The scent of a man makes lab mice nervous

Scientists’ inability to replicate research findings using mice and rats has contributed to mounting concern over the reliability of such studies.

Now, an international team of pain researchers led by scientists at McGill University in Montreal may have uncovered one important factor behind this vexing problem: the gender of the experimenters has a big impact on the stress levels of rodents, which are widely used in preclinical studies.

In research published online April 28 in Nature Methods, the scientists report that the presence of male experimenters produced a stress response in mice and rats equivalent to that caused by restraining the rodents for 15 minutes in a tube or forcing them to swim for three minutes. This stress-induced reaction made mice and rats of both sexes less sensitive to pain.

Female experimenters produced no such effects.

“Scientists whisper to each other at conferences that their rodent research subjects appear to be aware of their presence, and that this might affect the results of experiments, but this has never been directly demonstrated until now,” says Jeffrey Mogil, a psychology professor at McGill and senior author of the paper.

The research team, which included pain experts from Haverford College and the Karolinska Institutet in Sweden and a chemosensory expert from Université de Montreal, found that the effect of male experimenters on the rodents’ stress levels was due to smell. This was shown by placing cotton T shirts, worn the previous night by male or female experimenters, alongside the mice; the effects were identical to those caused by the presence of the experimenters, themselves.

Further experiments proved that the effects were caused by chemosignals, or pheromones, that men secrete from the armpit at higher concentrations than women. These chemosignals signal to rodents the presence of nearby male animals. (All mammals share the same chemosignals).

These effects are not limited to pain. The researchers found that other behavioural assays sensitive to stress were affected by male but not female experimenters or T-shirts.

“Our findings suggest that one major reason for lack of replication of animal studies is the gender of the experimenter – a factor that’s not currently stated in the methods sections of published papers,” says Robert Sorge, a psychology professor at the University of Alabama, Birmingham. Sorge led the study as a postdoctoral fellow at McGill.

The good news, Mogil says, is that “the problem is easily solved by simple changes to experimental procedures. For example, since the effect of males’ presence diminishes over time, the male experimenter can stay in the room with the animals before starting testing. At the very least, published papers should state the gender of the experimenter who performed the behavioral testing.”

The material in this press release comes from the originating research organization. Content may be edited for style and length. Have a question? Let us know.


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10 thoughts on “The scent of a man makes lab mice nervous”

  1. This study has been carried out very well, but in order to fully understand how scent affect these animals I believe more tests should be done. The stress that the mice or rats feel have been limited to male and female scents. What would happen if the male experimenter sprayed himself with a floral woman scent? Would this decrease the stress? This study can also be conducted where researchers are told not to put on any antiperspirant or body spray and the results are recorded. Whether the results are the same or not, woman should wear something of a male partner and visa versa to see the animals’ reaction. Each persons body and sweat reacts differently to antiperspirant, so each person has an individual smell.

    I believe the results from this testing could possibly show what specifically has the animals panicking when men are around. We as humans also rely on smell somewhat to give us an initial “feel” for a person. Have you ever seen someone and you were afraid immediately? I think smell has the same effect. When a person has lost the use of a specific sense, the others become stronger. I believe a blind person can judge a persons character firstly through smell, then feel and talking to that person. Humans are able to hide their initial reaction to a certain smell, whereas animals can not. as the animal grows used to the smell, they calm down. Humans do the same.

  2. This is a very interesting article. It draws attention quickly because of the topic and it can sort of relate on humans as well.
    I recently found another article that stated: ‘A scientist and his team gave pain-inducing injections to anaesthetised mice and rats. When the animals awoke, the team recorded their facial grimaces, a measure of pain intensity. When a male investigator sat in the room with the rodents, they grimaced 35 per cent less, on average, than when no one was in the room. There was no significant grimace difference when a female investigator sat in the room than when it was empty. The reduced sensitivity to pain shown when the men were present is a common side effect of stress, as it allows for the preparation of the fight or flight response.”

