"Look, a donkey that flies!" it is said to make fun of a sucker. But if someone told you "look, a fish on a tree!" could also be in good faith. Because a little fish with this curious habit exists. It lives in the swamps of the eastern coast of North and South America, has a passion for mangroves and is called Kryptolebias marmoratus, for friends "mangrove killfish". To remedy the periodic drought of its natural habitat, the little one - at most 7.5 centimeters long - has learned to get by even in the dry, and when the water withdraws it remains holed up in damp shelters like rotten leaves or in small holes in the wood carved by insects. Thus, by breathing through the skin, it can survive even a few months, waiting for better times.

In such conditions, in the open air and with dry scales, the last thing one would think of is sex. And in fact the mangrove fish does not think of it at all: when it wants to reproduce, it does everything by itself. In fact, this vertebrate has both male and female sexual organs. At the right time, just release sperm and eggs at the same time to make fertilization happen. Hermaphroditism among fish is a frequent condition: usually, however, even hermaphrodite fish mate with other specimens to mix genes and generate stronger offspring. In the populations of Kryptolebias marmoratus there are some males, with whom from time to time other fish mate to cross DNA. But the norm, for these aquatic creatures, is self-fertilization. After a few generations, therefore, the mangrove killfish become homozygous, that is, with two identical copies of each gene. This feature has prompted the ethologists of the University of Exeter, United Kingdom, to study the behavior of these populations, to understand if the identical genetic makeup also corresponds to the same behavior.

With some surprise, the researchers discovered that genetic affinity does not correspond to a marked behavioral similarity. These fish are not framed in rigid character patterns, on the contrary, they are incredibly malleable and able to adapt to the most disparate environmental situations. A winning strategy in evolutionary terms: in the presence of a scarce chromosomal richness, the marked behavioral elasticity makes them able to cope with any new danger or circumstance.

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