From hooked to amoebas

Dec 15, 2007 12:20 GMT  ·  By

Sperm? That's simple: a cell with a flagellum that moves through a liquid called semen and attempts to fecundate an egg. Well, this is the human case, but nature can amaze you!

1.Which are the performances of human sperm compared to our relatives? Humans, like animals, experience a fierce competition for sex. And this competition does not stop with mating, as a woman can be promiscuous. That's how sperm competition emerges. A new research has tried to see how sperm speed connects to the species' sexual behavior, while placing us amongst other primates.

A 2006 research made at the University of California in San Diego compared sperm of humans, gorillas, chimpanzees and rhesus macaques. The sperm samples from humans had a travel speed of about 0.2 km/hour.

The sperm from chimpanzees and macaques had a speed of 0.7 km/h. Chimps and macaques live in groups where males form coalitions, that's why any male can mate with any female, and vice versa. A chimp female can have multiple sex partners in one hour, thus the sperm competition is much stronger in this case.

But in the case of the gorillas, the sperm speed was of just 0.1 km/h. Gorillas live in a groups formed by one male, several females, and their offspring. Females have just one sex partner at a given time.

The chimp and macaque sperm also move more powerful, at about 50 piconewtons, while human sperm develops just about 5 piconewtons, and gorillas some lousy 2 piconewtons.

The speed and power developed by the sperm is due to inner cell structures that power the movement of their Tails. These structures' activity can be influenced in humans by some chemicals, like caffeine and anti-impotence drugs.

2.Hooked sperm. Sperm competition is particularly fierce in those species where the female promiscuity level is high, like monkeys, bats or rodents (rats and mice). Sperm cells coming from various males must compete.

In this case, what about sperm trains? Most mammals have paddle-shaped headed sperm cells, but the sperm heads of many rat and mouse species are curved like scythes, forming a hook.

In the European woodmouse, these hooks permit a sperm cell to attach to more of its brethren, allowing them to form groups of up to 100 sperms. These "sperm trains" are faster and stronger than sperm swimming alone, increasing their chance of fertilizing the egg.

Only the highly specialized design of rat and mouse sperm permits this form of sperm cooperation.

In the case of rodents, individual sperm cooperate with one another in order to out-compete sperm of rival males and the larger the testes (thus the amounts of ejaculated sperm), the sharper the hooks, because more sperm means more competition between sperm to reach the egg.

When the pressure from rival males is high, individual sperm will cooperate with one another to ensure that at least one of their siblings successfully reaches the egg.

3.Some sperms must pass through a gut to fertilize. Oral sex is practiced by many people also as a contraceptive measure. Not few still believe that sperm would get through the digestive system to fertilize the eager egg. But, there are some catfishes in which sperm passes through the female's digestive tract to the eggs, the sperm drinking type of fertilization, ending into an external fertilization. These sperm drinking species belong to two South American families of catfish: driftwood catfishes (Auchenipteridae family), inhabiting rivers from Panama to Argentina, and armored catfish (Callichthyidae family).

The male of this species has a pair of seminal vesicles (absent in most other fish species), and they release the sperm in the form of discrete bundles (spermatozeugmata).The main function of the seminal vesicles is the secretion of mucin, a sticky combination of sugars and proteins, which has the role to protect the sperm while passing through the female gut.

During the courtship, a male shows its abdomen to the female. Before expelling the eggs, the female attaches her mouth to the male's genital opening (there is no penis in any known catfish species!), and directly ingest his sperm. The sperm cells go unharmed through the female's gut and are released together with the eggs into the "pouch" made by her pelvic fins (thus, in the end, fertilization is external, not inside the female's body). This way, the eggs get in contact with fresh non-dispersed sperm in an enclosed space, ensuring a more effective insemination, not disturbed by the water flow.

Humans should not try this. The digestive tube will take advantage of the proteins and vitamins of the sperm.

4.Still, what about sperm that has to pass through the blood for fertilizing? This is the case of the African bat bug. Male bat bugs do not have a "taste" for the vagina, but they pierce the female's abdomen instead and ejaculate directly into the blood stream, where the sperm then moves to the ovaries to fertilize the eggs.

Females seem not to enjoy much being stabbed for sex and they have evolved a defense structure, a paragenital tissue on their abdomen that decreases the damage by heading the male's sharp penile tip into a spongy area crowded with immune cells.

5.If you thought only men buy women a drink to win her favors, that's wrong. There are beetle females that will mate just for quenching their thirst. It's the case of the bean weevil Callosobruchus maculatus, which feeds on stored pulses, which may contain a maximum of 10 % water.

With such a dry diet, the male's ejaculate is a precious water source for females. Females kept on beans and lacking water sources mate with more males, to get the water in the seminal fluid. They are more likely to mate if they are thirsty. Males invest 10 % of their body weight in semen. It buys them time before the females mate again and their sperm have to compete with that of other males.

6.Not all the sperms have just one flagellum. For example, the most primitive termites, Mastotermes in northwestern Australia and close islands, resembling cockroaches, have sperms with 100 flagella, a totall record in the animal world!

7.Some sperms are totally devoid of flagella (tails). For example, the nematode worms, crayfish or horseshoe crab sperms looks like amoebas, lacking the flagella. These sperms crawl towards the eggs.