Seasonal Update (March 2024)

March heralds the start of meteorological spring as the days are noticeably longer and warmer. - Credit: Marc Baldwin

After a mild January, February started on a similar note for much of the UK, and there was record heat across Europe during the second week. A brief cold pulse brought some snow to Scotland and northern England, but Wales along with central and southern England remained mild wet and windy. This was the really the picture for whole of February, with any significant wintery weather failing to materialize. We did see some cooler nights, with some overnight frosts even here on the south coast, but daytime highs remained in double figures.

Website Update

In what feels like a semi-momentous occasion, I finished writing the Chinese water deer article (at least, as far as any of my articles are “complete”). This is something I began writing back in May of 2019 and ended up at a fairly comprehensive 150 pages. The remaining sections will come online as they progress through my heroic proofreaders, and I will now turn my attention to refreshing some of the existing content on the site, a great deal of which has been neglected for far too long.

News and discoveries

A recent poll of Gardener's World readers found that hedgehog sightings had increased by 2% in the last year, although this may not reflect the general trend in the UK. - Credit: Marc Baldwin

Urchins on the up? Over the past few years, we've seen stories in the news about how hedgehogs are declining across large areas of Britain, particularly in the countryside. A recent survey of readers of the BBC's Gardener's World magazine gives some small cause for optimism. The results of the survey, published in the February issue of the magazine, point to 62% of respondents appreciating hedgehogs in their garden and being willing to do more to help them. Moreover, following two years of decline, sightings of hedgehogs were up by 2%, and more were seen in rural than urban gardens.

Helpful hybrids. Since the emergence of COVID 19, a substantial amount of work has been undertaken to better understand how species that are able to act as reservoirs for diseases that can infect humans, such as bats, are able to carry pathogens without developing symptoms. A recent complex genetic study of the swarming of Myotis bats suggests that hybridization between related species is key. Scientists found that during swarming events (basically, social events for bats, where related species come together, chatter, interact and sometimes mate), immune genes were among the most frequently exchanged. This intimates that such social grouping, and the mixing of closely related species that they involve, are key to helping drive the evolution of the chiropteran immune system.

Badger blitz. Just over a quarter of a million badgers have been killed as part of government attempts to tackle bovine tuberculosis cases in England to date. Official government policy had been set to allow up to 70% of badgers in an area to be shot as part of control efforts, but an article published in The Independent on 17th February suggests that the government will announce at the National Farmers' Union conference that permission will be given to cull 100% of badgers under certain circumstances - in other words, local populations can be eradicated - from January 2026, if the consultation is approved.

Adulterated Avon. At a meeting about the health of the River Avon, held in August last year, the Environment Agency reported that the water quality in the river had not declined in the past five years. According to an article published by The Guardian at the end of last month, however, data collected by The SmartRivers Programme, a citizen science initiative run by the charity WildFish, reported strong declines in invertebrates in the river that were linked to “chemical pressure”. The data suggest a decline in invertebrate biodiversity and abundance in the river between 2019 and 2023 at 11 sites on the upper Avon.

Seasonal highlight – Spawning behaviour of frogs and toads

We're now well into the breeding season for many of our amphibians. Many areas saw early spawning from frogs, presumably in response to it having been such a mild winter, with several reports of spawn during December.

Common frogs with spawn. - Credit: Kentish Plumber (CC BY-NC-ND 2.0 DEED)

Here in Britain, we have six species of frog and three species of toad living in the wild. The pool frog (Rana lessonae) is thought to have been native to the fenland pools of Norfolk until the 1990s and, although there are still some hanging on in isolated pockets of eastern (particularly south-eastern) England, these are considered to be survivors from introduced animals. The marsh frog (Rana ridibunda) was introduced to England, first at Romney Marsh on the Kent-East Sussex border during the 1930s, and is now found in parts of Surrey, London, Kent, and Sussex, while the agile frog (Rana dalmatina) is now found only on Jersey, where it is native. Edible frogs (Rana ;kl. esculenta) are found in small colonies in, predominantly eastern, England (e.g., Yorkshire, Norfolk, Surrey/Sussex/Kent, Somerset, and Worcestershire) and are most probably an introduced species. There are also sporadic reports of American bullfrogs (Rana catesbeiana) in southern England but, to the best of my knowledge, there are no documented breeding colonies. There are a few isolated, largely coastal, populations of the Natterjack toad (Bufo calamita) found in Britain and Ireland, particularly eastern Cumbria, western Norfolk/Suffolk, and southwest Ireland. Similarly, there are a few small populations of midwife toad (Alytes obstetricans), a secretive non-native species, in central eastern England (i.e., Bedfordshire, Cambridgeshire, Lincolnshire).

