Especially since Dubbo, Monarto and Werribee will likely be the only holders of Common hippopotamus for the foreseeable. It’s no coincidence that these three holders are all open range zoos; and while new Pygmy hippopotamus holders are likely to arise (e.g. Australia Zoo), I doubt we’ll see any zoo invest in Common hippopotamus anytime soon.
In order to maintain a viable population of Common hippopotamus, some effort will need to be made to increase the capacity of the region. There’s indications Werribee are making modifications to increase their holdings (i.e. to house two bulls); and while I acknowledge Dubbo and Monarto each have their own priorities, they have the potential to do the same.
In an ideal world, Monarto and Dubbo would each import a bull to increase genetic diversity; but at a minimum, we can expect any bull calves born at Werribee to transfer out to Dubbo/Monarto. If Werribee are to import and retain two bulls, they certainly won’t want to retain bull calves born on site in addition to those bulls.