Running in the heat

There’s something I love about running in the heat of the day, being in the full blaze of the afternoon sun, feeling the salt crystalizing on my skin. It seems to draw into clearer relief just how well-adapted our bodies are for the heat. 

Yesterday afternoon, I headed up to UVM’s track to do mile repeats (a workout where I run 1 mile at my 10k pace, jog a lap and repeat for 4 miles). I did the same workout last week when it was almost 80º, and I was keenly aware of how much easier yesterday’s workout felt. 

This wasn’t all that surprising as it only takes 4-5 days to acclimate to the heat and it’s been in the 80s since Saturday (and about 2 weeks to become fully acclimated; here’s a great synopsis of what heat acclimation looks like for endurance runners). While I was running, I was thinking about my baseline body temperature and wondering how this compared to the lethargic squirrels and panting robins.  

Boots McGovern

Variability in body temperature

My body temperature hovers somewhere around mid- to high-97°F (my thermometer is precise if not accurate). But humans are on the low end in comparison to other homeotherms (animals that maintain a roughly constant temperature). Most placental mammals have stable body temperatures outside of hibernation around 97–104°F. Marsupials tend to fall on the lower side of this. Opossums, for example, hover around 96°F (source). Coincidentally, this low body temperature also makes them poor hosts for rabies (source). Contrary to marsupials, birds find themselves on the other end of the spectrum, with body temperatures in the 106–109ºF range. 

And if you're curious about how to take a rectal temperature of an animal, here's the visual guide from Review on Practical Guidance of Veterinary Clinical Diagnostic Approach
Black-capped chickadee taking a bath (Backyard, Burlington)

Experience of heat

Having a different baseline body temperature makes for a different subjective experience of “being hot.” While running in the heat yesterday, my body temperature was likely somewhere between 101 and 104°F (link). Being acclimated to the heat staved off most of the symptoms of heat stress (which can occur in humans when body temperatures rise above 100.4°). Had I kept pushing it (or had I been less acclimated), I likely would have exhibited some of those symptoms of heat stress (headache, nausea, etc.) earlier in the workout. 

If I’d been out running with my opossum friend (remember: their body temperature is typically around 96°F), they would feel hot at a much lower temperature and start to exhibit symptoms for heat stress around 98°F. If I could coax my chickens into joining us, they would’ve fared far better as their heat stress zone is closer to 110°.

But the equation isn’t quite that simple. As I said at the top, we’re adapted for exerting ourselves in the heat of the day. This isn’t because our normal body temperature is higher than other animals but rather because we’re capable of shedding heat faster than other species. While my dog’s body temperature is a few degrees warmer than mine, Boots struggles to run in anything about 70°. This goes back to all those amazing adaptations we have to limit the rate of heat gain (e.g. vertical posture, thick hair on the top of the head and nowhere else) and increase the rate we shed heat (e.g. sweating, long lanky limbs, etc). Other vertebrates tend to lack the confluence of these adaptations and so struggle to avoid body temperatures rising into a critical zone as ambient temperatures rise.

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