It turns out that pigeons are as smart as monkeys--and that's getting pretty close to home! At least when it comes to counting things when there are fewer than ten, according to a paper in the Dec 23 Science, as reported in the NYT. We blogged about smart corvids -- crows, jays, ravens and jackdaws -- and their sense of self a few weeks ago, but pigeons aren't corvids, and they're smart too? Starts to sound like a trend.
|Pigeons performing math tests. When it pecks a shape, a box |
appears. Photo from the New York Times.
As the authors note in their paper,
In a landmark study, Brannon and Terrace showed that rhesus monkeys could not only discriminate stimuli differing in numerosity but that they could also acquire abstract numerical rules. The monkeys were trained to order stimuli containing one, two, three, or four elements in ascending order. To assess whether the monkeys had learned simple nominal categories or an abstract rule, Brannon and Terrace tested the monkeys with pairs of novel values outside of the training range. The monkeys were able to order the novel pairs, suggesting that they had learned an abstract numerical rule that was not tied to the training numerosities (Fig. 1B). In addition, the monkeys displayed distance effects, with accuracy increasing (Fig. 1C) and response latency decreasing (Fig. 1D) as the numerical distance between the paired items increased. The monkeys' performance was dependent on the ration of the paired items (Fig. 1E). As Brannon and Terrace noted, their data suggests that "monkeys represent numerosities 1 to 9 on an ordinal scale."Further,
Pigeons were trained to order 35 three-item numerical lists (Fig 1A). Each list contained stimuli consisting of one, two, or three elements, and subjects were trained to respond to them in ascending order. Subjects were then tested on pairs of numerosities drawn from the range of one to nine.
(A) Stimulus sets used in training. (B) Performance on the test pairs. Error bars indicate SEM. (C) Accuracy as a function of distance. (D) Response latency as a function of distance. (E) Accuracy as a function of ratio. The dashed lines represent the best-fit linear models. Pigeons (N = 3) completed 10 test sessions. The monkey data were redrawn from Brannon and Terrace.
That is, they trained some pigeons to peck images on a computer screen, in order, from the smallest to the largest number. And, it turned out that the pigeons did this just as well as monkeys. The authors suggest that there are one of two explanations for this -- it's either convergent evolution, that is, numerosity evolved independently in both lineages, or numerical competence derived from an ancestor shared by both birds and primates. This would have been a common ancestor about 300 million years ago, before dinosaurs and mammals. So long ago as to seem unlikely, which is why Dr Scarf says that if he had to guess he'd say the trait evolved separately in birds and primates.
But hold on. Alfred Russel Wallace, co-discoverer of evolution by natural selection with Charles Darwin, got off the boat when it came to the evolution of the human brain because he couldn't understand how humans could do something like calculus when calculus wasn't around when the brain would have been evolving. That is, he believed we could do things that we couldn't possibly have been selected to do, like calculus, or music. His conclusion was that therefore God must have made us. But this is incorrect. Apparently by similar logic -- that is, incomplete understanding of evolution -- the ability of birds and primates to count must be because they evolved a specific ability to do so, and that has to be explicable. But what about ants? They don't do calculus, but they do calculate; Darwin himself marveled at the remarkable problem-solving abilities of ants, with their pin-point sized brains. How can this be? Does numerosity have to be a trait that itself evolved?
As we've said numerous times before, one trait that arose so early in evolution that probably all organisms have it is facultativeness, the ability to adapt to changing circumstances, such as temperature, or food supply. Facultativeness also means the ability to learn, from concrete things like finding and exploiting new food sources to abstract things like learning how to read, or in the case of these pigeons, how to count.
The brain is a prime example. The human brain has the ability to make sense of all sorts of things we've never seen before, never mind that didn't exist when it was evolving, and the same can be said for the brains of ants and bees and birds and non-human primates. Birds are currently being found or trained to do all sorts of things no one thought they had any right to be able to do, from tool use to understanding that another bird might steal cached food, and now to having an ordinal sense.
There are limits, we hasten to add -- even if they had the hardware to do so, non-human primates aren't ever going to be as prolific with language as we are, no matter how diligently they're trained. Even if orangs can learn to use iPads.
And birds aren't going to reinvent calculus. We're straying far afield here, but we assume the limits are largely a function of the structure of the brain and its synapses. But the fact that there are constraints doesn't mean that brains evolved to meet only the specific problems and challenges that the early ancestors of birds, ants or humans had to confront early in our evolution.
Undoubtedly there are many genetic pathways shared between us, that are involved in neurotransmission systems, as is the case with genes associated with light reception and many basic developmental genes, and much more. But problem solving undoubtedly involves many different genes and combinations of variants in those genes, even within a species--such as among humans (or ants), so that invoking 'parallel' evolution for such a complex trait as problem-solving may stretch the idea of any precise homology.
We see no need to invoke specific separate adaptive scenarios for the ability to count in birds and mammals (and ants). What evolved, perhaps, was 'just' the ability to assess environments and make decisions about them. Counting and ordinal relationships may be a specific ability or a manifestation of general problem-solving; we're not qualified to judge that (JohnV?). But general problem-solving would be involved in survival, for sure, and the more general problem-solving ability is, perhaps the more robust in terms of survival.