Thesis Title

Thyroid Hormones and the Onset of Hibernation in the 13-Lined Ground Squirrel, Spermophilus Tridecemlineatus

Date of Graduation

Spring 1996


Master of Science in Biology



Committee Chair

Thomas Tomasi


None of the various endocrine glands thought to play a role in hibernation have been more thoroughly studies with more conflicting results than the thyroid gland. In most species, the thyroid gradually becomes less active throughout late spring and summer, and virtually ceases activity by the time the animal initiates hibernation. If a reduction in thyroid hormones is a physiological cue or prerequisite for entering hibernation, then blocking thyroid hormone secretion and/or activation may induce premature hibernation. Conversely, keeping thyroid hormones elevated may prevent hibernation, or delay its onset. Thirteen-lined ground squirrels, Spermophilus tridecemlineatus, (n=7 per treatment) were given propylthiouracil (PTU: 0.01%) in their drinking water to block thyroid function, in combination with a long photoperiod or naturally decreasing photoperiod, and either cold (10°C) or warm (25°C) temperature. Control animals (n=7-9 per treatment) were kept in the same environmental conditions without the PTU. In a second study, 12 ground squirrels were injected with either 500 ng or 5000 ng of L-thyroxine daily (as determined by a preliminary study) in an attempt to delay the entrance into hibernation. Entrance into hibernation was defined as 48 continuous hours of torpor. Blood samples were taken initially, and once hibernation started, by cardiac puncture to determine serum concentrations of free and total thyroxine (T₄), and free and total triiodothyronine (T₃). Juveniles had higher hormone levels during the summer due to the difference in adult females versus juvenile females. PTU lowered all hormones measured, however, it only lowered T₄ and free T₄ significantly. Even though PTU affected hormones levels, it did not make the ground squirrels enter hibernation sooner than those that did not receive PTU. Injecting the animals with L-thyroxine did increase the amount of time it took the animals to hibernate but the different treatment groups were not all significantly different.

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