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How Some Birds Evolved to Build Protected Nests

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Birds' nests are as diverse as the creatures who build them. The variety of locations, shapes, and sizes they’re built in and the materials they’re made from can be staggering. Hamerkops, for example, spend weeks assembling thousands of twigs into a massive nest that’s insulated and waterproofed with mud. Then there’s the horned coot, which makes its nest on top of an island of pebbles that it piles up in shallow lakes. Meanwhile, the edible-nest swiftlet builds its home entirely from hardened layers of its own saliva.

Even for someone more focused on birds’ brains, like neurobiologist Zach Hall was during his Ph.D. work, the diversity of nests didn’t go unnoticed—nor did the fact that the evolution of such wildly different nest types isn’t well researched. With a new study, Hall thinks he’s figured out how one variety of nest, a protected dome-shaped one, evolved. As they say in real estate, it’s all about location, location, location, and dome nests appear to have evolved as some birds transitioned from nesting in trees to nesting on or near the ground and needed a different kind of nest more suited to the new terrain. 

It’s not exactly a new idea, Hall says. Biologist Nicholas Collias suggested it nearly 20 years ago to explain why some some birds from a family called the Old World babblers build dome nests while others build open, cup-shaped ones. At the time, though, Collias didn’t have enough information or the right techniques to test his hypothesis. With a more complete picture of babblers’ evolutionary history and relationships now available, Hall and his team decided to see if the idea held up. 

The researchers gathered descriptions of the nests built by 155 different species of babbler and mapped their heights and structures (either cup- or dome-shaped) to the birds’ family tree. They found that Collias was on to something. Across the babbler family, species that build dome nests live closer to the ground than their cup-building relatives, and as the birds diverged from ancestors that built cup-shaped nests high in the trees, dome-shaped nests coevolved with nest height as some species ventured closer to the ground.

While they confirmed that nest height and structure go hand in hand, the researchers are still left with the question of which trait arose first and influenced the other. Nesting closer to the ground could have driven some species to add domes to their nests for protection against predators, as Collias suggested (though the team says it can’t yet rule out other influences, like parasites or the need to keep nests warm on shady forest floors), but the domes might have also come first and allowed the species that built them to stake out new nesting sites. The team notes, though, that the first situation is more likely. Changing nest height is an easier leap than changing nest shape, and more consistent with what researchers have seen in other birds. 

There’s still more work to be done, but Hall’s team is confident that their approach can answer the remaining questions about babblers and be used with other birds to uncover the reasons they build their nests they way they do.

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Big Questions
What Makes a Cat's Tail Puff Up When It's Scared?
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Cats wear their emotions on their tails, not their sleeves. They tap their fluffy rear appendages during relaxing naps, thrash them while tense, and hold them stiff and aloft when they’re feeling aggressive, among other behaviors. And in some scary situations (like, say, being surprised by a cucumber), a cat’s tail will actually expand, puffing up to nearly twice its volume as its owner hisses, arches its back, and flattens its ears. What does a super-sized tail signify, and how does it occur naturally without help from hairspray?

Cats with puffed tails are “basically trying to make themselves look as big as possible, and that’s because they detect a threat in the environment," Dr. Mikel Delgado, a certified cat behavior consultant who studied animal behavior and human-pet relationships as a PhD student at the University of California, Berkeley, tells Mental Floss. The “threat” in question can be as major as an approaching dog or as minor as an unexpected noise. Even if a cat isn't technically in any real danger, it's still biologically wired to spring to the offensive at a moment’s notice, as it's "not quite at the top of the food chain,” Delgado says. And a big tail is reflexive feline body language for “I’m big and scary, and you wouldn't want to mess with me,” she adds.

A cat’s tail puffs when muscles in its skin (where the hair base is) contract in response to hormone signals from the stress/fight or flight system, or sympathetic nervous system. Occasionally, the hairs on a cat’s back will also puff up along with the tail. That said, not all cats swell up when a startling situation strikes. “I’ve seen some cats that seem unflappable, and they never get poofed up,” Delgado says. “My cats get puffed up pretty easily.”

In addition to cats, other animals also experience piloerection, as this phenomenon is technically called. For example, “some birds puff up when they're encountering an enemy or a threat,” Delgado says. “I think it is a universal response among animals to try to get themselves out of a [potentially dangerous] situation. Really, the idea is that you don't have to fight because if you fight, you might lose an ear or you might get an injury that could be fatal. For most animals, they’re trying to figure out how to scare another animal off without actually going fisticuffs.” In other words, hiss softly, but carry a big tail.

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Animals
10 Notable Gestation Periods in the Animal Kingdom
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The gestation periods of the animal kingdom are varied and fascinating. Some clock in at just a few weeks, making any human green with envy, while others can last more than a year. Here are 10 notable gestation times for animals around the globe. The lesson? Be thankful that you’re not a pregnant elephant.

1. ELEPHANTS: 640-660 DAYS

Elephants are pregnant for a long time. Like really, really long. At an average of 95 weeks, the gestation period is more than double the length of a human pregnancy, so it shouldn't come as a shock that female elephants don't often have more than four offspring during their lifetimes. Who has the time?

2. HIPPOS: 8 MONTHS

A photo of a mother hippo and her baby in Uganda

Yes, it takes less time to make a hippopotamus than it takes to make a human.

3. GIRAFFE: 14-15 MONTHS

Baby giraffes can weigh more than 150 pounds and can be around 6 feet tall. Another fascinating tidbit: giraffes give birth standing up, so it's pretty normal for a baby to fall 6 feet to the ground.

4. KILLER WHALE: 17 MONTHS

There’s a reason for the long wait: after that 17 months, Baby Shamu emerges weighing anywhere from 265 to 353 pounds and measuring about 8.5 feet long. Yikes.

5. OPOSSUM: 12-13 DAYS

A baby opossum wrapped up in a blanket

Blink and you'll miss it: This is the shortest gestation period of any mammal in North America. But since the lifespan of an opossum is only two to four years, it makes sense.

6. GERBILS: 25 DAYS

Hey, they get off pretty easy.

7. GORILLAS: 8.5 MONTHS

It's not a huge surprise that their gestational periods are pretty similar to ours, right?

8. BLACK BEAR: 220 DAYS

A pair of black bear cubs

Also less than a human. Interestingly, cubs might only be 6 to 8 inches in length at birth and are completely hairless. 

9. PORCUPINE: 112 DAYS

This is the longest gestation period of any rodent. Thankfully for the mother, porcupine babies (a.k.a. porcupettes) are actually born with soft quills, and it's not until after birth that they harden up.

10. WALRUS: 15 MONTHS

Baby walruses? Kind of adorable. They certainly take their sweet time coming out, though.

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