Frog sex

Although most frogs advertise their readiness to mate very loudly in swamps and streams at night, we’re only just starting to get a glimpse of the sex life of frogs. It appears that no two frog species take the same approach to mating. Indeed, there’s an utterly enormous amount of variation in how frogs ‘do it’. Research into frog mating behavior is also revealing the creative ways that frogs have evolved to survive, and breed, in different kinds of habitats all over the world.

There’s still so much that we don’t know about frogs, and this lack of knowledge extends into the realm of reproduction. Given that most frog reproduction occurs in waterbodies at night, and there are almost 7700 species of frog known, many in really remote places, it’s perhaps not that surprising. From what we do know, though, it’s becoming clear that frogs don’t all do the same thing when it comes to sex.

The first step in reproduction is to find a partner. In most frog species, males initiate this contact via advertising their presence and readiness, loudly. Males call, usually from a possible breeding site such as a pond, stream or swamp. Each frog species has a different call and female frogs can recognize the call of their own species. When females are ready to breed (when they are full of eggs), they move towards calling males, and pick the male that they prefer. Female frogs are fussy. In the Wrinkled Toadlet (Uperoleia rugosa) from eastern Australia, females were found to wander around calling males for 3-4 nights before selecting her mate. Heavier males with deeper-pitched calls were deemed the most attractive by females of this tiny frog.

Once a female frog approaches her chosen mate, he will typically embrace her in a position known as amplexus. This most often takes the form of the male jumping on the back of the female, grasping her either behind the arms or around the waist. Because the vast majority of frog species fertilize their eggs externally, this positioning helps ensure that the eggs of the female are fertilized by the male when they are released.

While two amplexus positions are most typical in frogs, there are seven distinct positions known. The newest of these, the “dorsal straddle”, was recently discovered in Bombay Night Frogs (Nyctibatrachus humayuni) from India.

Common Rain Frogs (Breviceps adspersus) from southern Africa face a particular challenge when attempting to mate due to their round body and short arms. Their rotund body shape is spectacular for surviving in arid habitats- enabling them to survive being buried underground during dry periods without losing too much water. However, it presents a unique challenge when it comes to mating as males are much smaller than females and their short arms can’t reach around the female. To solve this problem, males glue themselves to the female with mucus skin secretions! This unusual position is known as “glued amplexus”.

There are also a few different groups of frogs that have evolved internal fertilization. The most well-known of these are the Tailed Frogs (Ascaphus) from western North America. In these species, males have evolved a copulatory organ- possibly to help stop sperm washing away in the swift streams in which they breed.

Mating is a little more complicated in some frog species, especially when they only have a limited time to breed and there’s not much time to find a partner. For example, some Asian treefrog species (Polypedates) rely on heavy rains to fill puddles and so need to breed as soon as they can if they want to give their tadpoles enough time to turn into a frog before the puddle dries out. In these species, multiple individuals often mate at once. When time is limited, males may also make a mistake and briefly grab on to the wrong species. It’s usually not long before they realise their error, though, and move on to find another potential mate.

So, how long do frogs stay in amplexus? Amplexus duration varies greatly among frog species and has been reported as short as five seconds to several months! Of course, the actual fertilization doesn’t take a month, so there’s often a lot of time in amplexus without much going on. This can be a bit of a burden for female frogs as they may have to cart their mate around on their back the whole time!

Amplexus is just a small part of the reproductive behavior of frogs. After amplexus, eggs are deposited in different kinds of habitats and develop into frogs in different ways- with some species even skipping the usual tadpole phase. In many cases frogs also look after their young, building and guarding nests or even placing eggs in skin pouches where they can develop safely. And there’s still so much we don’t know about all of this. What we do know is certainly worth another blog though!

Given that the reproductive behavior of hundreds, if not thousands, of frog species is unknown, there are likely to be some really wonderful mating adaptations out there. These adaptations have allowed frogs to spread across all corners of the earth and fill the night with their amorous advertisement calls.

Dr Jodi Rowley
Curator, Amphibian & Reptile Conservation Biology, Australian Museum Research Institute & AMRI.

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• Willaert, B., Suyesh, R., Garg, S., Giri, V.B., Bee, M.A. & Biju, S.D. (2016) A unique mating strategy without physical contact during fertilization in Bombay Night Frogs (Nyctibatrachus humayuni) with the description of a new form of amplexus and female call. PeerJ, 4: e2117.