How do amphibians migrate?🐸
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| the pool frog, a semi-aquatic frog released into Norfolk |
In the UK, we have seven species of native amphibians: the common toad, natterjack toad, common frog, pool frog, smooth newt, great-crested newt and the palmate newt. However, the pool frog wasn't considered native here until recently, and soon after the discovery they were reintroduced to Norfolk. Additionally, three other species - the moor frog, agile frog and European tree frog - were found to be native, yet none have been reintroduced formally. All three went extinct on the mainland, though a small population of agile frogs remained in critical condition on Guernsey, where recent conservation measures have saved them. However, the general amphibians you see - the widespread, not habitat-fussy species - are most likely common frogs, common toads, and smooth newts. These are the generalist species - the ones you might see in your garden pond, under pile of rotting logs or crossing the road on a warm, wet spring night. Why, you may ask, do you see these amphibians crossing roads though? Well, that is a great question - one that we will explore today; a story of breeding instincts, magnetic fields and epic migrations.
Why do amphibians migrate?
The first thing we need to know to uncover the reason amphibians cross roads is... why? Well, contrary to popular belief the majority of frogs - excluding the 'water frogs', for example pool and moor frogs - don't live in water for most of their life like a fish. While pool and moor frogs stay in ponds, wetlands, and slow areas of river (such as
oxbow lakes and beaver-created pools) to breed, feed, sleep and do pretty much everything in water, unless on a bank or lily pad, common frogs and toads only need to enter water to breed. For the months of February to April, they come together to ponds, ditches and rivers to breed - or maybe even, if you're lucky, your own garden pond! Mating is chaotic and can be tough for the females - with often multiple males clinging on to her, each hoping to be in the right position at the right time for when she releases her eggs, passing on his genes to the next generation - of up to 4,000 young for common frogs, and 6,000 for the toads! No wonder they're so enthusiastic! After the rest of the males are shaken off or surrender due to exhaustion, one male remains - and him and his female go into a mating position called
amplexus, which can last many hours!
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| Common toads in amplexus |
Unfortunately, though, some females die of exhaustion before they can make it to laying eggs. For the lucky ones who aren't pestered by the males so excessively, she goes on to lay hundreds, if not thousands of eggs, called spawn (frogspawn or toadspawn specifically, depending on species). How to tell them apart in a pond, you may ask? Well, frogspawn - the most common that you'll likely see, and the most likely in a average-sized garden pond - is in large clumps, like a ball. Toadspawn, however - more often seen in larger ponds and rivers - in straight lines, one egg thick, often stretching multiple metres. What about newts, you may ask? Well, unlike their larger frog and toad cousins, all newts, from the widespread smooth newt, to the localised great crested and palmate newts, to invasives like alpine newts, lay only individual eggs - wrapped in a small aquatic leaf or reed stem. Unlike the chaotic approach of frogs and toads, however, newts have a more polite, calm approach to mating. First, the male locates a female, and then begins a courtship dance to impress her, before fanning pheromones to her with his tail. If it is one of the rare great crested newt, he will show off his crest to the female to impress her. If it is successful, they then mate - but not in the same sense of frogs. Instead, the male deposits a
spermatophore, which the female then picks up with her
cloaca to fertilise the eggs internally.
Dissimilar from the aggressive mating strategy of the frog world, newts show an alternate breeding strategy - with no wrestling, dog-piling and endurance battles in sight! Despite this, there is one big thing that both newts and frogs (which includes toads too) share: an epic migration! So now we know the
why, the next logical question is
how?
An epic migration
Like Arctic terns, the wildebeest of Africa and swallows from Europe, toads, frogs and newts also take on migrations of their own; though no so famous or spectacular. They range from an average of 300 metres for newts to 500 for frogs and 1 kilometre for common toads - though they can push much further! Newts have been recorded travelling distances up to 1 kilometre, common frogs 2 kilometres and the common toad, the long-distance specialist, has reached over 3 kilometres before. Perhaps even 4 kilometres, maybe even 5 or more - of course not every toad migration is recorded! What has been recorded, however, is the remarkable record journey of the natterjack toad - a whopping 5 kilometres! While this doesn't sound like much to us, that's actually around 7,000 hops for frogs, and it would take 10,000 newts nose-to-tail to match their migration distance! A newt's migration takes around 2-4 weeks on average, because, despite the small distance, they are smaller, slower, and regularly stop in damp micro-climates. Great-crested newts are the slowest, because of their larger size, around 18cm and double the size of of their smaller cousins (causing them to be much slower), and more moisture-dependant than others. Common frogs, moving around 50-200 metres a night, often take around 10 nights on average, though for longer distances they can take up to 20 nights or more, moving on wet and mild nights. Toads, on the other hand, move 200-500 metres a night. This causes them to take around 1-3 nights for small migrations, a week for normal migrations and up to two weeks for longer migrations. On the other hand, tree frogs, being much smaller than other native or formerly native species, often have only small-scale migrations, at an average of 200 or so metres, only taking a couple of days. However, some may go up to 1 kilometre, which could a week or more. Moor frogs migrate far, especially compared to their arboreal counterpart, going up to twice the distance of tree frogs - while agile frogs only migrate a maximum of 1 kilometre. Conversely, pool frogs - true 'water frogs' - often brumate (the amphibian version of hibernation) mere metres away from their ponds, with a maximum of 250 metres. The absolute opposite of the common toad, in migration terms at least! While small migrations like those of the pool frog are easy to remember the route of, how do moor frogs, common frogs, toads and others remember their way home? Well, along with visual and spatial clues, and smell once they get close to the pond (e.g. of pond water, chemical signatures of the soil, humidity gradients, even the smell of vegetation types), the main way they find their way around it magnetic fields. Like birds, salmon, turtles and painted-lady butterflies (though they use it in different ways) they use the magnetic fields of the Earth to sense the direction. How? Well, it's complicated. It's not a physical feeling like pain or touch, or just 'know' like how we know when we're tired or hungry. In truth, they have magnetite - tiny iron crystals - in their tissues, which physically rotate and align with the magnetic field, as if they had tiny compass needles inside them. When they shift, nerve cells can detect the movement. Furthermore, ever heard of cryptochrome? I didn't think so! It's a protein in their pea-sized eyes, which means, in the blue light of dawn and dusk, they can literally SEE the magnetic field overlapping their surroundings. This allows them to migrate huge distances (for their tiny bodies, at least), without forgetting which way their pond is!
