Women often outperform men in odor-sensing tests.
The study was led by a team from the Federal University of Rio de Janeiro in Brazil, where another group of researchers had developed the "isotropic fractionator" - a fast and reliable method of measuring the number of cells in a given brain region such as the olfactory bulb.
The olfactory bulb is the first region of the brain to receive signals about odors sensed via the nostrils. The ability to discriminate among odors and scents varies widely among individuals. Also, studies show marked differences between men and women, with women often outperforming men in many kinds of odor-sensing tests.
There are theories that sex differences in smell are due to cognitive and emotional influences rather than perceptual ability.
Previous studies that have looked for biological explanations for women's apparent superior sense of smell have used brain scans to look for structural and volume differences. These have led to mixed results and left many questions still unanswered.
So Roberto Lent, a professor in the Institute of Biomedical Sciences at Rio's Federal University, and colleagues set out to measure the biological evidence more directly - by counting the number of cells in women's and men's olfactory bulbs.
To this end they examined post-mortem brains from seven men and 11 women who were all healthy and aged over 55 when they died. None of the subjects had worked in jobs that required them to have exceptional sense of smell such as cookery or coffee-tasting.
Study found women's brains have up to 50% more olfactory neurons
Using the isotropic fractionator, the team calculated the number of cells in the olfactory bulbs of these individuals and found that, on average, the women had 43% more cells in this brain region than the men. When they included only neurons in the count - that is leaving out other cells like glial or structural cells - this figure went up to nearly 50%.
The olfactory bulb is the first region of the brain to receive signals about odors sensed via the nostrils.
Image credit: Roberto Lent
The authors acknowledge that just finding this difference is not enough to prove that women have a superior sense of smell - it is not even enough to explain the findings of previous studies about differences in ability to differentiate, identify and remember scents and odors. However, Prof. Lent suggests:
"Generally speaking, larger brains with larger numbers of neurons correlate with the functional complexity provided by these brains. Thus, it makes sense to think that more neurons in the female olfactory bulbs would provide women with higher olfactory sensitivity."
Since the brain does not accumulate many more cells as we grow, it would seem that women are equipped with these extra olfactory cells from the day they are born.
There are still many questions to explore, including why women should have this ability hard-wired into their brains, and what mechanism produces this greater quantity of olfactory cells in the female brain.
One theory is that a superior sense of smell helps mother and child to bond after birth; another is it also influences females' selection of potential mates.
In October 2012, Medical News Today learned of another PLOS ONE study from the University of Pennsylvania that suggested losing sense of smell could be an early sign of Parkinson's or Alzheimer's disease, or another neurodegenerative disorder.