Friday, 7 April 2006

Roads take heavy toll on toads

Heading out of Thetford on Sunday I came across the sad sight of a dozen or so common toads squashed on the road. The corpses were all concentrated along one twenty-foot stretch, a crossing point used by the toads on their annual migration to a local breeding pond. Over the coming nights it seems inevitable that other corpses will be added to the sickening toll, as more toads reach the road and attempt to cross. One of the reasons why common toads seem so susceptible to motor traffic is their habit of crawling a few feet and then resting for several minutes before continuing. Although the scene will be repeated on other stretches of road close to breeding ponds, there have been efforts to reduce mortality levels through the erection of warning signs and the designation of registered toad crossings. It is hard to estimate the impact of traffic-related mortality on the toad population but, from the handful of studies that have been carried out, it appears that many populations can compensate for the annual losses since the populations remain stable over time.

We have our ‘own’ toad that frequents the patio and borders on warm summer nights and frightens our over-inquisitive dogs by adopting a defensive posture. I do not know where this particular individual breeds but it may use the pond next door, where toad spawn was in evidence last year. Our own smaller pond is deemed unsuitable and just supports the local frogs (spawn first appeared on 26th March).

Common toads are only dependent on water for breeding and outside the breeding season spend little time in the water. Instead, they search for prey in gardens, rough grassland and even woodland. This means that your best chance of viewing these creatures is during late March and early April when they gather at the breeding ponds. The smaller males outnumber females at these gatherings because not only do they mature at a younger age but they arrive first and remain longer at the ponds. Sometimes there may be ten males to each female, which means that competition for a mate can be intense. Males attempt to grasp a female and position themselves so that they can fertilise her spawn. Other males try to break the embrace and manoeuvre themselves into position so it can all get rather chaotic, with over-enthusiastic individuals grasping at other objects including goldfish, vegetation and other males. Unlike frog spawn, toad spawn is formed in strings and is wrapped around the vegetation. The emerging tadpoles, like their parents, use toxins to deter would-be predators and this may be why they are successful in ponds containing fish, where frogs usually fail.

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