Intelligence is hard to define exactly: it can be interpreted as a mental feature, or a set of features, that involve reasoning, planning, goal-oriented behavior, learning from experience, abstract thought and projection, problem-solving and intentional self-adaptation to the circumstances.
A great number of speculative-biology projects, especially those about extraterrestrial life, try to include high, human-like intelligence for at least one species, for it's necessary to derive from biological interactions the extremely complex structures and behaviors that form a civilization such as ours.
Features of IntelligenceEdit
Of course, living organisms cannot simply be divided as intelligent and not-intelligent. Being as complex as it is, it can be broken up in many characteristics, degrees and specializations. Xenology mentions two basic, not-too-anthropocentric sets of criteria to analyze intelligence. Edward Wilson's "behavioral hierarchy" divides it into three general levels of complexity, the last two of which are limited to animals, since they need a nervous structure to carry information:
- Stimulus → reaction process: simple biochemical mechanisms produce an automatic reaction to each relevant stimulus. Bacteria and protozoa can orient themselves in the environment and look for nourishment by following chemical and luminous signals; animals at this level include sponges, coelenterates and flatworms].
- Directed learning: arthropods, cephalopods and most vertebrates have a complex nervous system that allows the individual development of sterotyped behaviors as a response to familiar situations; however, they still rely often on genetic-based inherited behaviors (instincts).
- Generalized learning: primates, canids, corvids and few other animals (see "Intelligence on Earth") retain a wide range of memories, allowing the formulation of complex behaviors that can be modified and adapted to new situations and the generalization of patterns. Only a few basic biological functions are still committed to instinct.
The aerospace computer scientist Roger MacGowan has produced a list of five basic criteria that he considers both necessary and sufficient to determine intelligence itself in any conceivable lifeform:
- Input: the sensory input from the environment, deeded as raw material for intelligence to work on;
- Storage: conservation of information as memories for a future need, necessary for learning;
- Deduction: the ability to insert currently received information in learned categories based on memories;
- Induction: the extension of past experience to expected future events;
- Output: physical or mental activity as response to processed information.
Among the numerous capabilities that make up intelligence, these seem to be the most important:
A capability of inductive categorization is necessary to learn about the environment: an animal stung by a wasp learns to avoid everything that looks yellow and black. More complex is the formation of categories based on abstract concepts: in the Vaughan experiment, some pigeons exposed to images randomly distributed in two arbitrary sets learned to peck, rewarded with food, only the images of a particular set, considering as relevant property the membership to it, and not any perceptible feature.
The California sea lion Rio (1986-) has shown a great ability to create categories, associating pictures of animals and objects according to similarity (e.g. different crabs), abstract qualities (e.g. "food"), logical transitivity (if A→B and B→C, then A→C) and even distinguishing letters from numbers.
Memory is in turn a combination of many different features. The most known is the spatial memory, well expressed by animals that stockpile food, such as Clark's nutcracker, tits, jays and squirrels. Despite their extremely tiny nervous system, bees can remember for days data met only once, and for all life data met at least three times.
The classic labyrinth test is solved in different species with different abilities: ants and bees mark with chemicals the places where they've already been, pigeons commit to memory the environmental features, rats get to the treat at the centre of a radial labyrinth (a central platform with a number of arms, generally 8, only one of which contains food; it forces the rat to start anew from the center each time) visualizing its geometric structure. Even the slime mold Physarum polycephalum, a colonial amoeba, is able to find the shortest path to exit from a labyrinth by scouting it with tendrils.
Reasoning and Problem SolvingEdit
Over the course of several experiments, chimpanzees proved to be able to understand the operation of a structure and use it to their own advantage, for example to get food, not simply through trial-an-error or previous training, but through preemptive reasoning; New Caledonian crows appear to be able to comprehend and implement cause-and-effect relationships, even combine more tools (also see below).
Self-ConsciousnessEditAlso see: Consciousness
Self-consciousness is generally understood as the capability of an individual to distinguish itself from the environment (including other individuals) without a direct intervention of sense organs. It's very difficult to test this capability in animals: the mirror test, in which it's controlled whether an animal is able to see in a mirror a mark on its body (and therefore recognize the image in the mirror as its own image, and not another individual) has been passed by chimpanzees, gorillas, European magpies, some species of cetaceans and an elephant, but not monkeys. While of some significance, the mirror test is thought to be incomplete and biased towards sight.
