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The Common Frog: Hopping for a good spring!

Ben DolphinThe first frogspawn of the year usually takes me by surprise. It shouldn’t of course, because it’s an annual event as reliable as the first green shoots of grass rising from the barren, brown hillsides. But when I see those first clumps of jelly I am usually on a cold and windswept hillside, bent double into a raging gale, perhaps trying not to slip on week-old ice, or more likely eyeing-up the sky for the first tell-tale snowflakes of an incoming cold spell. Spring therefore feels like it is still months away……but then there it is, crammed into a puddle on the path like bubble-wrap stuffed to bursting into a cardboard box.

As with most natural phenomena, frogspawn appears at different times depending on where you are geographically. Generally it appears in January in the far southwest of England or, in the worryingly mild years of late, as early as December or even November! The first sightings then spread north up the UK and, judging by the various online maps generated by public reports this year, the first sightings in Scotland were at the end of February and into the beginning of March.

Frogspawn in a puddle on a path

And indeed there it was last week, high up in the Campsie Fells near the base of Dumgoyne, half filling a large flooded stretch of the path. It just seemed so horribly exposed up there in that dirty puddle, windswept by an icy wind and with yet another cold spell on its way. I couldn’t help wondering how on earth the eggs, or the frogs for that matter, can survive those flip-flopping conditions or manage to hatch out in such an unlikely location?

Laid in the thousands by the female and immediately fertilised by the male, the tiny eggs initially sink downwards into the water before their protective jelly expands, becomes buoyant and rises to the surface as a clump. The frogspawn then needs light and warmth, both of which are things that can be in short supply in Scotland at the best of times.

The uppermost spawn in a clump will likely die in a severe frost but this is to be expected in most years. Similarly the frogs themselves can die over the winter if the water body in which they are hibernating freezes over for a prolonged period. Not all frogs hibernate underwater, many do so in damp, dark places on land such as compost heaps, but those that do use ponds can actually stay below the water for months if necessary, slumbering in the silt and taking in oxygen through their skin. But when a layer of ice cuts the pond off from the atmosphere, or a layer of snow on top of it prevents underwater plants from photosynthesising, the concentration of oxygen in the water can decrease. The frog’s woes can be compounded if there is a large quantity of dead vegetation or fish faeces in the pond, as the toxins and gases that are released by the decomposition process cannot escape the pond and can subsequently build up to toxic levels, which can kill the frogs.

Frogs lay their spawn in clumps (left), whereas toads lay their spawn in lines (right – photo by Fiona Cousland)

Again, none of this is unusual and a natural mortality of frogs, as with most species that spend winter in Scotland, is to be expected. But while winter happens every year within a broadly similar time frame, as we know all too well its tail-end can be both a lamb and a lion, sometimes within days of one another. And sometimes, as has been the case in 2018, the lion can roar well into spring. It’s the irregularity and swings between the two that can catch wildlife out, although such swings are to be expected even normal winters and springs, whatever those are these days!

As for my frogspawn in that dirty puddle, or the many eggs you will likely see in tyre tracks or flooded fields, sadly the odds are against them. Frogs aren’t as wedded to their ancestral ponds as toads are but they do still have a tendency to lay their spawn in the same place that they hatched out. If that pond has disappeared or they are unable to reach an equally suitable water body, they will use whatever is to hand. The simple fact is some of their chosen sites just aren’t suitable and most if not all of the eggs will perish when the puddles dry out. However, in more favourable still-water habitats such as ponds, lakes and canals most of the eggs will, as every good school kid knows, successfully hatch out into tadpoles.

Tadpoles a few weeks old

Initially, tadpoles are sustained by eating the protein-rich jelly in which they were laid, but when that runs out they start to eat algae and other plant matter in the pond. As they grow larger they become meat eaters, picking off water-borne insects or even eating one another! This increasingly ravenous diet powers an extraordinary physiological change, which we perhaps don’t marvel at enough if I’m honest. When they first hatch out, tadpoles breathe through their gills, but because they will need to survive out of the water in later life they lose these and develop lungs instead. This is why you see older tadpoles repeatedly swim to the surface to take a gulp of air.

The speed of tadpole development varies depending on factors such as temperature and availability of food, but generally speaking the hind legs start to appear after about a month along with speckled markings, and then front legs and a frog-shaped head, before finally the tadpole tail is absorbed back into their bodies. After spending three or four months in the water a lucky few ‘froglets’, which resemble impossibly exquisite miniatures of the adults, successfully hop out of the pond and off into the big bad world.

