When we talk about "intelligent animals", who do we think of? Is it the border collie who “is a border collie and a dog is a dog”, the gorilla, which is “human’s cousin”, or the killer whale which “knows teamwork when hunting and has rescued a diver from attack”?

Humans living in Australia may have a different answer: the sulfur-crested cockatoo.

This famous ornamental bird has beautiful white feathers and a yellow crest that looks like a sunflower when it blooms. Because of their successful artificial breeding and excellent learning ability, they can often be seen in domestic zoos. However, in Australia, the happy home of the sunflower cockatoo, swarms of parrots are annoyance to local residents - they have learned to open different styles of garbage bins in various neighborhoods to find food, and have learned the skills through social learning. Spread the word within the community.

A paper published in the journal Science shows that sunflower cockatoos "open food boxes" "The behavior spread rapidly, rapidly expanding from three Sydney suburbs before 2018 to 44 in 2019. The researchers even observed geographical patterns created by forest barriers, with each area having slightly different methods of unboxing the sunflower cockatoos. These smart birds have successfully forced their human neighbors to come up with various technological means to protect their trash cans, which is a wonder in the relationship between man and nature.

You may have noticed that when people think of "smart", many people do not immediately think of birds. In contrast, mammals that are more closely related to humans tend to attract our attention more. But thinking back carefully, from "parrots" to "crows drinking water", the "little cleverness" of birds is indeed full of our cultural life. Ethological data also show that corvids and parrots have cognitive abilities comparable to those of nonhuman primates in many areas. Not only do they possess spatial and episodic memory, but they can also understand cause and effect, delay gratification, and plan for the future. They even possess theory of mind (the ability to understand their own and other people's mental states).

So, why are these little creatures so smart?

1. “Size” is not important, “content” is the key

If you want to understand wisdom, you must first study the brain. The intelligence of birds challenges a long-standing stereotype: bigger brains mean smarter. Although scientists cannot predict the intelligence level of a species or individual based on the weight of the brain alone, within the same taxon, larger and heavier brains do provide more neurons and computing power, thus making their owners Smarter than other "cousins"[4]. This is seen in nonhuman primates, cetaceans, and birds. But in horizontal comparisons between more distantly related species, the absolute weight and relative size of the brain are less important: with brains of only 5-20 grams, corvids and parrots can compete with gorillas with 400-gram brains. Higher and lower.

So, how do these walnut-sized brains do it?

In 2016, a study published in PNAS pointed out that although the brains of birds are small, the density of neurons is very high [5]. Bird brains contain twice as many neurons per unit volume as primates and four times as many as mice. The researchers suggest that such a high density of neurons means that the forebrain of birds has a comparable number of neurons to that of primates. Neurons are the basic units of information processing and calculation in the brain. Although the upper limit of the brain's computational power cannot be determined alone, comparable numbers of neurons provide the basis for birds to develop information-processing abilities similar to those of primates. In addition, the proportion of pallial neurons in the cerebral cortex of birds is much higher than that of primates: the crow and the marmoset have similar brain weights, but the number of pallial neurons in the former is three times that of the latter. Cortical neurons are particularly relevant for flexible cognitive abilities because of their ability to coordinate different cognitive processes to achieve a common goal. Rooks, with their large cortical neurons, are smarter than marmosets.

Another type of neuron related to flexible cognitive regulation is associative neurons. Contact neurons exist between the perceptual system and the motor system and are closely related to associative learning and motor learning. Compared with chickens, pigeons and ostriches, the New Caledonian crow has a large number of contact neurons, which is almost as many as those in the gorilla's prefrontal cortex [6]. New Caledonian crows have indeed shown higher intelligence - they use leaf stalks to make tools to hook longhorn beetle larvae in narrow gaps to eat.

Therefore, high neuron density, a large number of cerebral cortical neurons and contact neurons provide amazing computing power for the small-sized bird brain, and provide a certain physiological basis for the intelligence of birds.

2. "Sufficient and unnecessary" neocortex

Birds and mammals began to evolve separately hundreds of millions of years ago, and eventually formed very different forebrain structures. In mammals, the dorsal cortex (dorsal pallium) develops into the cerebral cortex, much of which is isocortical. "Isocortical" means that all parts of this part of the cerebral cortex look similar. The "isocortex" is better known as the "neocortex" because this part of the cerebral cortex is unique to mammals and is evolutionarily newer and more recent. Not only that, the mammalian neocortex contains all areas related to perception, movement, and communication. It can be described as a comprehensive brain structure that is extremely important for cognition and learning abilities. Because of this, the neocortex has been considered the source of mammalian intelligence for a long time. However, clever birds have once again challenged this view.

In birds, the dorsal cerebral cortex, which is homologous to mammals, evolved into the hyperpallium. Unlike the neocortex of mammals, the supracortex of birds only contains sensory areas and is not functionally comparable. Most of the remaining avian cortical nuclei (pallial nuclei) are located below the lateral ventricles, collectively known as the dorsal ventricular ridge (dorsal ventricular ridge). The dorsal ventricular crest has no homologous structure in mammals, but it functionally complements the supracortex, processing sensory information, and also contains motor and communication areas. Recent studies have shown that the part of the dorsal ventricular ridge that processes sensory information exhibits a hierarchical structure and information processing routes similar to those of the mammalian cerebral cortex [7], while the motor and communication areas are still a nuclear arrangement. ).

