Evolution can be hard to wrap your head around, sometimes.
Like, it’s the process that turned a small, hoofed land-mammal into a diverse group of
whales over millions of years as some individuals did a better job of getting into the gene
pool than others, which sounds absolutely wild.
So it can seem like a thing that just happened in the distant past or in a lab somewhere,
not something happening right now, all around you.
But what if I told you that you might not need thousands of lifetimes, or fossils, or
a lab full of bacteria to see evolution start to happen?
That you could track it in a few short years in a population of a big, living animal?
This is a story about what might be the very beginning of that process in an unusual raptor,
and the invasive snail that may be driving it.
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Today’s Bizarre Beasts is a two-for-one about two animals that share part of their name.
The first is the snail kite, a small bird of prey found in South and Central America,
the Caribbean, and the Florida Everglades.
And it’s endangered in Florida due to habitat loss caused by rising and falling water levels.
Water levels are super important to snail kites.
If water levels are too low, then their nests, which they sometimes build on bullrushes or
cattails, collapse as the vegetation dries out.
Predators like raccoons and snakes also have an easier time sneaking into low nests.
And high water levels are a problem for basically the same reasons.
Submerged plants can’t hold up a nest, and swimming animals like marsh rats and even
alligators can reach the nests more easily.
But being threatened by the draining of the Everglades isn’t exactly bizarre, a lot
of animals have this problem.
What makes snail kites special is their very picky eating habits.
Snail kites, as you might guess, almost exclusively eat snails.
You could say they’re not really into fast food.
And just like any food connoisseur, they have special tools for preparing their food.
They have long lower legs, which in birds is actually a part of their foot called the
tarsus, that they use to reach down and snatch
snails up as they glide over the surface of the water.
They also have delicate, tapered, hooked beaks, which is where their genus name comes from.
“Rostrhamus” combines ‘rostr’ from rostrum, meaning snout or beak,
and ‘hamus’ meaning hook.
The kites use those beaks to extract snails from their spiral shells.
Now, the way that the size and shape of bird beaks can adapt to particular food items is
a really interesting area of evolutionary biology.
It goes all the way back to Charles Darwin, himself.
But being super specialized can be risky, and the kites usually don’t do so well when
marshland habitat isn’t available to them.
Although they were numerous in Florida in the early 1900s, snail kite populations plummeted
to no more than 75 birds in the 1950s,
and human impacts on the marshes are still a threattoday.
Picky eaters struggle when access to their preferred food drops, either through habitat
loss or other environmental changes that make it hard to hunt, like an increase in sediment
muddying the waters, and so the snail kite population declined.
In 1967, they were listed as endangered after only 10 individuals were found in surveys
focused on key habitats in Florida.
Conservation measures like managing water levels to protect snail kite habitats and
food sources helped, and by the mid ‘90s, snail kites were on the rise, though they’re
still very threatened, with only 700 left in the wild in Florida.
And then a new challenge appeared: the island apple snail.
These baseball-sized mollusks from South America probably got introduced to the US through
the aquarium trade where they’re sometimes called mystery snails.
And they’re widely recognized as a troublesome invasive species in North America and Asia.
They have a voracious appetite for plants, including crops like rice and taro.
Like other members of their family, including the much smaller Florida apple snail, they
have both lungs and gills, so they can breathe underwater and in air.
This gives them a foot up in a habitat like the Everglades where water levels can be unpredictable.
When times are tough, they can also seal off the inside of their shell.
If the weather is humid enough, which it is almost everyday in Florida, they can hide
out for weeks in their sealed shells until conditions improve.
And they lay way more eggs at a time than native species, like the Florida apple snail.
And it’s this ability, combined with the traits that make them a good fit for the environment
of the Everglades, that had conservationists worried.
As island apple snail populations expanded in the Everglades, concerns grew around what
this would mean for the snail kites.
The bulkier apple snails are difficult for young kites to handle, and snail kite populations
plummeted by 80% in the early and mid-2000s.
But then something unexpected happened: snail kite populations increased, and juvenile survival
rates even went up in wetlands invaded by non-native snails.
What was going on?
One way that organisms can change in response to their environment is evolution.
Because it takes place in a population over many generations, evolution isn’t usually
something we can watch happen in real time, unless the organism we’re studying multiplies
quickly in the lab so we can get lots of generations in a short period of time.
For the same reason, we don’t know a lot about how long-lived animals, like snail kites,
respond to big, rapid changes in their environment or the role evolution can play in these responses
when it takes so long to go from one generation to the next.
In 2017, researchers from the University of Florida reported that average snail kite beak
size had grown over the last eight years, and that the bigger beaks were more common
within a year of the island apple snail invasion.
And beak sizes only started growing after
island apple snails got a foothold in the Everglades.
It’s tempting to think that juveniles with bigger beaks were more likely to survive their
first year because they had an easier time eating bigger snails, and therefore, were
more likely to pass their big beak genes down to their offspring.
But not so fast!
Generation time in snail kites is about 5 to 8 years in the wild, there’s no way that
there would be enough action in the gene pool in one year for them to all evolve bigger
beaks.
There was something else going on.
And that something is phenotypic plasticity, the ability of an animal to change during
its lifetime in response to its environment, without a change to its genes.
The juvenile snail kites grew larger overall
and had disproportionately large beaks compared to their parents.
The larger beaks likely helped with the problem of island apple snails being difficult for
them to handle, meaning the juveniles with the largest beaks
were more likely to survive their first year.
And a really intriguing possibility is that the larger beaks were also the reason that
the snail kites got bigger, since they had easier access to an extremely abundant food source.
And remember that access to food has been a concern for the snail kites for decades.
It could be that the island apple snails help the kites stave off extinction, at least in
the short term, but finding evidence for that, like evolution, will take time.
The Bizarre Beasts pin club subscription window is open again,
from now through the end of June 3rd.
When you sign up, you’ll get this pin in the middle of the month and the pins after
that around the time each new video goes live.
This pin, we made sure, has both the snail kite and the snail!
And you might’ve noticed that we dropped a second episode this past month,
featuring a ghostly jellyfish.
It’s a new format we’re trying out, so please check it out and let us know what you think!
And, as always, profits from the pin club and all of our merch go to support our community’s
efforts to decrease maternal mortality in Sierra Leone.
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