A froglet that has just left its pond

Interestingly, not all tadpoles will complete their metamorphosis in their first summer. If conditions are not conducive to development, tadpoles can suspend the transformation and remain in the pond over the winter. Assuming they survive the winter they will be at an advantage in the spring, able to eat the plentiful food (likely consisting of newly hatched tadpoles) and emerging from the pond in a larger, fitter state.

However, even in the most favourable environments the odds on a frog’s egg reaching the finish line and escaping the confines of its pond are slim. Of the thousand or so eggs laid by a single frog, it’s reckoned that only between 1% and 5% will complete the transformation into a froglet. Fewer of those will reach adulthood and, because it takes a further three or four years to reach sexual maturity, even fewer adult frogs will actually return to give birth to another generation.

This is because frogs and their eggs are food for many other creatures both in and out of water, and pretty much everything in the pond wants to eat the hatched tadpoles, not least the rather terrifying-looking Great Diving Beetle larvae. Watching one of those devour a tadpole (or a fish, for that matter) is the stuff of nightmares, believe me! This perhaps all sounds rather dire for the frogs’ chances of successfully reproducing, but when the odds are that poor and the parents effectively abandon the young to their fate, the reproductive strategy has to be one of sheer numbers to ensure that at least some will actually make it to be adult frogs. And it works.

Meet the parents

Scotland only has one species of frog, the Common Frog, and unlike some Scottish creatures that share the ‘Common’ prefix, in this case it is fairly accurate. The common frog is found right across the country in a wide range of habitats, from the coast to our high mountain summits, and I dare say most of us will have encountered one in our gardens or on a walk.

An adult frog doing its best statue impression

Because they can breathe through their skin as well as their lungs, they can dehydrate easily and therefore tend to stay out of sight in damp, shady places (such as underneath logs) to avoid drying out. Mild wet evenings draw them out to hunt slugs, snails, insects and spiders, which reveal their local profusion to us due to their unfortunate habit of congregating on roads in vast numbers. Certainly where I stay in Fife I have to be very careful on such evenings when I’m driving the last kilometre home, slowly steering around the tiny white shapes on the road.

The common frog can come in a variety of different colours such as olive-green, brown, yellow or even red, but grey-green and brown tend to be more commonplace. Most of the specimens you encounter will be around 6 or 7cm in length (not including their outstretched hind legs), but some of the larger females are surprisingly bulky specimens and can be more like 10cm.

If you’re anything like me you probably encounter the bulk of your frogs when they’re sitting on or just next to an upland path, which might seem a strange place to find them but they actually spend most of their lives away from water. Like all amphibians, they lead a double life in that regard, able to survive both on dry land and in the water, and indeed the etymology of ‘amphibian’ tells you as much – apparently it comes from the ancient Greek words ‘amphi’, meaning ‘both’ or ‘double’, and ‘bios’, meaning ‘life’.

That said, frogs tend to stay within a few hundred metres of a water body, a fact that has puzzled me on numerous occasions when I’ve found frogs in very dry habitats, after which I’ve stood there with a furrowed brow, trying to think where on earth the closest body of water is. But the frogs will of course know, and when they reach sexual maturity after a few years they return to the water to mate, and the cycle begins again.

Frog or toad?

Frog on the left (photo by Fiona Cousland), toad on the right. Note that not all frogs have dark blotches on their backs.

Though similar in appearance and size, toads live subtly different lives to frogs and are deserving of an article in their own right, so I won’t go into the details here. However, because the two are often confused I’ll finish with a list of the number of fairly easy ways you can (usually) tell the difference between the two.

Frogs

• Mostly smooth skin, wet or slimy in appearance.
• Very long hind legs, usually with dark bands.
• Moves by hopping and leaping.
• Two ridges along its back
• Dark patch behind the eye
• Pupils tend to be somewhere between oval and circle-shaped.

Toads

• Skin looks rough and warty, but more importantly it looks dry.
• Moves by walking rather than hopping.
• Pronounced linear bulge (which is a gland) behind each eye.
• Pupils tend to be horizontal slits or perhaps rugby-ball shaped.
• Eggs are laid in long lines of spawn rather than clumps.
• Not everyone will agree, but I think toads look more grumpy than frogs!




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