Plasticity and repeatability of activity patterns in free-living Arctic ground squirrels

In indigenous arctic reindeer and ptarmigan, circadian rhythms are not expressed during the constant light of summer or constant dark of winter, and it has been hypothesized that a seasonal absence of circadian rhythms is common to all vertebrate residents of polar regions. Here, we show that, while free-living arctic ground squirrels do not express circadian rhythms during the heterothermic and pre-emergent euthermic intervals of hibernation, they display entrained daily rhythms of body temperature ( T b ) throughout their active season, which includes six weeks of constant sun. In winter, ground squirrels are arrhythmic and regulate core body temperatures to within ±0.2°C for up to 18 days during steady-state torpor. In spring, after the use of torpor ends, male but not female ground squirrels, resume euthermic levels of T b in their dark burrows but remain arrhythmic for up to 27 days. However, once activity on the surface begins, both sexes exhibit robust 24 h cycles of body temperature. We suggest that persistence of nycthemeral rhythms through the polar summer enables ground squirrels to minimize thermoregulatory costs. However, the environmental cues (zeitgebers) used to entrain rhythms during the constant light of the arctic summer in these semi-fossorial rodents are unknown.

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The secret life of ground squirrels: accelerometry reveals sex-dependent plasticity in above-ground activity

Royal Society Open Science ◽

10.1098/rsos.160404 ◽

2016 ◽

Vol 3 (9) ◽

pp. 160404 ◽

Cited By ~ 13

Author(s):

Cory T. Williams ◽

Kathryn Wilsterman ◽

Victor Zhang ◽

Jeanette Moore ◽

Brian M. Barnes ◽

...

Keyword(s):

Activity Patterns ◽

Ground Squirrels ◽

Thermal Exchange ◽

Active Season ◽

Below Ground ◽

Overall Dynamic Body Acceleration ◽

Second Year ◽

Arctic Ground Squirrels ◽

Daily Activity Patterns ◽

Variance Explained

The sexes differ in how and when they allocate energy towards reproduction, but how this influences phenotypic plasticity in daily activity patterns is unclear. Here, we use collar-mounted light loggers and triaxial accelerometers to examine factors that affect time spent above ground and overall dynamic body acceleration (ODBA), an index of activity-specific energy expenditure, across the active season of free-living, semi-fossorial arctic ground squirrels ( Urocitellus parryii ). We found high day-to-day variability in time spent above ground and ODBA with most of the variance explained by environmental conditions known to affect thermal exchange. In both years, females spent more time below ground compared with males during parturition and early lactation; however, this difference was fourfold larger in the second year, possibly, because females were in better body condition. Daily ODBA positively correlated with time spent above ground in both sexes, but females were more active per unit time above ground. Consequently, daily ODBA did not differ between the sexes when females were early in lactation, even though females were above ground three to six fewer hours each day. Further, on top of having the additional burden of milk production, ODBA data indicate females also had fragmented rest patterns and were more active during late lactation. Our results indicate that sex differences in reproductive requirements can have a substantial influence on activity patterns, but the size of this effect may be dependent on capital resources accrued during gestation.

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Changing seasonality and phenological responses of free-living male arctic ground squirrels: the importance of sex

Philosophical Transactions of the Royal Society B Biological Sciences ◽

10.1098/rstb.2012.0480 ◽

2013 ◽

Vol 368 (1624) ◽

pp. 20120480 ◽

Cited By ~ 24

Author(s):

Michael J. Sheriff ◽

Melanie M. Richter ◽

C. Loren Buck ◽

Brian M. Barnes

Keyword(s):

Snow Cover ◽

Ground Squirrels ◽

Free Living ◽

Male And Female ◽

Reproductive Maturation ◽

Changing Patterns ◽

Arctic Ground Squirrels ◽

Northern Alaska ◽

The Impact

Many studies have addressed the effects of climate change on species as a whole; however, few have examined the possibility of sex-specific differences. To understand better the impact that changing patterns of snow-cover have on an important resident Arctic mammal, we investigated the long-term (13 years) phenology of hibernating male arctic ground squirrels living at two nearby sites in northern Alaska that experience significantly different snow-cover regimes. Previously, we demonstrated that snow-cover influences the timing of phenological events in females. Our results here suggest that the end of heterothermy in males is influenced by soil temperature and an endogenous circannual clock, but timing of male emergence from hibernation is influenced by the timing of female emergence. Males at both sites, Atigun and Toolik, end heterothermy on the same date in spring, but remain in their burrows while undergoing reproductive maturation. However, at Atigun, where snowmelt and female emergence occur relatively early, males emerge 8 days earlier than those at Toolik, maintaining a 12-day period between male and female emergence found at each site, but reducing the pre-emergence euthermic period that is critical for reproductive maturation. This sensitivity in timing of male emergence to female emergence will need to be matched by phase shifts in the circannual clock and responsiveness to environmental factors that time the end of heterothermy, if synchrony in reproductive readiness between the sexes is to be preserved in a rapidly changing climate.

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