Another factor currently under study is metacognition ("knowing to know"), the perception of one's own knowledge: in a experiment, some rhesus monkeys were able to evaluate the tests' difficulty by expressing uncertainty for the most complex ones, instead of trying a random answer.
Many animals are able to distinguish different quantities: angelfish seem to be able to recognize the large set, provided that it is at least twice as large as the other; pigeons and other birds can sort sets of objects from the smallest to the biggest; rhesus monkeys can recognize the smaller between sets whose elements look different, and the same number of visual and auditive stimuli; African elephants are able to perform simple additions, computing the total number of apples left in buckets in different times.
The capability for abstraction is extremely rare in the animal kingdom: most species reacts to an object's depictions only if realistic enough. It's strictly related to the abilities of categorization and prototype formation; for example, a human can see a face in a simple : ), identifying two dots and a curved line as the basic elements of a human face. The recognising of abstract symbols, that do not resemble at all to the symbolised object, has probably been developed with the association between animals and their footprints. Also see categorization.
Using complex languages requires symbolic abstraction to associate concepts with words (see below).
The capability to attribute mental states (knowledge, beliefs, intentions, desires...) to other individuals is called mind theory, and it's likely the single biggest incentive to intelligence in a social species. A baboon does not warn others about a danger they can't see, since it ascribes to everyone the same knowledge it has, as 3-years old children do. Chimpanzees, however, often take food for themselves only when they know individuals of higher rank can't see them, and they can deceive others giving them false notions: for example, pretending to be hurt or sick to get more food.
Many of the abilities above described exist, even well-developed, in organisms that are not thought to be especially intelligent, such as spatial memory in migratory birds and seed-stockpilers, or the orientation and complex language of bees. They're, however, highly specialized skills; we can give a new definition of "intelligence" as the capability of integrating and coordinate these skills together. This capability belongs, above all, to primates, cetaceans and some groups of birds (see Intelligence on Earth).
Encephalization quotient, or EQ, is a way to measure the brain's development in an animal species (or, more properly, a vertebrate, since different animals have very different nervous systems and the comparison might not be possible). It could be thought that mere brain size would be indicative of intelligence, but let's consider this list of animal species, ordered by brain mass:
|Animal||Brain mass||Body mass||Ref|
|Sperm whale||7800 g||40 000 kg|||
|Killer whale||5600 g||6000 kg|||
|African elephant||4800 g||6600 kg|||
|Humpback whale||4700 g||39 000 kg|||
|Bottlenose dolphin||1600 g||160 kg|||
|Human||1400 g||70 kg|||
|Walrus||1100 g||670 kg|||
|Homo erectus||900 g||55 kg|||
|Giraffe||680 g||1200 kg|||
|Hippopotamus||580 g||1600 kg|||
|Horse||530 g||450 kg|||
|Gorilla||500 g||250 kg|||
|Chimpanzee||420 g||70 kg|||
|California sea lion||360 g||350 kg|||
|Lion||240 g||200 kg|||
|Tyrannosaurus||200 g||7400 kg|||
|Brachiosaurus||190 g||78 000 kg|||
|Pig||180 g||70 kg|||
|Baboon||140 g||30 kg|||
|Manta ray||120 g||170 kg|||
|Rhesus macaque||95 g||9 kg|||
|Dog (beagle)||72 g||10 kg|||
|Giant octopus||68 g||14 kg|||
|Kangaroo||56 g||35 kg|||
|Raccoon||39 g||4.3 kg|||
|Whale shark||35 g||1400 kg|||
|Nurse shark||32 g||330 kg|||
|House cat||30 g||3.3 kg|||
|Stegosaurus||23 g||3100 kg|||
|Troodon||20 g||45 kg|||
|Rabbit||12 g||2.5 kg|||
|Grey parrot||9 g||0.4 kg|||
|Alligator||8.4 g||360 kg|||
|New Caledonian crow||6.3 g||0.28 kg|||
|Fowl||3.8 g||1.7 kg|||
|Nile monitor lizard||2.4 g||7.5 kg|||
|Rat||2 g||0.4 kg|||
|Pigeon||2 g||0.3 kg|||
|Sparrow||1 g||0.03 kg|||
|Vampire bat||0.94 g||0.028 kg|||
|Tortoise||0.4 g||0.99 kg|||
|Elephant-nose fish||0.3 g||0.01 kg|||
|Bullfrog||0.24 g||0.7 kg|||
|Hummingbird||0.2 g||0.0048 kg|||
|Tree frog||0.1 g||0.018 kg|||
|Goldfish||0.097 g||0.01 kg|||
|Honeybee||0.00069 g||0.00012 kg|||
This is not entirely useful. Obviously bigger animals need bigger brains to operate at the same level of cognition: due to its sheer mass, a hippopotamus needs a bigger brain than a cat. On the other hand, a man and several other animals have roughly similar brain mass but wildly different body sizes. We need a more refined approach.
