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.