The Seasonality Of Reproduction, Body Composition, And Energy Expenditure In Northern Red-Backed Voles (Myodes Rutilus)

Abstract

Arvicoline rodents (voles and lemmings) inhabit high-latitude environments and undergo pronounced seasonal changes in their physiology and behavior. They are an important prey resource in circumpolar regions, and their population numbers can affect the survival and reproductive fitness of many predator and secondary prey species. I studied the effects of seasonality and environmental factors on reproduction and energy allocation in the northern red-backed vole (Myodes rutilus), an arvicoline rodent in Alaska known to have bred in winter. My overall aim was to measure the effects of season and environmental factors on the reproductive axis, body composition, and energy expenditure of this animal. I validated a dual-energy X-ray absorptiometry (DXA) apparatus for use in determining fat and lean tissue, body water, protein, and mineral content in M. rutilus (R2 = 0.65 to 0.98, p < 0.001 for all parameters). Absolute fat, but not percentage fat changed seasonally. Reproductive organ masses reached peak levels in spring (females) and early summer (males), and significant co-variates were photoperiod, temperature, snow cover, body mass, and percent fat (depending on breeding period and gender). I found one instance of late-summer male non-responsiveness, but no winter breeding. However, 28.2% of captive, lab-raised male voles were non-responsive to short days (ad lib. food and water at 20�C), which was within the 20-40% frequency range known for lower latitude species. Differences were found at the gonadal level and pituitary level (testosterone and luteinizing hormone (LH) either varied by group and/or were correlated with testis mass), while differences at the hypothalamic level (gonadotropin-releasing hormone immunoreactivity (GnRH-ir) and gonadotropin-inhibiting hormone (GnIH)-ir cell counts) were inconclusive. Body composition and relative visceral organ mass changed seasonally, and significant covariates were photoperiod (mass, %protein, %mineral), gender (intestines), and temperature (heart). Field metabolic rate did not differ by breeding period, but was significantly correlated with temperature. Bone mineral density (BMD) of voles was highest in early summer and lowest in winter, whereas the BMD of two hibernating mammals did not change during winter torpor. These findings could help to identify the mechanisms underpinning arvicoline rodent population cycling and to predict physiological and ecological responses of small mammals to different climate change scenarios.