Mouse Dating Game
“It turns out that male mice do actually belong to two kind of categories: the dominant and the subordinate. The dominant male mouse will in fact bully, scratch and push around the subordinate male mice,” says neuroscientist Samuel Weiss, director of Hotchkiss Brain Institute in Canada.
“Females choose dominant mice, male mice, over subordinate male mice in order to mate with the dominant male mouse so that more of her babies will be dominant as well,” explains Weiss.
Weiss and colleagues wanted to find out the mechanism behind female mouse preference for alpha males.
Weiss wondered if female mice’s choice of dominant males had something to do with pheromones, chemical signals found in the urine of male mice. (Pheromones have been found in mouse tears as well by other researchers.) He says that dominant mice and subordinate mice have what he calls different “pheromonal signatures.”
First, Weiss took virgin female mice and exposed them for a few days to a mix of the bedding from both dominant and subordinate mice, because the bedding contains the male pheromones. He found that that had no effect on the brains of the females.
Weiss then exposed the females to either the dominant or subordinate pheromones.
“When females were then exposed to dominant male pheromones there was a huge spike in the production of new brain cells (neurogenesis) in both the olfactory bulb and in the hippocampus,” Weiss says. “While if they were exposed to subordinate male pheromones, no change in neurogenesis.”
The olfactory bulb is where the sense of smell is mediated, and the hippocampus stores new memories.
The researchers then proceeded to ask “If they have this increased neurogenesis, what about a month later? When they’re exposed to either a dominant or a subordinate male mousewill they know the difference?’” says Weiss.
As they reported in Nature Neuroscience, the group of females that had previously been exposed to alpha male pheromones chose the dominant mice. The female mice that had been exposed to subordinate male pheromones did not show a preference for either dominant or subordinate.
“This suggested that there was a link between exposure to dominant pheromones, production of new neurons in these two structures, the formation of a memory of the dominant male pheromone, and then the ability to select later on a dominant versus subordinate male,” Weiss says.
For the final part of the experiment, the researchers interfered with the production of new brain cells in females when exposed to dominant pheromones. Without the new brain cells, the females had no memory of the dominant pheromones and, later, when exposed to dominant and subordinate male mice, showed no preference.
“These findings together suggest that pheromones can spark the production of new brain cells, which form a memory of the dominant male pheromone, which will ultimately allow the female to choose a dominant male over a subordinate male,” says Weiss.
Weiss and his colleagues found that the alpha male pheromones triggered the female mice to produce certain female hormones.
“Two of those hormones are prolactin and luteinizing hormone, which then in turn go through the blood and cross back into the brain and that’s how they increase brain cell production,” he says.
Could similar mechanisms be occurring in people?
“Given that the same brain cell growth [olfactory bulb and hippocampus] goes on in humans, the same hormones are in humans, it seems reasonable to suggest that there may also be a link between those hormones and brain cell growth in humans,” says Weiss, adding that this hasn’t been tested yet.
As to whether people’s choice of mate might have something to do with pheromones, Weiss says that’s a bit of a stretch without evidence, but he wouldn’t rule it out.
“At some point down the road, functional magnetic resonance imaging (fMRI) may actually allow us to understand whether or not certain parts of the brain will light up when exposed to one pheromonal signature or another,” he adds.
PUBLICATION: Nature Neuroscience, July 1, 2007 online
AUTHORS: Gloria K. Mak, Emeka K. Enwere, Christopher Gregg, Tomi Pakarainen, Matti Poutanen, Ilpo Huhtaniemi and Samuel Weiss
RESEARCH FUNDED BY: Alberta Heritage Foundation for Medical Research, the Canadian Institutes of Health Research and the Canadian Stem Cell Network
Footage of martial artists in this news story was taken at Danzan Ryu NYC Jujitsu.
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