Abstract
Partial migration, a phenomenon wherein only some individuals within a population migrate, is taxonomically widespread. While well-studied in birds and fish, partial migration in large herbivores has come into the spotlight only recently due to the decline of migratory behavior in ungulate species around the world. We reviewed studies describing these trends to explore both population- and individual-level mechanisms for partial migration in ungulates. We addressed how density-dependent and -independent factors, alone or together, could maintain both migrants and residents within a population. We then searched for evidence that intrinsic and extrinsic factors could combine with genetic predispositions and individual differences in temperament or life experience to promote migratory tendencies of individuals. Despite the long-held assumption that migration is a fixed behavior of individuals, evidence suggested that changes in migratory behavior result from state-dependent responses of individuals. Data are needed to demonstrate empirically which factors determine the relative costs and benefits to using migratory versus resident tactics. We outline what types of long-term data could address this need and urge those studying migration to meet these challenges in the interest of conserving partially migratory populations.
Highlights
Dramatic declines in populations of migratory ungulates and the disappearance of migratory behavior in many ungulate species are recognized as a global conservation challenge (Berger, 2004; Bolger et al, 2008; Tucker et al, 2018)
Partial migration through an evolutionarily stable state (ESSt) could not happen if there is switching between tactics, which has been reported in deer (Odocoileus virginianus, Nelson, 1995), elk (Cervus elaphus, Eggeman et al, 2016), impala (Aepyceros melampus, Gaidet and Lecomte, 2013), moose (Alces alces, White et al, 2014), pronghorn (Antilocapra americana, White et al, 2007), Sierra Nevada bighorn sheep (Ovis canadensis sierrae, Spitz, 2015), and Svalbard reindeer (Rangifer tarandus platyrhynchus, Hansen et al, 2010; Meland, 2014; Table 3)
The age at which this hypothesis might occur in ungulates could be quite old; Eggeman et al (2016) showed potential evidence that elk became more likely to migrate with age in Alberta, Canada, but migrants rarely switched to a resident tactic after aging (>15 years old)
Summary
Dramatic declines in populations of migratory ungulates and the disappearance of migratory behavior in many ungulate species are recognized as a global conservation challenge (Berger, 2004; Bolger et al, 2008; Tucker et al, 2018). Partial migration through an ESSt could not happen if there is switching between tactics, which has been reported in deer (Odocoileus virginianus, Nelson, 1995), elk (Cervus elaphus, Eggeman et al, 2016), impala (Aepyceros melampus, Gaidet and Lecomte, 2013), moose (Alces alces, White et al, 2014), pronghorn (Antilocapra americana, White et al, 2007), Sierra Nevada bighorn sheep (Ovis canadensis sierrae, Spitz, 2015), and Svalbard reindeer (Rangifer tarandus platyrhynchus, Hansen et al, 2010; Meland, 2014; Table 3).
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