Translating PUFA omega 6:3 ratios from wild to captive hibernators (Urocitellus parryii) enhances sex-dependent mass-gain without increasing physiological stress indicators

Abstract

Omega 3 polyunsaturated fatty acids (PUFAs) are well-documented for their influence on health and weight loss. Recent studies indicate omega 3 PUFAs may exert a negative impact on cellular stress and physiology in some hibernators. We asked if physiological stress indicators, lipid peroxidation and mass gain in Arctic Ground Squirrels (AGS) were negatively influenced by naturally occurring dietary omega 3 PUFA levels compared to omega 3 PUFA levels found in common laboratory diets. We found plasma fatty acid profiles of free-ranging AGS to be high in omega 3 PUFAs with balanced omega 6:3 ratios, while standard laboratory diets and plasma of captive AGS are high in omega 6 and low in omega 3 PUFAs with higher omega 6:3 ratios. Subsequently, we designed a diet to mimick free-range AGS omega 6:3 ratios in captive AGS. Groups of wild-caught juvenile AGS were either fed: (1) Mazuri Rodent Chow (Standard Rodent chow, 4.95 omega 6:3 ratio), or (2) balanced omega 6:3 chow (Balanced Diet, 1.38 omega 6:3). AGS fed the Balanced Diet had plasma omega 6:3 ratios that mimicked plasma profiles of wild AGS. Balanced Diet increased female body mass before hibernation, but did not influence levels of cortisol in plasma or levels of the lipid peroxidation product 4-HNE in brown adipose tissue. Overall, as the mass gain is critical during pre-hibernation for obligate hibernators, the results show that mimicking a fatty acid profile of wild AGS facilitates sex-dependent mass accumulation without increasing stress indicators.

Keywords: Ground squirrel; Hibernation; Omega 3; PUFA.

Fig. 1

The ratios of omega 6:3 in plasma of wild AGS in summer (n = 9), wild AGS in early fall (n = 16) and fall captive Balanced Diet AGS (n = 9) are significantly lower than Standard Rodent Chow fed AGS in summer (n = 10) and early fall (AGS n = 18) (p < 0.001, ANOVA, Tukey’s post-hoc test). Feeding a balanced omega 6:3 diet reduces the omega 6:3 ratio in plasma compared to captive AGS fed Standard Rodent Chow and mimics the plasma lipid profile of wild AGS. Different superscripts (a or b) signify that groups are different (p < 0.001). Data incorporates previously published data on Fall Balanced Diet (n = 9) and Fall Standard Rodent Chow AGS (n = 9) (Rice et al. 2021). Summer sampling occurred in very early July and fall sampling occurred in August, autumn time in the Arctic. Data shown are means ± SEM

Fig. 2

Diet did not influence cortisol in plasma or 4-HNE in brown adipose tissue (BAT), but influenced white adipose tissue (WAT) cortisol a Plasma cortisol measured in pre-hibernation and various stages of hibernation (early torpor, late torpor and arousal) did not differ between AGS fed either Standard Rodent Chow or Balanced Diet (mixed-effects analysis, FDR corrected). b Cortisol levels in WAT were higher in AGS fed the Balanced Diet prior to hibernation, but higher in Standard Rodent Chow AGS during the arousal phase of hibernation (*p < 0.05, mixed-effects analysis, FDR corrected). c Neither diet significantly increased 4-hydroxynonenal (4-HNE), a marker of lipid peroxidation, in AGS BAT (two-way ANOVA, FDR corrected). Some AGS 4-HNE measurements fell slightly below the standard curve base. No significant differences were found between sexes, the data shown are means ± SEM. Fall Standard Rodent Chow n = 11, Fall Balanced Diet n = 12; Early Torpor, Late Torpor and Arousal are n = 3–4 per diet

Fig. 3

a The average body mass of females shows a significant increase in pre-hibernation for a Balanced Diet compared to Standard Rodent Chow AGS (*p < 0.05, two-way repeated measures, FDR corrected). b The average body mass of males tracked before and during the hibernation season do not differ significantly between the two diets (two-way repeated measures, FDR corrected). Data shown are means ± SEM, Standard Rodent Chow n = 11, Balanced Diet n = 12