© 2006 Nature Publishing Group
Vol 439|16 February 2006
803
Invasion and the evolution of speed in toads
Cane toads seem to have honed their dispersal ability to devastating effect over the generations.
Cane toads (Bufo marinus) are
large anurans (weighing up to
2 kg) that were introduced to Aus-
tralia 70 years ago to control insect
pests in sugar-cane fields. But the
result has been disastrous because
the toads are toxic and highly
invasive. Here we show that the
annual rate of progre ss of the toad
invasion front has increased about
fivefold since the toads first
arrived; we find that toads with
longer legs can not only move
faster and are the first to arrive in
new areas, but also that those at
the front have longer legs than
toads in older (long-established)
populations. The disaster looks set
to turn into an ecological night-
mare because of the negative
effects invasive species can have
on native ecosystems1,2; over many
generations, rates of invasion will
be accelerated owing to rapid
adaptive change in the invader3,
with continual ‘spatial selection’
at the expanding front favouring
traits that increase the toads’
dispersal4,5.
Introduced to Queensland in 1935, cane
toads have since expanded their range to
encompass more than a million square kilo-
metres of tropical and subtropical Australia6.
We have radio-tracked toads (for methods, see
supplementary information) at the invasion
front 60 km east of Darwin and confirmed the
astonishing locomotor performance in these
animals, which move up to 1.8 km per night
during the rainy months — far further than
previously studied anurans7. Does this
remarkable ability result from selection for
enhanced dispersal during the toads’ Aus-
tralian colonization history?
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:
r0.44, n29, P0.02). But, more impor-
tant, longer-legged toads moved further over
24 h (maximum displacement of radio-tracked
toads versus relative leg length: r0.46, n
21, P0.04) and over three days (r0.58,
n21, P0.006; Fig. 1a). Longer legs there-
fore facilitate more rapid dispersal.
If the invasion process has been assisted by
the evolution of improved dispersa l ability
among toads at the front, three consequences
would be expected. First, longer-legged toads
should be disproportionately common among
the first wave of arrivals at any site. As the toad
invasion front passed our study site, we mea-
sured relative leg lengths of all toads encoun-
tered over a 10-month period. Longer-legged
toads were the first to pass through, followed
by shorter-legged conspecifics (order of arrival
versus relative leg length: r0.34, n552,
P0.0001; Fig. 1b). Longer-legged toads
therefore moved faster through the landscape.
Second, toads at the invasion front should
be longer-legged than toads from older popu-
lations. As predicted, longer-term historical
analysis within Queensland populations
sho ws t hat r ela tive leg len gth is g reat est in n ew
arrivals and then declines over a 60-year
period (Fig. 1c; r0.23, n139, P0.008).
Third, the rate of progress of the toad inva-
sion front should increase through time. As
predicted, rates of frontal progress have con-
sistently increased (Fig. 1d; time
versus annual rate of spread, Pear-
son’s r0.96, P0.005). Toads
expanded their range by about
10 km a year during the 1940s to
1960s, but are now invading new
areas at a rate of over 50km a year.
Accordingly, previous pred ictions
about the time course of future
expansion of the toads’ range9seri-
ously underestimate their actual
rates of movement.
These rapid shifts in toad mor-
phology, locomotor speed and
invasion velocity indicate that
conservation biologists and man-
agers need to consider the possi-
bility of rapid adaptive change in
invading organisms. If there is no
fitness disadvantage to individual
organisms at the invasion front,
evolutionary forces are likely to
fine-tune organismal traits in
ways that facilitate more rapid
expansion of the invading popu-
lation10. Hence, control efforts
against feral organisms should be
launched as soon as possible,
before the invader has had time to
evolve into a more dangerous adversary.
Benjamin L. Phillips, Gregory P. Brown,
Jonathan K. Webb, Richard Shine
School of Biological Sciences A08, University of
Sydney, New South Wales 2006, Australia
e-mail: rics@bio.usyd.edu.au
1. Crossland, M. R. Ecography23, 283–290 (2000).
2. Smith, K. G. Biol. Conserv. 123,433–441 (2005).
3. Cox, G. W. Alien Species and Evolution(Island, Washington,
2004).
4. Simmons, A. D. & Thomas, C. D. Am. Nat. 164,378–395
(2004).
5. Travis, J. M. J. & Dytham, C. Evol. Ecol. Res.4, 1119–1129
(2002).
6. Lever, C. The Cane Toad. The History and Ecology of a
Successful Colonist (Westbury, Otley, West Yorkshire,
2001).
7. Smith, M. A. & Green, D. M. Ecography28, 110–128
(2005).
8. Choi, I., Shim, J. H. & Ricklefs, R. E. J. Exp. Zool. 299A,
99–102 (2003).
9. Freeland, W. J. & Martin, K. C. Aust. Wildl. Res.12, 555–559
(1985).
10. Thomas, C. D. et al. Nature 411,577–581 (2001).
Supplementary information accompanies this
communication on Nature’s website.
Received 24 November 2005; accepted 24 January
2006.
Competing financial interests:declared none.
doi:10.1038/439803a
BRIEF COMMUNICATIONS
–1,000
–500
0
500
1,000
1,500
–0.16 –0.12 –0.08–0.04 0 0.04 0.08
Relative distance moved (m)
Relative leg length (ln mm)
–0.15
–0.10
–0.05
0
0.05
0.10
0.15
0.20
010203040506070
Relative leg length (ln mm)
Time since colonization (yr)
–0.15
–0.10
–0.05
0
0.05
0.10
0.15
0 200 400 600 800
Relative leg length (ln mm)
Order of arrival
1945–54
1955–64
1965–74
1980–84
2001–05
0
10
20
30
40
50
60
ac
bd
Radial increase in range
(km yrǁ1)
Period
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Figure 1 |Morphology of cane toads in relation to their speed and invasion
history. a, b,Compared with their shorter-legged conspecifics, cane toads
with longer hind limbs move further over 3-day periods (r20.34) (a), and
are in the vanguard of the invasion front (based on order of arrival at the
study site; r20.11) (b). c,Cane toads are relatively long-legged in recent
populations, and show a significant decline in relative leg length with time
in older populations (r20.05). d,The rate at which the toad invasion has
progressed through tropical Australia has increased substantially since
toads were first introduced in 1935 (r20.92).
16.2 brief comms MH 9/2/06 4:15 PM Page 803
Nature Publishing Group
©2006