Why an octopus's main heart stops while it swims

3 hearts

Swimming stops an octopus’s main heart

An octopus has three hearts: two pump blood through the gills, while a systemic heart circulates blood through the body. That systemic heart stops beating during swimming, which helps explain why octopuses tire quickly and mostly crawl.

A giant black hole can have the average density of water

≈ density of water

A black hole can contain an enormous mass while having a surprisingly low average density inside its event-horizon-sized region. For a black hole with 100 million times the Sun's mass, that average density is roughly similar to water.

A high-altitude gene variant spread quickly among Tibetans

≈3,000 years

Over roughly 3,000 years, a gene variant that helps people live at high altitude became very common among Tibetans. It is an example of how quickly natural selection can change a human population when the environment strongly favors a trait.

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Why an octopus's main heart stops while it swims

Swimming stops an octopus’s main heart

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