Brain size:
One of the factors influencing cognition is the brain size. The high level of perception and cognition related to brain size in many radiations is the main reason of their evolutionary success (Amiel, Tingley et al. 2011). Since the cost of increasing the size of brain is high, in the species that have increased brain size, the high amount of perception and wisdom and the benefits that may have been gained are balanced or overpower the related metabolic costs. (Martin 1981, Aiello and Wheeler 1995, Isler and Van Schaik 2006).
One sign of the greater cognitive ability of larger-brained species is the ability to create viable populations based on the knowledge they gain from environmental events; this is seen in mammals (Sol, Bacher et al. 2008), birds (Sol and Lefebvre 2000) and reptiles (Amiel, Tingley et al. 2011). Interestingly, fish (Drake 2007) do not fall into this category.(Kotrschal, Buechel et al. 2015). The brain of most invertebrates is small. Hence, brain size is not an influential factor in their cognition analyses. (Elwood 2011) For example, Insects have a miniature brain. Their brain is unable to produce and process complex behaviors with few neurons. (Giurfa 2013). However, some small species which have a relatively large brain. Generally, brain is enclosed, meaning that morphological modifications are relatively difficult to happen. Nonetheless, some miniature animals ignore many morphological aspects in accordance to their large brain. (Eberhard and Wcislo 2011) the octopus has a large and complex brain that is even larger than some vertebrates with a huge number of neurons (Elwood 2011). As a result, it demonstrates complex behaviors and possesses extensive cognitive abilities. (Elwood 2011)
Most bony fishes, except few, have the same relative brain size while agnathans, have larger brain size for the same body size (Northcutt 2002). However, relative forebrain size of hagfishes is almost four times larger than lampreys; their forebrain is almost in the same level with bony fishes and amphibians in terms of development.(Northcutt 1981)
Generally, among amphibians and reptiles, those species which are more successful invaders or have better adjustment in establishing in new environments, have a relatively larger brain. The forebrain of amphibians and reptiles, is smaller than mammals and birds, therefore they are believed to have simpler behavioral complexity. However, their ability in adjusting in novel niches is certainly a reflection of the increase in brain capacity, not just forebrain function. (Amiel, Tingley et al. 2011).
In reptiles, the brain and telencephalon size is approximately two to three times larger than the amphibians with the same body size (Northcutt 2002, Northcutt 2013). Interestingly, the relative telencephalon size in lizards and crocodilians almost overlap with mammals and birds (Northcutt 2013). There is not enough information about the size of midbrain and forebrain in primitive reptiles or mammallike reptiles, because their brain cases are not completely ossified (Hopson 1979). Generally, however, the largest change in reptiles’ brain is in pallium. Also studies on tetrapods’ brain indicate that most of the variations in brain size is in telencephalic and cerebellar hemispheres.(Northcutt 2013)
Birds with larger brains establish in new environments more successfully.(Sol and Lefebvre 2000). The relative brain size of birds is approximately 10 times larger than reptiles’ brain with the same body size during evolution(Meek and Nieuwenhuys 1998, Northcutt 2011, Dicke and Roth 2016, Shimizu, Shinozuka et al. 2017). Forebrain’s size in birds is curiously linked with their population declines, invasion success and some other aspects of social cognition. (Healy and Rowe 2006). In some birds species (Molothrus bonariensis, M. rufoaxillaris, M. badius ) brood parasitic screaming and shiny cowbirds need better spatial memory to remember where host nests are and when they might be ready for parasitic eggs to be laid so they need larger hippocampus’s than non-brood parasitic bay-winged cowbirds to process these spatial information.(Reboreda, Clayton et al. 1996). Among other reasons of the birds’ brain enlargement is that they achieved several adaptations during the evolution of their flight that led to reduced metabolic costs.(Gill 2007)
In some mammals such as chimpanzees (Pan troglodytes, Papio anubis ) having a larger neocortex relative to their total brain size is associated with better spatial problem solving and tool use.(Pawłowskil 1998, Schmitt, Pankau et al. 2012) the largest anterior cerebrum volume (part of the frontal cortex)enable spotted hyenas (Crocuta crocuta, Parahyaena brunnea, Hyaena hyaena,Proteles cristata ) to solve a puzzle box that striped hyenas cannot, allows them to have the a more complex social system and hunt larger prey.(Sakai, Arsznov et al. 2011, Holekamp, Dantzer et al. 2015). (Moore and DeVoogd 2017)
However, the question still remains that whether animals with more complex cognitive abilities necessarily have larger brains or not. The relative size of brain does not necessarily link to complex behaviors. For instance, although the octopus’ nervous system is very complex and their neurons are even more than some mammals such as rats, (Hochner 2010) rats possess more complex cognitive abilities.
Furthermore relative brain size of anuran amphibians is almost equal with many reptiles and also in cartilaginous fishes it overlaps that of reptiles, birds, and mammals (Northcutt 2013)but it’s obvious that these radiations do not have the same level of behavioral complexity.