You have an interesting area of expertise. How did you end up in this field?

I’ve always been interested in reproduction and birds. And I’ve managed to combine my two passions into one career. I’ve researched bird reproduction for 30 years, with a particular emphasis on sperm. There’s a lot of interest in this area, partly because of a change in biological thinking in the 1960s.

Your first choice is a paper by Gustaf Retzius called Die Spermien der Vogel, published in his book Biologische Untersuchungen.

Retzius is one of my heroes: a well-trained polymath from a dynasty of scientists. He had a fortunate break in his thirties – he married the daughter of a newspaper magnate. She was so rich that he never had to work again.

That was lucky.

He bought the best microscope on the market and published his results in the way he wanted to. He spent his career doing fundamental zoological anatomy using the microscope. This book is one of a series. He was a very skilled draftsman and did beautiful drawings.

What period was this and where did Retzius come from?

This was in the late 1800s. He was Swedish, and was nominated 12 times for a Nobel Prize. He never quite made it.

What is this particular paper about?

When he was about 60, Retzius became interested in the microscopy of sperm. He started buying specimens – including a chimpanzee pickled in brandy – and dissected them. He was amazed at the tremendous diversity of sperm in different animals. For those of us working in the field today, Retzius was the first spermatologist and the first person to make an encyclopedic directory of animal sperm. He didn’t do much interpretation. He just described in tremendous detail, and with great accuracy, what he saw.

As a layperson, I can’t imagine how sperm could be particularly diverse. Can you give me an example?

Many of us have a basic image of sperm in our heads: a tadpole with an elongated tail.

My image is of Woody Allen in a tadpole suit.

Some insects produce sperm that look like discs, with no tails at all. The bird sperm I study have augur-shaped heads and helix-shaped tails. Fruit flies, just two or three millimetres long, have sperm that stretch up to ten centimetres because they are in rolls like balls of thread. That’s phenomenal. 

Let’s move on to the ecology book by Krebs and Davies.

Behavioural Ecology is the first textbook in a field that emerged in the late 1960s and 1970s. It focuses on the fact that natural selection doesn’t operate in groups or species, but in individuals. This refinement of Darwin’s idea caused a revolution in biology. With its four subsequent editions, it had a massive effect on the way people viewed the natural world.

It’s a collection of essays from the key players in the field. One of them, Geoff Parker, was a pioneer in ‘sperm competition’. When females mate with more than one male, Geoff noticed that the sperm would compete to fertilise the eggs. He wrote a groundbreaking paper in 1970 and he provides a chapter on the subject in this book. For a few years people thought the concept only applied to insects – Geoff’s speciality. In the mid-1970s I realised it could relate to birds. In the following decade, there was an incredible realisation that female promiscuity was largely ubiquitous in the animal kingdom. Darwin had said exactly the opposite – that most females were strictly monogamous. Geoff’s chapter summarises what we knew until this point.

So you are saying that the competition for the female carries on after ejaculation.

Exactly. Darwin assumed that the competition stopped once a partner was acquired. Geoff’s idea was that sexual selection continues after insemination, with males competing for fertilisation and not simply for a partner. Females are particularly promiscuous in the breeding season. 

That topic continues with Bob Smith’s Sperm Competition.

Parker was invited to a US symposium organised by Bob Smith. Bob brought together everyone in the field and got them to write essays on key topics. This is a set of reviews on promiscuity and sperm competition across the animal kingdom. It was the first broad overview and recognition that sperm competition and promiscuity were ubiquitous. Smith’s own chapter on human sperm competition was exciting, stimulating and controversial.

Why?

Until that point, we were rather reserved about these things. The implicit assumption was that female humans were monogamous, even though males were not. There’s a section on testes size in primates. It turns out that the relative size of testes in any animal group strongly correlates with the degree of promiscuity in that species. Therefore, gorillas have tiny testes for their body size, whereas chimpanzees have enormous ones. In gorilla societies, you have huge silverback males who can afford to produce relatively few sperm because they don’t need to compete. By contrast, male and female chimpanzees copulate with everyone in the troupe, every day. The males need huge quantities of sperm to do that.

How exhausting.

Exhausting, but exhilarating. What Bob Smith did was to fit humans into the data for primates. Humans are at the lower end of the scale. We’re not as exciting as chimpanzees but not as dull as gorillas. 

William Eberhard looks at things from a different perspective in Female Control. 

A criticism of early pioneers like Geoff Parker and me was that we were sexist because we only studied males.