Archive for the ‘Human interaction’ Category

Research update

June 2, 2011

Science is ever moving and ever changing, and we’re always finding new things. In this article I’m revisiting some of my past topics with some recent research.

The structure of skin
One of my first articles on thatscienceguy discussed the structure of human skin and how the sun’s radiation affects it. Skin is the largest organ in the human body, and has a critical role in protecting our body from external threats and stopping excessive water loss. The outermost layer, called the stratum corneum, can actually act like a sponge and absorb quite large amounts of water depending on the humidity of the surrounding environment. This ability to absorb water means it needs to be quite flexible, however it needs to balance this flexibility with being robust enough to be able to protect the deeper layers of skin and organs underneath.

Researchers from the Australian National University examined the structure of the stratum corneum to try to understand how its structure allows these dual roles. They found that the keratin filaments which provide skin its structure have a remarkable three-dimensional weave which allows individual fibres to wind and unwind. While the fibres can individually wind and unwind, they do so cooperatively to allow the stratum corneum to breathe without losing structural strength.

The weaving of keratin in its condensed form. From Evans M.E. and Hyde S.T. 2011

The weaving of keratin in its expanded form. From Evans M.E. and Hyde S.T. 2011

Male motor skills
The study which I rated as the strangest of last year investigated the perfect male dance moves to attract women – they even produced videos which demonstrated these moves. Needless to say, it was quite a popular topic!

I explained the importance of that study by likening it to courtship displays by other animals – the males will put on a ‘show’ to demonstrate their prowess to the female, and in the case of humans, dancing may well be one of our courtship displays. But the question remained why exactly do animals (including humans) put on these courtship displays, what exactly are they exhibiting?

Studying the manakin bird, researchers from the University of California Los Angeles found that the female birds preferred to mate with males who performed the courtship display at greater speeds, and were able to tell differences measured in the milliseconds. The speed and energy exertion required by the male to do this courtship display means they have extremely fast heart rates. From this the authors suggested that the courtship display is actually a demonstration of the male’s motor skills, coordination and cardiovascular qualities, and so being able to do it faster shows that the male is stronger and has better quality genes.

And for those wondering what the manakin bird is, from QI:

Sexual attraction
Back at the start of April I blogged about the science of sexual attraction, and in the intervening two months new research has been released which is worth examining.

In the original articles I wrote about the effects testosterone has on males and their attraction to women, and attractiveness to women. Now, new research has shown that men who have higher testosterone are flirtier. Remembering back, testosterone is important for competition between males, so researchers increased men’s testosterone levels by making them compete in computer tasks. The men who showed the highest increases in testosterone as a result of the competition subsequently showed more interest in the woman, made more eye contact with her, smiled more and talked more about themselves. So the testosterone increases induced by competition makes men more attentive to women – maybe this means the best plan before a big date is to do something competitive.

Males have also been found to be able to distinguish whether a female is fertile just from looking at her face. Back in the original articles I wrote how oestrogen levels, which rise during ovulation, slightly change the shape of a woman’s face, making it more rounded and considered as slightly more attractive. Using macaques (a species of monkey), new research has shown that men can recognise these signs of a female’s fertility, but only in faces they are familiar with. Researchers showed male macaques images of females faces which had been classified as being pre-ovulation, during ovulation, or post-ovulation (they found these stages out from measuring the female’s hormones). The male macaques were able to tell the difference between the faces of during ovulation and pre-ovulation, however they could only tell the difference if they were familiar with the particular female. When showed images of an unfamiliar female, they couldn’t tell the difference.

Little is known about the molecular reasons for sexual preference, but research published recently in Nature has investigated how chemicals in the brain may affect who we find attractive. Serotonin, also known as 5-HT, is known to have a huge effect on mood – in fact the most common drug treatment for depression works by making serotonin last longer in the brain. The researchers found that male mice normally prefer female over males as mates. However, when the same breed of mice was modified to make them unable to produce 5-HT, the males lost their sexual preference. When these mice had their 5-HT production restored to normal levels they regained their preference of females over males. This research is the first to show that 5-HT may be involved in sexual preference, and raises the question of whether other brain chemicals are involved in sexual preference.