Each of the aforementioned species has their own particular breeding biology, but that which follows is a brief overview of the breeding (i.e., mating and spawning) behaviour of common frogs (Rana temporaria) and toads (Bufo bufo); those most likely to be found in your garden pond.

Hop it to your local pond...

Contrary to popular misconception, frogs and toads are actually highly terrestrial and although frogs may spend large parts of the year in or around a pond, they also spend a good deal of their time hunting or resting on land, often in leaf litter and long grass, during the day. Toads spend most of their lives on land, and I commonly come across them under logs in the New Forest. Indeed, in northern parts of their range where winter temperatures drop below about 10C (50F), toads can often be found torpid under logs or in leaf litter. Frogs can hibernate underwater, in silt at the bottom of ponds, and although toads appear physiologically able to do the same, they seem to prefer to hibernate on land. Both frogs and toads must, however, return to water to spawn, because the first stage of their lifecycle is entirely aquatic.

A common toad among leaf litter in the New Forest. People are often surprised to find that common frogs and toads spend much of their time on land, even some distance from water. Many will hibernate among leaf litter or under logs. - Credit: Marc Baldwin

Even in fairly cold winters, frogs start getting amorous towards the end of the year and they can be seen moving around even under ice. Spawn may be laid in January, but often succumbs to frost. More generally, the breeding season for common frogs begins in February, with large numbers congregating in good breeding ponds. If the frogs didn't overwinter in ponds, the journey back to them begins in February and most of the travelling happens overnight. Frogs may travel up to 10 km (6 miles) back to the pond in which they were born to spawn, and researchers now know that the smell of certain types of algae (to which frogs appear particularly sensitive) is what guides them to the pond.

As well as needing warmth and light, frog and toad tadpoles, as we shall see, have two dietary stages to their development, which means that not all ponds are suitable for frogs to spawn in and may help explain why even ponds that have frogs most of the year seldom see spawn. Certainly, while I was in the New Forest a few years ago, I visited four ponds all within easy amphibian reach of each other; three were apparently devoid of spawn, while the fourth had lots. That said, I have found frogspawn in ditches and what can be described as little more than puddles that seem very unlikely to successfully support the tadpoles, suggesting there's more to this picture than we can see at the moment. Nonetheless, fish can be voracious predators of spawn and tadpoles, explaining why garden ponds with goldfish are also often bereft of spawn, even if the occasional frog is present. Newts are also predators of tadpoles, which means that—although the two amphibians can co-exist—ponds with lots of newts often have few, if any frogs. While frogs seem happy to lay their eggs in a variety of different water bodies, toads appear more discerning, preferring mature ponds with plenty of vegetation among which to deposit their spawn, although they have also been recorded spawning in fairly fast-flowing rivers and even behind weir gates on a canal.

Can't we just cuddle?

The precise trigger for spawning in frogs is still something of an enigma, and even male frogs seem unable to tell precisely when the female will release her eggs. Spawning seems to be closely linked with the temperature between January and March. In a paper to the journal Global Change Biology during 2009, a team of biologists—led by Tim Sparks at the NERC's Centre for Ecology and Hydrology in Cambridgeshire—presented the results from their analysis of nearly 70,000 records of frogs spawning collected in the UK between 1998 and 2007. Sparks and his colleagues found that the annual mean dates of spawning (i.e., the dates on which most frogs spawned) varied by more than two weeks, with populations in the south-west spawning first, followed progressively later by those farther north and east. On average, the biologists found that the “spawning wave” took about 7.5 days to move 100 km (62 miles) east, and five days to move 100 km northwards. Furthermore, a 1C (almost 2F) rise in central England temperature during January to March brought forward the average spawning date by five days. Spawn can be laid between January and April in the UK, with frogs tending to spawn earlier than toads.

