Larger brains, which typically contain more neurons, store and transfer more information (Tehovnik and Chen 2015), but the precise relationship between number of neurons and information has yet to be deduced. Although some have argued that a greater storage and transfer of information is related to intelligence/cognitive capacity (Herculano-Houzel 2017; Horschler et al. 2019), we would suggest that a larger number of neurons is required to control a larger number of cells making up a large-bodied animal, which also happens to have a protracted longevity since the mitotic addition of cells for complete development takes time (Finlay 2019b; Hofman 1993; West et al. 1997). In fact, it is common to assess biological time according to the number of cell divisions per lifetime (see Footnote 1, Hackfield and Moorhead 1961). Indeed, the bowhead whale has a large brain to accommodate its large body, and it can live for 200 years, requiring the first 25 years to become sexually mature (George et al. 1999). At the other extreme of information transfer is the mayfly, which has a pinhead of a brain as well as a pinhead of a body. These animals live for up to one day and they become sexually mature within hours (Hoell et al. 1998). As for intelligence, the evolutionary duration of a species should be the ultimate metric of its success (rather than whether it has been able to decode the genome), while the storage and transfer of information by the brain is related to the number of somatic cells under its control, which requires more learning time before reaching sexual maturity. In vertebrates, learning time covaries with the number of telencephalic-cerebellar loops added during development prior to sexual maturity, while factoring in the uniqueness of each vertebrate class, whether fish, amphibian, reptile, bird, or mammal (Tehovnik, Hasanbegović, Chen 2024). Each vertebrate class has a different amount of parental investment with fish, amphibians, and reptiles making a low investment, and with birds and mammals making a high investment, e.g., whales are known to bond with and teach their offspring for years before the offspring become independent (George et al. 1999). And raising children to adulthood is seen by every culture as an important duty, for we all know what happens when this goes wrong.
Footnote 1: Each time a cell undergoes mitosis, the telomeres on the ends of each chromosome shorten slightly (Hackfield and Moorhead 1961) thereby producing a biological stamp of age. Some animals with long lifespans: Greenland shark (272 years/sexual maturity 150 years), Bowhead whale (200 years/sexual maturity 25 years), Tortoise (175 years/sexual maturity 15 years), Giant Tube worm (170 years/sexual maturity 2 years), Giant Salt Water clam (160 years/sexual maturity 15-20 years), and so on. Some animals with short lifespans: Mayfly (24 hours/sexual maturity < 24 hours), Drone ant (3 weeks/sexual maturity ~ 3 days), Housefly (1 month/sexual maturity ~ 24 hours), Dragon fly (4 months/ sexually mature ~ 1 week), mouse (1 year/sexual maturity ~ 5 to 8 weeks), and so on. The average lifespan for humans is about 80 years, becoming sexually mature at 13 years. Typically, the larger the body the longer an animal needs to live to complete development (but there are exceptions in the invertebrate world, e.g., the Giant Tube worm), and a large-bodied animal generally has a larger brain to control the greater number of somatic cells. (Information obtained from Wikipedia)
As we can see the life was unicellular at the starting point and by the evolutionary development and cell division life become now multicellular that seems to be more intelligent. As we see one more factor that ratio of brain and body if brain has high ratio, then it indicates high intelligence. In case of human the requirement of oxygen is 25 % more than other part of body consequently more circulation of blood and fast development of brain. The speed of vertebrates one more factor indicate the highest intelligent. As per as the diversified intelligence development is concern it depends fastest diversify and evolutionary approach of life that match only with the human. Because the animals are also intelligent but for specific not diversify.
This is not one question, but several, as can be seen from the convoluted nature with which it is written. As written it would take a long time to untangle the various threads, but I can tell you what my experience of 45 years as a behavioral scientist has shown me. All animals have intelligence, which is related to their ecological situation and how they cope with it. Asking which animal is an empty question, because it is usually used to justify the "superiority" of humans, who have large brains and flexible behavioral repertoire in complex ecological situations that they create. I have seen fish do very clever and insightful things and mammals do incredibly stupid things.
Part of this question is about how well animals are able to express the intelligent behavior of which they are capable. This relates well to how they feel about their life situation. Animals under stress, show less flexibility, and much smaller repertoires than happy confident, well fed ones. A major aspect of what we call intelligence is personality, if an animal is outgoing and confident, we regard it as more intelligent than an animal that is withdrawn and timid.
I think we should stop talking about intelligence, which is concept invented by humans to make themselves feel good. Instead we should discuss social and ecological flexibility, but at the same time we should recognize that this is not necessarily the same thing as well adapted. Animals that show limited behavioral repertoires may be quite flexible but if they live in a good situation they may not need to shown their flexible activities.
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