A lone bottlenose dolphin swam into the harbour at Tory Island off the north coast of Ireland in April 2006. Her playfulness quickly earned her an appreciative following among the locals who nicknamed her Duggie. Her biggest fan was a Labrador retriever named Ben, who began swimming out to meet her on a daily basis.

During a typical encounter, Ben would paddle frantically with his tail wagging happily above the waves while Duggie blew giant bubble clouds around his canine companion. Ask most people which of the two species in this touching tale is the most intelligent and there is unlikely to be much of a contest. Dolphins - with their sociability, communication skills, playfulness and ability to understand the complex commands of trainers – are widely considered to be the second most intelligent of all animals after humans.

Yet recent years have seen a growing interest in canine cognition. Researchers at institutes such as the Duke Canine Cognition Center, North Carolina, US, and the Dog Cognition Centre at the University of Portsmouth, UK, are concluding there is far more going on in the minds of dogs than we previously thought. In fact in some cognitive tests they outwit both great apes and dolphins. So what does the latest science say? Could dogs really be the intellectual rivals of dolphins?

“There's lots of evidence that dogs are more skilled than primates at thinking about a person's communicative intentions,” says Laurie R. Santos, a psychologist at Yale University in New Haven, Connecticut, US, who studies primates and canines to shed light on the human mind. “They understand that people are trying to communicate information and use such communicative signals way better than primates do.”

Understanding us

Dogs’ abilities to comprehend human communicative signals make them one of the only animals to understand what people mean when they point at something. Even our cognitive cousins the chimpanzees don’t look past the tip of an outstretched finger when a human points them in the direction of food. A dog can use pointing as well as eye-direction cues to locate objects in the distance. These abilities are rare in the animal kingdom, although dolphins have them too, so on this test alone dogs don’t best them in a contest of wits.

Dolphins comprehend thousands of different sentences

There is however one cognitive feat at which dogs, somewhat unexpectedly, outshine almost every other non-human animal. Chaser, a Border Collie trained by US psychologists, is reported to be able to understand over 1,000 words used by her trainers to label her toys. When asked to fetch Bamboozle, a stuffed orange horse, she is easily able to pick it out of a lineup of other toys. She is even able to understand simple sentences consisting of a preposition, two nouns, and a verb, like “to Frisbee take ball” (take the ball to the Frisbee). With research showing the famous language-trained apes like Kanzi the Bonobo top out at vocabularies of less than half those of Chaser, and the dolphin language prodigies from the Kewalo Basin Marine Mammal Laboratory only managing about 40 words, dogs come out top when it comes to learning the meaning of individual symbols.

More than words

But there’s more to language comprehension than just vocab. Although Chaser was able to understand some basic syntax-like structure, scientists working with dolphins have succeeded in combining up to five words in one sentence containing complex concepts like “through” or “imitate.” This allows dolphins to comprehend thousands of different sentences that involve much more complex behaviour than simply “fetching.”

And unlike dogs, it seems that dolphins are keen problem solvers.

In one experiment, dolphins were tasked with retrieving a series of weights scattered around their pool which they had to place on top of a box in order to release a food reward. Almost immediately they hit upon the idea of collecting all of the weights at the same time instead of placing them on the box one at a time. This apparent flash of insight suggests the dolphins were planning their behaviour and turning over possible solutions in their mind’s eye; a hallmark of complex thinking.

On top of this, dolphins are among the handful of animals that understand they are looking at themselves when put in front of a mirror (others include chimpanzees, elephants, and magpies). Researchers have used the mirror self-recognition test to determine whether animals understand that they exist as separate entities in the world, with their own thoughts and minds. This level of self-awareness opens the door to greater flexibility in the face of new problems, or when trying to figure out what’s going on inside the minds of other animals. In this particular test of intelligence, dogs don’t make the grade.

Sometimes it just doesn't make sense to make comparisons

At first glance then, perhaps dolphins’ superior word-crunching skills, problem solving, and self-awareness mean the gut instincts of those who assume they outsmart dogs are supported by the evidence. But relying on our gut to evaluate intelligence is problematic.

For one thing, the concept of human intelligence itself is an ill-defined amalgamation of various different cognitive skills, and efforts to evaluate it such as IQ tests are plagued by cultural bias. Is someone who is good at solving algebraic equations more intelligent than someone who can quickly determine the motivations of others? Is being able to remember facts a mark of cleverness, and is it more or less important than logical reasoning?

The difficulties such questions pose help explain why many scientists object to subjective notions of human intelligence being applied to non-human animals.

Furthermore, even if it were possible to agree on a constant and universally-accepted definition of human intelligence, why should this apply to non-human animals?

