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Simple question I think, I have some Idea of the answer but would like the ol' second opinion. Vertebrates use olfactory glands in their nostrils (or thereabouts) to sense smell, as well as the roof of their mouths in mammals. For the most part arthropods use antennae.

Currently, one major group (phylum) of animals on my alien planet has a nostril-like system (gill-like rows of slits in front of their eyes) as I assumed larger animals would likely have such a structure instead of antennae - much like exoskeletons and compound eyes. After some thought however, I couldn't definitively say that's right, especially since the reason large animals (i.e. vertebrates) all have nostrils is more likely because they all are or came from fish, which had nostrils.

Question incoming

Is it feasible that a large animal (e.g. a scent-reliant predator the size of a rhino) would be able to make-do with antennae-like structures like an insect? To clarify, would antennae on a large animal be equal to or less efficient/viable/useful?

This question is in regards specifically to olfactory sensing, and not the other sensory purposes of antennae/nostrils. Also, it is not in regards to one specific species but an entire phylum.

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  • $\begingroup$ Cool question. To clarify, you’re asking specifically about the olfactory functions of antennae? Other functions might be sensing touch, air motion, heat, or vibration/sound $\endgroup$
    – Dubukay
    Commented Jun 30, 2018 at 13:17
  • $\begingroup$ yes, antennae for olfaction. $\endgroup$
    – XenoDwarf
    Commented Jun 30, 2018 at 13:32

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Part of this would have to depend on the evolution of your creature. Modern animals have their olfactory organs inside their nose (and taste sensors inside their mouths) because they help creatures find food, and sense danger. In the very ancient past (500 MY ago) everything lived in the water, so antenna like sensor organs were fine. As creatures evolved, putting the sense organs close to where the action was (the mouth) or where you would filter larger amounts of fluid (the nose; air is a fluid) to detect even faint traces of scent molecules had great evolutionary advantages.

Insects use their antenna for scent (among other things) because they breath through spiracles along the sides of their bodies. Filtering lin large quantities of air isn't being done, so external antenna provide more surface area and are exposed to more air, providing a much better ability to detect scent molecules.

This does not necessarily need to be confined to antenna. A very ancient creature related to Hallucigenia has been reconstructed with sense organs all along the feeding arms.

Meet Ovatiovermis:

enter image description here

Ovatiovermis reconstructed. It clings to high objects and uses its arms to sense food particles and capture them

However there will also be scaling effects, after a certain point antenna cannot become larger or they could be easily damaged. As well, antenna might make it difficult to localize where scent is coming from, and of course you can't just "take a deep breath" to isolate, localize or even capture more of a faint scent.

So thinking more deeply about how the creature functions in the ecosystem will give you ideas about why it might have antenna and how it would use them.

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  • $\begingroup$ First question is gold, I somehow hadn’t thought of breathing method. $\endgroup$
    – XenoDwarf
    Commented Jun 30, 2018 at 14:37
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    $\begingroup$ Ovatiovermis looks like a nightmare fuel horror from the beyond that just really wants a hug. $\endgroup$
    – user8827
    Commented Jun 30, 2018 at 16:08
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Snakes (and some other reptiles) use a forked tongue for smelling

There's a real life equivalent for what you are asking:

As the entry in Wikipedia puts it:

The advantage to having a forked tongue is that more surface area is available for the chemicals to contact and the potential for tropotaxis. The tongue is flicked out of the mouth regularly to sample the chemical environment. This form of chemical sampling allows these animals to sense non-volatile chemicals, which cannot be detected by simply using the olfactory system.

This increased ability to sense chemicals has allowed for heightened abilities to identify prey, recognize kin, choose mates, locate shelters, follow trails, and more.

So even for a creature with working nostrils and lungs, there is a clear advantage in having a larger, bifurcated appendage for smelling the environment. Those "feathery tentacles" nicely fit that description.

Also, while snakes have to flick their tongue into their mouth to sense anything (the vomeronasal organ - responsible for analyzing scents - is at the roof of their mouth), nothing prevents those tentacles from having the "analysis organ" right on them, rather than having to flick it in and out of their mouth equivalent.

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    $\begingroup$ I suppose a retractable antennae would work, I know about snake tongues but never likened them to antennae before. $\endgroup$
    – XenoDwarf
    Commented Jul 1, 2018 at 14:15
  • $\begingroup$ @XenoDwarf - I'm saying the opposite: there are evolutionary reasons for the disconnection between the "sample collector" organ and analysis organ in snakes (they evolved from creatures with nostrils responsible for scent) - but that's not mandatory. Your creatures can collect and analyze the scent on thier tentacles, they don't have to be retractable. $\endgroup$
    – G0BLiN
    Commented Jul 1, 2018 at 14:23
  • $\begingroup$ @XenoDwarf also, while antennae and forked tongues are very different, for this case they serve the same purpose - increase the distance between sensores to improve the ability to determine the direction of stimuli. $\endgroup$
    – G0BLiN
    Commented Jul 1, 2018 at 14:26
  • $\begingroup$ Oh no I understand that, I was just saying that I never noticed the similarities. I also understand that the antennae don’t need to be retractable in a large animal, however it would be more plausible if they were (protection from elements for example). $\endgroup$
    – XenoDwarf
    Commented Jul 1, 2018 at 15:31
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In your question, you said you were not interested in the non-olfactory uses of nostrils. However, I do not think they are seperable. Nostrils are used for olfactory sensing because they are already used for other things. Specifically, the other main usage of nostrils already involves moving air over the surface at decently respectable windspeeds.

And if I need to put an olfactory sense organ somewhere, the most efficient place to put it is in a location that already sees significant airflow.

1) It's easier and faster to detect chemical compounds in a highly turbulent flow, versus a still fluid. This is because it mixes much more rapidly, and your sensor only is in contact with a small fraction of the total volume.

2) It's more efficient energy-wise: You're already expending significant amounts of energy to create this airflow. Lining the inside of it with a sense organ is less costly than building some new structure to house it.

Consequently, it seems to me that organisms which breathe heavily will tend to smell with their noses. If you want your fauna to smell in some other way, give them a metabolism which does not require drawing large amounts of fuel from the atmosphere.

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  • $\begingroup$ While it does not devalue your answer in any way, I meant sensory purposes specifically - so breathing is relevant to the question. I’ve edited it for clarity. Thanks for the great answer. $\endgroup$
    – XenoDwarf
    Commented Jul 1, 2018 at 10:48

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