Figure 1 – The Northern quoll. Photo by B. Leue.
The genes for ‘toad-savviness’ are heritable, and they may be the
key to protecting Northern Quolls from Cane Toads.
Social media and news outlets are filled with stories about species becoming extinct. These stories
highlight what are among most concerning issues of the 21st century. The complexity of natural
systems and the abundance of introduced species in many parts of Australia can make these problems
seem insurmountable. The good news is that some native animals are dealing with these problems
already. Researchers have recently conducted a ground breaking trial aimed at conserving quoll
populations through targeted gene flow – that is, the transfer of animals with desired genetic traits
from one location to another (Kelly and Phillips, 2019).
The Northern Quoll (Dasyurus hallucatus) (Figure 1) is a carnivorous marsupial, sometimes described
as being like a ‘native cat’. However, at around half a kilo in weight, the northern quoll is much smaller
than cats and is the smallest of all quoll species. While northern quolls usually have around seven
offspring annually, almost all adult males die off each year after the energy-intensive breeding season
(Department of the Environment, 2016).
Enter: the Cane Toad (Rhinella marina), an introduced
species whose toxins can easily kill anything unfortunate
enough to eat it (Figure 2). Alongside the yearly die-off of
male quolls, habitat loss and fragmentation, the cane toad
poses a huge threat to northern quolls as it is present across
almost all of the quolls’ distribution (Figure 3). The
combination of these threats means that the northern quoll
is now listed as endangered (DEE, 2016).
Figure 2 – The introduced cane toad.
Photo by D. Metters
Cane toads are ground-dwelling and are often present in huge numbers, so naturally, quolls frequently
encounter them and eat them, causing swift and deadly poisoning. Many northern quoll populations
have already experienced massive declines after invasion by cane toads; and in some areas the result
has been total extinction.
One innovative study aimed at dealing with this problem involved ‘conditioned taste aversion’, in
which captive northern quolls were fed cane toad sausages which had been laced with a nauseainducing compound (Indigo et al., 2018). This was very successful in captivity; quolls that had eaten
the sausages later showed half as much interest in dead cane toads compared to the quolls who hadn’t
been fed sausages. In the field, this didn’t translate quite so well. While the sausage-fed quolls were
much less likely to prey upon cane toads and so survived for slightly longer, they fell prey to dingoes
quite quickly, showing how difficult it can be to solve complex ecological problems.
Researchers then started to look at examples of how other species have adapted to cane toads though
natural selection (yes, this is Darwin in action). The Red-Bellied Black Snake and the Common Tree
Snake have both undergone physical changes since the arrival of cane toads (Phillips et al., 2004).
Because snakes eat by swallowing their prey whole, they can only devour what fits within their jaws –
this is called being gape limited. This means that bigger snakes can eat bigger prey (such as the hefty
cane toad) or if they do eat smaller cane toads, the dose of poison that they consume is much lower.
As a result, snakes with larger heads are often fatally poisoned in areas where cane toads are common
– this is a process of negative selection. Snakes with heads too small to eat cane toads don’t fall victim
to this same fate and can survive to reproduce. Many generations later, these species of snake have a
smaller average head size.
Figure 3 – Distribution of northern quolls and cane toads. From Kelly and Phillips (2019).
So, can threatened species adapt their behaviour in the same way they adapt their physical features?
Fortunately, there are a few examples of species already making behavioural changes. Torresian
Crows have been observed eating cane toads by carefully flipping them onto their back, avoiding the
toxin-producing glands (Wilson, 2018). The Yellow-Spotted Monitor (a large lizard similar to a goanna)
also appears to have learned not to prey upon cane toads (Llewelyn et al., 2014). The ‘sausage’
experiment also shows that we can train individual quolls to avoid the poisonous toad.
The best way to help a population adapt is to find
individuals with a genetic advantage and then help this
trait to spread more quickly than it naturally would. This
process is called targeted gene flow (TGF) (Kelly and
Phillips, 2016). What if there were wild quolls already
surviving alongside cane toads? And what if there was a
genetic basis to their survival? The beauty of TGF is that
it can be done by simply moving breeding individuals who
have the desired trait into the area where we want it to
spread. Kelly and Phillips (2019) did a pilot study on this
idea and got some very promising results.
Under their experiment, northern quolls from a few different locations were bred in an environment
without cane toads. Importantly, some of these quolls were from Astell Island which is toad-free, while
the others were from areas that have been toad-invaded for quite some time. These quolls were bred
to produce three different types of offspring: pure ‘toad-naïve’ quolls from Astell Island parents, pure
‘toad-exposed’ quolls from Mareeba and Cooktown parents, as well as hybrids of the two. The young
quolls were raised in a controlled environment without any exposure to cane toads to prevent the
opportunity for their mothers to teach them aversion behaviour. When these offspring became
independent of their mothers, they were tested.
First, each quoll was exposed to a dead cane toad enclosed in a cage. They were later given the option
to eat cane toad legs, which do not contain the poisonous glands that harm predators. While all of the
quolls spent a similar amount of time investigating the dead toads, their choices on whether to eat
the legs were very different. The quolls bred from toad-exposed parents were substantially less likely
to eat the cane toad legs than the quolls bred from toad-naïve parents. Toad-savviness had been
inherited by the offspring! Most excitingly, the hybrid quolls showed a similar response to the toadexposed group. This suggests that the genes for ‘toad-savviness’ are not only heritable, but also
dominant. This is very positive news; it means that wild toad-exposed quolls could be introduced to
areas where the local quolls are naïve and vulnerable to cane toads, and any hybrid offspring would
be likely to inherit the toad-savvy traits.
While this is great news for quolls, a little more research needs to be done before TGF is implemented
in the wild. In particular, one important question still remains: could the introduction of new quolls to
an existing population cause outbreeding depression? This occurs when genetically distant individuals
breed and produce offspring that are less ‘fit’ than either parent. The researchers think that this is
unlikely in northern quolls because of the relatedness between populations, but further investigation
is needed to rule out the possibility.
Figure 4 – A quoll inspecting a cane toad.
Ecological problems can be hard to solve, especially with a species as invasive as the cane toad. While
it can be hard to predict how our efforts to help a species will play out, it’s worth remembering that
some animals are managing to cope with threats without us. It’s our responsibility first of all to look
after these animals, but also, to help the other members of their species where we can. The good
news for the northern quoll is that their best chance to survive toad invasions could be hiding in the
genomes of their ‘toad-savvy’ friends.
This article follows the study by researchers Ella Kelly and Ben Phillips, titled ‘Targeted gene flow and
rapid adaptation in an endangered marsupial’ (2019).
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Figure 1 – The Northern quoll. Photo by B. Leue.