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One of my fictional planets (which is largely terran in nature) is notable for an unique animal. Said animal is a dangerous predator that is notable for several things:

• It is physically separated into three parts: one "hunter", one "digestor" and one "reproducer". All three parts share the same DNA and arise from a single egg and yet have individual circulatory systems and lungs. Only the hunter is mobile while the other two parts are largely static.


• The hunter's body contains a mouth and a pouch for storing prey, where it is only "pre-digested" (if at all). Other than that, it has eyes, feet, claws, teech and everything else a successful predator requires.


• After hunting, the hunter returns to the nest and gives the prey to the digestor, whose body lacks eyes and an ability to move. The digestor basically consists of one giant stomach rooted to the ground with deformed appendages.


• To eat, the hunter returns regularly to the digestor and harvests a nutrient juice through a separate, smaller "mouth".


• The reproducer is a modified digestor which uses its stomach to grow eggs before dying off.


• Only the digestor has a well-developed liver and kidneys. In turn, it lacks a large brain (the hunter has it).

While a process of "evolutionary assimilation" is presumed to be the origin of chloroplasts and mitochondria, the formation of a creature like described above is certainly the result of a reverse process, in which an animal's organs are separated from its body and become semi-independent in their function while de facto staying parts of it. All three parts - the hunter, digestor and reproducer (and any arbitary additional "castes") are dependent on each other in the long term, as most vital organs are "separated" among them.

The only organs every part will have will be those required for immediate survival. Among the already mentioned circulatory systems (with individual hearts, of course) and lungs, each part would probably also have an excretion orfice.

My question is:

• What would force an animal to evolve in such a way? What would be the advantages and disadvantages?

Note: it can be assumed that the immobile parts protect themselves from enemies by means of a strong venom or hunters who stay in the nest for security purposes.

Sounds like a miniture bee swarm. You have highly specialized individuals who all work for the survival of the whole swarm.

According to your description, you will end up with a slightly bigger swarm than just 3 individuals:

• One egg hatches one "hunter", one "digestor" and one "reproducer"


• The first reproducer creates more eggs and dies, but the hunter and digestor are still alive


• In every generation, some hunters and digestors survive the reproducer. Their numbers will increase while the number of reproducers stays low.

And that's exactly how it works with bees, ants and termites. You only need one reproducer hatching egg after egg. You only need enough digesters to feed your swarm, no need for a 1:1 ratio. What you need are many of the hunters. They bring nutrition to the digesters and defend the stationary individuals from physical danger.

The advantage of this specialization is a reduced energy demand and reduced risk for the whole species. Solitary mammals have to hunt their own food, find a good mate, nurse their offspring while hunting enough food to produce enough milk and if the mother dies, the offspring often dies as well.

Your specialized animals share the risk of reproduction. Only one individual has to ingest enough food to produce eggs. The hunters are not burdened with embryos and if one hunter dies, the offspring will survive nevertheless. If all hunters have a bad day and don't catch anything the digesters can still feed the swarm for a limited time.

From an evolutionary point of view this species has the added advantage that every individual only needs the energy to grow its very specialized body. The hunter has no use for a womb, so the energy to grow one is saved. The digester has no need for eyes or legs, so the energy needed for them is saved.

One disadvantage is that if all reproducers of a swarm are killed, the swarm will eventually die. Insect swarms produce a new queen, but this seems not possible for your aliens.

Another disadvantage is the location of reproduction. Since 2 of 3 casts are stationary, the swarm cannot relocate to another territory when the food sources run out or when natural catastrophes endanger the survival of the stationary individuals. If you make the hunters spread the eggs in their territory, the swarm will grow to a size that is not managable anymore. Some digesters or reproducers might be forgotten and die of starvation simply because they hatched too far away from the center of the territory.

What you describe sounds like the Portuguese man o' war

The Atlantic Portuguese man o' war (Physalia physalis), also known as the man-of-war, is a marine hydrozoan of the family Physaliidae found in the Atlantic, Indian and Pacific Oceans. Its long tentacles deliver a painful sting, which is venomous and powerful enough to kill fish or, rarely, humans. Despite its appearance, the Portuguese man o' war is not a true jellyfish but a siphonophore, which is not actually a single multicellular organism (true jellyfish are single organisms), but a colonial organism made up of specialized individual animals (of the same species) called zooids or polyps. These polyps are attached to one another and physiologically integrated, to the extent that they cannot survive independently, creating a symbiotic relationship, requiring each polyp to work together and function like an individual animal.

the Portuguese man o' war is composed of three types of medusoids (gonophores, siphosomal nectophores, and vestigial siphosomal nectophores) and four types of polypoids (free gastrozooids, gastrozooids with tentacles, gonozooids, and gonopalpons), grouped into cormidia beneath the pneumatophore, a sail-shaped structure filled with gas. The other three polyp types are known as dactylozooid (defense), gonozooid (reproduction), and gastrozooid (feeding).

