DISCUSSION
In this study, we first demonstrated only a large Japanese field mouse,A. speciosus, preyed on snails, but the predation frequency was
not correlated with shell colour (Figure 3; Table 1). To clarify the
persistence processes of the diversified shell colour in snails, we then
compared the colour variation and selection pressures of the snails
between the mainland and the island. The frequency distributions of the
shell colour showed that the colour variation was monomorphic on the
mainland, while it was dimorphic on the island (Figure 2). The
mark-recapture experiments showed that the survival rate excluding
predation effects of bright-shell snails was the highest on the
mainland, where predation pressure is higher than on the island (Figure
4A; Table 2).
The result of the survival rate of bright-shell snails being the highest
suggests that weak stabilizing selection acts on shell colour from a
factor other than predation on the mainland. This result differs from a
previous study, which showed that disruptive selection acts on the shell
colour on the island (Ito & Konuma, 2020). In the previous study, the
disruptive selection estimated from the Bayesian method did not exclude
predation effects. However, the proportion of dead snail shells from
predation was relatively low on the island. Thus, it has been suggested
that the cause of disruptive selection is also a factor other than
predation. Hence, both stabilising selection and disruptive selection
are caused by a factor other than predation. The stabilising selection
favours bright snails, which could be why the shell colour variation is
monomorphic on the mainland. In contrast, the disruptive selection
favours dark and bright snails over intermediate-coloured snails, which
could be why the shell colour variation is dimorphic on the island. The
difference of the selection pressures would then result in the
difference of shell colour diversity between the mainland and the
island.
Strong disruptive selection acted only on juveniles on the island, and
no natural selection acted on adults (Ito & Konuma, 2020). This pattern
is opposite to that of the mainland. The survival rate on the mainland
was lower among adults than juveniles, whereas that on the island was
higher for adults than juveniles. Therefore, natural selection might
only act on shell colour when the survival rate is low. The reason why
natural selection acted in juvenile only on the island might be that
post-zygotic isolation occurred (Wade, 2002; Coughlan & Matute, 2020).
The distribution of shell colour was bimodal, in which darker and
brighter snails existed sympatrically. The genetic relationship of the
snails was close on the island, and hybrids of darker and brighter
snails showed intermediate colour (Hayashi & Chiba, 2004; Ito &
Konuma, 2020). In contrast, there was a unimodal distribution of shell
colour on the mainland, in which almost all of snails were a brighter
colour. These results implied that the hybrid between dark and bright
shells exists only on the island and that they have a lower survival
rate since there is a genetic incompatibility between them.
In adults, one of the hypotheses for the cause of natural selection is
that shell colour has a function of thermal regulation (Di Lelliset al. , 2012; Schweizer et al. , 2019). For example, the
brighter shell could help to maintain an optimum body temperature when
exposed to sunlight in open environments, while a darker shell could be
advantageous when exposed to less sunlight in closed environments. The
altitude of the distributed area is higher on the islands than the
mainland (Figure 1). Thus, this geographical feature of the islands
might create complex environmental conditions such as heterogeneity of
habitats and lead to shell colour diversification (Ożgo & Schilthuizen,
2012). In contrast, predation from birds is an alternatives hypothesis
because bird predators promote shell colour diversification (Allenet al. , 1988; Kraemer et al. , 2019). In a predation
scenario, for example, a brighter shell colour has an advantage of
camouflage in an open habitat, while a darker shell would have the
opposite effect. Although no birds preyed on the snails that we studied,
some birds were filmed in our trail-camera experiments. Hence, this
possibility is lower than the environmental hypothesis for this snail,
as in the case of polymorphism in Cepaea nemoralis (Schilthuizen,
2013).
The high predation pressure would be a primary factor in reducing the
survival rates on the mainland. Our trail-camera experiments showed that
field mice were the main predators of the snails. The number of the
predated snails was not correlated with the shell colour in the
mark-recaptured experiments, so the field mice are unlikely to predate
snails based on dark or bright shells. In fact, rodents like this mouse
do not recognise colour because they are colour blind with two cone
types (Conway, 2007), so predation pressure from mice could occur
randomly regardless of shell colour (Rosin et al. , 2011). Our
results are consistent with this idea, so it is suggested that predation
from field mice can be only a factor that reduces the survival. In
addition, previous studies reported that field mice are poor at climbing
trees and dwell on the ground all year (Imaizumi, 1978; Nishikata,
1981). Thus, the mainland snails would face constant predation risk from
field mice on the ground. The adult snails need to come down from trees
to lay eggs on the ground (Inoue & Nakada, 1981), so the evidence of
predation would have been more frequently observed among adults than
juveniles on the mainland.
However, predation could be an indirect driver of persistence in
shell-colour monomorphism. Constant predation can result in restricted
habitat or microhabitat use to avoid predation (Losos et al. ,
2004; Lapiedra et al. , 2018). In such a case, a trait is fitted
to the adaptive optimum according to the habitat (Eklöv & Svanbäck,
2006). In contrast, a restricted trait is diversified when a release
from predation occurs (Stroud & Losos, 2016). In the Izu Islands, a
release from predation could have triggered the use of a variety of
habitats or microhabitats and then shell colour diversification.
In conclusion, our findings suggested that monomorphic shell colour
persisted in the source population due to stabilising selection. In
contrast, disruptive selection acted on shell colour on the island,
which was diversified in derived population. Although it is unknown
whether natural selection could be changed without predation effects,
the direct cause of changing natural selection was a factor other than
predation, such as environmental effects. In some land snails on
islands, adaptive radiation can occur, and then shell colour is rapidly
diversified (Davison & Chiba, 2006; Kraemer et al. , 2019). The
change of natural selection from stabilising selection to disruptive
selection that we have presented in this study could explain the
ultimate factor of such shell colour diversification in the process of
adaptive radiation, especially in an initial stage.