Saturday, April 3, 2010

Post Mating Competition

Coyne talks of post mating competition on page 151 where even after a male has mated with another female, other males may still try to mate with the inseminated female. Other males would first remove the sperm already inside the female reproductive tract and attempt to fertilize the female himself and steal his paternity. For example the Royal Entomological Society published an article where Drosophila melanogaster were able to "extensively displace previously stored sperm". If the females have spent so much time selecting a proper mate (we know they do because of sexual selection where males compete for females), why is it that the females have not developed a defense mechanism against such devious "weapons" --as Coyne likes to call them-- so that they may keep the revered semen of the mate they chose? Why even choose if all the ejaculate may just be stolen away by another probing male?

One answer I have is that the newer male that has the opportunity to remove semen from the genital tract and introduce his own, then the newer male must be more stronger and more fit to pass along its genes. But then does this not destroy the whole point of sexual selection if stronger and better males can just come along and steal another's paternity? How does post mating competition affect sexual selection overall? Would it be advantageous or disadvantageous for the species?

Why Evolution is True by Jerry Coyne


  1. It’s no secret that the natural world is full of competition. Competition for food, competition for survival, competition for mates, and, in this case, competition for paternity. Sexual selection is a “’special case’ of natural selection” (UC Berkeley) where species go to “extreme lengths for sex” (UC Berkeley) so then post mating competition could be considered a more special case. Post mating competition, in my opinion, is merely an extension of the fight to pass on one’s genes. Impressing the female enough is the first step, and that ensures that the male’s genes, if not removed by another male, are passed on as they obviously did something right to earn a mate in the traditional sense. This leads to “sexual dimorphism of body size in many species…due to competition between males for access to females” (Coyne 150) because the bigger, stronger males outcompete the smaller, wimpier males for a mate. That’s easy enough to see, yet things get even more interesting when getting into the details.

    After the deed is done and the male has mated with the female, a lot of really cool behaviors can come into play to keep the male’s paternity because it wants to pass on its genes. Some examples include “a pair of dragonflies attached to each other” (Coyne 151) to prevent other males from inseminating the female, a Central American millipede riding the female to prevent other males from showing up (Coyne 151), and even certain snakes having ejaculate that “contains substances that temporarily plug up a female’s reproductive tract” (Coyne 151). Of course, these traits were selected because, in the past of these species, the males could compete for paternity and that necessitated extra measures to ensure paternity. Sexual selection is a very fickle thing, with the slightest differences allowing traits to pass from one generation to another.

    In the end, because of this great complexity, many answers have been given to explain the advantages behind such competition. One that particularly interested me was the notion that when females mate with many males, it “can enhance offspring survival” (Fisher et al) because the male with the best traits will also have the best ejaculate, so that sperm will survive and fertilize the female’s egg the best. Then, why don’t females have mechanisms to prevent insemination? The answer is: it is not up to them to select traits! If everything were up to the female, the biggest, strongest males would not necessarily always be the ones to pass on their genes! Nature is a really brutal place, and, as I said, competition is everywhere. Competition can take the form of two males fighting, or their sperm fighting for fertilization of the female egg. After all, “competition breeds success”, and even though this quote originated in sports, it applies to nature as well. For the female to interfere in that competition would take away from the future success of the species, and is why post mating competition continues without female intervention.

    Page 150 of the book
    Page 151 of the book

  2. Using the way Mike defines sexual selection, a female mating with more than one male doesn't destroy the whole point of sexual selection; in fact, it reinforces it. Post mating competition allows another male to mate with that female and, as mike said, lets the most fit sperm fertilize the egg of that female. The reason animals such as fruit flies are able to displace the ejaculate of prior males' sperm is another strong indicator for sexual selection; if a female finds a male that is more fit or able to care for children, she should be able to mate with that male, thus selecting for his traits instead. In fact, there are some cases in which females display cryptic choice, in which a female removes a males ejaculate without the male's knowledge ("Female control: Sexual selection by cryptic female choice"). Females haven't developed mechanisms to prevent another male's insemination because it wouldn't allow a female to select for the fittest male after she mated once; this would be much more disruptive to the sexual selection process because it wouldn't necessarily be the fittest male that spreads his genes, but the one who got to the female first ("Have Only Men Evolved?"). Post mating competition is very beneficial for a species because it allows a female to choose and mate with a fitter male after the fact, thus producing more fit offspring.