Hot-headed squirrels use eye fat!

Our paper has just been published, this one a collaboration led by Amanda MacCannell from Dr. James Staples lab at Western University.

Here’s a summary (cribbed from the abstract):

We discovered an orbital lipid depot in the 13-lined ground squirrel during the first ever magnetic resonance image (MRI) of this common experimental model of mammalian hibernation. The volume of this depot increased in the autumn and decreased in the spring, suggesting an endogenous circannual pattern. Water-fat MRI revealed that throughout the year this depot is composed of ∼40% lipid, similar to brown adipose tissue (BAT). During arousal from torpor, thermal images showed higher surface temperatures near this depot before the rest of the head warmed, suggesting a thermoregulatory function. This depot, however, does not contain uncoupling protein 1, a BAT biomarker, or uncoupling protein 3. Histology shows blood vessels in close proximity to each other, suggesting it may serve as a vascular rete, perhaps to preferentially warm the eye and brain during arousals.

And a link to the paper:

http://jeb.biologists.org/content/early/2019/01/23/jeb.195750

Thermal video time lapse of a 13-Lined ground squirrel arousing from hibernation. Note the warm region behind the eye (1990 s) corresponding to the orbital lipid depot. Warm surface temperatures provide hints toward changes in relative blood flow and metabolism in the underlying tissue during the rapid rise in body temperature from 5 to 37C.

Reprint requests can be sent to umamac@leeds.ac.uk.

Citation:

MacCannell, ADV, Sinclair, KJ, Tattersall, GJ, McKenzie, CA, and Staples, JF. 2019. Identification of a lipid-rich depot in the orbital cavity of the thirteen-lined ground squirrel. Journal of Experimental Biology, 222: jeb195750 doi: 10.1242/jeb.195750

One thing I found really interesting about this was the fact that brain temperatures are kept warmer than body temperature, something I noticed back in 2009 studying Columbian ground squirrels (something also noted by Heller in his seminal work in the 1970s). At the time, I assumed this was related to higher metabolic heat production of the brain, and yet we observed rapid transient changes in hypothalamic temperature during hypoxic transitions that might readily have been explained by changes in blood flow. Further study in other hibernators seems to be required to corroborate Amanda’s neat findings!