This list gives the same species ordered according to a new feature, the brain-to-body mass ratio, that is, the brain mass divided by the body mass:
|New Caledonian crow||1:44|
|California sea lion||1:970|
|Nile monitor lizard||1:3100|
|Nurse shark||1:10 000|
|Whale shark||1:40 000|
That's better, but there are still many issues. Small animals, such as the hummingbird, are clearly too high, while whales are all found on the bottom; chimpanzees are found below pigeons, and even humans are below several birds and a fish. Clearly this method does not work, either.
This is due to allometry, a relationship between organs in different-sized body expressed with an exponential function. Since changing the size of an object with a constant shape doesn't leave unchanged its properties, to keep the same functions, the brain has to be scaled differently from the body: the exponent can vary, according to different estimates, from 0.3 to 0.75, with a likely average of 0.66 for mammals (for other animals it should be somewhat lower or higher depending on their metabolism, and for invertebrates a wholly different method might be needed due to their fundamental differences in the nervous system relative to vertebrates except, perhaps, in the case of cephalopods). That means that, if the body becomes n times heavier, the mammalian brain needs to become n0.66 heavier to perform effectively the same work. For the same reason, the brain-to-body ratio privileges small organisms.
Let's thus define encephalization quotient as a relationship between the brain mass and the 2/3 power of the body mass: Q = 10·E/S2/3, where E is the brain mass and S the body mass.
|New Caledonian crow||1.46|
|California sea lion||0.75|
|Nile monitor lizard||0.063|
Much, much better. We get a reasonable-looking ranking, that still holds many surprises (but, then again, EQ doesn't perfectly correlate with actual intelligence). Still, the 0.66-method is generally well supported by known data. Generally, amphibians rank below 0.1 EQ, fish hardly go beyond 0.5 EQ (with some notable exceptions), while the most prodigious reptiles, birds and mammals usually go from 1.5 to 2.5 EQ.
As an average, herbivores (with the notable exception of the elephants) and insectivores stay under 1.0 EQ, while carnivores (especially pinnipeds and toothed whales) and omnivores (especially primates) are above (see also here); social animals rank higher than solitary animals (dogs higher than cats, horses and lions higher than rats). Only a few species approach, reach or exceed 3 EQ, with both extinct and modern hominids at the very top.
Notice, however, that the EQ is only a rough approximation: small changes in the estimate of body and brain mass can easily change its value. Its min function is to compare different group of animals to see which, on the whole, are more compatible with intelligence. Besides, the development of brain is not a guarantee of intelligence even in the proper scale: the elephant-nose fish, for example, uses its unusually large brain to interprete electrical signals, but it's not otherwise much more intelligent than other similar fish.
It could be said, if "tool" means simply an object extraneous to the body that an organism uses to extend its influence of the environment, that instinctive tool use is extremely widespread throughout the animal kingdom. Archerfishes spit water on their prey, reduviid bugs camouflage with remnants of killed preys, termites build their nests with mud and detritus, striated herons bait fishes with leaves and feathers, some crocodiles bait birds with sticks used to build nests.
There are also many learned and seemingly reasoned form of object use: octopi protect themselves with coconut and mollusk shells; dolphins use shells to trap fish and sponges to protect their snout; an alaskan brown bear has been observed using a stone to get rid of patches of moulting fur; elephants use branches to eliminate parasites, open passings in electric fences with rocks and cover water pools with bark to prevent evaporation; gorillas and orangutans measure with sticks the depths of the water streams they wade, and at least one orangutan has been seen trying to fish with a spear. The Galapagos woodpecker finch extracts larvae from bark using cactus spine, often breaking them to make them more manageable, obtaining up to half of their food with this method.
