberry Fragaria virginiana the smaller hermaphroditic flowers emitted signifi-
cantly more odor, which resulted in more visits by pollinators compared to con-
specific females [34]. However, these authors suggest that the odor of hermaph-
roditic flowers is preferred due to the production of unique compounds produced
by the anthers, rather than due to quantitative differences in odor production.
A positive association between scent concentration and attractiveness was also
found in earlier experiments, where higher odor concentration amplified the re-
sponse of the pollinators in wind tunnel bioassays with Hadena bicruris [17].
Schiestl (2004) showed that larger amounts of a biologically active floral odor
compound attracted more pollinators of the orchid Chiligottis trapeziformis [35].
In natural populations of S. latifolia, preferences of pollinators for male flowers
could be the result of learning, since male flowers produce higher sugar con-
centrations and thus higher quality rewards [13]. As we used naïve pollinators
for our experiments, learning should not have influence the preferred choice of
male flowers. Alternatively, preference of stronger odor sources may be due to the
