Raw Animal Sexuality

John Merck

We have read in Zimmer that among the most rapidly evolving parts of the genome are those parts dealing with sex, so sex should be a good thing to look at for real world examples of evolutionary forces at work. But....

Q What is sex, exactly?

Gametes: Remember meiosis, the process by which gametes are generated. Genetic recombination is facilitated by:
  • Crossing over
  • Separation of homologous chromosomes.

Plasmids: This arrangement didn't appear overnight, but precursors are apparant. Prokaryotes (E. G. bacteria) which lack distinct chromosomes nevertheless recombine genes by swapping plasmids, small loops of DNA. This fact:

  • Enables them to share genes that confer increased fitness
  • Necessitates great care in the judicious use of antibiotics

From Tokyo Medical and Dental University


From The Daily Texan Online

What is the point?

Asexual reproduction: Any gardener knows that many species can reproduce asexually, budding off pieces of themselves that grow into a new organism. That's a good "quick and dirty" method of establishing a population from a single pioneer. Yet almost no species completely forgoes sex. Those very few who do, including complex animals like several "species" of whiptail lizard, show why this is a poor approach.

Such parthenogenic species occasionally result from hybridizations and accidents in meiosis in which females end up with more than two sets of homologous chromosomes. These can sometimes reproduce asexually, yielding clones of themselves.

Easy enough. The hassle - the parthenogenic ratchet. Deleterious mutations simply accumulate and cannot be selected out of the population by recombination. Thus, parthenogenic species apparantly last only a few thousand years.

Indeed, the lizards' need to engage in pseudocopulation in order to ovulate reveals their fundamentally sexual nature. Even what looked like textbook examples of parthenogenic species have recently been shown to sex after all. Consider the virginal shrimp Vestalenula.

The current asexual champs are Bdelloid rotifers, which seem to have gone without for the last forty million years. We'll see.


The advantages of sex:
  • Remember our "genetic drift" card game. Through sexual recombination of genes, it is possible harmlessly to remove a harmful allele from a population.
  • Resistance to rapidly evolving disease and parasites can be evolved rapidly through recombination of alleles.
  • Parthenogenic species are all clones of their founding ancestress. In a population, it's good to have variation, so that in case of a selective challenge, someone will have the genes to meet it.
  • In social organisms, we can take this one step further. The group may depend on its members having a variety of diverse abilities. E.g.:
    • New World monkeys tend to show several variants of color vision abilities in a single troop. thus different individuals are adapted to seeing different food sources or predator camoflague.

From Rutgers University


From Mark Rothery's Biology Web Site

So why are we female and male?

Complex species with three or more genders remain the domain of SciFi, however there are many unicellular organisms:

  • without physically distinct sexual morphs
  • with large numbers of physically similar "mating types."
  • with complex life cycles in which haploid and diploid stages alternate. Dinoflagellates may spend many generations as haploid "schizonts" that occasionally fuse to form diploid "zygotes" which in turn give rise to schizonts.

But for multicellular organisms, there seem to be two optimal strategies for making gametes:
  • The nurturing approach - Make a big, fat gamete with plenty of nutrients to support the new organism.
  • The sneaky approach - Make lots of minimalist gametes with DNA, flagella for moving around, and just enough cytoplasm to support them.
I.e. ova and sperm.

Why this is so is not absolutely clear. It might be to manage the transmission of cytoplasmic components of the genome. (After all, if you got genetically dissimilar versions of the organelles with their own genomes like mitochondria and chloroplasts, from both parents, they might waste time competing rather than doing their jobs.)

Nevertheless, if you make ova, you are female. If you make sperm, you are male. NOTE: These are NOT mutually exclusive catagories. Creatures may be:


Every organism has one evolutionary goal: to maximize the representation of its genes in the next generation.

How one goes about that varies greatly between species and between genders in a species. Despite their great diversity, one great pattern emerges from organismal sex practices:

The battle of the sexes.

You would think that by "putting all their gametes in one basket," males and females would have an incentive for complete cooperation, but this is not so. In fact, males and females each have their own agendas that can, at times, be so divergent that they actively sabotage one another. The paradox of sex (from an evolutionary sense, at least) is that while it requires intimate cooperation with another individual, it is utterly selfish.

For the female

  • The limiting gender: Typically, a male who choses his mate badly has lost a few minutes and a little effort. The female is usually the limiting gender - the one whose large investment in reproduction is the limiting factor for success. She must choose more rigorously. Females are not always the limiting gender. Examples of males as limiting:

    With a more limited number of gametes, maximum fitness comes from optimizing the survival chances for each ovum. That typically means:

    For the male

    With a lots of gametes to spread around, maximum fitness comes from maximizing the number of ova fertilized. That typically means: Other issues: Of course, the "battle of the sexes" is constrained and moderated by:


    The female strategy

    From dkimages.com

    Maximizing one's female fitness - being the optimal nurturer - requires:

  • Selecting optimal mates:


    These choices can be heavily weighted by peculiarities of the reproductive systems or environment.

