Why does the
Temminck’s Tragopan have highly conspicuous plumage? Why would the male Himalayan
Monal Pheasant want to attract prey with his iridescent attire? Long- tailed
widow birds as the name suggests, have tail feathers that extend further than
the length of their body. How could these feathers improve the survival and
fecundity of these birds? They make the species easier to catch, and would take
high amounts of energy to produce and maintain; energy that could go into
producing offspring (Pryke& Anderson 2005).
If these animals
are failing to survive due to detrimental traits, then they are unable to pass
on their genes to future generations. According to the theory of Natural
Selection, ridiculously long tails and conspicuous plumage would be selected
against. So what is the answer to this confounding cost of having these traits?
... SEX!
Organisms need not only to
survive longer and reproduce, they also need to convince the opposite sex that
they are the one and only answer to all their copulatory dreams.
The scientific term
for this is Sexual Selection, another theory of evolution which states that if
there is a heritable variation in a trait that affects an organism’s ability to
obtain mates and reproduce, then the trait leading to more success in
accomplishing this will be selected for and become increasingly common over
successive generations (Freeman & Herron 2014).
Watch the following video to
see the Bulwer's pheasant (Lophura bulweri), Temminck's tragopan (Tragopan temminckii), Himalayan Monal pheasant (Lophophorus impejanus),
the Great Argus pheasant (Argusianus argus) and the Long-tailed or African Widowbird (Euplectes progne) in the action of seducing
the opposite sex.
Sexual selection may occur within the same sex (intrasexual selection),
where individuals of one sex compete directly for mates of the opposite sex
(for example through combat or sperm competition). It may also be intersexual,
where individuals of one sex (usually, but not always, the females) are picky
in selection of mates of the opposite sex. This selection usually depends on
the male’s sexy appearance or behaviour, and these attractive traits sometimes
pose some risk for the individual.
In several species, there is a
difference in factors such as appearance and behaviour between males and
females, which is termed sexual dimorphism. For example; Swordtail fish have a
colourful and elongated caudal fin (called a sword), which is absent in females
(Basolo, 1990). In male Narwhals (Common name: Unicorn of the Sea, Scientific
name Monodon monoceros), one
of the two teeth that they have grows out of the jaw into a long tusk that that
is most likely used to impress females with their evident fitness superiority
over the other males.
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| Female and Male Swordtail fish Photo sourced from http://fishyaquaria.wordpress.com/2010/02/03/keeping-and-breeding-the-swordtail-fish/ |
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| Male Narwhal Photo sourced from http://www.bbc.co.uk/nature/life/Narwhal |
So wait. Hold up... Why on
earth do the males get the pretty colours and cool extra structures??? How did
we, the females, end up with the bland colours, shapes and sizes??? Maybe the
following diagram will help you understand:
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| Sexual dimorphism - the difference between males and females Image by Mathilde Gordon |
It is important to note that
sometimes there are sex role reversals, however for the simplicity of this
blog, and because sexual selection is a sub-category of what I will be talking
about, I will not go into too much detail on this. In general, members of the
sex which endure strong sexual selection will be competitive, members of the
sex subject to weak sexual selection will be choosy.
There are different selection
pressures on females and males which cause sexual dimorphism. Sexual dimorphism
is often associated with traits that appear to contradict natural selection.
However, when sexual selection puts pressure on the organisms in the opposite
direction, and the benefit of attracting a mate outweighs the risk of predation
and energy inefficiency, then the overall reproductive success is worth this
cost, and the species lives on.
Now, if YOU would like to understand the following diagram, tune in for the
next post!
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| Image by Mathilde Gordon |
References:
- Basolo, A.L., 1990, ‘Female preference for male sword length in the green swordtail, Xiphophorus helleri (Pisces: Poeciliidae,)’ Anima Behaviour, No. 40, pp332-338
- Freeman, S., Herron, J.C., 2014, Evolutionary Analysis, Pearson Education Limited, Edinburgh.
- Pryke, S.R. & Andersson, S. 2005, ‘Experimental evidence for female choice and energetic costs of male tail elongation in red-collared widowbirds,’ Biological Journal of the Linnean Society, No. 86, pp35-43
- http://www.bbc.co.uk/nature/life/Narwhal (23rd March, 2014)
- http://fishyaquaria.wordpress.com/2010/02/03/keeping-and-breeding-the-swordtail-fish/ (23rd March, 2014)
A very interesting topic, I'll be waiting to read all about what Austin Powers has to do with that diagram!
ReplyDeleteDo you know if sexual size dimorphism is subject to the same selection pressures as other types of sexual dimorphism? Or is the difference in size between males and females in some species a static trait?
Hi Rhys, sorry for the late response, unfortunately I only just noticed your comment.
DeleteThere are a few types of selection pressures that have been proposed as an explanation for size dimorphism. One is male combat, where often large size increases the success of males when it comes to fighting, and hence also increases their fitness.
A potential explanation for small males is dispersal. That is, the most mobile males may be able to maximise their fitness by putting more energy into locomotion rather than anything else. So in a lot of cases, smaller males leads to more mobility.
Current evidence for sexual size dimorphism also suggests that it could arise as a result of the different reproductive roles of males and females. In a previous post I talked about females often having a higher parental investment than males. So there is a hypothesis that states females in some species will have a larger body size as this increases their fecundity (perhaps they have more babies at a time and can put more effort into looking after them), whereas there won’t be any particular reason for the males to have a large body size.
This article explains it quite nicely, relating it to turtles and the different body sizes of males subject to different environments I hope that answered your question in one way or another.
Berry, J.F. & Shine, R. 1980, ‘Sexual Size Dimorphism and Sexual Selection in Turtles (Order Testudines),’ Oecologia, Vol. 44, No. 2, pp. 185-191
Great that you’ve incorporated your own images! It’s interesting that we always tends to use birds as the iconic example of sexual dimorphism when discussing sexual selection, most probably because of the numerous awesome examples out there (such as those in the video). Are there any cool insect or reptile examples of sexual dimorphism in relation to sexual selection? Great blog.
ReplyDelete