Energy regulation in context: Free-living female arctic ground squirrels modulate the relationship between thyroid hormones and activity among life history stages

Thyroid hormones (THs), key regulators of lipid and carbohydrate metabolism, are likely modulators of energy allocation within and among animal life history stages. Despite their role in modulating metabolism, few studies have investigated whether THs vary among life history stages in free-living animals or if they exhibit stage-specific relationships to total energy expenditure and activity levels. We measured plasma total triiodothyronine (tT₃) and thyroxine (tT₄) at four, discrete life history stages of female arctic ground squirrels from two different populations in northern Alaska to test whether plasma THs correlate with life history stage-specific changes in metabolic rate and energy demand. We also tested whether THs explained individual variation in aboveground activity levels within life history stages. T₃ peaked during lactation and was lowest during pre-hibernation fattening, consistent with known changes in basal metabolism and core body temperature. In contrast, T₄ was elevated shortly after terminating hibernation but remained low and stable across other life-history stages in the active season. THs were consistently higher in the population that spent more time above-ground but the relationship between THs and activity varied among life history stages. T₃ was positively correlated with activity only during lactation (r²=0.50) whereas T₄ was positively correlated with activity immediately following lactation (r²=0.48) and during fattening (r²=0.53). Our results support the hypothesis that THs are an important modulator of basal metabolism but also suggest that the relationship between THs and activity varies among life history stages.