Some more info:
This is going to be somewhat subjective, but I’ve done a massive amount of research in this (it’s a personal obsession that goes far beyond most researchers in animal intelligence that I’ve talked to), so I’ll share my thoughts.
For an incomplete list of resources I’ve collected – see What are some good resources for animal behavior/animal intelligence/convergent evolution of animal intelligence?
Also, regarding brain evolution: Alex K Chen’s answer to What’s the best book about brain evolution?
Bear in mind that animals are different, and you can’t always objectively say that one species is smarter than another. But the difference is clear in many cases.
In some cases, animals have very low behavioral flexibility but very high speed at learning what they’re super-specialized at doing (this is particularly true for insects). My rankings are mostly done with behavioral flexibility in mind – however – curiosity (particularly in the case of the kea) can sometimes make an animal appear smarter than it otherwise would be
In a few cases, I will draw inferences based on the brain to body mass ratio data (and graphs) that I have – many of them from the book Principles of Brain Evolution. But they may not be equally accurate for all animals, and I even suspect that they are wildly off for some particular animals. In some other cases, I will draw inferences based on the anecdotal data I have (as well as the information I have about the complexity of the animal’s social interaction). I’ve also kept an informal list of particularly ingenious cases of insight learning.
It will be very hard to make a comprehensive ranked list though, since in many cases, you have gradients within lineages that overlap with each other. Monkeys, in particular, have a huge intelligence gradient between species (some may be no smarter than the average mammal, others are extremely intelligence)
I’ll import it from https://simfishthoughts.wordpress.com/animal-intelligence/. My explanation is at bottom . I’ve collected numerous papers, books, and interviews, but it will be hard to sort them all out (and will take a long time).
How about listing them in order of decreasing intelligence?
Tier 1: highly cultural species – usually means mirror self-recognition, evidence of insight learning, and sophisticated social structure + information communication even to individuals who have not directly witnessed the experience (information travels beyond just
family groups – in fact – it’s amazing how fast information about
hunting can spread between populations of crows and elephants)
- top: lots of dolphin species: orcas, bottlenose, others. http://www.timesonline.co.uk/tol/news/science/article6973994.ece
- sperm whale, other bony whales, also, northern whales. It’s possible that some dolphins belong in their own tier, but I’ll be conservative for now. See What are some good journals or studies on the intelligence of whales?
- orangutan (smarter than chimps – http://www.timesonline.co.uk/tol/news/uk/article1654998.ece)
- bonobo (smarter than chimps – http://news.scotsman.com/world/Bonobos-found-to-be-smarter.6815886.jp)
- other cetaceans, smaller brained dolphins like the river dolphins
Cannot be ranked, but definitely in tier 1
- elephants (they’re slow learners, but intensely emotional, possibly even more so than humans). More at What can an elephant do?
- gorillas (Koko is extremely impressive). Traditional tests are shown to be pretty biased against them
- baleen whales (low brain to body mass ratios and impossible to study in captivity, but anecdotal evidence shows a very surprising degree of self-awareness)
- smarter corvids (raven, new calendonian crow)
- kea (see What is the smartest bird?)
Regarding theory of mind – Which animals have a theory of mind?
Tier 2 – exceptionally cognitively sophisticated, but not to the level of Tier 1. At this point, it’s impossible to rank-order within tiers – I can only do it between tiers
- smartest monkey species (rhesus macaques, capuchin monkeys). rhesus macaques pass mirror test
- gibbons (said to not be as smart as some Old World monkeys – also cannot pass mirror test, but still far more alert than dogs)
- Some cetaceans
- 2nd tier of avians: cormorants, african gray parrot, various amazon parrots and macaws, other corvids (magpie, etc..), harris hawk, pied currawongs
- bears. See Alex K Chen’s answer to Are wolves the most intelligent land predators?
- sea lions (google shows many impressive articles)
- wild boar (most likely smarter than pigs, since domestication does shrink brain sizes)
Tier 3 – smarter than average for birds and mammals
- herons, pelicans, woodpeckers, seagulls, skua
- some monkeys
- hyenas – can count up to 3 (weak compared with parrots though), and much more behaviorally flexible than lions. See 
- prairie dogs (communication system decoded – see What are some good resources for animal behavior/animal intelligence/convergent evolution of animal intelligence? )
most other birds and mammals. some more intelligent than
others. donkeys are smarter than horses and hippos seem pretty high up (though I’m basing this just on two Youtube videos – their brain to body mass ratio is quite low). horses, in turn, seem smarter than most ungulates. rats seem smarter than mice.