Lapping dogs
And finally, another update from the strangest research studies from last year, this time the study which examined how cats drank. They found then that cats used the back of their tongues, skim over the surface of the liquid, and then pull rapidly upwards into their mouth. The surface tension would lift the liquid with their tongue straight into their mouth. This seemed much more refined than the simple scooping method that dogs use.

But do dogs really just scoop liquid? It turns out that comment was premature, as new research has now found. Using high speed video it has now been found that dogs too use a very similar method as cats, picking up liquid with the back of their tongue and relying on surface tension and inertia to keep the liquid in place. The liquid travels with the tongue through the oral cavity into the oesophagus, with the tongue then pressing up against the roof of the mouth to prevent the liquid from falling out.

You can see it all in action in these videos:

This is a 300 fps video of a dog lapping. It seems to show spooning of liquid into the mouth but X-ray video tells a different story. From Crompton A.W. and Musinsky C. 2011

This video shows that, contrary to published accounts, dogs do not scoop liquids into their mouths with a spoon-shaped cavity that forms in the ventral surface of a backwardly directed tongue tip. As in cats, an aliquot of liquid adheres to the dorsal surface of the tongue tip and is transported into the oral cavity as the tongue is rapidly withdrawn. From Crompton A.W. and Musinsky C. 2011


Sexual attraction – Part 2

April 7, 2011

Last post talked about the effects that the sex hormones oestrogen (females) and testosterone (males) have on attractiveness to the opposite sex. While the do have a major role in assisting our search for appropriate mates, there are other factors.


Physical features

The effects of sex hormones on facial features has already been described, however it was believed for a long time that the waist-hip ratio was a prime determining factor for measuring attractiveness of women. Body fat is an indicator of fertility – too little or too much reduces fertility, possibly explaining why very thin or overweight people are not considered generally attractive. A ratio between waist and hip size of 70% has been thought of as optimal. However, waist-hip ratio is an overly simplistic way of determining physical attractiveness, and it is now considered to be a combination of 25 measures which describe physical attractiveness, one example being leg length.


This idea of physical measurements determining attractiveness is not confined to humans. The female widow bird for example will preferentially seek a male with a longer tail. In fact if the male birds’ tail is artificially shortened, they will then be less attractive to females.  So while there are thought to be around 25 measurements that define human attractiveness, the widow bird’s tail is the major determinants of their attractiveness.


Gene matching

In nature there are instances where animals will preferentially seek out animals which have similar genes. The Gouldian finch is found with one of a number of head colours, and when it comes to mating will prefer to partner with another animal of the same head colour. In fact, if it mates with an animal of different colour, the bird will get rather stressed out about it. This mechanism exists to maintain a proper balance in the gender of the offspring, mating with a different head-colour bird will produce more male offspring, so this distaste for mating with different coloured birds has evolved to maintain proper population size and gender ratios.


In humans it is somewhat similar; we tend to look for someone who is similar and not too different from ourselves as that means our genes will be a good match. However, we also look for someone who isn’t exactly the same, there is a balancing act involved, and there are several mechanisms to help that. For example, part of the immune system is called the major histocompatibility complex, or MHC, which is slightly different from person to person. Women can actually (subconsciously) detect the MHC-type of a male, and will preferentially choose a male who has a slightly different MHC than their own. This is for two reasons, firstly some similarity means the two people are genetically similar, and secondly, a slightly different MHC will mean their immune system is slightly different, giving offspring a potentially greater variety in their immune system. Strangely however, women on the pill are unable to pick up a male’s MHC.


These features which are attractive between men and women are also present between homosexual couples. What straight men look for in women, gay men look for in men. Similarly, gay men actually prefer the smell of gay men over the smell of a straight man, so there does appear to be a difference in pheromones.


These olfactory mechanisms play an important part in maintaining genetic variability by acting as an incest avoidance system. There are several mechanisms which exist, not only in humans, to maintain genetic variability by effectively finding someone closely related ‘unattractive’, including the MHC-type detection. Again, humans look for someone who is similar but not the same, and different but not wildly different, so these mechanisms exist to maintain that divergence.


