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Multiple Mating in the Common Toad Bufo bufoby Kate Hutchence About toads
There are three orders of amphibian, the Urodeles (newts and salamanders), the Apodans (caecilians), and the Anurans (frogs and toads, including Bufo bufo). Anurans show a wide diversity of mating behaviour.
At one extreme are the prolonged breeders where males typically guard territories or breeding sites and wait for females to select them over a relatively long breeding period. Females are thought to choose males on the basis of their territories, croaking, or visual displays. Some anuran males also show 'sneaky' or 'satellite' behaviours where they intercept and mate with females drawn to a rival male with attractive qualities such as a strong call.
At the other end of the spectrum are explosive breeders, where males greatly outnumber females in a temporary breeding zone. There is scramble competition for females and as a result very little opportunity for females to choose a mate.
Species can rarely be categorised perfectly into these groupings, but they provide a useful guide to mating behaviour.
Pictures L-R: the smooth newt (a urodele), a caecilian, a tiny red-eyed tree frog (Agalychnis callidryas), the common frog (Rana temporaria), the Australian frog (Crinia georgiana), and the common toad (Bufo bufo).
Almost all anurans are external fertilisers. External fertilisation is where the transfer of sperm to eggs takes place outside of the female body. As a female ejects her eggs males will release their sperm either directly onto the eggs or into the water around them. Most anurans breed in water, although there are a number that breed on land.
During mating males typically grasp females in a behaviour known as amplexus (see picture on right). Amplexus is thought to help to align both male and female reproductive tracts and help to achieve successful fertilisation. It is probably also a form of mate guarding by the male, reducing the risk that other males will be able to fertilise the females offspring.
Because Bufo bufo is an explosive breeder and there is high competition for females more than one male may grasp onto the female forming multiple amplexus or a mating ball (see picure on left). In some cases (including in Bufo bufo) females can be swamped and drowned by eager males.
In such a mating system where the sperm of many males is being released over eggs, it seems likely that multiple fathering of egg strings occurs. So far, multiple paternity has been genetically demonstrated in the red-eyed tree frog (Agalychnis callidryas), the Australian frog (Crinia georgiana), and the common frog (Rana temporaria).
Picture above: a pair of common toads in amplexus
Picture left: diagram of a mating ball with two males attempting amplexus with one female
The common toad
The common toad is effectively nocturnal, preferring to move and hunt at night, and can be found throughout the countryside of most of Western Europe. Toads are carnivorous and will eat a wide diversity of invertebrate species including insects, worms, and slugs.
Breeding takes place in early spring during mild nights and wet weather. Although land-living these toads travel to water to reproduce, often covering large distances to reach a particular breeding pond. Males arrive first and always outnumber females.
Females are larger than males, and males can be distinguished by thumb pads (see picture below) which become more pronounced during the breeding season and are used to grip females during amplexus, or sometimes to remove rival males already in amplexus with a female.
The common toad is categorised as an explosive breeder, but males will call to attract unpaired females and deter other males, and there is some evidence that females may choose males with the deepest croak.
Spawning takes place at dusk in water, usually within a small area of the pond. Eggs are produced in long strings of jelly and are wrapped around pond weed, twigs, or other stable material by females. Eggs hatch into tadpoles after about 10 days, and suffer less predation than frog tadpoles due to being bad tasting.
Why the common toad?
I used Bufo bufo in my study because it is prevalent in Britain and its mating habits, sites, and timing are well known. These factors, as well as its commonness, make it relatively easy to locate and witness spawning. In addition multiple paternity has never been genetically investigated in this species before.
Picture above: the common toad (Bufo bufo)
Picture left: the nuptial thumb pads of the common toad (in black)
