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>> The Role of Pheremones in Sex and Aggression
Living organisms use a variety of means to communicate with
one another. The bared teeth of a growling dog, the cry of
a startled bird, and the wail of an infant all send clear
messages, such as "back off," "danger,"
or "help me!" Likewise, many organisms use pheromones
to signal sexual receptivity or territoriality. For example,
when a female cat is in estrus, tomcats can smell the pheromones
that signal a readiness to mate; similarly, dogs micturate
along fences, trees, and other objects in their territory
to mark the area as "occupied."
How such signals are perceived is an area of active research
among psychological scientists, who have studied the effects
of pheromones on behavior through the use of "knockout
mice"-laboratory animals specially bred to eliminate
a specific gene or genes deleted from their DNA. Scientists
have designed all manner of knockouts; some have a predisposition
to alcohol while others an aversion; some are prone to kidney
or skin diseases. Knockout mice provide an important tool
for investigating diseases in humans.
Dr. Catherine Dulac at Harvard has used knockout mice to
study how a specific brain center called the vomeronasal organ
(VNO) controls rodent sexual behavior. Dulac and her colleagues
found that they could create a knockout mouse that was missing
an important functional pathway in the VNO known as the TRP2
ion channel. Her knockout mice were therefore normal except
for a dysfunctional VNO. How would these knockout females
react to a normal, sexually-mature male?
As it happens, gender recognition is largely based upon olfactory
cues that are present in the urine of adult males and females.
These pheromones are detected through the VNO. Under normal
conditions, a male mouse will react aggressively when a second
male is introduced into its established territory. An attack
by the aggressor is typically preceded by rigorous tail rattling.
Thereafter, the aggressor typically lunges toward the other
male, biting the victim's flanks and/or hindquarters. By contrast,
a male mouse will not attack a female that is introduced into
its territory, but will instead attempt to mate with her.
Dr. Dulac's findings are demonstrated in a series of short
video clips, presented here. In the first research condition
a normal male (left) is paired with a castrated male that
produces no pheromones.
Video reproduced with permission of Dr. Catherine Dulac and
AAAS.
Video reproduced with permission of Dr. Catherine Dulac and
AAAS.
The castrated mouse shown in the first condition provokes no
response from the resident male, suggesting that something other
than the mere presence of the mouse signals a territorial threat
to the intact mouse. However, in the second condition, the resident
male attacks the castrated intruder carrying intact male urine.
While the mice are the same in both trials, trial 2 differs
in that the castrated male intruder is carrying normal male
pheromone. These two experiments suggest that sexual identity
is signaled not by vocalizations, behavior, or appearance in
male mice, but by pheromones instead. In the absence of the
male pheromone, the resident feels no need to defend his territory.
In the third research condition, a TRP2 knockout resident
male mouse is introduced to a castrated intruder that has
been swabbed with urine from an intact male mouse. Will the
knockout mouse defend its territory? In fact, a knockout mouse
may react to a male intruder in a fashion not typically shown
by intact males.
Video reproduced with permission of Dr. Catherine Dulac and
AAAS.
Rather than attacking the intruder, the TRP2
knockout attempts to mount the other male. Moreover, the knockout
mice also emitted ultrasonic courtship vocalizations toward
male intruders. Males typically make such vocalizations only
during encounters with females. Dulac and her colleagues interpret
these findings to mean that without the TRP2 ion channel,
the VNO of the knockouts was unresponsive to pheromonal cues;
thus, the knockouts were unable to discriminate male from
female intruders. This interpretation was supported by the
observation that neuronal activity in the VNO that is triggered
in response to male or female urine was not observed in the
VNO of knockouts. Furthermore, the investigators suggest that
the most adaptive strategy for the knockouts to adopt during
encounters with a strange mouse of indeterminate gender was
to initially treat all intruders as females.
Discussion Questions:
Web Links:
To read the original article in Science Daily on
Dulac and the TRP2 gene, visit www.sciencedaily.com/releases/2002/02/020201074842.htm.
Two brief summaries of the work of Catherine Dulac are available
online:
The Howard Hughes Medical Foundation: www.hhmi.org/news/dulac.html.
The Harvard Gazette online: www.news.harvard.edu/gazette/2002/02.07/01-mice.html Primary References:
To read more about pheromones and menstrual synchronicity,
see K. Stern and M. McClintock, "Regulation of ovulation
by human pheromones," Nature 392 (1998): 177-79.
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