In the early 1990s researchers at the University of Copenhagen analysed the scientific literature that had appeared between 1938 and 1991 on
the quality of sperm, which had involved 15,000 men. They found that in 1940 the average number of spermatozoa per millilitre of seminal fluid was 116 million. By 1990 that had fallen to 66 million. The amount of semen (sperm cells and seminal fluid) produced per ejacu­lation had also fallen. The average for 1940 was 3.4 millilitres, for 1990 only 2.75 millilitres. These two findings pointed to a decline in male fertility. The researchers assumed that exposure to toxins like dioxins, alkylphenols, pcb and ddt played a significant part in this.

The standards for ‘normal’ sperm have been adjusted over the course of time: nowadays 20 million sperm cells per millilitre counts as the lower limit. If there is to be a chance of fertilization there must be a minimum of 5 million spermatozoa present in each millilitre of sem­inal fluid, but a minimum of 10 million is desirable and as was said in 2007, a minimum of 20 million was regarded as normal.

Spermatozoa are incredibly small: from the head to the tip of the tail they measure 0.05 mm with a maximum diameter of 0.0025 mm. The sperm cells of men, horses and zebras resemble each other, having the same shape of head and a long tail. They share these characteristics with the lancelet fish, the escargot and the water flea. A rat’s sperm cells have a sickle-shaped head, while the hermit crab has beetle-like, exploding spermatozoa. If any of them touches an egg it leaps up and launches its genetic cargo into the interior of the egg.

The human male tadpole really is a miracle of design. It consists of three parts: the oval head, in which the genetic material is transported, an oblong thickened central section housing the engine room, and a tail which enables the sperm cell to steer towards the ovum. The fuel is sugar, which in the engine room is converted into adenosine phos­phate. Very many spermatozoa die in the extremely acid vagina, and 40 per cent of the survivors are rejected at the entrance to the womb. Then half of the survivors swim into the wrong Fallopian tube, until finally one victor emerges in the sperm competition. Arriving at the ovum the chosen spermatozoon sheds its cap, or acrosome, which contains a special protein, releasing enzymes that enable the sperm cell to pene­trate the ovum. So while a large number of sperm cells are required to make fertilization possible, quantity alone is not decisive: quality is also of great importance. This includes having a normal head and being able to swim fast in one direction, not swimming circuits but a long­distance race. If little of the sperm is up to standard, doctors speak of oligo-terato-asthenospermia. ‘Oligo’ indicates an insufficient quantity, ‘terato’ an excessive number of abnormal heads and ‘astheno’ poor mobility. If no spermatozoa are found in the ejaculate, it is called azoospermia.