What's your opinion on the concept of marsupial intelligence? People said that marsupials are dumber simply because of the brains they built, but I highly believe they are just as smart as placentals because there are biases in those studies on marsupial development. What would the hidden potential of marsupial intelligence would be like?
Interesting question. There are fewer marsupials than placentals, most of them are concentrated in the continent with the lowest primary productivity (after Antarctica), and IIRC they tend to have a slower metabolic rate, which might artificially against marsupial intelligence.
However, Weisbecker and Goswami (2010) make two interesting observations:
The difference in relative brain development between marsupials and placentals is distorted by the inclusion of Brain Georg among the latter (i.e. primates). Once you exclude those the two clades aren't that different; placentals also have small-brained subgroups (e.g. Xenarthra).
Marsupials may actually have less constraint on max brain size than placentals because of their extremely precocial birth, and -- perhaps because their development is spread over more time -- it's also less limited by the mother's metabolic rate. It's not clear to me they have made much use of this reduced constraint, though.
Ashwell (2008) examines in detail the relative brain size of Australasian marsupials, and interestingly finds that small marsupials have the same endocranial volume (proxy of brain size) as placentals of similar size, but larger ones fall short. Pygmy possums, honey possums, and sugar gliders apparently have similar e.c.v. as similarly-sized prosimians (with whom they share tree-dwelling, grasping hands, and sugar-rich diet). They also find no relation between e.c.v. and the lushness/aridity of the environment, which I find vexing.
Karlen & Krubitzer (2007) also finds a relative brain size in some possums that rivals some primates, and note that similar specializations in the cortex can be found in marsupials and placentals with similar lifestyle; Rasmussen & Sussman (2007) explicitly describe phalangeroid possums as marsupial analogues of prosimians. The brushtail possum (Trichosurus vulpecula) is already fairly close to a marsupial monkey, or at least a marsupial lemur.
The most obvious difference in brain structure is the lack of a corpus callosum in marsupials, but there is an anterior commissure with the same function. Interestingly, per Dos Santos &al (2016), Australasian marsupials have a cortex-to-rest of brain ratio of neurons "comparable to artiodactyls and primates", whereas American marsupials have a less concentrated cortex.
Having a different brain organization compared to placental mammals is not necessarily a negative; birds don't even have a neocortex, strictly speaking, and this doesn't prevent parrots and corvids from being impressively smart.
Tl;dr I'm not convinced marsupial brains as such are in a poorer state than placental ones.
@morgrimmoon, have you thoughts on this matter?
I'll second some practical experience with possums: both brushtail and ringtail possums can be quite smart. I'd put them at roughly similar levels to a pet cat?
Quokkas (a small wallaby famous on social media) have learnt how to open zips and velcro, which we know because there's a group of them on Rottnest Island who have become somewhat infamous for sneaking into people's tents to steal food. I don't think that quite counts as tool-using, though. Quokka have also learnt how to manipulate tourists, but again that may be more a sign of cunning than smart.
A hand-reared kangaroo can learn its name and maybe one or two other verbal commands, but that's about it. Admittedly, they're not a very vocal species and don't tend to respond to food rewards, so training them is hard.
Some marsupials are not very intelligent, though. Wombats are either oblivious or simply too stubborn to learn, in some cases, and will do things like repeatedly shock themselves on an electric fence. (There's speculation they're trying to intimidate the fence. This does not help their case.)
Thanks for the addition! I admit I was so focused on brain size and structure that I neglected to look much at the behavior, heh.
Large herbivorous diprotodonts were indicated by some of the papers as the marsupials with the (relatively) smallest brain, so that gels with wombats (including the infamously smooth-brained koala, though that seems a peculiar adaptation to an exceptionally crappy diet and to frequent fall injury). Though in theory that should include kangaroos and wallabies as well, so I'm no longer so sure of that.
(Funny that "looking cute" is now a viable evolutionary strategy. Wonder what the biosphere would look like if it lasted for a few million years.)
Still, if Earth were to get a marsupial sophont in the future, my money would be on a brushtail possum descendant that went down the tool-using route. They might need to become more social, though.
I heard a story from the 2019-20 fires, from friends in wildlife rescue:
How it goes is kangaroo joeys often survive the collision that kills their mum, cushioned in the pouch by her body. Human volunteers raise them, then release them by slow transition on bush blocks. Some disperse, some stick around and their carers see them raise joeys of their own.
In early January 2020, when a string of hot days had reinvigorated the NSW fires, the shops were cleared out of n94 masks and air purifiers, and the smoke was showing up on the rain radar, a volunteer went outside to find her driveway and garage full of kangaroos.
Most she didn't recognise. But one in particular she did- a male she'd released, Billy. Males tend not to stick so closely to their release site, they range more. But Billy had been back around at intervals like many a human grown-up son.
This time he'd brought all his exhausted, frightened mates.
"We'll all go to mum's. She'll know what to do".