The challenges they face - and how we can help
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Unfortunately, thousands of toads die every breeding season |
You likely have seen some form of amphibian - most likely a smooth newt, common toad or common frog - in a place it doesn't seem to belong in the last few months. Maybe your drive, a road or even a housing estate. The reason is - like the stubborn Zambian elephants, who annually migrate through a hotel reception for one mango tree in the courtyard - toads don't often change their migration routes, even if buildings, roads or pretty much anything is in their way. Thousands per annum die on roads, and you've likely seen some squished too. This, added on to habitat loss and pollution, is proving a disaster for our amphibians. So, what is being done to help? First, lets look at road closures. Everything Spring since 2003, Charlcombe lane in Bath shuts for 6 weeks. This 6 week closure may be annoying for some drivers, but is placed directly in the migration season for toads, frogs and newts - February to March. Volunteers help them across, helping an estimated 2,500 - 4,000 amphibians a season. However, many make it across the road without the help of volunteers, going uncounted - leading many to believe the real number they help is between 6,000 and 10,000 amphibians - including common frogs, common toads, smooth newts and palmate newts. In total, this means they helped 150,000 - 200,000 of our native, wetland-wonders as they've migrated to their spawning point. Why is it such a bottleneck for migrations, you may ask? Well, the Charlcombe lane runs directly through the ancient migration route - thousands of years old - of toads, frogs and newts from woodlands and hills to winter in on one side, and wetlands and ponds on the other. However a road closure isn't possible for all major roads, so, what else can be done? On a road in Henley-on-Thames, Oxfordshire, an underpass was built in 1987 - the first amphibian underpass in the UK, still used by thousands annually. An underpass is a small, wide tunnel under the road, often 50-100 metres in diameter, allowing toads, frogs and newts to move through, when migrating to their breeding grounds, back, or just dispersing juveniles. So, what should you do if you see a amphibian crossing the road? Well, if it is safe - often advisable on small country lanes - gently scoop it up (making sure not to squeeze) and place it on the other side of the road, in the direction it's heading. Don't put it back where it came; they're stubborn and will most likely try to cross again. However, always make sure their is no danger to you! Or, if you want to have a bigger impact, join a toad patrol! Their is over 150 'toad patrol' groups in the UK, which are groups of volunteers who help amphibians to cross the road. For more information of groups in your local area, head to the Froglife website*. Additionally, to benefit amphibians in different ways why not make a garden pond**, or leave a deadwood pile or overgrown corner? All of these methods can give a helping hand to our declining amphibians.
Thank you for reading today, and I hope you have learned something about Britain's wonderful amphibians! At the time of writing, in mid-May, any toads, frogs or newts seen on the road are likely heading back from their spawning point, or are late breeders. Every year, I get toads underneath the concrete slabs which our bins rest on, and smooth newts under wooden slabs in our back garden! In today's degraded Britain, not only are our present species declining, but we have lost many more. Today a mere 13 species of herps are considered native Here. However, many other species - including Aesculapian snakes, fire salamander, European pond turtles, tree frogs and midwife toads - are considered non-native or even invasive. However, our fossil record for these species are notoriously terrible, with most of them having bones the size of matchsticks, and often being eaten whole by predators or dying in acidic soils where they quickly decompose. Their ranges extend right up to Northern France and Western Belgium, and they had thousands of years to reach us via Doggerland. So why couldn't they be native? Nevertheless, we need to help our present species too - so why not take action for our amphibians today? Thanks for reading,
Isaac, Per[Nature]🌿
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Fire salamander - a 'non-native' amphibian that I believe is just another of Britain's long-lost fauna |
Oxbow lakes: a lake formed when a bend in a river is disconnected with the flow, resulting in a separate water body; a sign of truly wild, free river.
Amplexus: the mating embrace of some amphibians where the male takes position on the female's back and grips her tightly with his hind legs.
Herps: reptiles or amphibians.
Links:
*Froglife toad patrol locations: https://www.froglife.org/what-we-do/toads-on-roads/tormap/
**How to make a garden pond for wildlife: https://www.youtube.com/watch?v=lK1qTLwq2E4
Image credits:
“Male Pool frog (Rana lessonae) croaking” by Viridiflavus, licensed under CC BY 2.5.
Toad warning sign: T Eyre, CC BY‑SA 2.0
Fire salamander: Ryzhkov Sergey, CC BY‑SA 4.0
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