Rejoinder: challenge and opportunity in the study of ungulate migration amid environmental change

On the adaptive benefits of mammal migration

“…By tracking locations 3 with vegetation growing at a fast rate, a migrating animal could better obtain a continuous supply 4 of young nutritious re-growth. This pattern of habitat use is consistent with migratory behavior in 5 many large herbivores , having been shown to well explain seasonal elegantly demonstrated that some migrating red deer (Cervus elaphus L., 1758) in Norway 10 follow the wave of vegetation green-up as it moves from the coastal regions to the mountains, 11 whereas others 'jumps ahead' of the wave, reaching their summer ranges well before the arrival 12 of spring. 13 Energy intake is often a function of local consumer density.…”

“…It is the scale and 8 repeatability of these spatiotemporal patterns that dictate the class of movement they give rise to 9 (Mueller and Fagan 2008; Mueller et al 2011). According to this view, migratory behavior 10 should arise if occupying specific ranges at specific times of the year results in higher fitness 11 gains than range residency (occupying the same range year-round) or nomadism (occupying 12 different ranges with no annual pattern). 13 A minimalist view of the evolution of migration that is particularly applicable to most 14 mammals has been formalized recently (Holt and Fryxell 2011;Fryxell and Holt 2013).…”

“…Thermal costs are often 6 difficult, however, to tease apart from other factors, such as food availability, due to the inherent 7 co-variation between productivity and climate. One clear example of thermoregulatory costs on 8 migration is the annual migration of tree dwelling insectivorous bats from their northern 9 breeding ranges to spend the winter in milder southern climates (Fleming and Eby 2003; 10 Rodrigues and Palmeirim 2008; Popa-Lisseanu and Voigt 2009). Like non-migratory bats, these 11 migratory bats spend most winter in torpor hibernation.…”

“…Parasites are known to 21 inflict severe metabolic costs on their hosts, either directly (i.e. via consumption of nutrients and 22 or tissue) or, in the case of some ectoparasites, indirectly, through ear flapping, tail wagging and 23 reduction in time spent foraging or resting (Hughes et al 2009 10 in the energy budget of many marine mammals (Geraci and St Aubin 1987), little is known about 11 transmission dynamics and host density dependence and we know of no established links to 12 migratory behavior. 13 From a mechanistic perspective, migratory escape from parasites may be possible due to 14 the inability of most ectoparasites to adapt to different environmental conditions characterizing 15 each seasonal range occupied by the host.…”

“…In practice, however, it will often be difficult to tease apart causal relationships between parasites and host migratory behavior as both are strongly 1 influenced by seasonality (Altizer et al 2006). Alternatively, moderate levels of risk en-route should still allow the evolution of migratory 10 behavior if there is sufficient reduction in predation pressure or increase in energy gain over the 11 entire annual cycle. 12 Predation has been elegantly demonstrated as the main driver of diel vertical migration in 13 zooplankton (Gliwicz 1986) but less is known about its role in shaping seasonal migratory 14 behaviors in mammals.…”