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.