Using rocks to get food from hard-shelled objects seems to be an especially common skill: chimpanzees and capuchin monkeys do this to nuts, sea otters to the mollusks and sea urchins they carry on their chest, wrasses to bivalves; egyptian vultures and seagulls let respectively bones and oysters fall to the ground.
Chimpanzees, after man, may be the most skillful tool users. They extract termites from their nests and honey from beehives with sticks, use moss and chewed leaves as sponges to carry water, rocks and branches to fend off predators, and even wooden spears to hunt bushbabies. New Caledonian crows manage not only to manipulate food, but also to build more tools, for example bending wire to make a hook, to use tools to safely examine possibly dangerous items, and even to get other secondary tools, something that not even chimpanzees are able to do.
Human language has a number of features that distinguish it from most other forms of animal communication:
- Arbitrariness: symbolic words are associated to concepts they have nothing in common with, and in fact different languages can identify any concept with any possible word (though there is a psychological relationship between some sounds and concepts: for example, o and u sound "bigger" than i and e).
- Cultural transmission: language is taught to andividual by another; any individual can learn many languages very different from each other.
- Discreteness: words are composed by a limited number of signals (phonemes, in the case of spoken language; characters, in written language) that can be combined in vitually infinite ways.
- Displacement: language can be used to talk about objects or events that are not close in space and/or time to the speaker.
- Duality: the language has a surface, mechanical level (the cluster of discrete elements) and a semantic level (the expressed concepts).
- Metalinguistics: the language can talk about itself.
- Grammar: the order in which expressed concept appear in a sentence, and often particles that modify the words (but meaningless on their own), change their meaning.
The gorilla female Koko (1971-) and the chimpanzees Nim Chimpsky (1973-2000) and Washoe (1965-2007) learnt to convey simple concepts using American Sign Language, showing they're able to associate a meaning to abstract symbols. More specifically, some of them were able to combine the meaning of different words to create new terms: for example, identifying a swan, never seen before, as "water-bird" for Washoe and a ring as "finger-bracelet" for Koko. Moreover, Koko can use words as proper names (therefore as pure symbols), calling, for example, one of her pet kittens "All Ball". The bonobo male Kanzi (1980-), who communicates through graphic symbols (lexigrams), learnt some of them by watching a video of Koko, and is believed to fully understand human spoken sentences.
There's even the case of a bird, the grey parrot Alex (1976-2007), to whom a vocabulary of about 150 words was taught, was able to combine them as Washoe and Koko did, and used them to accurately describe shape, colour and number of the objects shown to him, or to ask water and food. Just before his death, he was apparently beginning to understand the concept of "nothing" or zero. What is more, he didn't need to use sign language: being a parrot, he was able to speak.
See here a more complete catalogue of animal communication.
An interesting reflection proposed in Xenology is that intelligence, as a form of data storing and elaboration, could not necessarily require an individual memory, or a developed nervous system inside each body. There are three known ways in which biological systems can store information: the genome (sub-individual) the nervous memory (individual) and culture (super-individual). Each of these might possibly lead to intelligence, and to self-consciousness, that is, perception of the self as a coherent entity distinct from the rest of the universe, receiving information from the environment, with voluntary control over (part of) its own body.
In this form of consciousness, information is stored in the genetic pool of a population, and it's modified over many generations by natural selection. A rudimentary (?) form of this kind of consciousness is found in social insects such as ants, termites and bees, and in other colonial organisms like slime molds. Ants and termites have only a very simple nervous system, slime molds have none, but they can develop complex behaviours in an instinctive and ereditary form: the architecture of anthills, termite nests, beehives and spider webs is not designed from time to time, but "hardwired" in the genome (Richard Dakwins gave the name "extended phenotype" to gene-dictated features external to the body).
An extremely simple form of genetic consciousness exists as adaptation in every organism, but it has its most high expression in social insects. A hypothetical advanced genetic intelligence would likely have its self-awareness extended to the entire colony, rather then each body (single worker ants and even queens don't show any self-regard, but colonies as a whole appear fully aware of their distinct identity and even fiercely territorial). Since genetic "memories" are innate in all the individuals, these would have a specific social role based on a specialized morphology (workers, queens, drones, soldiers...); such a society could have a complex technology.
