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Eyes, ears and nostrils are shaped to facilitate their function. So what is the optimal shape for a humidity sensing organ? A paper about fruit flies (link here) explains that the insects are adapted to retain and seek water.

"The mechanical deformation of a specialized little organ inside the antenna, called the sacculus, could tell the brain about humidity levels."

The organ expands as humidity levels rise, giving the animal information about the amount and location of water. A useful trait not just for insects but also desert dwelling animals. For them, it would make sense to have an organ (presumably on the face) entirely dedicated to sensing bodies of water. The organ would be more developed in smaller animals that retain less water than larger ones. However, I wasn't sure about it's shape...

EDIT: JBH, your extensive feedback has been noted. I will do my best to describe the animal that will serve as my example.

A semi-quadrupedal reptile roughly the same size as a coyote (25 inches tall or 63.5 cm) and with the general anatomy of an iguana. It is diurnal and is most active at dusk and dawn. It's long and powerful hind legs allow it to hop on the sand with ease much like a kangaroo. The animal has webbed feet covered in setae which help it move on the sand but also make it a good swimmer. It requires water especially in the early stages of its life. The humidity sensing organ replaces the pit organs found in vipers, located in between the eyes and nostrils. Water heated by the sun evaporates during the evening making it easier to detect potential waterholes from a long distance.

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    $\begingroup$ For which body plan and size? What is optimal for a fly is not necessarily optimal for an elephant $\endgroup$
    – L.Dutch
    Oct 17, 2022 at 19:16
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    $\begingroup$ I'm tempted to VTC this question. Ears and noses are not organs. They're specialized skin formations that support organs (like the antennae). In the case of ears, they direct atmospheric compression waves toward the inner ear where the sensing organ is. The nose's purpose isn't to smell, but (simplistically) to keep us from drowning since it's the olfactory neurons that actually smell. When I think about organs, most are shaped to fill the space they inhabit (lungs, kidneys, liver, brain) and are not shaped due to their purpose. What's wrong with the shape of the fruit fly's organ? $\endgroup$
    – JBH
    Oct 17, 2022 at 19:22
  • $\begingroup$ Let's put that another way. Let's assume we're talking about a human with this organ. It obviously must have contact with outside air to do its job. So, is it associated with the ear canal? (What shape fits in the skull behind the ear, or in the neck under the ear?) Is it associated with the sinuses? (What shape would fit in the upper cheeks or above the soft pallet?) Is it associated with the mouth? (Maybe not... too moist.) How unaffiliated it must be with the body's moisture is very important. I think you're asking the wrong question. I think you should be asking (*continued*) $\endgroup$
    – JBH
    Oct 17, 2022 at 19:26
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    $\begingroup$ OK. Frankly, I really think you should delete this Q and focus on the location of the organ on one specific creature and not the shape at all. But, VTC for lack of details since you don't describe the creature and are assuming that it could be applied generally to any creature. The more I think about it, the more that's not true. $\endgroup$
    – JBH
    Oct 17, 2022 at 19:30
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    $\begingroup$ Vote retracted. I still think you should focus on location and not shape, but the creature details allow for specific answers and that's good enough. Thanks! (BTW, remember to leave a note in comments tagging people. I didn't know you'd made your change and just happened by to see what had happened.) $\endgroup$
    – JBH
    Oct 18, 2022 at 20:02

2 Answers 2

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Nose, trunk or antenna shaped.

This is yet another case of the ever running show "nature beat us to it". You're completely right that the ability to find water is something useful to have, especially if you're an animal that lives in a desert or dry climate region, or if you can't reproduce without a body of water. In fact, it's so important that camels and mosquitoes can do just that. Camels can smell water over 75 km away by smelling the spores of microorganisms that thrive in the water bodies, carried far away by the desert winds. Elephants are also on this list, with their potent noses capable of smelling water 20 km away.

Mosquitoes meanwhile, lack noses, but instead have antennae filled with special hairs capable of detecting water vapor, allowing them to find the water they need to lay their eggs in.

So apparently, there's no need for complicated swelling organs. A pair of sensitive antennae or potent noses with plenty of olfactory receptors, prehensile or not, are two examples of effective, natural-selection-approved water detecting sensitive organs (and they can even detect the smell of other things too! Talk about neat).

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    $\begingroup$ Welp... if it ain't broke, don't fix it. $\endgroup$ Oct 19, 2022 at 8:35
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There are a number of options available to an organism though figuring out the evolutionary pathways to any of them is likely a major undertaking, but several means of measuring relative humidity exist that are well within a biological entity's materials palette.

1. Twisted hairs This is how pinecones work: opening when the air is dry, closing when it is wet. Bundles of tiny hairs that are coiled around each other will slacken when moist, loosening up; contract when dry, tightening down. From there any conventional nerve bundle that responds to the tension of the system will produce a proportionate response to the level of dryness. It's got a long delay from change to stimulus, and it's approximate at best - but aren't all organic sensory organs?

2. Biologic Psychrometer Psychrometers work by measuring the difference between wet-bulb temperature and dry-bulb temperature. Assuming the creature can tell how hot/cold the outside air is, all it needs is for some of those heat-sensing organs to be submerged in water, either by finding an external source of water that it's adapted the instinct to dipping one set of temperature-sensors into, or excreting something that covers a portion of the sensor organs. The brain is then adapted to interpreting the difference between those two temperature signals in order to produce an understanding of the relative humidity. As long as there's an evolutionary advantage to doing so, it'll happen on its own sooner or later. Binocular vision works in a similar fashion: two distinct signals are merged/cross-interpreted.

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