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Episode 332: Hunting Partners and Mutualism

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Thanks to Vaughn and Jan for their suggestions this week! We’re going to learn about mutualism of various types.

Further reading:

The odd couple: spider-frog mutualism in the Amazon rainforest

What Birds, Coyotes, and Badgers Know About Teamwork

Octopuses punch fishes during collaborative interspecific hunting events

An Emotional Support Dog Is the Only Thing That Chills Out a Cheetah

Buddies [picture from the first link above]:

The honeyguide bird:

Cheetahs and dogs can be friends:

Show transcript:

Welcome to Strange Animals Podcast. I’m your host, Kate Shaw.

This week we’re going to learn about a topic that I’ve been wanting to cover for a long time, mutualism. It’s a broad topic so we won’t try to cover everything about it in this episode, just give an overview with some examples. Vaughn suggested symbiotic behavior ages ago, and Jan gave me a great example of this, also ages ago, so thanks to both of them!

Mutualism is similar to other terms, including symbiosis, often referred to as “a symbiotic relationship.” I’m using mutualism as a general term, but if you want to learn more you’ll quickly find that there are lots of terms referring to different interspecies relationships. Basically we’re talking about two unrelated organisms interacting in a way that’s beneficial to both. This is different from commensalism, where one organism benefits and the other doesn’t but also isn’t harmed, and parasitism, where one organism benefits and the other is harmed.

We’ll start with the suggestion from Jan, who alerted me to this awesome pair of animals. Many different species have developed this relationship, but we’ll take as our specific example the dotted humming frog that lives in parts of western South America.

The dotted humming frog is a tiny nocturnal frog that barely grows more than half an inch long from snout to vent, or about 2 cm. It lives in swamps and lowland forests and spends most of the day in a burrow underground. It comes out at night to hunt insects, especially ants. It really loves ants and is considered an ant specialist. That may be why the dotted humming frog has a commensal relationship with a spider, the Colombian lesserblack tarantula.

The tarantula is a lot bigger than the frog, with its body alone almost 3 inches long, or 7 cm. Its legspan can be as much as 8 and a half inches across, or 22 cm. It’s also nocturnal and spends the day in its burrow, coming out at night to hunt insects and other small animals, although not ants. It’s after bigger prey, including small frogs. But it doesn’t eat the dotted humming frog. One or even more of the frogs actually lives in the same burrow as the tarantula and they come out to hunt in the evenings at the same time as their spider roommate.

So what’s going on? Obviously the frog gains protection from predators by buddying up with a tarantula, but why doesn’t the tarantula just eat the frog? Scientists aren’t sure, but the best guess is that the frog protects the spider’s eggs from ants. Ants like to eat invertebrate eggs, but the dotted humming frog likes to eat ants, and as it happens the female Colombian lesserblack tarantula is especially maternal. She lays about 100 eggs and carries them around in an egg sac. When the babies hatch, they live with their mother for up to a year, sharing food and burrow space.

This particular tarantula also gets along with another species of frog that also eats a lot of ants. Researchers think the spiders distinguish the frogs by smell. The ant-eating frogs apparently smell like friends, or at least useful roommates, while all other frogs smell like food. Or, of course, it’s possible that the ant-eating frogs smell and taste bad to the spider. Either way, both the frogs and the tarantulas benefit from the relationship–and this pairing of tiny frogs and big spiders is one that’s actually quite common throughout the world.

Mutualism is everywhere, from insects gathering nectar to eat while pollenating flowers at the same time, to cleaner fish eating parasites from bigger fish, to birds eating fruit and pooping out seeds that then germinate with a little extra fertilizer. Many mutualistic relationships aren’t obvious to us as humans until we’ve done a lot of careful observations, which is why it’s so important to protect not just a particular species of animal but its entire ecosystem. We don’t always know what other animals and plants that animal depends on to survive, and vice versa.