This is the only kind of consciousness of which we have a concrete example of advanced development (us). In it, data are stored (also) in a centralized nervous system that manages actions and sensorial information of a single individual, leading to a strong self-awareness (and, from this, empathy for others, at least in social species). The social structure, if it exists at all, is only partially innate, and roles in the community are learnt by each individual; there is no morphological specialization, and each of them can fulfill any role, though they have to undergo a later psychological and social specialization, that usually still allows mobility.
It can arise in solitary organisms too, but the development of individual intelligence seems linked to the complexity of societies (see development of intelligence). Given the high self-regard of its members, this is the only kind of consciousness whose society requires a government to mantain order.
This is the most speculative kind of consciousness, of which we don't have any example. Here, data are stored in the common cultural heritage, allowing a much more extensive accmulation and faster elaboration. By extrapolating from the other two kinds, we can suppose that members of such a society wouldn't have a grerat specialization either morphological or social, and could fulfill indifferently any social role; they'd be able to perceive sociopolitical processes of their community with the same immediacy as their individual biological functions.
Examples in Speculative BiologyEdit
- ↑ Robert Freitas, Xenology, chapter 14. <http://www.xenology.info/Xeno/14.0.htm>
- ↑ Cook, Robert G., ed. Avian Visual Cognition. N.p.: n.p., 2001. N. pag. Tufts University. Web. 8 Dec. 2013. <http://www.pigeon.psy.tufts.edu/avc/huber/exemplar.htm>.
- ↑ Crab, Charlene. "Rio, the Logical Sea Lion." Discover 1 Feb. 1993. Tufts University. Web. 8 Dec. 2013. <http://discovermagazine.com/1993/feb/riothelogicalsea184#.UqTs1uLEZMh>.
- ↑ Jason G. Goldman. "Forget Elephants; Sea Lions Never Forget!" The Thoughtful Animal. ScienceBlogs, 27 October 2010. Web. 8 December 2013. <http://scienceblogs.com/thoughtfulanimal/2010/10/27/forget-elephants-sea-lions-nev/>
- ↑ Couchman, Coutinho, Beran, Smith, "Beyond Stimulus Cues and Reinforcement Signals: A New Approach to Animal Metacognition", Journal of Comparative Psychology, 10 May 2010. <http://www.apa.org/pubs/journals/features/com-124-4-356.pdf>
- ↑ "Fish Can Count... Up to Three", Discovery News, 09 January 2011. <http://news.discovery.com/animals/angelfish-counting-math-110109.htm>
- ↑ Jennifer Viegas, "Pigeons Are Brilliant in Math", Discovery News, 22 December 2011. <http://news.discovery.com/animals/zoo-animals/pigeons-math-animals-111222.htm>
- ↑ Michael Tennesen, "More Animals Seem to Have Some Ability to Count", Scientific American, 15 September 2009. <http://www.scientificamerican.com/article.cfm?id=how-animals-have-the-ability-to-count>
- ↑ James Randerson, "Elephants have a head for figures", The Guardian, 21 August 2008. <http://www.theguardian.com/science/2008/aug/21/elephants.arithmetic>
- ↑ 10.00 10.01 10.02 10.03 10.04 10.05 10.06 10.07 10.08 10.09 10.10 10.11 10.12 10.13 10.14 10.15 10.16 10.17 10.18 10.19 10.20 10.21 10.22 10.23 10.24 10.25 "Brain Facts and Figures", University of Washington. <http://faculty.washington.edu/chudler/facts.html>
- ↑ 11.0 11.1 11.2 11.3 11.4 Harry Jerison, "Dinosaur Brains", Encyclopedia of Neuroscience. <http://hjerison.bol.ucla.edu/pdf/dinobrain2.pdf>
- ↑ 12.0 12.1 Csilla Ari, "Encephalization and Brain Organization of Mobulid Rays with Ecological Perspectives", The Open Anatomy Journal, 2011. <http://benthamopen.com/FULLTEXT/TOANATJ-3-1>
