Science of Sex, Smell and Pheromones

13 February 2005
Presented by Chris Smith, Kat Arney


This show is about Cupids Chemistry - the science of sex, smell and pheromones - with Cambridge University olfaction and pheromone researcher Dr. Peter Brennan, who joins us to discuss how the nose picks up smells, Prof. Steve Jones, from University College London who describes why animals and plants have sex, Keele University chemist Dr. Graeme Jones who discusses the role of pheromones, and how ants and other insects use smell to find their way back to their nest, and Dr. Steve Yanoviak drops in from the rainforest canopy in Peru to talk about ants that can glide...

In this episode

How Long Does it Take To Fall in Love ?

It seems that not even falling in love is safe from the clutches of science. Demonstrating that scientists can be trendy, psychologists at the University of Pennsylvania in America have been investigating the phenomenon of speed-dating. After studying over 10,000 speed-daters they've come up with some surprising results. Some people say they're looking for one kind of person, then choose another. Other people say that don't even know what they're looking for. But the researchers found that, however it happens, people know love - and spot it quickly - when they see it. The scientists found that people generally understand their own worth on the dating market, so they are able to judge each others' potential compatibility within moments of meeting. During speed dates, the participants have three minutes to get to know someone. However, the American researchers found that most participants made their decision based on the information that they probably got in the first three seconds. Psychologists often tell us that relationships are a bit like shopping - people select mates based on the qualities they have to offer, such as power, money, nice car and so on. But the Pennsylvania scientists found that when people meet face-to-face, things like smoking preferences and bank accounts don't seem to figure. Somewhat surprisingly, factors that you might think would be really important to people, like religion, education and income, played very little role in a person's choice of who they would like to see again. It turns out that in a speed-dating situation, people use their hearts and go on first impressions, rather than using their heads to work out if it's a good idea. Ahhh, how romantic!

Dying of a Broken Heart

Researchers in the US have found that bad or shocking news really can break your heart. They found that after a trauma like this, people can suffer days-long surges in adrenalin and other stress hormones which "stun" the heart. This can even be mistaken for a heart attack. The researchers examined patients who came into hospital complaining of symptoms similar to those of a heart attack - chest pains, fluid in the lungs, shortness of breath and heart failure. But on closer inspection, these people - who were mainly older women - had no other signs of a heart problem. But all the patients had experienced some kind of severe emotional shock just before they got ill. For example, half had just learned of the death of a partner or relative. One person had been the victim of an armed robbery, while another had been the victim of a surprise party. And when doctors investigated further, they found that the patients had very high levels of stress hormones, particularly adrenalin and noradrenalin, in their blood. This was even higher than levels of these hormones found in genuine heart attack patients. The researchers also found high levels of another heart hormone, confusingly called brain natriuretic peptide. It's these stress hormones that can be toxic to the heart, say the researchers, effectively stunning it. Luckily, these broken hearts can be mended - using MRI scanning and other techniques, the doctors found that the damage caused by stress is temporary, usually lasting just weeks. So there's no need to cancel that surprise party just yet.

Horses Have Handedness

Knowing whether a horse is right or left handed might help you to beat the bookies in future, according to Irish researchers from the University of Limerick. Riders and trainers often report that their mounts respond better when turning or jumping in a particular direction, but whether this was just down to training, or an underlying innate cause, was unknown. To find out, Jack Murphy and his team studied 40 untrained horses destined to become show jumpers or dressage competitors. They watched which leg the animals preferred to use when stepping forward, and which direction they chose when detouring around obstacles or rolling over on the floor. The researchers found that females preferred the right sides, whilst male horses preferred their left. About 10% of the horses showed no preference. As a well-balanced mount is the most desirable, these results could help trainers and jockeys to develop their mounts' weak sides, and they could help punters better predict a horse's competition performance because the direction and bends in a race will suit some runners better than others. Bad news for the bookies !

What Is a Smell, And How Does The Nose Work ?

Chris - Your research is orientated towards the sense of smell and olfaction. What is a smell?