Love and sexual attraction can be hard to define and quantify, and a large reason comes down to the physiological mechanisms which have evolved over many generations and have become ingrained. Cues such as hormonal cycles which subtly change the appearance and attractiveness of someone, subconsciously assessing someone’s fertility, or detecting the MHC-type of a partner play a role which we never notice and never consider. But nevertheless they’re there and while other factors such as personality play a role, inevitably it’s these unconscious reasons which will determine whether or not we find someone attractive and an ideal mate.


Thanks go to Bill von Hippel from the University of Queensland and Rob Brooks from the University of New South Wales

Sexual attraction – Part 1

April 6, 2011

Apologies for the late posting of this fortnight’s articles, I needed to wait for an embargo to lift prior to publishing.

Finding a mate is one of the most important tasks for every organism which reproduces sexually. While people say they have different ‘tastes’ in an ideal mate, in fact there is a large amount of commonality between what humans define as attractive, and at the risk of taking the ‘fun’ out of finding a mate, there are specific physiological reasons for sexual attraction.


Attractiveness may have hormonal reasons

One of the major hormones in the reproductive system of women is oestrogen, with the male equivalent testosterone. As well as having vital roles in regulating the reproductive system, they also affect the attractiveness of a person.


A female with high oestrogen levels will usually have a rounded face and large eyes, two factors which are generally regarded to increase attractiveness. When an audience is shown 2 computer generated faces with one having these features but otherwise identical to the first, nearly 70% of people find the “high-oestrogen” face more attractive, with the proportion of women and men who find it attractive being around equal.


Humans aren’t the only animals to have facial features altered depending on fertility state. The face of female rhesus macaques (a type of monkey) darkens when they are fertile, with the changes similarly linked to hormonal cycles. Interestingly, males can actually recognise the fertility states of female partners from these features. The more familiar a male is with a particular female increased their knowledge of fertility states from these cues.


Similarly, a male with high-testosterone will usually have the attractive square jaw and angular face, and when an audience is shown computer generated faces and asked to choose which is more attractive, the “high-testosterone” face comes out as the preferred face, again by nearly 70% of the people. Interestingly however, while the same number of men and women found the “high-oestrogen” face attractive, men were less likely to find the “high-testosterone” face more attractive than the “low-testosterone.”


There is an evolutionary reason for an increased attractiveness as a result of high levels of sex hormones. Women with high oestrogen have high fertility levels, and so are desirable mates due to an increased ability to produce offspring. Similarly, testosterone levels in men are a sign of high quality genes, which are sought after to increase the chances of offspring survival (remember all these processes evolved long ago when living conditions were much harsher). Testosterone in men actually acts as an immunosuppressant – it reduces the activity of the immune system. While this may seem a disadvantage, the ability of the male to have survived to procreation age shows that they must be genetically strong to have been able to overcome any infections with the reduced immune system. For these reasons, high sex hormone levels are considered to be desirable traits in a mate.


Supporting this idea that high testosterone is a sign of genetic strength, when women ovulate they tend to become even more attracted to the high-testosterone males, showing there is an evolutionary mechanism to try to choose mates with ideal genetic characteristics.


Although testosterone is a sign of genetic strength, it does however also increase some undesirable traits when choosing a mate. High testosterone does increase aggressiveness, making the male less nurturing towards offspring, and also increases the male’s desire for what is called sexual novelty; in effect the male will be more likely to be unfaithful as they tend to seek different sexual partners. So for women there is a trade-off to be made between high and low testosterone, to balance the advantages and disadvantages, and it may also explain why some women in particular seem to like “bad boys”, they would like have high testosterone levels. For men however, there is no trade-off, they will generally just seek high-oestrogen (and hence fertile) females.


For these reasons, women may tend towards a lower testosterone male for the reasons of producing offspring as although the offspring may be of lower genetic quality, they may be more likely to be provided for by the father. However, often testosterone levels in men will drop after producing offspring – a mechanism to reduce aggressiveness, promote nurturing behaviour, and increase faithfulness to a single partner. Women’s preferences also tend to change slightly with age, with older females more likely to prefer lower testosterone males.


As these effects have been developed due to evolutionary pressures and are linked to genetics and hormones, they are consistent across cultures. No matter where people come from, they tend to look for the same features in a mate.


Thanks go to Bill von Hippel from the University of Queensland and Rob Brooks from the University of New South Wales