As a school project, I am designing an extraterrestrial animal, and I would like to include a multiple-hearted mammal-like organism. Is there any set of circumstances in which having 3 hearts would be beneficial to a mammal-like creature, such as the hematocrit or possibly the complexity of the organism?
Well... we already have some 3-hearted organisms here on the earth. Octopuses have 2 "respiratory" hearts, that they use to increase the flow of blood running through their gills, and one "main heart", which keeps the blood-flow on the body.
You can do something similar for your mammalian creature, having your extra hearts do work for some specific but intensive task, keeping the main heart free to focus on the flow for the "main" parts of the body.
I used a concept similar to this for the dragons of my fantasy world - they are dual-hearted, and have two completely separated circulatory systems with different functions. Each heart serves one of those systems.
You creature can have two sets of lungs, each set tied to one heart and providing oxygen to a different body section. A third, segregated heart could be localized on a central part that serves as a meeting point for those two body sections, pumping blood between those halves as a bidirectional pump.
Take a look at our own organs. We have 2 lungs, 2 kidneys for example. We COULD survive with 1 lung or 1 kidney. We may not be able to function as WELL, but we could survive. A creature in turn, who has multiple hearts may be able to have far superior endurance because their body can feed oxygen to the muscles much more efficiently. The 3 hearts would also share the workload of 1 heart significantly reducing the wear and tear of a tired heart that will eventually cause death.
Now, depending on how you work it, as I stated in my first few sentences, you can survive losing 1 of a set of an organ. The implication is that if someone with 3 hearts were to lose one due to an attack (stabbed, gunshot, etc), or because one went bad, a surgical procedure or an internal process could then bypass the dead/failed heart and continue living while the 2 other hearts pick up the slack.
Your creatures probably live on a planet with a really strong gravity.
Our body already fights gravity by having valves inside our veins, however on a planet with strong gravity these valves might not be enough to prevent back flow, so the creatures could have developed more hearts which work in parallel to increase blood flow in order to decrease the chance of back flow.
Another solution is segmentation. Some organisms have multiple body segments, each with its own set of organs. Although vertebrates are segmented, we only have one or two of most of our organs. But a more strongly-segmented organism could have one complete set of organs for certain segments. There wouldn't even need to be a strong reason, it could just be imposed by their evolutionary history (like the fact that human segmentation is laid out for an animal that walks on all fours).
In STTNG Worf had duplicate organs. So they thought it worked and the audience accepted it.
My own heart has four chambers, they could be separate in another species, I'd think. Perhaps if one or two were harmed/destroyed the others could do the work. Ruminants have more than one stomach. Humans survive with one lung or one kidney after an accident. It works if you make it work. Some species regrow organs (humans/partial livers) or (lizards) limbs.
If a human man can potentially lactate LINK then I'd say it's possible.
The benefits of a redundant system seem fairly obvious. You can survive when 1 fails.
In the Mass Effect games, the Krogan species is often mentioned having several redundant organ systems, developed to survive an astonishingly hostile home world. From the Mass Effect Codex entry on the Krogan:
The krogan evolved in a hostile and vicious environment. Until the invention of gunpowder weapons, "eaten by predators" was still the number one cause of krogan fatalities. Afterwards, it was "death by gunshot".
A sufficiently large organism (e.g., the size of large dinosaurs) might have secondary hearts to assist circulation, even if there isn't a question of gravitational impedance. The more blood a single heart has to move at each beat, the more stress is being placed on that heart. Under such circumstances, two or three hearts at relatively large separation may be more efficient than a single heart, even one significantly larger, and the organism may well be able to live longer.
According to an authoritative source, earth has 3 creatures with multiple hearts:
- Cephalopoda have 3 (1 main plus 1 at each gill)
- Earthworms have 5 ("aortic arches" are simple pumps that effectively hearts)
- Hagfish have 4 (1 main 3-chambered, plus 3 boosters)
The creature is arbitrarily large as a result of higher rates of evolution, and has multiple hearts, lungs, etc. to avoid negative consequences of the square-cube law. Having multiple respiratory/cardiovascular systems could provide oxygen to the larger area, which a single system may be unable to cover (because the creature needs more of its mass to be muscle to support its higher weight.[These creatures evolved very rapidly in this scenario])