I have a suggestion for the noise that electric cars should be forced to make:

A hissing sound like that of a snake, for three simple reasons:

(1) It is not a sound that you would hear in an urban environment so it will be unmistakable.

(2) Many animals are genetically predispositioned to avoid snakes as predators so they will avoid cars which may in turn reduce the amount of animal MVAs

(3) The noise is easily replicated with a broken whistle (a standard one with a metal barring inside of it)

Thanks for the good question, it got me thinking...

Thank you so much for including and answering my question about colour this week.

I was thrilled, and so interested to hear what Professor Ron Douglas had to say.

I shall continue to listen enthusiastically to your great podcast, all good luck with it.

Chris -   We said there shouldn’t really be a theoretical maximum for an airplane.  It will be down to the materials that are used to design it and whether you can make a frame that’s big enough and cost-effective enough to put in the air, and he says, “Yes, planes could be made bigger but there is a worldwide gate size limit, in other words the air terminal, of 80 meters wing span.  The A380, that’s the new massive airplane, it’s got something like four or five hundred people on it - a 79.8-meter wing span for that one and this restriction is to stop collisions apparently.  But he says, “To make a larger plane would require a longer wing span which should be greater than the legal limits.”  Apparently, Howard Hughes did build the largest plane ever.  It was in 1947, it was called the H4 and it was nicknamed the ‘Spruce Goose’ and had a 97-meter wing span.

Kat -   I wouldn’t like to try and park that one!

I just wanted to write to congratulate you on a fantastic show.  I listen every week.  You are carrying on the tradition of people like Carl Sagan who bring science to laypeople like myself.  The show is fascinating, entertaining, and makes for great cocktail party fodder.  Keep up the good work!

Kat -   I wish I could come to some of your cocktail parties where they talk about science!

We put this question to Dr Cathy Allen:

Cathy -   Well, sort of indirectly, yes and you do need to have the proper genes, acting in a synchronized fashion for the proper hormonal and cycle to occur, but there’s a lot more to it. For example, there are women who had have very high oestrogen levels but still reduced fertility because of the patterns of the hormones, and peaking and troughing during the month would be incorrect.  So yes, if you’ve got very, very low oestrogen levels, it’s a reflection really of what’s going on with the ovaries and how they’re functioning.

Kat -   Well this is an interesting one.  I’ve been doing a bit digging around and actually, there is some evidence that some animals can have post-natal depression.  The main animals that have been looked at are rats and mice because they've mainly been used as laboratory models for humans and they do find that rats can show depressive symptoms, things like poor nursing, signs of stress and anxiety after birth and they were also quite useful, you can manipulate the hormone levels in rats to bring on post-natal depression type symptoms and there’s some labs that are doing this and are trying to test interventions and ways to help reduce this because it’s definitely thought that changing hormone levels, increases in cortisol, drops in females sex hormones help to bring on this kind of problem.  There are some interesting mice models as well; particularly mice who have faults in their GABA receptors in their brain on their brain cells and the nerve cells are much more likely to show these kind of depressive symptoms after birth and a terrible experiment has been in done rats they find that if they take their puppies away for short lengths of time, well not like you know days, if they take their pups away it does induce depression like symptoms.  I can’t find any evidence about larger animals but there are certainly some anecdotes out there about dogs and cats.  Some of them showing post-natal depression types symptoms like not nursing properly and not really being themselves.

Chris -   It’s interesting because the same mechanisms that lead to another mother baby bonding in humans also work in animals don’t they?

Kat -   Absolutely.

Chris -   There are big changes in hormones which we know affect the mood of animals and in the way in which they engage or link or associate; bond with their offspring.  So it’s not so unlikely that the same hormones that trigger the mood changes in us to cause post-natal depression could also occur in another mammalian species.

Kat -   Yes I would certainly expect it to be the case and certainly definitely in the lab rats and mice can show post-natal depression.

Chris -   I’m not sure I know the answer to that.

Kat -   No, I do think so.  I don’t know when the cats go through puberty - is there cat puberty?  If any listeners have any cats and would like to tell us when their cats started sounding like a grown-up cat.

