Cane
toads (Bufo marinus) were introduced
to Australia in 1935 to help farmers control the pests in farmlands. But the
population of cane toads in their new territory expanded quickly, and cane
toads became one of the most noxious invasive species in Australia. Large
predators in Australia, such as monitors, snakes and crocodiles, could not efficiently
eliminate cane toads, because the potent cardiac toxins stored in the glands on
toads’ shoulders could kill most animals that tried to eat them.
A
study led by evolutionary biologist Ben Phillips and PhD supervisor professor
Richard Shine of the University of Sydney revealed an evolutionary adaptation
of some of the native Australian snakes which was caused by the expansion of
cane toads. Two species of Australian native snakes, red-bellied black snakes (Pseudechis porphyriacus) and green tree
snakes (Dendrelaphis punctulatus),
are highly sensitive to toxic chemicals secreted by cane toads. Phillips found
that the average length of the two sensitive snake species had increased by 3%
to 5% in the past 70 years. Further study showed that the snakes that have
larger ratio of body-length to head-size were less affected by the toxic chemicals
secreted by cane toads. Because the larger snakes with relatively smaller heads
could not swallow the giant adult cane toads that could secret enough toxins to
kill them. The invasion of cane toads caused a very strong selection pressure
to the snake populations in Australia. The individuals of the two native snake
species with small body sizes and relatively large head sizes were wiped out
because they were more easily to be killed by the toxins of cane toads. And the
individuals with larger body lengths and relatively small heads survived and
reproduced.
Other
two Australian native snake species, swamp snake (Hemiaspis signata) and keelback snake (Tropidonophis mairii), have acquired certain level of resistance to
the toxins of cane toads, and they became less vulnerable. The study of
Phillips and Shine showed that the length of these two species did not change
over the 70 years since the cane toads invaded Australia, which supported
Phillips’s theory that the changing of body lengths of the two sensitive
species was a rapid evolutionary response to the invasion of cane toads.
Posted
by Muchen Liu (Group B, Week 5)
I did not know anything about this so this was very interesting to read. What a great way to show a rapid evolutionary response compared to the usual evolutionary response which takes usually thousands of years.
ReplyDeletePosted by Danielle Bermingham
So the toads keep the poison in glands, but is it something that can actively be spewed out? Also, how fast does the poison act, because I'd imagine the effects would have to be produced pretty quickly or be very debilitating to survive attacks from such large predators.
ReplyDelete-Matt Murdoch
Cane toad has two swelling parotoid glands on its shoulder, or behind its two eyes. When a cane toad is threatened by a snake or an alligator, its glands will secrete potent cardiac venom. The venom can cause rapid heartbeat, excessive salivation, convulsions and paralysis. It cosumes a large amount of energy for a cane toad to secrete venom, so the cane toad often prefer to inhale air and plump up its body to threaten predators rather than use venom. However, if a cane toad is deeply threatened or harmed by a predator, it can even use its glands to spit venom to predator's eyes.
DeleteReplied by Muchen Liu
I think professor Alan Richmond has one adult cane toad. You may can go to see him if you want to know more.
DeleteReplied by Muchen Liu
This was quite fascinating to read. I wonder how this small, yet rapid, change in the two species has impacted the rest of the ecosystem and food web? Since the snakes can more easily consume the toxins secreted by the toads, do they build up in the snakes themselves? If so, do the predators of the snakes experience toxic magnification?
ReplyDeletePosted by Jamie Courtney
It is interesting to me how body length can be associated with poison resistance. I suppose larger body mass means more poison is needed for equal effect. But the fact that smaller heads are associated with bigger body length does not make any sense to me? How is it possible to have different body proportions insofar as the head of long snakes is actually smaller than the head of short snakes?
ReplyDeletePosted by "Takoda Nordoff"
The snake with larger ratio of body-length to head-size will have limited capacity to swallow the adult cane toads. The large snakes will still have larger heads than the small snakes have, but their heads are small compared to their whole body sizes. In fact, cane toad is a giant toad species. The adult cane toads can reach up to 20 cm in length. The large snakes cannot eat such a big prey. They may eat young toads, but as you said, the poison secreted by young toads is not enough to kill them.
DeleteReplied by Muchen Liu
It is amazing how such a small change to the head can allow these snakes to fend off such a dangerous and invasive species. I am also amazed how this change was able to take places in such a short span of time! Thanks for sharing such an interesting story!
ReplyDeletePosted by Alexandra Rios