I was first taught the “birds and bees” lesson about human reproduction in my last year of primary school. Before that I had given the matter no thought at all, unlike the kids of today who seem to know enough to write a PhD thesis on human sexuality by the time they are 10.

I can remember the lesson very clearly. While we were out on the playground playing our customary game of football with a tennis ball, Miss Howe had been busy drawing enormous diagrams of human genitalia on the blackboard and these diagrams became, naturally, the focus of our attention as soon as we returned to our desks.

This was a kind of awkward situation and I was keen to see how the other kids were going to react. Would they giggle? Would they blush? Did they look as though they knew all this stuff already? Most of them gave little away but I noticed something odd about Jimmy Simmonds.

Jimmy was one of those kids with thick lenses on his glasses which distorted his eyes and made it look as though he viewed the world through the bottom of beer glasses. What was immediately obvious about Jimmy, and this image I will never be able to erase from my memory, was that his reaction to the huge chalk diagrams could not be read from his eyes because the insides of his spectacles had completely misted over. The structures of the human reproductive organs were hidden from him by a thin layer of fog.

It’s funny how things work out. I am now a teacher myself and, Like Miss Howe, I find myself explaining to a new generation where babies come from.

It is difficult to think about sex and sexual reproduction without our natural bias towards reproduction in humans. If kids are told that the next topic is sexual reproduction they will assume it is about human sexual reproduction and turn up to the next lesson on time, or even early, and foaming at the mouth. But sex is, biologically speaking, what makes the world go round and is overwhelmingly common in just about every species you might care to think of. Even flowers and trees contrive to reproduce sexually in the majority cases although they have no awareness that they are doing so.

Biologists have often been puzzled by how commonplace sexual reproduction has become. It is not necessarily the easiest or most reliable means of producing offspring. For one thing you need to locate a reproductive partner or organise, as flowers do, for your sex cells to be delivered to one. But the benefits in terms of genetic reshuffling during the formation of sex cells, and the new genetic variability resulting from combing the genes of two parents, seem to outweigh the costs involved.

I usually introduce the sexual reproduction topic by getting the kids to think of organisms which are not human, and among my favourite are the fish. And, among the favourite fish of fly fishermen everywhere, and at the top of the list for many, are the salmonids.

Salmonids, which include char, trout, salmon and grayling, can be taken as pretty representative of fish in general in terms of how they reproduce. Sperm cells can swim quite happily in water they are released there by the male fish and fertilise eggs, which are also released into the water by the female. This typically happens in fast flowing, cold, well-oxygenated water in the upper reaches of rivers and streams. This external form of fertilisation is typical of fish and frogs and other animals that reproduce in the water. Of course, problems arise for animals that live on dry land and it is the absence of a liquid medium through which sperm can travel that drove the evolution of internal methods of fertilisation of land lubbers like us. In humans the sperm need to be deposited inside the reproductive tract of the female which results, among other things, in the giggling of the kids sitting in the back of the class in Year 9.

Before returning to the salmonids, here is an interesting thing that happened at work a few years ago. One of my colleagues came into my class in an agitated state. She was, at the time, teaching the “Sexual Reproduction” topic to Year 7 and was having to fend off probing and embarrassing questions on the topic by one of the boys. There seemed to be nothing he was not prepared to ask. I could only sympathise. Fearless kids like this can make teaching this stuff tricky. It was only some time later that I got wind of the fact that the other, more timid boys, were paying the other kid to ask the questions on their behalf. They were egging each other on, each trying to come up with a question to make the teacher most uncomfortable.

The going rate for questions about sex? One euro a pop.

Where were we? Okay, I remember, back to the salmon and trout. Once the fertilised eggs are deposited and covered by stones they are pretty much on their own. Mum and Dad are not going to be around to take care of them and, invariably, a punishingly high level of mortality results in very few of the offspring surviving even their first year. To offset these huge losses, large numbers of eggs are deposited and this, again, is quite typical of fish.

It is probably fair to say that it is the reproduction of salmonids which is best known by people with no particular interest in fish. The image of a salmon navigating obstacle and dangers to return from their ocean feeding and growing to the rivers of their own birth is vividly etched in the popular imagination. We have all seen the waterfalls they leap and the grizzly bears trying to hunt the fish as they push their way endlessly upstream. It is the salmon, and particularly the half dozen or so species of Pacific salmon that swum through our television sets and into our living rooms. They provide a spectacle that is unrivalled in natural history.

But the price to be paid for sex by these salmon is death. The Atlantic salmon may survive the rigours of spawning, although most do not, but this is not true of their Pacific counterparts whose spent bodies, post-spawning, litter the watercourses like the petals of flowers that have served their purpose and remain scattered on the ground. The nutrients within those remains are locally recycled and the whole migration can be seen as a swimming conduit of nutrients from the cold ocean to the interior of a continent hundreds or thousands of miles distant.

Reproduction of brown trout is essentially similar although the migratory distances will be relatively short. Some sea-going trout will have returned from their coastal wanderings to spawn, like the river relatives from which they are genetically indistinguishable, they are very likely will spawn in the rivers in which they were born.

Most of this drama is unseen by the fishermen who might have spent hundreds of hours chasing after trout during the fishing season. I came across some beautiful photos taken by Miguel Aguilar which show trout spawning. If you would like to see them, you might like to follow this link:


I got a present of a book by Bob Wyatt for Christmas. It is called “What trout Want”. Wyatt is a good writer and this is a fine book from which I learnt many things. It also threw light on a mystery: the giant trout of the back country rivers of New Zealand.

I have long been puzzled by these iconic fish. Their large size and the remote pristine landscape they inhabit probably draw thousands to New Zealand each year. The thing I found odd about these fish was that most of them seem to be males, often broad in the back and with the characteristic kype. Why is this? And how could such huge fish sustain their bulk in these headwaters?

The answer, according to Wyatt, is that these are fish which have remained in the reaches of the river close to their spawning grounds after the others have dropped back to the more hospitable reaches of the river further downstream, or into large lakes into which these rivers drain. The fish may indeed struggle to maintain their bulk here and many will, in due course, work their way downstream where the food supply is more likely to meet their demands. And there they may remain until the powerful reproductive urges drive them upstream to spawn again.

It is difficult for me to think about things like the back country trout and the powerful drive that push salmon to the spawning grounds following their ocean wanderings. It is enough to make my glasses steam up!

I found this lovely drawing of a male (top) and female brown trout (bottom) on a blog called The Weedbed Blog. It was made by a Canadian guy called Nick Lafferiere.