- ↑ 13.0 13.1 13.2 13.3 Robert Freitas, Xenology, chapter 3.3. <http://www.xenology.info/Xeno/3.3.htm>
- ↑ 14.0 14.1 14.2 14.3 <http://serendip.brynmawr.edu/bb/kinser/Size1.html>
- ↑ "Magpies Challenge Bird Brain Myth", ScienceBlogs, 19 August 2008. <http://scienceblogs.com/grrlscientist/2008/08/19/magpies-challenge-bird-brain-m/>
- ↑ Jiri Mlikovsky, "Brain size and foramen magnum in crows and allies", Acta Soc. Zool. Bohem., 3 October 2003. <https://web.archive.org/web/20151027010206/https://www.nm.cz/download/pm/zoo/mlikovsky_lit/161-2003-EncephalizationofCorvidae.pdf>
- ↑ 17.0 17.1 17.2 Donald W. Linzey, "Vertebrate Biology", 2012. <http://books.google.it/books?id=qpQ9y-vXovoC&pg=PA219&lpg=PA219&dq=Monitor+lizard+%22brain+weight%22&source=bl&ots=mi8XL7geeY&sig=bju6dJ0afb6lVK96fO111LczekY&hl=it&sa=X&ei=zr9rU9yPNeye7AaGi4D4Aw&ved=0CEgQ6AEwAw#v=onepage&q=Monitor%20lizard%20%22brain%20weight%22&f=false>
- ↑ James Kalat, "Biological Psychology, 11th ed.", 2013. <http://books.google.it/books?id=evbdjAQYA1cC&pg=PA117&lpg=PA117&dq=Elephant+fish+%22brain+mass%22&source=bl&ots=zNVDQzgSar&sig=g4d-EStftqMQOhxHmOMdx7JVoF4&hl=it&sa=X&ei=uK2oUuaNOOTQygOO2oKYCA&ved=0CE8Q6AEwAw#v=onepage&q=Elephant%20fish%20%22brain%20mass%22&f=false>
- ↑ Darren Naish, "Tool use in crocodylians: crocodiles and alligators use sticks as lures to attract waterbirds", Tetrapod Zoology, 30 November 2013. <http://blogs.scientificamerican.com/tetrapod-zoology/2013/11/30/tool-use-in-crocs-and-gators/>
- ↑ "Ingenious fishing method may be spreading through dolphins", Murdoch University, 24 August 2011. <http://media.murdoch.edu.au/ingenious-fishing-method-dolphins>
- ↑ Michael Marshall, "Wild bear uses a stone to exfoliate", NewScientist, 5 March 2012. <http://www.newscientist.com/article/dn21537-wild-bear-uses-a-stone-to-exfoliate.html>
- ↑ Craig Holdrege, "Elephantine Intelligence", The Nature Institute, 2001. <http://www.natureinstitute.org/pub/ic/ic5/elephant.htm>
- ↑ "Wild Gorillas Handy with a Stick", PLOS Biology, 1 October 2005. <http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0030385>
- ↑ Kambiz Kamrani, Primatology.net, 29 April 2008 (from Schuster, Smits, Ullal, "Thinkers of the Jungle"). <http://primatology.net/2008/04/29/orangutan-photographed-using-tool-as-spear-to-fish/>
- ↑ G. Bernardi, "The use of tools by wrasses", University of California Santa Cruz, 20 September 2011. <http://bio.research.ucsc.edu/people/bernardi/Bernardi/Publications/2011Tools.pdf>
- ↑ John Roach, "Chimps Use "Spears" to Hunt Mammals, Study Says", National Geographic, 22 February 2007. <http://bio.research.ucsc.edu/people/bernardi/Bernardi/Publications/2011Tools.pdf>
- ↑ Robert Winkler, "Crow Makes Wire Hook to Get Food", National Geographic, 8 August 2002. <http://news.nationalgeographic.com/news/2002/08/0808_020808_crow.html>
- ↑ Brandon Keim, "Clever Crows Use Tools in New Way", Wired, 5 January 2011. <http://www.wired.com/wiredscience/2011/01/new-crow-tools/>
- ↑ Elizabeth Norton, "How Dolphins Say Hello", ScienceNOW, 29 February 2012. <http://www.wired.com/wiredscience/2012/02/dolphin-greeting-language/>
- ↑ Leila Haghighat, "Baboons can learn to recognize words", Nature, 12 April 2012. <http://www.nature.com/news/baboons-can-learn-to-recognize-words-1.10432>
- ↑ "Researchers explore whether parrot has concept of zero", World Science, 2 July 2005. <http://www.world-science.net/exclusives/050701_parrotzero1frm.htm>
- ↑ Robert Freitas, Xenology, chapter 14.2 (and following pages). <http://www.xenology.info/Xeno/14.2.htm>