Cane toad invasion and evolution

The Hawaiian cane toad is a classic case of an invasive species, and its genetics have long been a subject of study for those interested in the spread of species into new habitat.

That's why it's so interesting to me that a new study in Nature(subscription) has shown morphological differences (LiveScience) between the initial wave of toads and later populations. The fastest colonizers have the longest legs:

From the 1940s through the 1960s, the toads were invading at a rate of about 6 miles per year; now they're taking over at a rate of about 30 miles a year.
To find out why the toads are spreading so fast, researchers stationed themselves about 40 miles east of Australia's port city of Darwin, in a region where the cane toads had not yet spread.
When the toads arrived, the researchers found that those in the vanguard of the invasion had legs that were up to 6 percent longer than average; shorter-legged stragglers followed. The study showed that newer populations of toads tended to have longer legs than those in long-established populations.

These are BIG toads, by the way -- weighing up to 2 kg. Leg length does lead to faster dispersal:

The morphological trait most often linked to locomotor ability in anurans is leg length, both among and within species8. Our trials (see supplementary information) confirm that cane toads with relatively long legs are indeed faster over a short distance (regressing time taken to cover 1 m against residual leg length: r=-0.44, n=29, P<0.02). But, more important, longer-legged toads moved further over 24 h (maximum displacement of radio-tracked toads versus relative leg length: r=0.46, n=21, P<0.04) and over three days (r=0.58, n=21, P<0.006; Fig. 1a). Longer legs therefore facilitate more rapid dispersal.

Of course, the idea that longer-legged toads would move faster and colonize more quickly makes perfect sense.

The mystery is: Why do leg lengths subsequently decline?

References:

Phillips BL et al. 2006. Invasion and the evolution of speed in toads. Nature 439:803. Full text (subscription)