Sometimes an individual animal will work together with an individual of another species to find food. This may not happen all the time, just when circumstances are right. Sometimes, for example, a coyote will pair up with a badger to hunt. The coyote is closely related to wolves and can run really fast, while the American badger can dig really fast. Both are native to North America. They also both really like to eat prairie dogs, a type of rodent that can run really fast and lives in a burrow. Some prairie dog tunnels can extend more than 30 feet, or 10 meters, with multiple exits. The badger can dig into the burrow and if the prairie dog leaves through one of the exits, the coyote chases after it. When one of the predators catches the prairie dog, they don’t share the meal but they will often continue to hunt together until both are able to eat.

Other animals hunt together too. Moray eels will sometime pair up with a fish called the grouper in a similar way as the coyote and badger. The grouper is a fast swimmer while the eel can wriggle into crevices in rocks or coral. The grouper will swim up to the eel and shake its head rapidly to initiate a hunt, and if the grouper has seen a prey item disappear into a crevice, it will lead the eel to the crevice and shake its head at it again.

Groupers also sometimes pair up with octopuses to hunt together, as will some other species of fish. Like the eel, the octopus can enter crevices to chase an animal that’s trying to hide. But the octopus isn’t always a good hunting partner, because if the grouper catches a fish, sometimes the octopus will punch the grouper and steal its fish. Not cool, octopus.

Birds have mutualistic relationships too, including the honeyguide that lives in parts of Africa and Asia. It’s a little perching bird that’s mostly gray and white or brown and white, with the males of some species having yellow markings. It eats insects, spiders, and other invertebrates, and it especially likes bee larvae. But it’s just a little bird and can’t break open wild honeybee hives by itself.

Some species of honeyguide that live in Africa have figured out that humans can break open beehives. When the honeyguide bird finds a beehive, it will fly around until it hears the local people’s hunting calls. The bird will then respond with a distinct call of its own, alerting the people, and will guide them to the beehive. This has been going on for thousands of years. The humans gather the honey, the honeyguide feasts on the bee larvae and wax, and everyone has a good day except the bees.

The honeyguide is also supposed to guide the honey badger to beehives, but there’s no definitive evidence that this actually happens. Honey badgers do like to eat honey and bee larvae, though, and when a honey badger breaks open a beehive, honeyguides and other birds will wait until it’s eaten what it wants and will then pick through the wreckage for any food the badger missed. But the honeyguide might lead the honey badger to the hive, we just don’t know for sure.

Humans sometimes even help other animals into a commensal relationship. Vaughn gave me an example of a cheetah in a zoo who became best friends with a dog. This hasn’t just happened once, it’s happened lots of times because zookeepers have found that it helps cheetahs kept in captivity. Cheetahs are social animals but sometimes a zoo doesn’t have a good companion for a cheetah cub. The cub could be in danger from older, unrelated cheetahs, but a cheetah all on its own is prone to anxiety. It’s so important for a cheetah to have a sibling that if a mother cheetah only has one cub, or if all but one cub dies, a lot of times she’ll abandon the single cub. If this happens in the wild, it’s sad, but if it happens in captivity the zoo needs to help the cub.

To do this, the zoo will pair the cub with a puppy of a sociable, large breed of dog, such as a Labrador or golden retriever. The cub and the puppy grow up together. The cheetah has a mellow friend who helps alleviate its anxiety, and the dog has a friend who’s really good at playing chase.

You can find Strange Animals Podcast at strangeanimalspodcast.blubrry.net. That’s blueberry without any E’s. If you have questions, comments, or suggestions for future episodes, email us at strangeanimalspodcast@gmail.com. If you like the podcast and want to help us out, leave us a rating and review on Apple Podcasts or just tell a friend. We also have a Patreon at patreon.com/strangeanimalspodcast if you’d like to support us that way.

Thanks for listening!

Episode 049: The Brantevik Eel and Friends

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This week’s episode is about some interesting eels, including the Brantevik eel.

A European eel:

A leptocephalus, aka an eel larva:

A moray eel. It has those jaws you can see and another set of jaws in its throat:

Episode transcript:

Welcome to Strange Animals Podcast. I’m your host, Kate Shaw.

This week, we’re going to learn about the Brantevik eel and some other eels, including an eel mystery.

The Brantevik eel is an individual European eel, not a separate species. Its friends knew it as Åle, which I’ve probably misprounounced, so I’m nicknaming it Ollie. So what’s so interesting about Ollie the eel?