Such an organism looks like an intermediate step between an unicellular organism and a true pluricellular one.

It has advantages with respect to simple unicellular organisms in that it has some specialization, however with respect to truly specialized organisms it doesn't look so extraordinary.

Your question makes me think of honeypot ants:

Honeypot ants, also called honey ants, are ants which have specialized workers (repletes, plerergates, or rotunds) that are gorged with food by workers to the point that their abdomens swell enormously. Other ants then extract nourishment from them. They function as living larders.

The queen is a reproducer, the repletes are the digesters, and everyone else is a "hunter" (more likely harvesters in the ants' case).

These ants could evolve to match your criteria, in two main steps. The simplest one is to have more than one ant being born from the same egg. The second may require aeons: ants evolved from non-social insects. They would have to go non-social again, so that a single egg contains all of the individuals.

Both changes may result from evolutionary pressure caused by food scarcity that makes the colony lifestyle less and less effective.

Notice that you will need mobile reproducers to fertilize the non-mobile ones. That, or the hunter may go gather the seed if their reproducer is female.

The actual analogue for this are social insects, where a meta-organism emerges from the collective behavior of the society - different development results in different 'organs' - workers, drones, queen.

EDIT: symbiosis is another direct analogue, though also not a segmented organism, but a dependent relation, of various sharing/benefit/detriment arrangement, between two distinct organisms

There is a quantum of minimal self-organization which the original question exceeds. A viable organism must be able to feed itself. Its gut must be connected to its mouth & anus. Any subdivision into segented parts must in each part also maintain the requisite sensory, cognitive & motile organs. Segmented organisms therefore require a prohibitive amount of redundancy, vulnerability, suffer from an absence of vital organic integration which would necessitate some handwavy substitute far from anything feasible/reasonable.

Remain Pure

I could imagine the hunter component and the reproductive component being kept separate due to hazardous conditions while out on the hunt. Perhaps there are elements of the flora and fauna that are carcinogenic or that degrade DNA over time. There would be an advantage to having the reproductive component remain pristine while the hunter suffers corruption over time. The reproductive component may have evolved to be better suited to a particular climate or elevation that makes use of its own unique defense mechanisms but is a less ideal environment for the hunter. With this consideration though, there are plenty of examples in nature of how some genders evolve differently to promote reproduction in the species.

That being said, I can't imagine a situation where a separate digestor component would be favorable to evolution. The hunter requires too much energy not to have instant access to food and the reproducer would need a constant supply over the long terms of reproduction. The best I could imagine is a "shared stomach" where the hunter travels with it during it's prime and then "sheds" it for the reproducer when the time comes, essentially passing the torch for the next generation.

Any third element to this odd trinity would probably be more useful in some maintenance role such as groom, or nursemaid. This would be similar to existing symbiotic relationships where the smaller critter gets the benefit of food and protection for services rendered.

While we have examples of similar things on earth, the question is what the pressures are to create such a thing at mammalian scale.

My initial thought was symbiosis between three species. The main problem there is that for breeding, you would somehow need all three to breed, even though there's only one reproducer.

But we already know that this can happen. Cells, for example, seem to be a union of two basic elements, probably each originally a separate organism, combining to become cell and nucleus. Butterflies, again, are likely a long-developed version of a parasite/symbiote and an organism that now entirely rebuilds from one form (hunter/digester) to another (reproducer) and passes on the genes of both.

So you COULD feasibly have three symbiotes, each very good at one thing (hunting stuff, converting hunted stuff to energy, and creating/protecting eggs), ending up specializing into those roles. The hunter, eating far simpler food (honey) needs spend less energy and body volume on digestion and reproduction.

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The other possibility, as others have mentioned, is specialization of roles within a single species as we see with hive animals, where in bees (simplistically speaking) we have exactly that breakdown: foragers, honey-makers, and the queen breeder.

The problem here is that it's difficult to argue a reason for the honey-maker and the breeder to be separate organisms if you only need one of each. In this case, I'd suggest that there instead be multiple "digesters" - perhaps even sacrificial in purpose, who fatten themselves, die, predigest themselves from the inside as if by spider venom, and are then eaten by the other two.

Reproducer needs to be able to create all three, and which one hatches may depend either on factors before laying (that is, the "type" of egg laid is internally determined by the reproducer, consciously or based on things like quality of diet, etc), or afterwards (the type of hatching is established by pheromones in the air, temperature, actions of whomever touches it, etc). The former gives complete control to the breeder, but in real life we instead find the latter - where eggs can be turned into queens through appropriate treatment by the drones.