RESULTS
In the mark-recapture experiment conducted on the mainland, we marked
and released 1714 snails in total. We re-encountered 337 of the snails
released in the field (257 living and 80 dead snails). We recaptured
some snails more than once, and the maximum number was four times. Most
of the snails on the mainland had bright shells, and the distribution of
the shell colour was unimodal (Figure 2A), while the shell colour
distribution of the island was bimodal (Figure 2B).
Some recaptured dead snails had evidence of shell crushing by predators
(Figure 3). There were 37 and 8 such snails on the mainland and island,
respectively. More snails were predated on the mainland than the island
(Figure S1; Fisher-exact test, odds = 9.46, P < 0.001).
In the GLMM analysis, the number of predated snails was not
significantly correlated with the shell colour in both study sites
(Table 1; z-value = 2.86, P > 0.05). However, the
number of predated snails was correlated with growth stage on the
mainland (Table 1). This result implies that predators attacked more
adult snails than juvenile ones on the mainland. We conducted
trail-camera experiments to identify the predators and filmed eight
mammals, four birds, and three reptiles within the monitoring snails’
camera range (Table S1). Of these, the only large Japanese field mouseApodemus speciosus was attacking the snails (Figure 3C; Movies
S1-S3 in Dryad).
The P-spline regression showed that the survival rates excluding
predation effects of mainland adult snails were not bimodal but
monomodal in terms of shell colour (Figure 4A). The adult survival rate
was estimated using a second-order polynomial function of shell colour
in the MCMC simulations based on the Bayesian framework, and the median
of the nonlinear selection gradient \(\gamma\) was -1.05 (Table 2). The
negative value of \(\gamma\) supports that the P-spline regression curve
was concave upward. Although the 95% Bayesian confidence interval (BCI)
of \(\gamma\) was [-2.47, 0.33], 94.2% of \(\gamma\) values
estimated in the simulation runs were negative. In contrast, the
survival rates of mainland juvenile snails were not monomodal (Figure
4A), and the 95% BCI of \(\gamma\) included 0 (Table 2). The other
parameters estimated from the Bayesian method are shown in Tables S2-S4.