First, let’s learn a little bit about the European eel in general to give some background. It’s endangered these days due to overfishing, pollution, and other factors, but it used to be incredibly common. It lives throughout Europe, from the Mediterranean to Iceland, and has been a popular food for centuries.

The European eel hatches in the ocean into a larval stage that looks sort of like a transparent flat tadpole, shaped roughly like a leaf. Over the next six months to three years, the larvae swim through the ocean currents, closer and closer to Europe, feeding on microscopic jellyfish and plankton. Toward the end of this journey, they grow into their next phase, where they resemble eels instead of tadpoles, but are mostly transparent. They’re called glass eels at this point. The glass eels make their way into rivers and other estuaries and slowly migrate upstream. Once a glass eel is in a good environment it metamorphoses again into an elver, which is basically a small eel. As it grows it gains more pigment until it’s called a yellow eel. Over the next decade or two it grows and matures, until it reaches its adult length—anywhere from two to five feet, or 60 cm to 1.5 meters. When it’s fully mature, its belly turns white and its sides silver, which is why it’s called a silver eel at this stage. Silver eels migrate more than 4,000 miles, or 6500 km, back to the Sargasso Sea to spawn, lay eggs, and die.

One interesting thing about the European eel is that during a lot of its life, it has no gender. Its gender is determined only when it grows into a yellow eel, and then it’s mostly determined by environmental factors, not genetics.

Until the late 19th century, everyone thought these different stages—larva, glass eel, elver, yellow eel, and silver eel—were all separate animals. No one knew how or even if eels reproduced. The ancient Greeks thought eels were a type of worm that appeared spontaneously from rotting vegetation. Some people thought eels mated with snakes or some types of fish. By the 1950s the eel’s life cycle was more or less understood, but many researchers thought the European eels never made it to the Sargasso Sea to spawn. It was just too far, so they thought the eels that arrived in Europe were all larvae of the American eel, which is almost identical in appearance to the European eel. The Sargasso Sea is off the coast of the Bahamas, so the American eel doesn’t have nearly as far to travel. These days we know from DNA studies that the American and European eels are different species. The European eel is just a world-class swimmer.

European eels are nocturnal and may live in fresh water, brackish water, or sometimes they remain in the ocean and live in salt water, generally in harbors and shallows. They eat anything they can catch, from fish to crustaceans, from insect larvae to dead things, and on wet nights they’ll sometimes emerge from the water and slide around on land eating worms and slugs. Many populations don’t eat at all during the winter.

Now, back to the Brantevik eel. Brantevik is a tiny fishing village in Sweden. In 1859, an eight-year-old boy named Samuel Nilsson caught an eel and released it into his family’s well to eat insect larvae and other pests. This was a common practice at the time when water wasn’t treated, so the fewer creepy-crawlies in the water, the better.

And there the eel stayed. Ollie got famous over the years, at least in Sweden. Its 100th well anniversary was celebrated in 1959, and children’s books and even movies featured it. But in summer of 2014, Ollie died. Its well is now on the property of Tomas Kjellman, whose family bought the cottage and its well in 1962. Everyone knew about the resident eel, which the family treated as something of a pet. In fact, they discovered it was dead when they opened the well’s cover to show the eel to some visiting friends.

Ollie’s remains were removed from the well and shoved in the family’s freezer, and later sent to be analyzed at the Swedish University of Agricultural Science’s Institute of Freshwater Research. That analysis confirmed that Ollie was over 150 years old.

In the wild, European eels don’t usually live longer than twenty years, and ten years is more likely. But in captivity, where eels don’t spawn, they can live a long time. A female European eel named Putte lived over 85 years in an aquarium at Halsinborgs Museum in Sweden.

What most people don’t know is that Ollie wasn’t alone. Another eel still lives in the well and is doing just fine, but it’s younger, only about 110 years old.

The larvae of European eels are small, only about three inches at the most, or 7.5 cm. Even conger eel larvae are small, only 4 inches long, or 10 cm, and conger eels can grow 10 feet long, or 3 meters. But on January 31, 1930, a Danish research ship caught an eel larva 900 feet deep off the coast of South Africa—and that larva was six feet 1.5 inches long, or 1.85 meters.

