Because this is interesting, I decided to do a little bit of research around some of these questions while procrastinating on my actual biology homework.
I would presume mermaids are mammals, based on their human-like upper half. For one thing, no fish have breasts, as only mammals breast-feed. Additionally, hair is only really found on mammals, and bare skin (non-scaled) is pretty rare in fish and reptiles. Due to the advanced vocalizations, intelligence, and possible social structures, I would say being mammalian would make sense.
Presuming that mermaids were mammals, they wouldnāt be able to have gills. However they would have the same adaptations as other aquatic mammals to allow them to store oxygen - for example, higher haemoglobin count in the blood. Depending on how anatomically sound you want to be, you may argue that mermaids could have lungs and gills like the lungfish, with a specially adapted circulatory system. We are talking about a fantasy creature, after all.
Going with the mammal assumption again, mermaids would likely reproduce like whales and dolphins. This means development of the foetus in a womb, and live birth, as well as suckling the young on milk. While weāre at it, we might as well address theĀ āhow do they bangā question - again, most likely like other marine mammals. Male sexual organs are probably kept internally like whalesā, to prevent the cold aquatic environment from rendering the sperm useless (hence why you donāt see any mermaid peen). There will be an opening somewhere in the abdomen that allows reproduction.
All mammals care for their young, because their offspring require milk during their early life. Itās likely that mermaids would suckle their young about as long as humans, perhaps longer. Depending on the social behaviour of the mermaids, young would probably either stay with their mother and family group, or leave to live on their own. How long mermaids take to mature would depend on various things, but if we base it on human maturation, young mermaids would probably stay with their mothers between 10-18 years.
(Personal note, I found myself wanting to call baby mermaidsĀ ācalvesā. This might be because Iām basing a lot of my ideas on whales and dolphins.)
Only a very few mammals have scales. Pangolins have keratin scales and plates; these are unrelated to reptilian scales. The musky-rat kangaroo has scaled feet and a scaled tail, and these scales are much closer to reptilian scales. Reptilian scales are distinct from fish scales, though, as they form in the epidermis, while fish scales form in the dermis. However, considering that there are semi-aquatic reptiles, and have been aquatic reptiles before, this probably wonāt affect whether or not the animal in question could survive in an aquatic environment. In short; it is possible mermaids could evolve scales.
I would presume mermaid skin would be evolved to cope with aquatic life. Most aquatic mammals have land-dwelling ancestors, so at some point their skin must have adapted to be waterproof. Even if the skin looks human, chances are it wouldnāt function or feel like human skin, instead being closer to either whale or seal skin. Similarly, mermaids would probably have blubber layers under their skin for insulation, though how much blubber they had would depend on environment.
The third eyelid may or may not be there depending on adaptations. It seems that most whales have lost their third eyelids, instead gaining other adaptations to help them see in water; however, seals, manatees and beavers, for example, do have them. This may be due to the amount of time spent out of the water, though I canāt say Iām sure. So the third eyelid would depend on your mermaid.
If mermaids are mammals, I would presume they could control when they excrete.
Due to the size of mermaids, itās unlikely they would have barnacles. Barnacles are only found on the large baleen whales, likely because these whales move slower than more agile species of whale, dolphin, or porpoise. Assuming the size of a mermaid is between normal human size and possibly large dolphin size, and that they are just as fast and agile, most likely barnacles would not form on mermaids. However, they may have other parasites like whale lice.
Marine mammals donāt have tear ducts, because their eyes donāt need to be moisturised. So itās likely that mermaids would not have tear ducts and would be unable to cry.
I hope you all enjoy the outcome of my procrastination.
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