    Does this effect of gender in mice then have effects on other studies that involves mice? How accurate can studies on mice and rats in labs be?

  3. This article was found to be rather reliable, than invalid results. As in this experiment, it was tested – Whether the gender of experimenters does play a role in the rise of stress levels of rodents. The results that were found suggests that the main reason for the lack of replication was due to the gender of the experimenter. This article is found to be reliable, because the gender was kept as the controlled variable, and the same results were found either or.

    This is really something to be on the lookout for when doing further studies with animals, and the impact man has on them.

  4. This research is hugely concerning because of the uncertainty it places on the validity of past experiments done with mice. It also demonstrates the importance of remaining unbiased during experimentation. This article states that the effects of men on mice have been suspected in the past, however researchers were happy to ignore their suspicions and thus skew the data based on results they expected to get. Eliminating bias eliminates the potential for skewed results. Accurate results in this case are particularly important because mice are the most frequently used lab animal for testing in the medical field. This is because their physiological responses are thought to be most similair to humans’. Factors such as stress can cause many changes, for example shifts in hormone levels, which could greatly affect outcomes of drug tests done on mice. In addition, reduced pain could lead to failure of detection of negative drug effects. Experiments are also not reliable if researchers of different genders worked with the mice – this would be an unfixed variable which would impact the respective results differently. When doing research, accuracy, validity and reliability should be of utmost importance to scientists, particularly when doing experiments which will have applications to human health.

  5. If the findings of this article are correct then how accurate are most of the experiments performed by males on lab rats and mice?. It is very interesting how lab rats and mice respond to male experimenters and yet most of the experiments carried out in the laboratories are carried out by male scientists, chemists and biologists and most of this experiments have been beneficial to the human race.

  6. This is not very surprising due to the differing levels of pherormones secreted by males and female. Since the problem is stress induced by the introduction of male pheromones could the introduction of counteractive hormones or chemicals be applied to the mice or rats before the experiment to eliminate the contaminant?

  7. I find it rather surprising that something such as the gender of an experimenter could have such an effect.On the other hand it is also quite concerning as the rodents responded differently to the same drug depending on who the experimenter was.This means that the results that were obtained were not entirely accurate and these are the clinical trials we rely on to obtain our medicine.However I am also fascinated with what further research could be done on the effectiveness of drugs depending on patients stress levels

  8. I found this article rather interesting as i have recently done research on why animal testing is not sufficient to predict human responses. This is a classic example thereof. The rodents exhibit fear and stress when smelling anything male, their reactions to the experimenter is therefore unreliable, as we have seen that the smell of another male does not make the human species stressed.
    You might want to consider looking into the project about the ‘organ-on-a-chip’ as this will remedy the problem that the animal testing poses with regards to inaccurate reactions.
    aside from these points, i found the article very interesting, as i have often wondered how the various synthetic perfumes affect the animal species of the world. You might want to consider looking into how various smells affect pets in the household.

  9. I think that this discovery could be the reason why certain, previously tested , drugs have yielded negative side-effects in experiments which humans.Since the male scent stress the rodents, leading to pain suppression the rodents, the effects of the drugs being tested on rodents could drastically be altered . The gender of the experimentor can now be considered a vital variable in experiments which can controled. A question which can be considered is: How does the stress levels of a patient alter the effectiveness of drugs?

  10. More awareness should be made concerning this finding as the gender of the experimenter has serious implications on the validity of the results.

    The stress levels which cause the rodents to suppress their pain, will negatively impact the drug trials done on rodents to test for new human medication. As the response of the rodents to the same drug varies depending on the gender of the experimenter, the results become more varied with new each experiment leading to inconclusive data and inaccurate results. This negative impact is time consuming and slows the progress of scientific research.

    Therefore I agree that stating the gender of the experimenter is an important variable to control in order to produce accurate results.


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