Scientists determined that birds lost their penises because of a biological program that triggers cell death. The discovery may offer clues about both evolution and the molecular biology behind birth defects. MORE
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Meet the animals that fascinated scientists in 2013 [Photos]

Scientists determined that birds lost their penises because of a biological program that triggers cell death. The discovery may offer clues about both evolution and the molecular biology behind birth defects. MORE

Scientists determined that birds lost their penises because of a biological program that triggers cell death. The discovery may offer clues about both evolution and the molecular biology behind birth defects. MORE

 (Horst Ossinger / AFP-Getty Images)

A closer look at the fossil remains of a South African reptile that existed 260 million years ago suggests turtle shells evolved from rib bones. The change forced turtles to come up with a new way to breathe. MORE

A closer look at the fossil remains of a South African reptile that existed 260 million years ago suggests turtle shells evolved from rib bones. The change forced turtles to come up with a new way to breathe. MORE (Scott Halleran / Getty Images )

Neurons in the brains of Japanese macaques are found to respond to images of snakes more strongly and quickly than they do to images of other objects. The discovery may explain why ophidiophobia ranks among the top fears of humans. MORE

Neurons in the brains of Japanese macaques are found to respond to images of snakes more strongly and quickly than they do to images of other objects. The discovery may explain why ophidiophobia ranks among the top fears of humans. MORE (Robert Sullivan / AFP-Getty Images )

The olinguito became the first mammal in the order carnivora to be discovered in the Western Hemisphere in 35 years. It was the culmination of a decadelong quest that began in Chicago's Field Museum in 2003. MORE

The olinguito became the first mammal in the order carnivora to be discovered in the Western Hemisphere in 35 years. It was the culmination of a decadelong quest that began in Chicago’s Field Museum in 2003. MORE (Mark Burney / Associated Press)

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