The island syndrome hypothesis is only partially validated in two rodent species in an inland–island system

Abstract

According to the island syndrome and island rule hypotheses, island isolation and reduced area lead to phenotypic shifts in ecologically relevant traits in insular populations compared to mainland ones. These hypotheses have been built up with oceanic islands in mind or islands where isolation is high and colonization rate relatively limited. This set of hypotheses, however, may not be applicable to other inland–island systems or recently fragmented landscapes. We investigated how island life leads to phenotypic changes in two rodent metapopulations: deer mice and red‐backed voles in a fragmented system on a river in Canada. From 2013 to 2016, we sampled 454 deer mice and 665 red‐backed voles spread across 10 islands and six mainland sites on river shores. We compared body mass, tail and hindfoot length, exploration and reaction towards predators, between individuals from island and mainland sites, and assessed the role of connectivity, isolation and area of islands. Insular individuals from both species were less aggressive towards potential predators and insular mice were more thorough explorers compared to mainland ones. Male mice were heavier and juvenile mice had longer tails on islands. However, contrary to expectations, we found negative relationships between aggressiveness and juvenile exploration score with patch connectivity for mice. Island connectivity significantly affected tail length of mice through a concave relationship. Finally, vole aggressiveness and exploration of male mice were positively correlated to island isolation. Our study supports only partially the predictions of the island syndrome hypothesis. It provides empirical evidence that inland–island life can modify traits in deer mice, but less so in red‐backed voles in a non‐oceanic system. It also raises questions about the direct causal factors of these changes in a naturally fragmented landscape, and urges for more predictive models about phenotypic/genetic divergence among populations in natural and artificial fragmented landscapes.

Epigenetics and island-mainland divergence in an insectivorous small mammal

“…Masked shrews do not follow the classic island rule (i.e., small-bodied mammals increase in size on islands) since some insular populations appear to have decreased body mass compared to their mainland counterparts (e.g., Folinsbee, 1971). BPI masked shrews occur in high densities (Downie, 1986; Stewart et al, 1989; Telfer, 1984), which should lead to small animals having larger body size to better compete for resources (Juette et al, 2020). However, the discrepancy with this expected phenotype for island shrews might be due to their diet.…”

“…The 'island rule' hypothesizes that smaller animals tend to increase in size, while larger ones tend to decrease in size on islands (Lomolino, 1985;Lomolino et al, 2013;van Valen, 1973). This pattern has been observed in mammoths (Palaeoloxodon falconeri) (Herridge & Lister, 2012) and deer mice (Peromyscus maniculatus) (Miller et al, 2021), but counter and ambiguous examples exist (Juette et al, 2020;Lokatis & Jeschke, 2018;Meiri et al, 2006Meiri et al, , 2008.…”

How to behave when marooned: the behavioural component of the island syndrome remains underexplored

“…[ 62 ] argued how dispersal-related environmental filtering could be responsible for the higher explorativeness in island common frogs and tadpoles [ 62 ]. By contrast, deer mice ( Peromyscus maniculatus ) on islands in the Canadian Winnipeg River tended to be less explorative than mainland conspecifics, possibly reflecting differences between inland and oceanic island systems [ 67 ]. Camperio Ciani et al .…”

“…The empirical evidence on changes in aggression towards (but not necessarily limited to) conspecifics go either way. Reduced antagonism in island populations, as the result of high densities that make the monopolization of resources ineffective [ 124 ], was reported in five studies on a few rodent species ( P. maniculatus [ 68 , 69 ], Myodes gapperi [ 67 ] and M. musculus [ 77 ]) and birds [ 11 ]. Interestingly, Baier & Hoekstra [ 68 ] found that the difference faded in the F1 generation, indicating an important role for phenotypic plasticity [ 68 ].…”

“…Brodin et al [62] argued how dispersal-related environmental filtering could be responsible for the higher explorativeness in island common frogs and tadpoles [62]. By contrast, deer mice (Peromyscus maniculatus) on islands in the Canadian Winnipeg River tended to be less explorative than mainland conspecifics, possibly reflecting differences between inland and oceanic island systems [67]. Camperio Ciani et al showed that human inhabitants of the Egadi islands are less open to new experiences than their compatriots in mainland Italy, an observation they attribute to disproportionally high emigration of individuals with neophilic personalities [81,82].…”