Peter - A smell is a chemical. The chemicals float around in the air around us and can tell us what is nearby. We have special receptors in the nose that can pick up these chemicals. This causes a response, which is then transmitted to our brain.

Chris - What about when there are lots of smells mixed up in the air? Your nose must get bombarded. How does the brain work out what it is smelling?

Peter - Coffee, for example, has around one hundred different odour components to it. When these go up someone's nose, lots of cells are activated. It's the pattern of these activated cells that the brain recognises as coffee odour. A slightly different pattern will show the difference between a normal coffee and an elaborate one.

Kat - I've noticed that food doesn't taste right when I have a cold. Is that related?

Peter - Despite what you would think, a lot of the flavour of food comes in through the nose. When you chew, chemicals are released into your mouth and some go up the back of your throat into your nose. Your tongue is not as refined at telling flavours apart, so your nose is an important part of tasting. When you have a cold, your nose gets blocked up and the chemicals from the food can't get through. This is why food can taste bland or different.

Chris - Why do dogs have such an acute sense of smell?

Peter - There are two ways in which a sense of smell can be better. One way is that the dog can be more sensitive and pick up smaller concentrations of a chemical, such as in a scent trail. The second way is to have more of the different types of receptors. This gives them a finer-grained ability, so they can tell things smell differently even when we might think they are same.

Chris - If you look at the brain of a dog, over a third of it is dedicated to smelling. This is in contrast to humans, which have sight as our main sense. Dogs really do live in a olfactory world.

Peter - Yes, and this is the same with most animals. Our sense of smell has actually been decreasing over evolutionary time. We have remnants of genes coding for these different receptors, but they don't work anymore. For that reason, they are called pseudogenes. Humans now have about 350 types of receptors. Rats might have about 1000 receptors picking up smells, which indicates the different smell worlds we live in.

Kat - I love the smell of my boyfriend's t-shirts. Does that mean that he's the man for me?

Peter - It could do. It's well known that people have different body odours, and it's these odours that dogs pick up on when trying to follow someone. It has also been suggested that people unrelated to you have particularly attractive body odours. Alternatively, when you get to know someone really well, you could just get to like everything about him, including the way he smells.

Why Do Plants And Animals Need To Have Sex ?

Steve - One thing about sex is that it is extremely expensive. It is particularly expensive for the female sex, as they do a very generous job of copying the male genes. Why do they do it? We know that a very tiny difference in the ability to copy your own genes can have dramatic effects.

So why do the females go into this altruistic business of copying somebody else's DNA ? The answer is we don't really know. There are a number of theories: one which is really popular is that it's like a Red Queen's race, which is like running up an escalator. Once somebody has started having sex, you've got no choice but to join in. If that somebody is a parasite or disease, once it takes up sex, it can reshuffle its genes very quickly. This is what sex is all about: recombination. If it can reshuffle its genes, then you as a host need to do the same thing otherwise it ill overwhelm your genetic defences. Therefore, we have a type of arms race going on. This may be as much an argument for the maintenance of sex as it is for its origin.

There's an interesting parallel between sex and DNA repair. DNA is actually damaged all the time. Many people think that mutations are rare and extraordinary things that hardly ever happen, but it isn't true. Even by the time people have finished listening to me talking, they will not be the same people. They will have undergone lots of changes and mutations; many of which will be very harmful. DNA is a very complicated chemical. To a chemist it seems almost impossible that it can maintain itself. Biology has come up with a very complicated system of repairing this damage with repair enzymes. A lot of this repair takes place when sex happens. Sex involves cutting and splicing DNA into new combinations and a lot of the mutational damage is fixed then. That may well have been how sex began.

There's an interesting spin on that, which is most the damage is actually done by males, because males make small sperm all the time. At each division, there is room for error, so most mutations happen in the male line. Females actually do a lot more DNA repair than males do, so it's the males' fault and the women put it right.

Chris - If we turned the clock back in time, were there males and females then, or was it something that came later?

Steve - I wasn't there at the time and it's fatally easy to speculate about what happened long ago, but it's very clear that you can imagine a universe that is entirely female. Indeed, there are plenty of species today that are entirely female. You can't imagine a universe that was entirely male as it's not one that would last for more than one generation. So I think it's pretty clear that for much of life, males weren't around. Thank god we made it!

