Endocrine rhythms in the brown bear (Ursus arctos): Evidence supporting selection for decreased pineal gland size

Abstract

Many temperate zone animals adapt to seasonal changes by altering their physiology. This is mediated in large part by endocrine signals that encode day length and regulate energy balance and metabolism. The objectives of this study were to determine if the daily patterns of two important hormones, melatonin and cortisol, varied with day length in captive brown bears (Ursus arctos) under anesthetized and nonanesthetized conditions during the active (March–October) and hibernation periods. Melatonin concentrations varied with time of day and season in nonanesthetized female bears despite exceedingly low nocturnal concentrations (1–4 pg/mL) in the active season. In contrast, melatonin concentrations during hibernation were 7.5-fold greater than those during the summer in anesthetized male bears. Functional assessment of the pineal gland revealed a slight but significant reduction in melatonin following nocturnal light application during hibernation, but no response to beta-adrenergic stimulation was detected in either season. Examination of pineal size in two bear species bears combined with a phylogenetically corrected analysis of pineal glands in 47 other species revealed a strong relationship to brain size. However, pineal gland size of both bear species deviated significantly from the expected pattern. Robust daily plasma cortisol rhythms were observed during the active season but not during hibernation. Cortisol was potently suppressed following injection with a synthetic glucocorticoid. The results suggest that melatonin and cortisol both retain their ability to reflect seasonal changes in day length in brown bears. The exceptionally small pineal gland in bears may be the result of direct or indirect selection.

Can offsetting the energetic cost of hibernation restore an active season phenotype in grizzly bears (Ursus arctos horribilis)?

“…Bears at the WSU Bear Center are trained for blood sampling year-round using honey diluted 12.5-25% with water (v:v) (Joyce-Zuniga et al, 2016; Ware et al, 2013). Bears are highly motivated to receive honey during blood sampling based on positive-reinforcement training and can be sampled without the use of sedatives or other drugs even during hibernation (Joyce-Zuniga et al, 2016).…”

Hibernating bear serum hinders osteoclastogenesis in-vitro

“…Gray et al [49] reported that treatment of dystrophin-deficient mice with black bear's PTH decreased osteoclastic surface and increased osteoblastic surface on the bone, leading to significantly higher hind limb bone density than control group [49]. ii) It has been reported that melatonin concentration during hibernation is about 7.5 times higher than summer active phase in brown bears (Ursus arctos) [50]. Melatonin has indicated inhibitory effects on osteoclastogenesis in-vitro.…”

Endocrine Regulation of Bone and Energy Metabolism in Hibernating Mammals

“…In greater Asiatic yellow bats, melatonin has been shown to enhance glucose clearance in the blood during pre-hibernation when the animals are at their maximum fat mass, and it may also protect the animal from hypoglycemia during hibernation (Srivastava and Krishna 2010). Furthermore, increased melatonin concentrations during hibernation compared to summer levels in brown bears (U. arctos) suggest that this hormone may play a role in metabolic suppression during the winter (Ware et al 2013). What turns off a hibernator's appetite and thus reduces circulating glucose to basal levels is unknown, but recent studies suggest that AMPK, NPY, and NPY-Y1 receptors, in addition to melatonin, may be involved in the mechanism (Dark 2005;Dark and Pelz 2008;Swoap 2008;Florant et al 2010).…”