Fig. 1.
Body temperature defines sampled states. Body temperature (Tb) of a laboratory-housed 13-lined ground squirrel (Ictidomys tridecemlineatus) over ∼8 mo. The hibernation label identifies the winter heterothermic period; the annual cycle of this species is completed by a spring-fall active, homeothermic period. Sample groups for the proteomics experiment were collected from eight defined states (n = 6 animals per state) and two transition states. Summer active (SA) and spring ground squirrels represented the homeothermic portions of the annual cycle [spring-cold (SpC) and spring-warm (SpW)]. Winter states representing the heterothermic period were defined by Tb telemetry as entering torpor (Ent) at 27°C > Tb > 23°C; late torpor (LT) at 80–95% previous bout duration; early arousing (EAr) at 7°C < Tb < 12.8°C; late arousing (LAr) at 18°C < Tb < 25°C; and 3–4 h after Tb stabilized in the interbout arousal (IBA) period. Spring animals were given food and water but remained in the hibernaculum (L:D 0:24). Spring-cold (SpC) were maintained at low ambient temperature (Ta = 4°C) and were killed 11–20 days after their last recorded torpor bout. Spring-warm (SpW) were collected 7–32 days after terminal arousal (Ta = 14–18°C). Samples were also collected during the highly variable periods of fall transition (FT; n = 28) and spring transition (SpT; n = 17) transition. Opportunistic histology samples were collected from fixed tissues as represented for the proteomics experiment with the exception of SA ground squirrels. SA tissues collected for the frozen tissue bank (proteomics) were obtained Aug. 4 and 5. Fixed SA tissues for histology were obtained June 24 and 25. Sample sizes from fixed tissues were SA (n = 4), FT (n = 4), Ent (n = 1), torpid for various durations (n = 4), EAr (n = 1), LAr (n = 1), IBA (n = 3), SpT (n = 2). To supplement the fixed tissues, we cryosectioned frozen samples from an additional n = 4 SpC, SA (August) and FT proteomics animals that had not yet used torpor.