Making no Bones About it: Bone Mineral Density Changes Seasonally in a Nonhibernating Alaskan Rodent

Abstract

High-latitude voles and lemmings undergo strong seasonal changes in their behavior and physiology, which may lead to concurrent changes in bone mineral density (BMD). We tested whether the BMD of northern red-backed voles (Myodes rutilus) in Alaska changed seasonally, and if so, whether these changes in their weight-bearing bones were correlated with seasonal changes in photoperiod (a mediator of activity and concentrations of reproductive hormones in high-latitude voles and lemmings), body mass, body length, or a combination of these. We used dual-energy X-ray absorptiometry to measure the BMD of the femur and humerus of voles collected in different seasons. BMDs increased dramatically from the start of spring to their peak level in early summer, and then decreased gradually to their lowest point in late winter. BMDs were significantly lower in fall and winter than in spring and early summer. BMDs of long bones were significantly correlated with both body mass and photoperiod, which accounted for 46.2% and 45.7% of the variation in the BMDs of femur and humerus, respectively. The strong changes that we observed in BMD are likely to be due, in part, to the combined effects of strong seasonal changes in body mass, activity, and baseline levels of reproductive hormones.

Raccoon dog model shows preservation of bone during prolonged catabolism and reduced physical activity

“…Previous histological data point to a reduction of bone formation in wintering raccoon dogs (Nieminen et al, 2011). The present results suggest a slight imbalance in bone remodeling with net bone loss, but it is not yet known whether raccoon dogs only minimize bone loss during passivity or whether they would also be able to recover their bone properties after remobilization (see Stevenson et al, 2009, for Myodes rutilus voles). The paradox of the bone resorption and formation imbalance while preserving bone remains unresolved in both bears and raccoon dogs.…”

The seasonality of reproduction in photoperiod responsive and nonresponsive northern red-backed voles (Myodes rutilus) in Alaska

“…This indicates that voles in this region do not typically time reproduction through plant secondary compounds (see Negus and Berger 1977;Berger et al 1981;Negus and Berger 1998) because they begin their reproductive change well before there are any freshly sprouted monocots available for them to ingest. Body mass and bone-mineral density in this species begin to increase at the end of late winter and rapidly increased in early spring (Stevenson et al 2009;Figs. 2A-2C).…”

“…Although a larger body size may, by virtue of its lower surface area : volume ratio, resist heat loss more effectively than a smaller one, the benefit of lower total-tissue maintenance appears more important to winter survival in some arvicoline rodents. Nonresponsive voles may, however, be able to take advantage of both a large body size and lower individual maintenance costs in winter through communal nesting (West 1977;Wolff and Lidicker 1981;West and Dublin 1984;Stevenson et al 2009). Voles huddle together and become one larger ''vole unit'' to resist heat loss, separating only to forage.…”

“…Locating voles in winter was challenging, even when subnivean nest boxes were used. Animals trapped in fall and winter were often found in pairs or groups (West 1977;Stevenson et al 2009), suggesting communal nesting.…”

“…The costs of gestation and lactation can more than double total energy requirements in voles (Kacmarski 1966;Migula 1969;Millar 1977;Innes and Millar 1981), and they can triple or quadruple energetic costs in other mammalian taxa (Bronson 1989;Clutton-Brock 1991). Both male and female arvicoline rodents live a much faster pace of life in spring and summer through increases in mating interactions, activity, foraging, and growth rates (Stebbins 1974;Batzli and Esseks 1992;Zuercher et al 1999;Getz et al 2005;Stevenson et al 2009). In fact, voles born in spring have been known to reach maturity and produce litters by the end of summer in the same year (Whitaker 1996).…”