Why do men repeatedly ejaculate more than 200 million tiny sperm when it takes only one sperm to fertilise an egg and produce a baby? You will find an answer to this and many other reproductive conundrums in Promiscuity . Not only will your curiosities about animal and human behaviour be addressed, but you will be informed by an unforgettable variety of facts about reproduction.
Did you know that a tiny fruit fly holds the record for the longest sperm and, at nearly 6cm in length, its sperm are longer than the male producing them and more than 100 times the length of our own humble human gametes? Did you know that a buck rabbit produces twice as many sperm in one ejaculation as a man, and a pig's ejaculation amounts to about half a pint? Or that male dragonflies have spiny genitalia that wipe out previous males' sperm from the female reproductive tract?
These are fascinating facts and you will be indulged with many more in this book, but Promiscuity is unique because it goes much further than astounding descriptions by explaining the hows and whys of reproduction. Tim Birkhead's logic follows Darwinian principles to explain clearly how an intriguing reproductive adaptation functions and why it evolved.
Darwin first conceived that life evolved through selection at a number of levels, and we now know that selection acts on genes. Every single organism exists to spread its genes into subsequent generations. Most organisms fail in this endeavour, but a select few succeed and their perfect genes are transmitted.
Reproductive success is an especially exacting challenge, which explains why sex and reproduction are so complex and so interesting. Sexual reproduction occurs when genes from a sperm and an egg unite and code for the development of a new organism. This union in itself is effected through a multitude of male and female adaptations, from opossum sperm that swim in tandem through to female spiders that cannibalise their mate and use the meal to make more eggs.
Promiscuity is especially revealing in demonstrating that while love and sex make the world go around, male and female interests do not always unite. Sexual conflict is an integral part of the route to reproduction, which stems from the difference in reproductive potential between the sexes. For males, reproductive quantity is achievable, while for females, it is quality that counts. Of course, females always know that they are the mother while males do not have that paternal certainty.
Promiscuity shows us that mating alone is not usually sufficient to guarantee paternity: if females mate with more than one male, their sperm are thrown into competition for fertilisation. This sperm competition operates via myriad mechanisms from simple attempts to outnumber the competition (is that why sperm are so numerous?) through to sophisticated seminal fluids that are toxic to rival sperm, or copulatory plugs that block the female tract.
Birkhead shows us that females are not necessarily shrinking violets in this contest. Apart from actually encouraging competition, females can play an active part in affecting who gets the egg. Hens, for example, will physically eject sperm of subordinate males to put them out of the competition.
So although it looks as if males and females have evolved to a common goal of reproduction, the truth is that it can be an evolutionary game of cat-and-mouse leading to ever more intriguing adaptations.
This is a captivating book that guides us through the history of our understanding of reproduction from early religious theorising to modern-day molecular genetics. Birkhead encourages an understanding of the real mechanisms and significance of fertilisation through ingenious experiments such as Spallanzani's frogs, which were dressed in contraceptive trousers to show that eggs must be mixed with sperm to produce tadpoles.
We are also introduced to early controversies of the "spermists", who believed that the male carried the seeds of life, which were merely nourished in the female's fertile garden. Many believed that a tiny "animalcule" was carried in the sperm head (one biologist arguing that he could distinguish between horse and donkey sperm because the tiny creatures in the latter had longer ears). The spermists were opposed by the "ovists" who regarded females as the real parents whose eggs were only activated by sperm. The relative influence of males and females in the dynamics of reproduction continues to be hotly debated today.
Scientists are all too often labelled as poor communicators who spend most of their time ensconced in laboratories, conducting self-indulgent and unintelligible research. If ever there was a book illustrating that good science need not be boring, irrelevant and inaccessible, Promiscuity is it. The subject matter is so absorbing that there is no need to sacrifice scientific truth to tell a good story and its content has relevance to absolutely everybody.
Birkhead takes us on a voyage of discovery through that most intriguing area of biology: reproduction. If you thought that human love and sex were complicated, then Promiscuity will show you that evolution has taken animals to even more astounding levels, and that there are remarkably logical reasons for this.
Matthew J. G. Gage is Royal Society research fellow in biology, University of East Anglia.
Promiscuity: An Evolutionary History of Sperm Competition and Sexual Conflict
Author - Tim Birkhead
ISBN - 0 571 19360 9
Publisher - Faber
Price - £9.99
Pages - 2