If you know anything about nightingales, it’s probably that they sing. Written about by the likes of ancient Greek playwright Aristophanes and English poet John Keats, they’ve also inspired such composers as Beethoven and Stravinsky to pay homage to their melancholy warbling. But not all of their songs are mournful: In fact, male nightingales use some of them to advertise their skills as fathers. One of the many factors female nightingales consider when assessing prospective mates is their suitors’ paternal potential, and a 2015 study on the subject showed that better male nightingale singers are known to feed their children more often than their less-talented peers. “Better” here means singing in a more orderly fashion — repeating the same song types over and over — and varying their song choices, with plenty of buzzes, trills, and whistles.

Most male birds are very involved fathers.

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In contrast to some other animals, most male birds — around 80% of all bird species, in fact — take an active role in raising their young. This begins before the chicks are born, as a father feeds the mother while she incubates the eggs, and it also includes keeping the nest safe from predators.

Whereas it was previously thought that the size of a male’s repertoire was the sole criterion a female considered, in nightingales, the kind of song is crucial as well. Overall, male nightingales are known to be doting fathers — they visit their chicks’ nest as many as 16 times every hour, which is about as often as their mothers do.

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New York City’s subway system is one of the largest in the world, operating 24 hours a day, seven days a week, in a city of more than 8 million people. Subway cars undergo a lot of wear and tear thanks to their near-constant use, and once no longer useful for human commuters, some of the trams have been sent to accommodate a new type of passenger: fish. More than 2,500 of New York City’s old subway cars have been dispatched to the bottom of the Atlantic Ocean, where they are used as artificial reefs. 

Most of the ocean floor contains coral reefs.

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Coral reefs are vital to ocean ecosystems, but they make up less than 1% of the ocean floor. Despite this limited size, scientists believe coral reefs support up to 25% of all marine species by providing food, shelter, and a safe place for young sea life to grow.

Normally, dumping scrap metal into the ocean would be frowned upon, but reusing old subway cars along the East Coast’s ocean floor has actually benefited some underwater ecosystems. New York’s Metropolitan Transit Authority (MTA) began the reef-building program for its decommissioned subway cars in 2001; the cars were stripped down to their metal hulls before barges dropped them into the ocean between New Jersey and Georgia. While some of the cars have disintegrated, at least 1,000 are made from anticorrosive carbon steel and can last indefinitely underwater, serving as homes for fish, invertebrates, and underwater plants. Proponents of the artificial reef system say the subway cars have allowed ecosystems to flourish where they might not have; the Atlantic’s coastal waters in the area are known for being particularly sandy and lacking many of the natural features necessary for quality aquatic habitats. While the MTA program ended in 2010, it was supported by the fishing and scuba diving tourism industries — and many scientists, who report that subway car reefs have provided 400 times more food for fish per square foot than previously existed.

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The most distinguishing feature about an octopus is its set of eight appendages — after all, they’re right there in the name. But don’t confuse that tangle of limbs for tentacles, because octopuses don’t have those — they have arms. For us armchair biologists, the two words seem interchangeable, but there’s an important difference. On animals such as squids, tentacles are usually longer and have suckers only on their clubbed ends; they’re primarily used for hunting. By contrast, a cephalopod's arms have suckers that smell, taste, and feel all the way down. Squids, for example, have both eight arms and two tentacles.

Octopus blood is blue.

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In humans, iron-rich hemoglobin delivers oxygen to tissues while turning our blood red. Octopus blood, however, contains hemocyanin, which uses a copper atom instead of iron to deliver oxygen to cells. This is what makes the blood of octopuses — and many other sea creatures — blue.

Of course, an octopus’s “arms” are nothing like the two appendages dangling from your torso. For one, octopus arms are entirely soft tissue, and that absence of bone makes octopuses incredibly flexible. But the most striking difference between octopus arms and our own is that all eight arms contain more than half of an octopus’s total neurons, cells that are usually concentrated in a central brain. This has led some scientists to theorize that octopuses essentially have nine brains — a centralized one and eight mini ones located in each arm. Recent research suggests that there might be more connection between these “brains” than previously imagined, and that an octopus’s arms are, at the very least, “clever” (a very peculiar adjective to describe an arm). Whatever the IQ of an octopus’s arms, it’s clear that they’re just as strange, and incredible, as the creatures themselves.

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