Chris - But you don't think we'll hang around?

Steve - The way we're made could change, but once we're here, we're embarrassingly hard to get rid of. There are plenty of creatures out there, especially in the plant world, which historically had two sexes and have gone on to only having one. What's interesting is that in nearly all cases, those lineages without males are at the tips of the branches of the evolutionary tree. Once you give up males, you lose your ability to adapt and evolve quickly. Maybe our job in life is to help females to evolve.

- Pheromones, And How Ants Find Their Way Home

The Naked Scientists spoke to Dr. Graeme Jones, Keele University

Pheromones, And How Ants Find Their Way Home
with Dr. Graeme Jones, Keele University

Chris - When women live in the same house, such as with groups of students, they often find their menstrual cycles start to synchronise. Why is this?

Graeme - We don't know for sure, but it might be a chemical from under the arms that can influence menstrual cycles. An experiment was carried out where chemicals from a woman in the early stage of her cycle were put on the upper lips of other women. This caused the other women's cycles to shorten. When the same experiment was performed using chemicals from a woman late in her cycle, the cycles of the women in the experiment seemed to lengthen.

Chris - Why would it be advantageous for women to have their periods at the same time?

Graeme - That's a good question but no-one really knows. If females were all to come and be receptive at one time, then it would mean that males are more likely to pair up with individual females and there'd be less likelihood of males having more than one female partner. Alternatively, it could have something to do with the possible rearing of the offspring. This is very difficult to see in terms of human biology. No-one really knows.

Chris - Part of your work is on social structure in insects and how chemicals like pheromones can change how different insects behave. Ants are a classic example of this.

Graeme - Yes they are. When you look inside an ant colony, you can see the queen and the ant workers. The workers are divided into castes: you might have foragers that go out and find food and you might have nurse ants that stay at home and look after the young. One of the things we can do is distinguish between the queen and the different types of workers by their chemicals.

The chemicals themselves don't make ants do different jobs. The roles each ant performs in the colony seems to depend on how old you are. Those that go outside the colony are going to be mature individuals. As workers are expensive to make, you want to get as much work out of them as possible. When they are young, it's best to give them jobs inside the colony and keep them safe. Only when they are older and more dispensable does it make more sense to send them out.

Lots of work has been done on bees. In a social insect colony, it's basically just a lot of women living together. The queen produces a chemical that controls the reproduction of all the other individuals by turning off their ovaries.

Chris - Are there analogous genes in people?

Graeme - No. It's a totally different social structure. Social insects are very specialised.

Chris - When you find an ant in your house, do you kill it or study it?

Graeme - I tend to study it, although it's not good if you find ants in your house.

Kat - Can you get rid of ant trails altogether so they stop coming in your house?

Graeme - No, not really. When an ant finds some food, it leads a chemical trail back to the nest for others to follow. While there's food there, the ants will keep on reinforcing the trail. Therefore it's very hard to get rid of them once they're there. The trails remain.

If you took the back end of a few ants and made a trail to send them back outside, you might end up getting rid of them! The trails are just made in chemical glands in back of the ant. As it drags its bottom along the floor, the compounds are released. Sometimes have a sting which almost acts like a nib. Once on surface, the compounds start to evaporate into the air. The antennae can then pick up these signals. Antennae are like ant noses.

Chris - How do they know which way is home?

Graeme - It's all about the angles at which the trails are being laid. When you leave the colony, you probably walk for a bit and then split off into two directions. Whenever they split, they choose an angle, usually about 51 degrees. This means that when the ants are walking away from home, they always find angles at 51 degrees. When walking back the other way, the angle is completely different. This allows them to know which way they're going.

- Gliding Ants in Peru

The Naked Scientists spoke to Dr. Steve Yanoviak, University of Texas

Gliding Ants in Peru
with Dr. Steve Yanoviak, University of Texas

Chris - Your work is normally carried out at the top of trees. Tell us your story about how you have discovered some very interesting behaviour in ants living in the canopy.

