This week during a class discussion about sea stars and their biology a teacher asked me the question how sea stars, which have no brain, know that they are hungry. Though it is a seemingly simple question, it actually sent me on an odyssee of reading articles about animal consciousness. Now, after hours of research on the topic and two Harvard publications later, I still can’t come up with a good answer.
Can a sea star feel hungry? If we assume that a sea star can feel hungry, it means that it can also feel satiated after a meal. We know that sea stars can sense the presence of food in their surroundings. For example, on feeding days at the touch tank the sea stars will move around more and crawl towards pieces of food. Does the presence and chemical signature of the food automatically make the sea star go after it, or does it actually need to have a certain state of nutritional need that triggers it to go? In the latter case a sea star that recently ate a meal would sit still even if there is food in the tank.
However, if we consider the case of the sea anemone, any recent larger meal actually makes it impossible for the anemone to eat another such meal, because its mouth is also where the undigestable components of the meal are released. Kids love it when I tell them that the sea anemone eats and poops through the same hole. At a certain age they love every topic that has anything to do with poop. In the sea anemones’ case this physical limitation means it has to digest its food completely before it can poop and has to poop before it can eat again. It seems that activating the tentacles for the next catch is a function of first emptying the stomach. Perhaps one could define that as feeling hungry.
In humans hunger is felt as an unpleasant feeling and can increase to real pain. In a book published by the Harvard press in 1916, which was the first hit on the topic in my google search, the author elaborated that the hunger pain is caused by the grinding or rubbing of the stomach walls against each other. He compares this to the sensation of touching a patch of tissue with the skin removed by an abrasion and the sensory nerve endings laid bare. Because the walls of a stomach could only touch if the stomach is lined with muscles that can actually contract it when it is empty, he argues that only those animals with muscular stomachs feel hunger pain and those that have only weak muscles lining their stomach would not feel those pangs. That would mean that snakes do not get ravenously hungry, but cats, dogs, rats and all other mammals do.
The article talked about many other factors that contribute to the feeling of hunger and it also defined in detail what the difference might be between hunger and appetite. Appetite, according to the authors deliberations, requires the memory of a pleasant sensation of eating something. Thus, only animals that possess the capacity for memory can have an appetite. Since sea stars do not have a brain, they can’t remember much. Not being driven by appetite is probably a good thing for the sea stars in the touch tank, because I can’t imagine anyone entertaining pleasant memories about the cut up chunks of frozen and thawed shrimp and herring that make up their food.
In a newer article about animal consciousness, also written by a Harvard scholar, I learned that octopusses are the only invertebrate animal which is credited with posessing consciousness. They can learn, distinguish, make decisions based on learned facts and they have individual personalities. The octopus that has been used for outreach this year at the Kodiak Fisheries Research Center for example loves to attach to the kids hands and give a hearty and wet handshake before it darts around in the tank, while the one we had last year would just sit quietly in a corner and move very little. The research project these octopusses are a part of is studying how much octopusses eat when they are hungry and how much they grow when they get an all-you-can-eat buffet every few days. It does seem reasonable to think that an octopus feels hungry when it starts to become active when the last meal has been a while, but could its relatives the snails or clams feel hungry? The snail actually does have a little brain. But can it actually think complex thoughts and make decisions like “The water is here now, I better go get a meal before the next low tide comes and I am stuck high and dry again”?
The question should be followed back to the simplest of one-celled animals. How can a single cell “decide” where to move and what to do? Is it just random movement and lucky encounters of edibles that keep microorganisms thriving? If yes, when does life “invent” conscious decision? Is pain like hunger pangs the driver for the first animals to get out and hunt for sustenance? I don’t think I will get any closer to these answers but I sure had a great time contemplating these questions. I still don’t know if sea stars feel hungry but writing this article has made my stomach growl. According to my research, the growling is caused by the release of gases during compression of the stomach when it is no longer full. My memory tells me that there is something in the refrigerator to satisfy that hunger feeling and make the negative sensation go away. Just as all conscious animals get restless when they get hungry, so do I feel the urge to finish writing now and move towards the food.