A pair of common frogs in amplexus, with a third looking on. It's not uncommon for two or three males to vie for a female and so-called "mating balls" can sometimes result in the female drowning. - Credit: Marc Baldwin

Males generally arrive at breeding ponds first, and there is good reason for the females being late. Part of the courtship and mating behaviour of frogs and toads is a period of amplexus, from the Latin amplecti meaning 'embrace'. Amplexus, as the Latin name implies, involves the male climbing on to the female's back and grasping her under her 'armpits' with his rough nuptial ('thumb') pads, which become enlarged just ahead of the breeding season. The male holds on to the female, sometimes with enough force to injure her chest, until they have spawned, at which point he lets go and goes looking for another suitable female. Males, particularly male toads, are very enthusiastic during the breeding season and will grab hold of almost anything, including passing fish and inanimate objects. The female cannot detach the male and, if she arrives at the breeding pond too early, she may have to carry him around for several days before she is ready to spawn. Potentially worse for her is that several amorous males may all grab her, resulting in a tangled mess of amphibian. (So called “mating balls” have been found to number 100+ animals, both males and females, although many more of the former than the latter, in some frog species.) I've not found a record for the longest amplexus, but in his fascinating and highly readable book 1985 book Frogs and Toads, Trevor Beebee notes:

Females which arrive early, or hibernate in the breeding pond, run a serious risk of being grabbed by one or more males as early as January and then held in tight embrace for weeks or even months on end.”

Common toads in amplexus, the female carrying the male on her back as she heads to the breeding pond. While this is a rather extreme example, it illustrates how females can be significantly larger than males. - Credit: Philip Hay (CC BY-NC-ND 2.0 DEED)

While Beebee's statement may sound almost amusing, for the female—who hasn't eaten since going into hibernation at the end of the previous year and will not eat until she has finished spawning—it can be a major problem and is one reason mortality during spawning is so high. In both species, the female is larger than the male, but a large male clasping a small female, or several males clasping a single female can drag her below the surface where both she and her potential suitors are at risk of drowning. Frogs and toads can 'breathe' through their skin—i.e., oxygen can diffuse across their semi-permeable skin into their blood—and, during the winter when their metabolism is low, this can easily see them through hibernation (hence the ability of frogs to hibernate on the bottom of ponds). As the weather warms up, however, their metabolism increases and they can no longer get all the oxygen they need from this passive diffusion, so they rely on breathing air through their lungs. Consequently, frogs and toads can drown if they get trapped underwater.


Frogs are what ecologists refer to as “explosive breeders”, which means females deposit all their eggs in one go, in large clumps. Toads, by comparison, are “protracted breeders” that lay a string of eggs over several hours, interrupted by breaks of about 15 minutes. The string contains two rows of eggs, one from each oviduct.

A male and female natterjack toad in amplexus during spawning, illustrating the strands of black eggs produced by toads. - Credit: Natural England / Peter Roworth (CC BY-NC-ND 2.0 DEED)

When the female is ready to lay her eggs, she moves into shallow water, generally no more than 30 cm (12 in.) deep, and the male palpates her stomach while feeling for the release of the eggs with his hind legs. If there are numerous frogs in the pond, there is often a continuous audible “purring croak” from the males shortly before and during spawning. When the female frog is ready to lay, she will generally invert herself in the water (i.e., hang head down) and her body pulsates as the eggs are pushed out at the surface; it takes only a minute or so to release a clump of eggs, after which the male releases a cloud of sperm. Toads tend to mate upright in the water but wrap their string of spawn around vegetation to keep it in place. Either way, the male has only a brief window during which he can fertilise the eggs.

Before the eggs (called oocytes) are expelled by the female, they pick up a thin coating of jelly from the walls of the oviduct. (For the biochemists among you, this is a mucoprotein composed of large molecules, approximately half protein and half the carbohydrates galactose, fructose and glucose linked in such a manner that confers considerable hygroscopy.) When first released, the oocytes are only about five millimetres (about one-fifth of an inch) in diameter, but the jelly coating is highly absorptive and within a few minutes (depending on the ionic concentration of the water) it has absorbed water and swollen to 10 mm and is now impenetrable to his sperm. The jelly helps protect the developing embryo from fungal infections and UV radiation, spaces them out and helps to keep them warm, about 0.5C (1F) warmer than the surrounding water.

