Whatever she wants – sexual selection

This post was translated by Igor Santos from the original published by Atila at the Brazilian blog Rainha Vermelha.



Sexual selection is not the freshest of news. Darwin’s grandfather Erasmus had already commented on it. [1]
But a very pertinent question is why it occurs, or at least, what is selected. After all, why does a female finds a male sexy?

Two hypotheses could explain that. Fisher proposed the sexy gene hypothesis. He argues that, whatever the characteristic chosen by females, if it comes from a gene, that gene must become common within a population just because it has been picked up. A female that chooses a male with the sexy gene will have offsprings with that very gene, which in turn will be chosen by other females, spreading it.
The other hypothesis is the good genes one. According to it, when females prefer a certain trace, they are actually opting for a male’s good health, reflected on that given trace. For instance, a male with bright red feathers that uses carotene to increase the redness. Only a very healthy male has enough carotene to “waste” on plumage, so whoever gets the redder male will have healthier offspring and will have been favored by sexual selection.

Whatever the reason, good genes or sexy genes, the selected characteristics in the Birds of Paradise make them amazing. Scenes from BBC’s Planet Earth.

Both hypotheses seem to be correct. It all depends on the cost involved in the choice. When its not too cost-intensive for the females to choose (e.g. birds that gather for “exhibition parades” with heaps of males, sexy gene might prevail, like in peacock’s tails). In other scenarios, where choice uses resoures, be it looking around for males or exposing their presence to predators for the exhibition, the selected characteristic must have added value, a good gene.

But neither hypothesis explain how a characteristic begins to be selected. Why do females prefer a certain color, a song or any other thing? Where does the preference come from?

Look at the Physalaemus pustulosus frog for instance: males have a longer call (sounding a bit like “wiiiinnnnn”) followed by a screech at the end. Kinda hard to put in words. Check out this video and enjoy the singing:

Like with most frogs, the song is used to attract partners. But in closer-related species, males don’t do the second part, they only chant the “win” bit. Now a bizarre fact.

When the P. pustulosus melody was played through loudspeakers to females of other close-related species, they like it better than their own species’ call! They prefered the screeching end, albeit they had never heard it before, since their own kind does not do it. Therefore, they already have the nervous circuit that make them like that kind of song better, but only the P. pustulosus males use them.

There is a bias preference amongst the females, who hear and prefer that frequency, but males of the other more than 40 species of the same genus can’t use it. [2]


Platy (left) and Green Swordtail (right). Same genus, but only one has a sword-shaped tail.

Other species follow the same path. In Xiphophorus fish, only the swordfish has its anal fin elongated. The longer the tail, more successful the owner. In close-related species, like platy, on which that long fin inexists, the same bias repeats itself. If artificial tails are attached to the males, thay become more popular. Again, females still prefer something they have never seen before. Probably because they prefer bigger males.

In more and more species, from birds with more intricate melodies to mite with specialized appendages on their pedipalps, we find characteristics in one species that are not present in others of the same genus, but are quite successful among them. It is about those who manage to explore a tendency already present.

The implication is that preference for a trait is inherent to females according to the more utilized senses. It could be the “eyes” on the pattern of a peacock’s tail, since they are vision-oriented birds and need to be in constant lookout for predators. And that crest is sure to attract attention. Senses and nervous circuits that are already in constant use for, say, searching food – like a spider’s sense of touch – are co-utilized or re-routed for sexual function. Which is expected, for our senses resources are limited. [3]

Nothing more fitting for evolution. Females preference is already there and males born with tiny variations. The one with a novelty-colored fin or a more refined song will be chosen and have more descendents rather than their rivals without those characteristics. With time, that gene becomes so ordinary as to level the success rate of all males, until one displays another interesting change, and the cycle starts over.

After all, the ladies like surprises.


[1] Smith, C. U. M. “Erasmus Darwin saw sexual selection before his grandson.” Nature 459, no. 7245 (Maio 21, 2009): 321. DOI:10.1038/459321d

[2] Ryan, M., Fox, J., Wilczynski, W., & Rand, A. (1990). Sexual selection for sensory exploitation in the frog Physalaemus
pustulosus Nature, 343 (6253), 66-67 DOI: 10.1038/343066a0

[3] Ryan, M. (1998). Sexual Selection, Receiver Biases, and the Evolution of Sex Differences Science, 281 (5385), 1999-2003 DOI: 10.1126/science.281.5385.1999

Discussão - 13 comentários

  1. boygenius disse:

    Proven fact: Females need a REASON to have sex, males just need a PLACE. 😉

  2. anony disse:

    you talk about females selecting males, but are there any examples of males selecting females?

  3. Atila disse:

    @ Anony,
    Yes, there are. It almost always follows the rule of the bigger investment. In species where the male spends more energy raising the babies, the male choses the females. This happens in some species of birds and fishs for example, like seahorses.

  4. anony disse:

    what you said fits with the social side of sexual selection too, like male apes exchanging food or grooming for sex from females. i've even heard the argument from a small number of academics that sexual selection is purely social, could these examples be in anyway social?

  5. seks disse:

    Connecting weather with climate is a tricky thing. Some thoughts... one very interesting result of GCMs would be the projected locations of low and high pressure areas with a higher tropospheric energy (stored there

  6. hiphop disse:

    McCulloch accuses Steig et al. of appropriating his ‘finding’ that Steig et al. did not account for autocorrelation when calculating the significance of trends. While the published version of the paper didn’t include such a correction, it is obvious that the authors were aware of the need to do so, since in the text of the paper it is stated that this correction was made. The corrected calculations were done using well-known methods, the details of which are available in myriad statistics textbooks and journal articles. There can therefore be no claim on Dr. McCulloch’s part of any originality either for the idea of making such a correction, nor for the methods for doing so, all of which were discussed in the original paper. Had Dr. McCulloch been the first person to make Steig et al. aware of the error in the paper, or had he written directly to Nature at any time prior to the submission of the Corrigendum, it would have been appropriate to acknowledge him and the authors would have been happy to do so. Lest there be any confusion about this, we note that, as discussed in the Corrigendum, the error has no impact on the main conclusions in the paper.

  7. anony disse:

    is the same sort of gene selection taking place when males are selecting?

  8. Atila disse:

    @ anony,
    Sexual selection happens outside social life too, like polen or sperm competition. About the gene selection, in some birds males take care of the offspring the females even get the good looking plumage. This makes me think the gene selection may be the same.

  9. Daryl McCullough disse:

    If it is not considered too rude to talk about evolutionary implications for humans, I have often wondered why (many) female humans are so good-looking. Was there significant sex-selection associated with human society when we were a bunch of hunter-gatherers (or more recently)? It doesn't seem to fit the normal pattern in the animal kingdom, because males don't invest much in sex (sperm is cheap). So there is no evolutionary advantage in human males being picky.
    Here's my theory (which is total speculation, I admit). In humans, most males aren't particularly picky, but high-status males are. So maybe there was an evolutionary advantage to a female being beautiful, because it made it more likely that you will be mated to a rich and powerful male, which in turn gave your offspring a better chance at survival.
    This crackpot theory would predict that beautiful women are associated with patriarchal societies. If men and women are treated equally, there is no particular advantage to women being beautiful, and in a female-dominated society, there would be more pressure for good-looking men.

  10. acı çehre disse:

    If men and women are treated equally, there is no particular advantage to women being beautiful, and in a female-dominated society, there would be more pressure for good-looking men.

  11. Thank's for sharing this
    This is really interesting

  12. acai disse:

    I have often wondered why (many) female humans are so good-looking.

  13. This makes me think the gene selection may be the same.

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