The genetics of morphological and behavioural island traits in deer mice

Animals on islands often exhibit dramatic differences in morphology and
behaviour compared to mainland individuals, a phenomenon known as the
"island syndrome". These differences are thought to be
adaptations to island environments, but the extent to which they have a
genetic basis or instead represent plastic responses to environmental
extremes is often unknown. Here, we revisit a classic case of island
syndrome in deer mice (Peromyscus maniculatus) from British Columbia. We
first show that Saturna Island mice and those from neighbouring islands
are ~35% (~5g) heavier than mainland mice and diverged approximately 10
thousand years ago. We then establish laboratory colonies and find that
Saturna Island mice are heavier both because they are longer and have
disproportionately more lean mass. These trait differences are maintained
in second-generation captive-born mice raised in a common environment. In
addition, island-mainland hybrids reveal a maternal genetic effect on body
weight. Using behavioural testing in the lab, we also find that
wild-caught island mice are less aggressive than mainland mice; however,
lab-raised mice born to these founders do not differ in aggression.
Together, our results reveal that these mice have different responses to
the environmental conditions on islands – a heritable change in a
morphological trait and a plastic response in a behavioural trait.