These data suggest that bird brains may partly exhibit similar structures and information processing methods to mammalian brains due to convergent evolution. At least when it comes to processing sensory information, these isocortex-like structures may have excellent advantages that are hard to replace. However, given that the dorsal ventricular ridge and the neocortex are not homologous from an evolutionary perspective, and the dorsal ventricular ridge does not fully adopt the processing methods of the neocortex, the neocortex may only be a "sufficient and unnecessary" condition for a smart brain.

3. Dopaminergic-driven "prefrontal cortex" of birds

We have discussed the reasons why birds are smart in terms of the number, type and brain structure of neurons, and found that they are related to Mammals are both similar and different. Next, we will continue to explore why birds are so smart in terms of brain function.

Among the structures arranged in a nuclear manner on the dorsal ventricular ridge, the nidopallium caudolaterale is particularly important. The caudal lateral nested cortex is located at the most caudal end of the dorsal ventricular ridge. It is very similar in function to the mammalian prefrontal cortex and is involved in almost all cognitive processes. What's more, both areas have the highest density of dopaminergic neurons in the brain and are connected to all communication areas and premotor structures in the brain. Similar to dopaminergic neurons in the prefrontal cortex, neurons in the caudal lateral nested cortex are also capable of categorically presenting perceptual information (such as length and number) and presenting stimuli in different temporal orders (prospectively or retrospectively) as needed. Even encoding executive functions (a set of abilities to manage and control abstract cognitive functions such as attention) and sensory awareness. Therefore, the caudal lateral nested cortex of birds has similar powerful functions to the prefrontal cortex of mammals, allowing them to coordinate and arrange complex cognitive processes and exhibit "smart" behavioral abilities.

4. Do birds have working memory?

Working memory (working memory) refers to the ability to store information in memory for real-time processing. Having working memory is at the core of all cognitive abilities and an important function of the prefrontal cortex. Multiple studies including brain injury, pharmacology, and neurophysiology have shown that the caudolateral nested cortex in birds is also involved in working memory in birds. For example, single-cell recording methods have observed “delay activity” in the caudal lateral nested cortex similar to that seen in the mammalian prefrontal cortex. Delay activity is the phenomenon in which prefrontal cortical neurons remain active during the time interval between the application of an external stimulus and the subsequent action [8]. The observation of delayed activity in birds means that they do have working memory, the basis of cognitive abilities. Not only in single neurons, but also in local field potentials (local field potentials) formed by the superposition of the activities of multiple neuron cells, similar neurophysiological activities in birds and mammals have also been observed. This evidence suggests that, despite lacking the layered structure of the neocortex, cognitive activity in birds produces similar neurophysiological fingerprints to those in mammals.

Research on working memory mainly focuses on the level of single cells and local brain regions, while research on sleep and dreams can provide evidence about the cognitive abilities of birds at the whole-brain level. In an experiment on pigeons undergoing REM sleep, a sleep state closely associated with dreams, researchers used functional MRI to scan the pigeons' brain activity. They found that, similar to humans, pigeon brains activated the limbic system and premotor brain areas, as well as visual and multimodal cortical areas during REM sleep [9]. Although it is not yet possible to reconstruct the content of dreams based on these brain activities, researchers still speculate that the pigeons may have dreamed about avoiding obstacles during flight. This evidence preliminarily suggests that the cognitive processes of birds, like those of mammals, are closely related to the activity of extensive neural networks.

In addition to similarities, the neurophysiological basis of bird cognition is also completely different from that of mammals. For example, sleep studies show that birds do not activate the hippocampus during sleep to consolidate memories the way mammals do. Exactly how they retain and retrieve long-term memory remains to be studied.

5. Why are birds so smart?

In this latest Trends in Cognitive Sciences opinion article, the authors discussed the composition of the "intelligent brain" starting from the question "Why are birds so smart?" So, what kind of brain can develop intelligence?

First of all, bigger brains are not necessarily smarter. The number, type, and interconnection of neurons within the brain may be more important. Unlike mammals, the high density and large number of cerebral cortical neurons and contact neurons in the bird brain allow birds to develop intelligence within limited brain size.

Secondly, the neocortex unique to mammals is not a necessary prerequisite for intelligence. The dorsal ventricular ridge of birds and the neocortex of mammals are not evolutionarily homologous, and their physiological structures and information processing routes are also different. However, the dorsal ventricular ridge, like the neocortex, is responsible for information processing including perception, movement and communication brain areas, making it possible for birds to develop intelligence. This shows that the neocortex is only a "sufficient and unnecessary condition" for the development of wisdom.

Third, the caudal lateral nested cortex of birds has dense dopaminergic neuronal connections and is involved in almost all cognitive processes. The caudal lateral nested cortex is very similar in function to the mammalian prefrontal cortex and is responsible for integrating information, encoding abstract cognition, and coordinating actions. Since the two are not evolutionarily homologous, this may be the result of convergent evolution. It also shows that a "control center" similar to the prefrontal cortex is extremely important for the development of complex cognitive abilities.

Finally, research on working memory and sleep dreams in birds shows that despite lacking similar brain structures, the neurophysiological fingerprints of bird cognition are very similar to those of mammals. This shows that in the process of generating intelligence, the cooperation mechanisms of neurons and neural networks may have essential and irreplaceable commonalities.

So far, we have used birds as an example to break the monopoly concept of "only mammals are intelligent" and discussed " More possibilities for “intelligent brain”. Of course, these are only the directions that are currently rich in research, and there are other "potential stocks" waiting for human research and discovery.

Mammals are not the only intelligent creatures on earth. Studying the brains of birds will give us a different perspective. While asking "why birds are so smart", humans are also exploring their own wisdom and boundaries, raising questions and thinking about the nature of life - or "creatures".