    Mate choice: We have discussed sexual selection in the form of conscious female mate choice, but it can also happen cryptically. E.G.:

    Environmental pressure: For the fig wasp, the environment is utterly hostile and barren unless you live in a fig, in which case it's like paradise. You can't find a mate unless you're in a fig, so it's worth it to stick with your fig, even at the risk of mating with your siblings who are likely to be the only ones around. In fact, insects like wasps, bees, and ants, practice haplodiploidy, a genetic mating system that minimizes the hazards of in-breeding. In haplodiploid animals, males are haploid, thus, any deleterious recessive gene is automatically phenotypically expressed, resulting in its quick removal from the gene pool. (See below)


    The male strategy

    From Richard Seaman's Dragonflies and Damselflies of Fiji Web Site

    A typical male dilemna: Ideally, a male would seek to mate with as many females as possible and, hopfully, exclude his rivals from so doing. But the problem: in most species, the female plumbing is arranged such that the last male to mate with her is the most likely to father her offspring. Strategies to deal with this include:

    • Mate guarding after mating, to deter the attention of other males. This can easily be observed in dragonflies and damselflies. On the other hand, the male will lose interest once the female starts to lay eggs and will zoom off in search of other females, thus optimizing his chances.
    • Sperm removal in which competators' sperm is removed from the female system prior to mating. E.G.: Cephalopod males mate by using a special arm to deposit packets of sperm (spermatophores) inside the female's mantle cavity. In some species, we see elaborate "cleanup" stages of courtship, prior to mating.
    • The "Sherwin Williams" approach In many species, especially those that mate frequently and promiscuously, one way to optimize the fitness of matings is simply to produce more viable sperm than the competition. For example:
      • Testicle size in chimpanzees vs humans
      • Sperm conjugation in Marsupials


    Maximizing coverage can lead to novel strategies.

    Male bedbugs are very strange:

    • They practice "traumatic copulation" in which the male ignores the female's genital opening and simply punches a hole in her abdominal wall and releases semen into her body cavity. The sperm are somehow drawn to her gonads.
    • But that's tame. The real weirdness is that a bedbug may inseminate another male in the same manner, then the sperm will be drawn to HIS gonads and mix with his own. When male number two mates with a female later own, he may pass along the first male's sperm with his own. In effect, the first male has mated by proxy.
    • Arguably, these mating practices are the selective evolutionary result of an exaptation in which sperm "know how" to swim to the gonads.

    From University of Minnesota Gardening Information




    Definitely not the norm no matter how much we, for cultural reasons, want it to be. Why would members of either sex forego access to multiple partners? Indeed, the spread of genetic testing has shattered our illusions about many species formerly thought to be monogamous. Seems that most even those that pair up for life as a social arrangement frequently sneak around when it comes to actual mating. But true monogamy happens in some unusual circumstances.

    From National Geographic News

    Danger: The male dragonfly has it easy in that there are typically plenty of females around to fight for. In some environments, potential mates are so rare, and the environment so hostile that one's chances of ever finding one mate before dying are not good. If you're lucky enough to find a mate, better to stick with him/her rather than risk dying alone. An extreme expression of this is in deep-sea angler fish, in which the male, in effect, becomes part of the female's body. The ultimate must be the Australian red-backed spider, whose male deliberately somersaults himself onto his mate's fangs, allowing her to start eating him while he is still busy copulating. (Male preying mantises, of whom we hear much, at least try to get away.)


  • Mutual-Assured-Destruction: The combination of demanding offspring, unusual life histories, and extreme environments can lead to very unusual levels of paternal involvement in the rearing of offspring, even to the extent that if either parent doesn't cooperate fully, reproduction will simply fail. Examples include hornbills (right) and the famous emperor penguins.
  • From Woodland Park Zoo



    Maybe the key to ending the war of the sexes is to give everyone the same equipment? Alas, no. Although there are many species of cross-fertilizing hermaphrodites in which each individual simultaneously uses both male and female systems, the matings of hermaphrodites are often downright violent. E.G.:

    From Pharyngula

    From Science News Online

    Among the more extreme expressions of hermaphrodite violence, Marine flatworms must fight over who gets to inseminate whom with caustic, ulcer-producing semen. The "loser" has to be the "girl," but at least "she" has made "her" mate pass a qualifying exam - a form of female choice.

    Snails routinely jab one another during mating with darts covered with pheromones that stimulate the recipient's female system.

    Maybe it's easier to be born knowing who gets to be the boy and who gets to be the girl and who's a potential mate and who's a potntial rival without having to fight over it. Link to more info.

    From Science News Online



    What would make a female evolve to be infertile? That's what happens in a handful of eusocial animals - those that live in complex societies in which most individuals are non-reproductive workers. Examples include:

    Why hymenopterans? They are the ones who practice haplodiploidy, in which fertileized ova develop into diploid females and infertile ones develop into haploid males. (Remember the fig wasps) You need to track where genes are coming from and who is most closely related to whom.

    Human (either gender)

    % of your genes shared (maternal) % of your genes shared (paternal) % of your genes shared (total)
    Mom 50% 0% 50%
    Dad 0% 50% 50%
    Sister approx. 25% approx. 25% approx. 50%
    Brother approx. 25% approx. 25% approx. 50%
    Child - - 50%

    Hymenopteran female

    % of your genes shared (maternal) % of your genes shared (paternal) % of your genes shared (total)
    Mom 50% 0% 50%
    Dad 0% 50% 50%
    Sister approx. 25% 50% approx. 75%
    Brother approx. 25% 0% approx. 25%
    Child - - 50%

    The result: Whereas a female shares 50 percent of her genes with her mother, she shares, on average, 75 percent with her sister. Thus, to maximize the representation of her genes in the next generation, her best strategy is to farm mom for more sisters.