“Absolute skull size in mammals principally increases until fully grown.” ~ American zoologist Scot LaPoint
“Postnatal size changes in most vertebrates are unidirectional and finite once the individual reaches full size.” ~ zoologist Javier Lázaro
Red-toothed shrews are common in Eurasia and North America. They have long, pointed snouts, and tiny ears which often cannot be seen. Poor eyesight has these wee creatures rely upon smell and hearing to hunt prey: mainly insects, worms, and grubs. Some shrews use echolocation, which is a cognitively demanding way to create mental images from a montage of audible feedbacks.
These shrews have a high metabolic rate, and scant fat storage. They must eat 80–90% of their body weight daily to stay alive.
Shrews cannot migrate to avoid winter, nor go into hibernation to save energy. The only adaptive recourse is to shrink.
“Brain tissue is energetically very expensive.” ~ Javier Lázaro
In anticipation of winter, shrew bodies shrink ~18%, and dramatically rebound in spring. Shrew brains also correspondingly shrivel as the cold comes on, and partly regrow in spring. This dynamic is known as Dehnel phenomenon, after its 1949 discovery by Polish zoologist August Dehnel.
Despite shrunken brains, these voracious hunters lose no mental acumen, as they must continue to eat their way through winter, so they can breed the following summer. If the brain were the organ of mentation, Dehnel phenomenon would be debilitating. But that does not happen. When a brain exists, it is a mere physical facsimile for the mind.
(Common shrews may live to 18 months, but mortality is high, as shrews are on the menu of owls, birds of prey, foxes, and other carnivores larger than shrews.)
“The brain is not an organ of thinking but an organ of survival, like claws and fangs.” ~ Hungarian physiologist Albert Szent-Györgyi