In the US, many of us are thinking about eating and digestion right now, after a holiday devoted to food. We usually think about digestion the way we do it, with food going in the mouth and into the gut, where the nutrients are extracted, and the waste products coming out the other end: a big tube, essentially. But we don’t have to resort to science fiction to come up with other ways to obtain nutrients: there are lots of options right here on Earth. Some of those methods might even inspire your next aliens.
Some very simple animals don’t have digestive systems at all. The single-celled Paramecium scoops in food particles and surrounds them with a membrane forming a vacuole, a space sealed off from the rest of the Paramecium cell. That separation is needed so the enzymes that break down the food particles don’t also digest the Paramecium.
Many sponges and other aquatic organisms do something similar, filtering food out of the water and getting the nutrients out without a distinct digestive system. This system doesn’t work if the animal doesn’t live in the water, or is big enough that all its cells aren’t directly immersed. Bigger, terrestrial, and faster animals (being active requires taking in more resources) had to come up with more efficient solutions.
Some of those solutions are pretty weird, at least from our perspective.
How about external digestion? Fungi do this, and spiders. Release digestive enzymes into the environment, let them do their thing, slurp up the predigested food. Fungi are unspecific about it, digesting all or most organic matter in the general area (decomposers, very important!). Spiders are more selective: they inject the enzymes into their prey and wait. That’s why a spider bite can be such a horrid experience: your flesh is being digested while you’re still using it.
Or how about coprophagy? Feces eating can be a normal part of the digestive process, not just a disgusting habit of your pet dog. Grasses are very hard to digest, since vertebrates can’t break down cellulose, so they need help from microbes that can break it down. Coprophagy is one method for making this system work. Rabbits partially digest their food, then excrete it. They eat the partially digested pellets and finish the digestive process. The feces after the second digestion is really a waste product, not an intermediate step, and isn’t eaten.
Ruminants such as cows took the opposite approach to having multiple digestions. They eat grass and start to digest it, then regurgitate the cud and chew on it for a while to further break it down. The second time it’s swallowed, it’s thoroughly digested and the nutrients extracted.
So which alien would you rather sit down to a diplomatic dinner with: a ruminant or a coprophage? And what would they find disgusting about our eating and digestion?
I don’t think a filter feeder is likely to evolve intelligence, but how about trying to design a system where that works. Nonsentient aliens with different digestions could be helpful or harmful to people trying to colonize a planet.