Five More Fantastically Freaky Frogs
Over the past couple of months, The Nature Nook has been looking at frogs that exhibit truly bizarre breeding behaviours. From the male that broods his offspring in his vocal sac to the toad whose tadpoles develop within pockets in the skin on their mother’s back, there seems to be no shortage of unusual ways in which anurans reproduce and take care of their young.
And in case you can’t get enough of them, here are five more fantastically freaky frogs. So if you want to find out about a frog that turns blue in the breeding season, or one that practises ‘reproductive necrophilia’ (yes, really!), make sure you read on.
1. Moor Frog
For most of the year, the moor frog from Asia and continental Europe looks unremarkable. Being a generally reddish-brown colour with hints of olive, it looks quite a bit like the common frog that lives here in the UK. But unlike the common frog, this species completely changes colour during the breeding season: the males turn an attractive shade of blue. It is thought that this change in colouration is not to attract a mate – females don’t seem to care whether their partners are blue or not – but is for identification purposes only.
Moor frogs gather together to breed in huge numbers. Hundreds or even thousands may cram together in a single pond. But in this confusing melee of frisky frogs, the lack of dimorphism between the sexes would traditionally make it difficult to tell whether a particular individual is male or female. Male frogs are extremely enthusiastic and will mount pretty much any other frog that comes within range, whatever the sex. However, if that frog happens to be another male, that’s valuable time and effort wasted. By turning blue for a short time, the males can minimise such mistakes; they know to avoid mounting other blue frogs and to instead concentrate all their effort on brown frogs, which are female.
2. Marsupial Frog
The male Darwin’s frog looks after his young inside his vocal sac. The gastric-brooding frog (before its extinction) safeguarded its eggs and tadpoles within the female’s own stomach. And my personal favourite, the Surinam toad, creates a small brooding chamber for each of her offspring inside the very skin on her back. Marsupial frogs aren’t quite as extreme as those examples, perhaps, but the females still provide a nursery for their young within their own bodies.
Like the mammals that these amphibians are named after, marsupial frogs have a pouch-like brood sac in which they raise their offspring during the early stages of infancy. But in the case of the frogs, the pouch is on their back rather than the front. During amplexus (the amphibian mating embrace), the female marsupial frog expels her eggs for the male to fertilise, whereupon he pushes them into the pouch on her back. When the eggs hatch, the tadpoles press their gills up against the blood-vessel-rich walls to receive gases and nutrients.
The number of offspring that a female marsupial frog can keep within her pouch depends on the species. Some produce only 20 eggs at a time, which means that there is enough room inside the pouch for all of them to develop into miniature frogs before they leave. Other species can lay as many as 200 eggs, although the lack of space within the brood sac means that they have to be evicted when they are still tadpoles, forcing them to finish their development in puddles on the forest floor.
3. Rhinella proboscidea
This little Amazonian toad is so obscure it doesn’t even have a common name. And it would probably be even more obscure if didn’t exhibit a very intriguing behaviour: it’s the only species known to practise reproductive, rather than accidental, necrophilia.
I know what you’re thinking. How can an animal mating with a dead individual result in successful reproduction? Well, in the vast majority of cases, it doesn’t. Adélie penguins, ground squirrels, lizards and ducks (among others) have all been observed indulging in necrophilia, but it is likely that most of these instances were the result of mistaken identity.
Not so when it comes to Rhinella. Like many frogs and toads, it is an explosive breeder. During the brief, frenzied mating season, males aggressively battle among themselves for access to females. They even attempt to steal females from other males already in amplexus. Occasionally, the struggling males accidentally suffocate and kill a female during the scrum.
But all is not lost. By mounting a recently-deceased female and massaging her abdomen, a male can still squeeze out her eggs in the usual way and then fertilise them. Both frogs, even though one of them is now dead, get to pass on their genes.
4. Tailed Frogs
Anura – the biological order to which all frogs and toads belong – literally means ‘without tail’. And indeed, unlike other amphibians, such as newts and salamanders, all anurans are completely tailess in their adult form.
Well, almost all.
In two species of frog, the males seem to possess tails and are therefore commonly referred to as tailed frogs. However, these so-called tails are, in fact, an extension of the male’s cloaca. Unlike almost all other anurans, which breed using external fertilisation, tailed frogs utilise internal fertilisation. This is because they live in turbulent, fast-flowing streams. Eggs and sperm released externally would likely be washed away before any fertilisation could take place. The ‘tail’ is therefore an organ for underwater copulation – an anatomical feature that minimises loss by allowing the male to deposit his sperm deep inside the female, thereby improving breeding success.
Eggs are laid in cold mountain streams and the hatching tadpoles have large, sucker-like mouths so they can attach themselves to the underside of rocks to prevent themselves from being swept away by the swift-flowing water. This species’ larval stage is one of the longest of any anuran, with tadpoles taking 2-4 years to fully transform into adult frogs. Even then, tailed frogs may not reach sexual maturity until the age of 8 or 9. Unsurprisingly, these two species are among the longest-lived of all frogs, with a lifespan of up to 20 years.
Very few frogs give birth to live young. Most of the ones that do belong to a genus called Nectophrynoides, which live in the forests and wetlands of eastern Africa. These amphibians retain their young inside their bodies in a way that is comparable to the techniques used by placental mammals.
Like the tailed frogs, Nectophrynoides are also highly unusual because they practise internal fertilisation. The male presses his vent against the female’s so that sperm can pass directly into her and fertilise her eggs. The eggs, rather than being laid, remain inside the female’s oviduct. The tadpoles hatch inside the female and feed within the oviduct on tiny white flakes secreted from its walls.
During this time, it is the dry season and the female Nectophrynoides lies low within a rocky crevice. When the rains return after nine months, she finally gives birth, to fully-formed juvenile toads. Her stomach and oviduct do not have muscles that can contract to expel her young; she must inflate her lungs so that they swell into her abdomen and physically squeeze the young out by pneumatic pressure.
I hope you’ve enjoyed our latest whirlwind tour of some of the freaky but fantastic frogs that we share our planet with. It’s time to move on from bizarre breeding behaviour now, though. Next week, we’ll be looking at frogs living in a very extreme environment, one that you wouldn’t expect to find any amphibians in: the desert.