If humans could change to become cold-blooded, would it be advantageous to us? (Assuming we changed instantly.)
And assuming some of us haven't already made the transition. I mean, try convincing me Vladimir Putin doesn't have at least a little reptile in him.
I kid, of course, but before getting to your question, I'll just note some research showing how humans can in fact become a little colder-blooded in a hurry: through social exclusion. In one study, after some subjects were excluded from what they thought was a communal computer game—frozen out, you might say—their skin temperature measured 1.3 degrees Fahrenheit lower than subjects who'd gotten to play. Other experiments have likewise confirmed that temperature influences our interpersonal skills, such that when folks are warmer, they're more likely to engage in social behavior; simply raising the temperature of the room can improve relations within a group. Need a converse data point? Take the Donner Party: when the going got cold, the cold ate each other.
These are minor, temporary fluctuations, of course, and you're apparently thinking bigger and bolder, Zayne. OK, but first we should be clear on what we mean. "Cold-blooded" is layperson-speak, and corresponds to several overlapping technical terms describing an animal's metabolism and how much variance in body temperature it can handle. The concept we're really interested in here, though, is ectothermy. Ectothermic animals—reptiles, amphibians, fish and invertebrates, basically—don't generate significant body heat; their external surroundings determine their internal temperature, which they can only regulate via behavior: seeking out sunlight or shade, burrowing, etc. (This limitation is called poikilothermy, but let's keep things moving.)
Endotherms, by contrast—birds and mammals, including us—maintain consistent body temperature using their own metabolic heat, and can regulate it physiologically as needed (by shivering or sweating, for example). The primary advantage endotherms have over ectotherms is the ability to thrive in a wider variety of climes, whereas the big advantage for ectotherms is lower food consumption, meaning a higher carrying capacity for the habitats they do live in.
So humans becoming ectothermic—out of the question, right? Not so fast. That first fish crawling out of the primordial sea was cold-blooded, and we evolved from it—suggesting that creatures can change teams, thermoregulation-wise, but it's likely to take a while. If somehow we were to manage it on the expedited schedule you propose, though, here are a few practical effects the switch might occasion:
Life would go by at a different pace. Because ectothermic creatures rely on external temperatures for energy, we'd have to spend some time lolling in the sun each morning before we were really able to get going—like drinking coffee, but cheaper. Wintertime? You might want to set up a few heat lamps in your house, lest you run the risk of descending into a state of torpor. On the other hand, your Facebook habits probably already primed you for this.
With torpor on the menu of metabolic options, though, space travel should be much easier for cold-blooded humans—kind of like the "cryosleep" you see in sci-fi (which, by the by, NASA-funded research really is exploring as a means of enabling long-distance missions—say, to Mars). A cold-blooded crew could survive at low temperatures for much longer than a warm-blooded one, allowing them to travel months, maybe years, on minimal resources. This won't make the trip to Alpha Centauri any quicker, of course, but time flies when you're torpid.
Come to think of it, combing the universe for other habitable locales might start to look like a pretty good idea. Back here on earth, land-dwelling ectotherms tend to do best in a temperature range of about 70 to 105 degrees Fahrenheit. So, provided we wanted to lead reasonably active lives, ectothermic people would likely gravitate toward latitudes close to the equator, and presumably give rise to the kinds of malign side effects that come with large-scale human migration: overcrowding, resource depletion, political destabilization. Think Cancun's packed now? Just wait till it's beset by lizard people.
Then again, there's global warming to consider. If the world gets too hot, those warmer regions might not end up being so attractive to the cold-blooded version of us after all, or for that matter to any ectotherms. We don't have to speculate alone on this topic: The co-author of a 2009 paper described the outlook for tropical ectotherms as "catastrophic," given the narrow range of temperatures in which they're comfortable. Too cold, they can't move; as it becomes too hot for them, though, they'll spend all their time searching for shady spots—which, with deforestation, are already disappearing—thus reducing the amount of time they're able to look for food or reproduce. We're talking about everything from crocodiles all the way down to insects; you can't take a swathe of creatures like that out of the food chain without some major repercussions. So not to get too cheery here or anything, but: If by some future miracle, humans are able to render ourselves cold-blooded, we'll already have foreclosed the possibility of living successfully that way on Earth.
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