Males have adapted to battle with competing sperm

February 9, 2007

In the context of sexual reproduction, natural selection is generally thought of as a pre-copulation mechanism. We are drawn to features of the human body that tell us our partner is healthy and will provide us a fighting opportunity to carry on our genetic lineage. But a new article appearing in the February issue of Current Directions in Psychological Science suggests that the human male has evolved mechanisms to pass on his genes during post-copulation as well, a phenomenon dubbed "sperm competition."

In their article, Todd Shackelford and Aaron Getz at Florida Atlantic University describe sperm competition as "the inevitable consequence of males competing for fertilizations."

For a monogamous species, sperm competition may seem unlikely. But according to the authors, extra pair copulations (i.e. affairs) appear to be a significant part of our ancestral history and could, evolutionarily speaking, spell disaster. Ultimately, a male whose female partner engages in extra-pair copulation is at risk of unwittingly investing his resources into a genetically unrelated offspring.

Competition may also affect sperm count, say the authors. When men spend more time away from their partners (time that their partners could have spent with other males), the number of sperm in their ejaculate increases upon their next copulation. In one study, the authors note, simulated phalluses that most closely resembled the human penis removed an ejaculate-like substance from an artificial vagina. This could potentially signify that that the penis developed its shape to act as an anatomical squeegee.

But sperm competition does not solely take place in the realm of our biological makeup. According to the authors, many sexual behaviors such as deep copulatory thrusting may function to remove rival sperm. Accordingly, sexual partners report that men thrust more deeply and quickly into the vagina following allegations of infidelity. The same periods of separation that increase sperm number in male ejaculates may also help to explain the increasingly lustful feelings human males develop after long periods of time apart from their mate. That is, the human male may want to copulate as soon as possible as insurance against possible extra-pair copulation.

As sperm competition comes to light as yet another facet of sexual adaptation, it raises the question of how many other ways have humans evolved in a world dictated by "survival of the fittest?"

"Sexual conflict between males and females," Shackelford and Goetz describe, "produces a coevolutionary arms race between the sexes, in which an advantage gained by one sex selects for counteradaptations in the other sex." Thus future research may move beyond male adaptations and, for example, attempt to identify mechanisms in which females increase retention of sperm inseminated by men with the most favorable genes.

Source: Association for Psychological Science

Explore further: Protein essential for spermatogenesis could lead to new methods of contraception

Related Stories

Protein essential for spermatogenesis could lead to new methods of contraception

September 20, 2017
"Infertility is generally perceived to be mostly a female problem, but this is wrong," says Philipp Kaldis, researcher from A*STAR. Kaldis studies the development of human sperm, his work could one day lead to the treatment ...

Scientists identify key regulator of male fertility

September 19, 2017
When it comes to male reproductive fertility, timing is everything. Now scientists are finding new details on how disruption of this timing may contribute to male infertility or congenital illness.

Genetic advance for male birth control

October 10, 2017
When it comes to birth control, many males turn to two options: condoms or vasectomies. While the two choices are effective, both methods merely focus on blocking the transportation of sperm.

Do men need sperm health supplements?

October 9, 2017
Infertility, defined as the inability of a couple to conceive after at least 12 months of regular, unprotected sex, affects about 15% of couples worldwide. Several factors can lead to infertility, but specific to men, infertility ...

Scientists think public opinion important before human gene editing

October 3, 2017
The public should be consulted before gene editing is used to treat human embryos, according to a survey of scientists published in the American Heart Association's journal Circulation: Cardiovascular Genetics.

Potential Zika vaccine protects against pregnancy transmission and testicular damage

September 26, 2017
For the first time, a collaborative team led by The University of Texas Medical Branch at Galveston has shown that a potential Zika vaccine quickly can protect fetuses against infection as well as protect males against testicular ...

Recommended for you

Large variety of microbial communities found to live along female reproductive tract

October 18, 2017
(Medical Xpress)—A large team of researchers from China (and one each from Norway and Denmark) has found that the female reproductive tract is host to a far richer microbial community than has been thought. In their paper ...

Study of what makes cells resistant to radiation could improve cancer treatments

October 18, 2017
A Johns Hopkins University biologist is part of a research team that has demonstrated a way to size up a cell's resistance to radiation, a step that could eventually help improve cancer treatments.

New approach helps rodents with spinal cord injury breathe on their own

October 17, 2017
One of the most severe consequences of spinal cord injury in the neck is losing the ability to control the diaphragm and breathe on one's own. Now, investigators show for the first time in laboratory models that two different ...

Pair of discoveries illuminate new paths to flu and anthrax treatments

October 17, 2017
Two recent studies led by biologists at the University of California San Diego have set the research groundwork for new avenues to treat influenza and anthrax poisoning.

A new compound targets energy generation, thereby killing metastatic cells

October 17, 2017
Cancer can most often be successfully treated when confined to one organ. But a greater challenge lies in treating cancer that has metastasized, or spread, from the primary tumor throughout the patient's body. Although immunotherapy ...

New method to measure how drugs interact

October 17, 2017
Cancer, HIV and tuberculosis are among the many serious diseases that are frequently treated with combinations of three or more drugs, over months or even years. Developing the most effective therapies for such diseases requires ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.