Scientists boggled at the thought that this six-foot eel larva might grow into an eel more than 50 feet long, or 15 meters, raising the very real possibility that this unknown eel might be the basis of many sea serpent sightings.

The larva was preserved and has been studied extensively. In 1958, a similar eel larva was caught off New Zealand. It and the 1930 specimen were determined to belong to the same species, which was named Leptocephalus giganteus. Leptocephalus, incidentally, is a catchall genus for all eel larvae, which can be extremely hard to tell apart.

In 1966 two more of the larvae were discovered in the stomach of a western Atlantic lancet fish. They were much smaller than the others, though—only four inches and eleven inches long, or 10 cm and 28 cm. Dr. David G. Smith, an ichthyologist at Miami University, determined that the eel larvae were actually not true eels at all, but larvae of a spiny eel. Deep-sea spiny eels are fish that look like eels but they’re not closely related. And while spiny eels do have a larval form that resembles that of a true eel, they’re much different in one important way. Spiny eel larvae grow larger than the adults, then shrink when they develop into their mature form.

So the six-foot eel larvae, if it had lived, would have eventually developed into a spiny eel no more than six feet long itself at the most, and probably shorter.

More recent research has called Dr. Smith’s findings into question, and many scientists today consider L. giganteus to be the larvae of a short-tailed eel, which is a true eel—but not a type that grows much larger than its larvae. So either way, the adult form would probably not be much longer than a conger eel.

But…we still don’t have an adult. So there’s still a possibility that a very big deep-living marine eel is swimming around in the world’s oceans right now.

The longest known eel is the slender giant moray, which can reach 13 feet in length, or 4 meters. Morays are interesting eels for sure. They live in the ocean, especially around coral reefs, and have two sets of jaws, their regular jaws with lots of hooked teeth, and a second set in the throat that are called pharyngeal jaws, which also have teeth. The moray uses the second set of jaws to help grab and swallow prey that might otherwise wriggle out of its mouth. The moray has a strong bite and doesn’t see very well, although its sense of smell is excellent. This occasionally causes problems for divers who think it would be fun to feed an eel and end up with a finger bitten off. Don’t feed the eels, okay? Not only that, but a moray can’t release its bite even if it’s dead, so if one bites a diver, someone has to pry the eel’s jaws open before the bite can be treated. And as if all that wasn’t warning enough to not feed wild animals, and frankly just stay out of the water entirely, research suggests that some morays are venomous. Oh, and the giant moray sometimes hunts with a fish called the roving coralgrouper, which grows to some four feet long, or 120 cm, which is a rare example of interspecies cooperative hunting.

Some people believe that at least some sightings of the Loch Ness monster can be attributed to eels—European eels, in this case. An eel can’t stick its head out of the water like Nessie is supposed to do, but it does sometimes swim on its side close to the water’s surface, which could result in sightings of a string of many humps undulating through the water. But while eels do live in and around Loch Ness, it’s unlikely that any European eel would grow much larger than around five feet, or 1.5 meters. Still, you never know. Loch Ness is the right habitat for an eel to grow to its maximum size, and while we have learned a lot about eels in general, and the European eel in particular, since Ollie was released into a well in Brantevik, we certainly don’t know everything about them.

One last note about eel larvae. Occasionally on facebook and other social media, well-meaning people will share warnings about a nearly invisible wormlike parasite that can be found in drinking water, with pictures of, you guessed it, eel larvae. Eel larvae are not parasites, are not found in fresh water at all, and even if you did accidentally swallow one, you’d just digest it and get a little protein out of the bargain. So you don’t need to worry about those clickbait warnings, the eels do.

You can find Strange Animals Podcast online at strangeanimalspodcast.com. We’re on Twitter at strangebeasties and have a facebook page at facebook.com/strangeanimalspodcast. If you have questions, comments, or suggestions for future episodes, email us at strangeanimalspodcast@gmail.com. If you like the podcast and want to help us out, leave us a rating and review on Apple Podcasts or whatever platform you listen on. We also have a Patreon if you’d like to support us that way.

Thanks for listening!