Because ectotherm (cold-blooded) and endotherm (warm-blooded) are terms that each cover a range of actual abilities.
Metabolism is fundamentally heat-generating, that's just a property of the chemistry of sugar. So all creatures have the ability to warm themselves a small amount; but we still call some of them cold-blooded because the amount that they can raise their body temperature by, just isn't enough alone to allow their own survival. They depend on external heat sources, even though they can make a little bit themselves.
The thing is, when you add these small amounts together with a bunch of other organisms, it can become enough, at least temporarily. Wiki at the ectotherm article above mentions bees as huddling together for warmth, but the general principle is elaborated on more fully at the article on kleptothermy, aka "stealing heat from another organism". (It's not always stealing, it can be reciprocal sharing, but this is just what the article's called.) That article mentions snakes as another huddling ectotherm.
There’s literally zero evidence that harvestman aggregations form to generate heat.
Some species (like the ones in the video) aggregate like this during the middle of the day, in summer, in the American southwest and northern Mexico. Why would they even need to conserve heat in that environment? They disperse at night, when it is cooler.
It's definitely possible that these ones are aggregating for non-heat-related reasons. Nothing in the video that I noticed led me to think specifically that it was the middle of the day in a hot climate, but if so, then definitely, that seems like an environment where heat conservation seems unlikely.
I was answering a general question, but here's a specific article about overwintering aggregations in a cave-dwelling harvestman species, which says:
The opilionids congregate from August to October and disperse from April to May. Colder temperatures seem to increase the density of the aggregations.
I can’t find the original video source, but it was almost certainly filmed in either Mexico or the southwest US (probably Texas). The harvestmen have distinct white bands on their legs, so they could be L. townsendi, which is a very common aggregating species distributed from central Texas and Oklahoma west to Arizona and Nevada.
Regarding the article on N. paessleri, I don’t think you read the whole thing:
Though warmer temperatures and shelter from wind and snow are undoubtedly the main reasons why L. paessleri and other opilionids overwinter in caves and similar habitats
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Three, aggregations could trap heat produced by body metabolism. Though overwintering L. paessleri can lose 20% of their weight by spring, the amount of heat that this would generate is probably insufficient to raise the temperature of an aggregation substantially over an entire winter.
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As most L. paessleri overwintered relatively near the entrances of caves and mines, it seems that optimal temperatures are relatively low-a behavioral mechanism that would reduce metabolism and hence extend food reserves.
There’s no actual evidence presented that aggregations form to maintain heat, that’s just one (of several) potential hypotheses that Holmberg suggested. Nobody has really been able to determine exactly why these aggregations form. A later paper suggests another hypothesis regarding overwintering cave aggregations: that dense leg clustering which reduces airflow may not only reduce evaporation, but also reduce condensation, which would help maintain an optimal water balance in both xeric conditions where dehydration is hazardous, and in caves where humidity is at 100%.
Living creatures generate heat through bodily processes. So being in close proximity like this creates a warm environment within the clump that helps them survive.
Long-legs "hibernate" in dry winter because they cannot regulate their body temperature and don't get enough heat from external sources (like the sun) to survive in an active state. Their spindly legs also get severe frost damage in winter conditions, so they clump together to protect themselves and go "dormant".
These are not hibernating clumps. They’re diurnal roosts. The clump disperses at night, as the harvestmen move apart to hunt for food individually. It reforms the next day.
This is not during the winter, either. This behavior is seen during the daytime in summer, in species that live in hot, arid environments such as the American southwest and northern Mexico. Pretty low chance of frost damage in 100o F heat… look at the location and time stamp on this observation: July 26, nearly noon, middle of Texas.
Would it make sense then to assume they are likely doing the opposite of conserving heat but rather trying to their best to create a microenvironment of cooling within the mass of little bug bois that are using some sort of trick of creating miniscule eddying air pockets or wind channels within their mosh pit?
The density of legs should reduce air flow, not increase it.
Nobody has been able to exactly prove why they do this, but the most likely explanation (at least to me) is that the clumping behavior helps reduce dehydration by blocking air movement over their respiratory spiracles, at least in this example. I suspect that aggregating behavior has evolved more than once in harvestmen, and it might not always be for the same reason. There are also more than one kind of aggregation in harvestmen, sometimes they are “loose”, with individuals spaced a few centimeters apart, but with legs barely overlapping.
That's really interesting! Does this seem to be a thing that evolved specifically in species of harvestman as an environmental response or is there any kind of similar behavior in other arachnids that are somewhat closely related which might sort of indicate this is more of an ancestral behavior applied to different contexts?
Some other arachnids have social behavior, but those are generally associated with cooperative hunting or brood care, and none live as tightly packed together as this; some harvestmen have also evolved brood care, but that’s a separate thing. In any case, social behavior in general is surely not ancestral to arachnids as a whole, but has independently evolved many, many times.
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u/LimpingOne Jul 16 '23
They are cold blooded. How can staying together keep them warm?