They also said the findings from their study of humans and 127 other mammal species offer further proof that breastfeeding is good for brain development and support the World Health Organization's recommendation that babies should be breastfed exclusively for their first six months of life, followed by continued breastfeeding (along with other foods) up to age 2 or longer.
The study was published in this week's issue of the Proceedings of the National Academy of Sciences.
The researchers found that brain size relative to body size was most closely associated with maternal investment; the length of time a mother spends carrying her offspring in pregnancy and how long a mother breastfeeds her young.
The length of pregnancy determines offspring's brain size at birth and the amount of time spent suckling affects brain growth after birth.
For example, humans have nine-month pregnancies and breastfeed their babies for up to three years. This is necessary to fuel the growth of the brains, which have an average volume of 1,300 cubic centimeters (cc) in adults. Fallow deer have about the same body weight as humans but are only pregnant for seven months, followed by a suckling period of up to six months. Their adult brain size is 220cc, six times smaller than the human brain.
"We already know that large-brained species develop more slowly, mature later and have longer lifespan, but what has not always been clear is why brains and life histories are related," lead investigator and anthropology professor Robert Barton said in a University of Durham news release.
"One theory is that large brains increase lifespan by making the animal more generally flexible in its behavioral responses to unpredictable challenges, permitting slower life histories. However, our findings suggest that the slow-down in life histories is directly related to the costs rather than the benefits of growing a large brain. The necessary benefits to offset these costs could come in other ways, such as improving specific perceptual and cognitive abilities, rather than through some generalized flexibility," he explained.