Steve - I've done a variety of different studies over the years, but the most dramatic part occurred two years ago. I was trying to catch a mosquito, and so had to sit very still so they would come up to me to feed on my blood. While sitting there, I saw some ants nesting in the branch. They weren't happy with my presence and so started to crawl all over me and bite! As the ants were spiny, I brushed them off the branch rather than squashing them. Instead of falling straight down to the ground, they made a J-shaped trajectory that took them straight back to the tree trunk.

Chris - How do they do that?

Steve - Good question! About 85 out of 100 ants will do it, but the mechanism is still a mystery. What we do know is that they do it by sight. We tested this by painting nail varnish over their eyes. When they were stopped from seeing, they couldn't get back to the trunk.

Chris - Why would ants need to do that?

Steve - Ant workers are important to their colonies. They forage for food and bring it back for the rest of the colony to eat. If a worker goes out and gets lost or eaten, the colony incurs a direct cost because not only do they have less food but they must also replace him. If an ant falls from the canopy into the leaf litter, it will have travelled the distance of about 3000 body lengths. From this distance, it may be difficult to find the chemical trails to lead them back again. Being able to fly back to your own leaf or tree trunk is therefore very advantageous.

Chris - How did you know it was the same ant that landed on the tree?

Steve - We painted the ants white so we could spot them on a video tape. This showed us that once they had fallen, it took on average about ten minutes for them to get back.

Chris - Where is this going to lead now?

Steve - It will go in a couple of directions. We want to find out some of the more specific details and the biomechanics behind it. How are they making the turns? How many different species of ants do this? What are the evolutionary origins? We are also going to look at other insects, including those in the Asian and African tropics.

- I lost my sense of smell. Can I get it back again?

I lost my sense of smell. Can I get it back again?

I lost my sense of smell. Can I get it back again?

There are various ways in which you can lose sense of smell. For instance, people can lose their sense of smell if they are allergic to something. These people can often regain it if the object they are allergic to is removed. In other people, events such as car accidents can cause a person to lose certain connections in their brains and stop being able to smell. It can sometimes come back after long period of time, but this usually depends on the type of accident the person was involved in.

- Do gay men respond to pheromones from other men, as men do from women and women from men?

Do gay men respond to pheromones from other men, as men do from women and women from men?

Do gay men respond to pheromones from other men, as men do from women and women from men?

It's still very controversial. We are not entirely sure whether 'the chemistry of love' even applies to humans. Even if we do send off pheromones, it might be hard to detect them under all the perfumes we use. Humans spend billions of pounds on aftershave and perfume each year.

- If smell is so important, how do we smell each other through deodorants and perfume?

If smell is so important, how do we smell each other through deodorants and perfume?

If smell is so important, how do we smell each other through deodorants and perfume?

Part of it might have been that pheromones were useful in the past when we didn't wash every day. It isn't clear whether by washing, we remove all these chemical cues. One of the main pheromone interactions is between men and women. No-one knows whether women are turned on by male pheromones or repulsed! Females may be repulsed at certain times of the month but attracted when it is particularly good to mate. Some research suggests that now many women are on the pill, they might be responding in a different way to pheromones, or even be desensitised.

- Why are more people shaving their public hair now?

I'm sure that when I was younger, personal hygiene wasn't such a big thing, especially shaving body hair.

Why are more people shaving their public hair now?

One of the functions for the hair under our arms is to act as a wick. They soak up some of the smelly chemicals under our arms. These chemicals have been shown to influence the timing of the female menstrual cycle.

- When a woman gets pregnant, why do some men suffer false pregnancies?

When a woman gets pregnant, why do some men suffer false pregnancies?

When a woman gets pregnant, why do some men suffer false pregnancies?

I've seen this once with a man who had funny stretch marks on his stomach. His wife was pregnant, and it seems he was showing sympathetic changes with the female body. We know women can affect other women, but the evidence that women can change men's behaviour is a bit controversial. It's certainly possible that the hormonal changes will affect the man, but we don't know if men can pick up the chemicals. Men and women are very different, so it's hard to see how such effects can happen. The answer is no-one really knows!


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