Survival of the fattest: How body fat and migration influence survival in highly seasonal environments

Abstract

1. Energy stores and migration are important adaptations for animals in seasonal environments, but their roles may vary relative to an animal's endogenous and exogenous environment. In partially migratory populations, migrants and residents experience different seasonal environments; thus, the influence of energy stores on survival may differ relative to migratory tactic, with potential consequences to survival and fitness.2. Using data from Sierra Nevada bighorn sheep (Ovis canadensis sierrae; hereafter, Sierra bighorn), we tested the hypothesis that body fat (energy stores) buffers animals against their environment, but that buffering capacity differs across environments experienced by high-elevation residents (using a single range yearround), traditional migrants (making 1 round-trip movement between high-and low-elevation ranges during winter) and vacillating migrants (making ≥2 round trips between high-and low-elevation ranges during winter). We predicted that: for animals with high levels of body fat, survival would be high regardless of migratory tactic; residents would require larger stores of body fat to survive than migrants; energy stores would be least influential to survival for vacillating migrants.3. High levels of body fat in autumn (≥14% for females and ≥19% for males) largely buffered animals against harsh environments (survival >0.90) regardless of migratory tactic. At lower levels of body fat, traditional migrants had higher survival than residents. Vacillating migrants exhibited nearly 100% survival with no detectable effect of body fat on survival. 4. Collectively, these results support the hypothesis that body fat buffers animals against harsh environments but that the buffering capacity differed relative to the environment and highly flexible behaviours (i.e. vacillating migration) can allow animals to decouple survival from body fat. 5. Our work reveals that synergies between physiological and behavioural adaptations of animals in highly seasonal environments carry potential fitness consequences for individuals and demographic consequences for populations.

Survival of adult mountain goats in Washington: effects of season, translocation, snow, and precipitation

“…Although accounting for body condition would ideally have helped refine our models here, we were unable to do so because it was measured for less than half the sample of animals, and for those animals, would only have been a useful covariate during their first year after release. Positive relationships between body condition and vital rates have been documented in bighorn sheep (Stephenson et al 2020, Denryter et al 2022), elk (Bender et al 2008), and moose ( Alces alces ; Cook et al 2021 b ).…”

“…Alternatively, hot summers could lead to increased lignification of plant cell walls, and thus reduced forage digestibility, in turn reducing goat body condition (Bo and Hjeljord 1991, Lenart et al 2002, Pettorelli et al 2007, White et al 2011. We hypothesized that mountain goats in poor body condition would be particularly susceptible to further environmental challenges such as deep snow, and thus more likely to succumb in subsequent seasons (Monteith et al 2014, Denryter et al 2022. Pettorelli et al (2007) and Hamel et al (2009Hamel et al ( , 2010, we hypothesized that the speed of green-up would affect survival.…”

Body mass change over winter is consistently sex-specific across roe deer (Capreolus capreolus) populations

“…This is likely an adaptation to buffer against severe winters, as further south, in the heart of its range, over-winter body mass is generally stable and may even increase slightly (Hewison et al 1996, 2002). Larger body size (Linstedt & Boyce 1985) and the capacity to store fat (Trondrud et al 202, Denryter et al 2022) have been widely interpreted as adaptations which increase fasting endurance in response to the dramatic fluctuations of resource availability in highly seasonal environments.…”

Sex and size shape the ontogeny of partial migration

“…Migration, defined here as a seasonal round‐trip movement between breeding and non‐breeding areas, is driven by the need to cope with fluctuating resource availability and conditions in seasonal environments (Denryter et al., 2022; Somveille et al., 2015). However, in some migratory populations, the propensity to migrate can vary between individuals, such that both migrants and year‐round residents co‐exist; this is known as partial migration (Lack, 1944; Terrill & Able, 1988).…”