The sex life of a cane toad is an interesting and intriguing subject. One of the questions we need to ask is "Cane toad sex - who does what to who?"
Like most animal species, cane toads reproduce sexually. So about half the toads are males, and half are females. The sex of a toad is determined by its genes, just like in humans. Male toads have two copies of the same sex chromosome (called the Z chromosome, so males are ZZ) whereas females have one copy of the Z and one of the W (so females are ZW). Researchers at the University of Queensland (Prof Peter Koopman and John Abramyan) studied the toad’s sex-determining system to see if they could change it to make the toads produce only sons. If it worked, that might help to control toad numbers – an all-male population wouldn’t produce any offspring. But it doesn't look like that trick will work with cane toads, although other researchers are still trying it with different pest species, like carp.
Male and female toads not only differ in maximum sizes (females get bigger) and general appearance (females have smooth brown skin, males have rougher skin and are often quite yellow) but also in habits. Males tend to hang around the breeding ponds, calling to attract females. Females spend lots of time wandering around the wider landscape, feeding so they can accumulate the energy they need to produce a clutch of eggs.
One unfortunate consequence of this sex difference in lifestyles is that by far the easier sex for people to collect are the males – because they are concentrated around the water, and draw attention to themselves by calling – so that hand-collections often are highly biased towards male toads. This doesn’t achieve much in terms of controlling toad populations, because a single male can fertilize the clutches of many females – so the few lucky males that are left behind may have a pretty exciting life, and the number of fertilized eggs laid in that pond isn’t reduced at all.
We don’t know why males suddenly decide to start calling around a pond, but it can be really dramatic. Even when we know there are hundreds of toads around, sometimes we don’t hear any calling for weeks on end. But then suddenly one night a group of males will converge on one particular pond and start calling. Local people understandably think that this is the arrival of the toad invasion front (because until they start calling, toads can be very difficult to find), whereas in fact the toads have been in the area for many weeks.
The sudden “decision” by male toads to start calling means that the first toads that people find are these males – and so, this has suggested to locals that the first wave of animals at the toad front are males, followed by the females. This simply isn’t true. Our careful nightly surveys at Fogg Dam showed that males and females were equally common among the first wave (the very first toad to arrive was a male, but the next few were all females). But because females are so much harder to find, people who just turn up to look for toads are likely to conclude (incorrectly) that the invasion front consists mostly of males.
there's another confusing aspect that may have led community groups to
make some mistakes about the sex ratio of toads they encounter. Males
and females are easy to tell apart during the breeding season, because
males have rougher skin and are often quite yellow in colour. But at
least at Fogg Dam, many of the males (even big adults) lose those
characteristics during the dry-season, and so can be really hard to
distinguish from females without dissection to examine the reproductive
Hear a Cane Toad Calling
When a female has amassed enough energy for a clutch of eggs, she goes to a waterbody with calling males. We don’t know how female toads choose their partners; research on other species of frogs and toads suggests that this can be a pretty complex issue. Despite their tiny brains, female frogs are able to work out the size of different males, the loudness of their calls, the exact “musical” structure of their calls, the suitability of their territory for egg development, and so forth. Cane toads may well do the same. Males are very enthusiastic when they are breeding, and seize the female under her armpits and hang on. They need to hang on tight, because other males try to push them off. Haley Bowcock’s studies at Fogg Dam showed that bigger males are stronger, and often are able to push the little blokes out of the way.
Because males are fairly indiscriminate about what kind of object
they grab, they sometimes grab other male toads by mistake. They soon
realize their mistake, however, because the other male gives a “release
call” to tell him he’s made a poor choice. Haley’s work showed that
female toads can’t give that call, so once they are grabbed it’s very
hard for them to convince the male to let go.
We’ll end by mentioning one REALLY peculiar aspect of toad sexuality. Unlike frogs, male toads not only have two testicles (inside their body, like in birds and fishes) but they also have two strange organs just above the testicles. These are called “Bidder’s Organs” and are almost as big as the testicles – sometimes even bigger. And they are basically ovaries – producing sex hormones just like female ovaries do, and containing tiny little egg-cells.
Under normal circumstances those egg-cells don’t ever develop further, but if a male is surgically castrated (that is, his testicles are removed), the Bidder’s Organ will grow larger and turn into a functional ovary and start producing eggs. Nobody really understands why toads have this weird system, but all toad species do have it whereas it’s never seen in frogs. Some recent research on cane toads in Florida (USA) suggests that pollution by agricultural chemicals can make male toads more like females in appearance and behaviour. Very peculiar!! One day, TEAM BUFO would love to do some research to find out what’s going on.