Breeding time for male frogs just out of hibernation
How do frogs know when it is time to come out of hibernation?
Tuesday, 22nd December last was the winter solstice, the shortest day of 2015 and Monday 20th June next will be the summer solstice, the longest day of 2016. Give or take a day or two either way, it has been that way for hundreds of millions of years. It has therefore been suggested that frogs, along with many other creatures, have probably evolved an inbuilt, instinctive clock based on day length.
There is evidence that frogs are more active near full moon so maybe lunar cycles play a role too. And air temperature must play a part as frogs emerge earlier in milder winters.
However they know, male frogs will be coming out of hibernation very soon and heading for their local breeding ponds. It is said that frogs know where ponds are by smell. Our Common Frog lives on land for all of the year but must return to water in springtime in order to reproduce.
Male frogs are smaller and slimmer than females. They emerge from hibernation first and wait at breeding pools calling for the larger and bulkier females to arrive. While they wait, the softly-croaking males grow special pads on their thumbs. Females arrive and enter the water. Males climb onto the backs of females, wrap their front limbs around their partners' bodies, clasp on with their special thumb pads and remain locked on in that position.
This nuptial embrace is known as amplexus and it is the trigger that stimulates the females to lay. The eggs emerge in a long sting. As they emerge the locked-on male secretes sperm over them thereby fertilising them externally before the eggs' coatings of jelly swell up to become the familiar clumps of frogspawn.
Couples stay in amplexus for a long period as mortality in tadpoles is very high. Tadpoles are eaten in large numbers by fish, Smooth Newts, dragonfly larvae and birds. As a result, very many tadpoles perish so each adult female produces a lot of eggs, over one thousand each spring.
There is no parental care. After breeding the adults hop off and won't come together again until the following year. Same time; same place. However they all know to come out of hibernation at the same time, their annual appearance at this time of year is one of the welcome signs that spring cannot be too far away.