HOW AND WHEN DO FROGS MATE?

Frogs usually go back to the pool where they have spawned. Natural selection means that this is a pond where they could grow to adulthood and therefore their tadpoles will be able to survive there. They will travel a mile or more to get to the pond. Their mating method is called “explosive” as it is often short and sharp. In the pond male frogs compete in a scramble for access to females.(Wells l977)

The exact timing of breeding season and frogs’ behaviour varies from one locality to another, according to local temperature, population size and health of the frogs. The minimum temperature for spawning seems to be about 5 degrees Centigrade (Haapanen 1982), though the males may start calling at lower temperatures (Elmberg 1990). Another influence is the moon. Large arrivals of frogs and big spawning events are more likely to take place during a full moon. It’s not clear why – extra light may help them find their way to the pond or may deter predators,  or possibly they have some kind of inner time clock (Grant et al., 2009).  While there is snow on the ground or ground frost, spawning will not take place. If the climate heats up, local frog populations may find it difficult to adjust (Phillimore et al., 2012).

The breeding season lasts 12-24 days within with there is 3-7 days of maximum chorusing from the males (Elmberg & Lundberg 1991). The males make a kind of brrp or purring noise. Mostly the chorus takes place at night but sometimes, if there is a lot of mating going on, it will go on during the day too. The male climbs on the fatter female and does the amphibian clasp, the amplexus. Actual spawning often takes place during the dark.

During the breeding season the throats of male frogs becomes brighter and more luminant, perhaps a sign to other male frogs to stay away from male-male mating (Sztatecsny, 2010). There are about two males to every female (Elmberg 1990) and males may try to dislodge males on a female.  The more males there are, the more aggression, and the greater the likelihood of amplexus interruptus. If females are in short supply several males may attach themselves to one female, drowning her (see photo above).

The female lays the eggs, and the male emits the milt which fertilises them. The spawn is usually laid among surface weed, which forms some protection against fish and larvae. The transparent jelly round the newly laid spawn expands in the water by about 50%. The embryo inside feeds on it before hatching. Within a clump of spawn, some embryos hatch earlier than others – possibly those at the top of the clump where it is warmer. If the embryo dies, it turns white.

Frogs lay an awful lot of spawn because the mortality is high. Tadpoles are eaten by fish, newts, dragonfly larvae and even birds which pick them out of shallow water. If the pond starts drying up, the tadpoles turn faster into frogs (Loman 1999, Merila et al., 2000). Of those that do survive to become adults, only one third of the males and only 16% of the females return to the pond the following year (Elmberg 1990). If they survive the first year, they are more likely to survive the next.

 

REFERENCES

Elmberg, J., (1990), ‘Long-term survival, length of breeding season, and operational sex ratio in aboreal population of common frogs, Rana temporaria L.,’ Canadian Journal of Zoology, 68, 121-127.  Abstract only.

Elmberg, J., & Lundberg, P., (1991),’Intraspecific variation in calling behaviour, time allocation and energy reserves inbreeding male common frogs (Rana temporaria), Annales Zoologici Fennici, 28, 23-29.

Haapanen, A.,(1982),’Breeding of the common frog (Rana temporaria L),’  Annales Zoologici Fennici, 19, 75-79

Grant, R.A., Chadwick, E. A. & Halliday, T., (2009)The lunar cycle: a cue for amphibian reproductive phenology?’ Animal Behaviour, 78, 349–357.

Loman, J., (1999), ‘Early metamorphosis in common frog Rana temporaria tadpoles at risk of drying: an experimental demonstration,’ Amphibia-Reptilia, 20, 421-430

Merila, J., Laurila, A., Pahkala, M., Rasanen, K. & Timenes Laugen, A., (2000), ‘Adaptive phenotypic plasticity in timing of metamorphosis in the common frog Rana temporaria,’ Eco Science, 7, 18-24

Ogurtsov, S. V. (2004), ‘Olfactory orientation in anuran amphibians, Russian Journal of Herpetology, 11, 35-40

Phillimore,Albert B., Hadeld, J. D ., Jones, O. R. & Smithers, R. J., ‘Differences in spawning date between populations of common frog reveal local adaptation,’ PNAS, 109, 1-13.

Sztatecsny, M.,  Strondl, C., Baierl, A., Ries, C. & Hodl, W.,(2010), ‘Chin up: are the bright throats of male common frogs a condition-independent visual cue?’ Animal Behaviour, 79, 779–786

Wells, K. D., ‘The social behaviour of anuran amphibians,’ Animal Behaviour, 25, 666-693