Recently-laid frogspawn, showing the egg surrounded by its protective jelly coating. - Credit: Marc Baldwin

Each female frog generally lays only a single clump of spawn (occasionally a couple of smaller clumps), containing anywhere between 700 and 4,500 eggs depending on the size of the female. Most spawn is deposited in the early hours of the morning (much happening around 3AM it appears) but may continue throughout the day at the peak of the spawning period. Much of the clump of spawn will be fertilised by the female's partner, but some can be fertilised by the sperm of other males being released into the water nearby. A large female toad will deposit between 3,000 and 8,000 eggs in a string that may be up to five metres (16.5 ft.) long. Biologists don't know why toads evolved to lay their eggs in strings, while frogs lay in clumps, although one suggestion is that it provides better aeration of the developing embryos. Whatever the reason, mortality of frog and toad embryos is very high, with only around 5% surviving to metamorphosis.

How long the eggs take to develop into tadpoles depends on the water temperature, with development being faster in warmer water. This partly explains why females prefer to lay eggs in shallow water, which takes less time to warm up. Development of the embryo begins two or three hours after fertilization, and it starts to lengthen after three to seven days. The gelatinous coating on the eggs dissolves slowly and tadpoles typically wriggle free (“hatch”) after two or three weeks, although they may remain attached to the yolk that will sustain them for a few days before they need to feed. Frog tadpoles may be largely inactive, spending prolonged periods resting on the bottom of their pool, while toad tadpoles often congregate and swim at the surface in large shoals, sometimes consisting of tens of thousands of individuals. At this stage, the tadpoles breathe through external gills, but the gills are covered by skin (i.e., the operculum fuses) and a spiracle develops around four weeks post-hatching.

Tadpoles are nourished initially by the yolk in the egg, but shortly after this they begin feeding on vegetation in the pond and, once the limbs start developing, turn to meat. Typically, carnivory takes the form of feeding on larvae and small flies in the pond, but they'll also eat one another and any carrion they can find. Feeding on carrion (or, in this case, a small cube of steak) can result in a "bait ball" forming. - Credit: Marc Baldwin

Once the yolk has gone, the tadpoles feed on algae in the pond until they're about seven or eight weeks old, at which point their hind legs—which began developing at around five weeks old—are fully developed, as are the lungs. At this stage the tadpoles switch to a carnivorous diet consisting mostly of aquatic insects, such as water fleas and fly larvae, although they'll take small flies and sometimes other tadpoles if food is scarce, as well as taking carrion. Tadpoles stop eating at about 12 weeks old and the skin is shed, which allows the mouth to widen, the eyelid to form and the tongue to grow. The final part of the metamorphosis is the resorption of the tail, and most tadpoles are ready to leave the water on damp nights between June and September. In cold climates, or areas with low food availability, some will over-winter as tadpoles and complete their metamorphosis the following summer.

When they emerge from the water, froglets and toadlets are only between seven and 15 mm long (one-quarter inch to just over half-inch) and are sometimes mistaken for insects jumping about in the grass. As they grow, frogs and toads shed their skin; the skin breaks along the back, the back legs are extracted, and the skin is pulled over the head and eaten. Most frogs, and many toads, will be able to breed by about three years old at around six centimetres (just over two inches) long, although some toads can take seven years to reach sexual maturity, depending largely on temperature and food availability. For most frogs and toads, old age is six to eight years, although they can live for ten years in the wild and this seems more typical of toads than of frogs. The oldest captive toad I've heard of was 36 years old.

The tail is the last part to be resorbed, at which point the froglets can leave the pond to hunt for food on land. How long metamorphosis takes depends on the water temperature and food supply, but froglets and toads often emerge on damp nights during July and August. - Credit: Marc Baldwin

I hope this introduction to the breeding biology of frogs and toads has piqued your interest. Our amphibians are really struggling at the moment in the face of increasing development, filling in of ponds, and water pollution, so anything you can do to support your local wildlife is fantastic. Amphibian and Reptile Conservation have a comprehensive FAQ document covering spawn and their site contains lots of information about British amphibians that's worth a look.

For a round-up of Britain's seasonal wildlife highlights for early spring, check out my Wildlife Watching - March blog.

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