At this point, I can only collect anecdotes for now (so that I can further sort things, maybe into subtiers in some point in the future).
Emperor penguins have the most complex communication system among penguins; juvenile gentoo penguins forage along with their parents (which Adelie penguins don’t do)
Dogs seem smarter than cats. But serval cats have very dog-like behavior. Regarding cats – see Cats: What are the smartest cat breeds?
Most exceptionally intelligent lower vertebrates: manta rays, monitor lizards (komodo dragon at top). It’s possible that they could belong in Tier 3 – we just need more research.
And maybe the octopus belongs here as well
Among rodents, naked mole rats are probably smarter than most (but don’t quite beat prairie dogs)
“below-average” birds and mammals: quail, chickens, basal mammalian and avian lineages, pigeons, mice.
“above-average” lower vertebrates: triggerfishes, cichild fishes, hammerhead sharks, numerous stingrays and sharks (great white sharks actually have a pretty low brain to body mass ratio though, as do groupers. But lower vertebrate body weight continuously grows as they get older, so that may skew results against particularly long-lived species – especially groupers and moray eels, which have been observed doing cooperative fishing)
Most reptiles and fish (amphibians, in general, actually have lower brain masses than fish).
But maybe the amphibians with larger brains (certain tree frogs) start belonging here
Salamanders, in particular, have especially small brains – smaller than most fishes.
Lungfishes and coelecanths also have particularly small brains
- smarter non-cephalopod invertebrates: mantis shrimp, portia spiders
- polychaete annelid worms (errantia in a newly constructed phylogenetic tree), velvet worms (similar to basal arthropods)
- lots of arthropods, probably with crustaceans, social insects, and
wolf spiders at top (crustaceans are probably higher on cognitive flexibility). most insects are really good at doing specialized
things (and can improve over time), but are horrible generalists.
- then the animals with ganglia and rudimentary nervous systems (flatworms, roundworms, annelids, other worms, water bears)
- then brainless animals (with nervous systems): echinoderms, etc..
- then the animals without nervous systems.
very preliminary. I am well-aware that you cannot objectively compare
the intelligences between animals, and that some animals are better at
some things than others (e.g. even chimpanzees can pass memory tests
humans fail). however, as a general principle, we can pretty much put
animals into various categories of how aware they are of the world, and
how this awareness translates into creative/novel action and quick
I often obsessively search for articles about the intelligence of
individual animals, so I may identify animals that only very few others
have identified for intelligence so far. e.g., woodpeckers, which are on
the top of Lefvrebre’s avian IQ index, but which a google search would
prove fruitless. Also, manta rays, which seem to be surprisingly aware
of how unusual humans are. Play is prevalent in birds and mammals, but
it might be present in several reptiles, several cartilaginous fishes,
and octopuses too.
Also, evaluating intelligence by evaluating animals at their
dumbest/lowliest moments doesn’t really work, since if we did that,
humans would have to be among the bottom of the scale. Seriously, you
need all sorts of laws and protections and social commands in order to
prevent humans from being tricked over and over again (and yet many
still get tricked; many still even manage to get tricked multiple
times). Yes, you can trick dolphins and elephants with the same tricks
multiple times too, but this is not a reflection of their intelligence
at their worst, since they don’t even have the sophisticated
communication systems that humans can use to learn from the mistakes of
populations where the sample size is N=millions of individuals (rather,
dolphins/elephants would have to learn from the mistakes much smaller
populations where sample sizes are of order N=small family groups). thus
you can use the same methods with different family groups cuz they dont
communicate with each other
Also, even when you’re catching lots of animals, you often only end
up catching the dumbest of them. This doesn’t reflect on the smartest of
them. If you wanted to catch humans and if humans didn’t have systems
of mass communication, then you’d probably be able to catch a lot of
humans too. in fact, pike fish often learn to avoid bait after they’ve
been baited the first time (source: Do Fish Feel Pain?).
One caveat though: crows may be capable of many things, but you have
to take them as individual species. If a New Calendonian crow isn’t
capable of something that an American crow is capable of, then don’t
just say that crows can do everything that both species of crows can do.
That’s like saying that humans are capable for both autistic savantism
and for having intuition for the emotions of other people.
Also keep in mind that most animals have far higher mortality rates
than humans so the average animal that you’re dealing with is most
likely younger than a human teenager (with orcas being the only possible
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