Dogs have been bred to thrive in a human-generated environment; they’re particularly adept at reading human social cues, and tugging on human heart strings. When we look into their eyes and see a glimmer of intelligence, we see a reflection of our own minds. The skills at which they excel – understanding pointing or object labels – are predominately a result of humans having spent thousands of years breeding them to understand concepts that are important to us. We are the architects of their minds in many ways, and we’ve shaped them to conform to our definitions of intelligence.

Dolphins, on the other hand, have evolved their cognitive skillset in a world untouched by humans. Yet they seem intelligent to us because their complex social behaviour reminds us of ourselves. We see reflections of humanity in their playful demeanour and intricate societies. And when scientists began digging into dolphin minds, they found abilities that closely resembled that of our fellow great apes, such as mirror self-recognition – an unexpected finding for an animal that looks more like a tuna than a chimpanzee. Dolphins are perhaps the most human-like of all non-primates, which is precisely why we consider them to be intelligent.

A question of intelligence

“We know animals are shaped for certain specific kinds of cognitive abilities by biology and evolution, so sometimes it just doesn't make sense to make comparisons,” says Santos.

A dolphin’s mind evolved to produce behaviour that helps a dolphin cope with dolphin problems, including finding fish buried in the sand with echolocation, or only sleeping one half of their brain at a time so they’re able to surface for a breath. The way a dolphin thinks is a direct result of their physical, social, and ecological needs. The same is true of dogs. And since each species has its own set of needs, each species has its own unique way of thinking.

Humans aren't necessarily ‘smarter’ than other primates

Humans have been shaped by the need to navigate complex social situations and pass along learned information to our peers. This has resulted in the evolution of language and culture, which coincided with the rise of the kind of complex critical thinking that resulted in things such as agriculture, technology and science, and ultimately space exploration and nuclear fission. Granted, these things are extraordinary by-products of the way we are able to think about information. But that doesn’t make exchanging learned information the best or most intelligent way of thinking.

“The idea is that humans aren't necessarily ‘smarter’ than other primates so much as we're good at learning from others,” argues Santos.

A good example of why intelligence is such a problematic term in this context is to consider how different species perform in tests of object permanence. This is the ability to understand that objects continue to exist even when out of view. In the most complex version of the test, an animal is shown an object such as a ball, which is then placed inside a container and covered with a sheet. The ball is then removed from the container while it is out of view, and the now empty container is pulled from behind the sheet. If the animal has a full understanding of object permanence, it should be able to work out what happened to the ball while it was out of view, and realise that it must now be behind the sheet.

Thinking differently

Human children work this out by the age of two, and other great apes have no trouble figuring this out. Dogs and dolphins however just don’t get it. So does this mean dolphins and dogs are not intelligent enough to have a full understanding of object permanence?

It could instead be a sign that dolphins and dogs think differently rather than less intelligently about the problem. Primates are a visual species, so our ability to visually track objects is how we relate to the presence or non-presence of things around us. But dogs live in a world of smells. Their understanding of objects in the world partly involves chemical trails that linger for hours or days. Perhaps dogs fail object permanence tests because objects that are no longer visually there might have a continued chemical presence, which makes it difficult for dogs to figure out what the researchers are asking of them.

Dolphins have visual systems that are similar to primates in many ways, but they also have an extra sense that renders object permanence a potentially troublesome concept. They are able to see through some materials by penetrating them with sound waves. So a fish hiding in the sand might be invisible to the naked eye, but be detectable to an echolocating dolphin. Perhaps objects rarely – if ever – simply disappear for dolphins.

It might then be that a dolphin’s brain cannot easily comprehend the idea of an object not being either visually or echoically accessible. Or maybe they have a totally different understanding of how objects move because they live in water where gravity sometimes plays second fiddle to buoyancy. A recent study found that dolphins can indeed pass the invisible displacement test using 2D objects projected on a screen. Scientists now think that dolphins were flummoxed in earlier studies by the idea of an object disappearing into a container – a scenario that almost never occurs in the wild. They seem to cope better with objects that pass behind – as opposed to into – a visual barrier.

It’s interesting to ask why dogs and dolphins have a hard time with object permanence, just as it’s interesting to wonder what Duggie and Ben’s friendship might tell us about the way dolphins and dogs think about the world. But ultimately, once subjective, human-centric value judgments are stripped out of the concept of intelligence, it makes about as much sense to ask which animal is cleverer as it does to ask whether a hammer or a screwdriver is the better tool. The answer is it depends on the task at hand.

Dr Justin Gregg is a research associate at the Dolphin Communication Project, and co-editor at the academic journal Aquatic Mammals.

He is also author of the book Are Dolphins Really Smart?

Follow Justin on twitter: @justindgregg