Chris -   Is it puberty?

Kat -   Oh I don’t know, maybe!  When does a cat become mature enough to make an adult sound?  Maybe when it’s fully grown, I don’t know, interesting one.

Do let us know if you have any ideas!

Chris -   Well the reason the body needs salt is because every single one of our cells contains large amounts of salt and, salt is the generic term for ions, charged particles, and most of the cells in our body in fact, all of the cells in our body are electrical.  In other words they pump these ions from one side of their cell membrane which is a lipid or oily substance and therefore an insulator so they pump ions from one side of that membrane to the other and this means there is an electrical potential difference across the membrane of a cell and this means that this gradient this electrical difference can be used by the cell to do other sorts of work.  So cells for instance do have channels that sodium can flow in to the cell and it comes down its potential difference in concentration gradient and the result is that it can be used to pull in glucose at the same time.  So sugars can get into cells.

So we need salts in our cells - that’s how they regulate their size by bringing water in by osmosis.  That’s how they regulate electrical activity. Nerve cells for instance couldn’t carry information without actually having this electrical gradient across the membrane because all that’s happening when a nerve cells fires off an impulse is that you get a sudden flood of sodium in to one patch of a nerve cell.  This brings in lots of plus in to that part of the cell and therefore an electrical signal goes whizzing goes down the nerve and gets built up and regenerated as it goes down the nerve and it travels at about 50 to a 100 meters a second so very rapid transmission of information.

So we need salts in our body, we take in salt in our diet, we absorb salts and those salts are also include important things like calcium to make your bone strong but you’re also losing salts all the time when you go to the toilet for example you lose calcium, you lose phosphates.  This is both in urine and faeces so you have to continuously top up the number of salts that you have in your body because you have obligate or insensible losses.

Kat -   But we hear so much about salt being really bad for you in your diet giving you high blood pressure.  So if you have a diet that’s too low in salt is that also pretty bad for you?

Chris -   Well the body is very good at scavenging salt from what you eat and what you drink so it’s very rare for people to get too lower levels of salt in the body based on diet alone.  Usually there’s something pathological going on.  Sometimes what happens is that people have a problem called syndrome of inappropriate ADH.  This is anti-diuretic hormone and the body saves too much water so it scavenges back water and as a result your blood can become too dilute and you have very low sodium levels and this can cause problems with your brain swelling.  It can also cause the accumulation of water elsewhere around the body and it can cause heart failure so a bad thing to have.  But that can cause low salt levels that can make people feel dizzy.

Kat -   Well, basically, this all boils down to a pigment called melanin.  This is the dark pigment - if you're fair skinned, you can see it in your moles; if you're dark skinned, you have a lot of melanin all over your skin.  And melanin is almost like the body's natural sunscreen, it helps to protect you against the damage that ultra-violet light can do.  This means that people from countries that have dark skin, they're actually much less likely to get skin cancer.  Us people with very fair, pale skin, when we're exposed to a lot of sun, a lot of ultra-violet radiation, our chances of getting skin cancer are actually much higher because the UV light from the sun can really penetrate into the skin.  Whereas if you have dark skin, the melanin helps to protect you.  There's also an interesting argument, some recent evidence, from Nina Jablonski; we heard her talking on the show recently about this, that very deep ultra-violet radiation can actually break down folic acid, folate, in your body.  Obviously, you need to protect yourself against this happening because folate is really important for healthy metabolism and also for making healthy babies.  So it's probably a natural defence mechanism that's evolved in people from countries where it's very hot to tend to have dark skin and lots of melanin to protect you.

Chris -   Just looking at Andrew Spence's paper in Science.  He was the guy talking to us earlier and actually pointed out the jockey contributes about 13% of the weight of horse and rider and when you load a horse up it does run slower.  They’ve got evidence to show that, so the jockey does slow the horse down but a good jockey by adopting this posture and also pumping the knees up and down, they’re saying could effectively contribute energetically to the running horse.  So, yes they could potentially make a contribution and that’s why a good rider has this skill.