Why do females care?

For a male to put in so much effort to impress a female, you would hope that it is all worthwhile. Why do females care in the first place about ‘advertisements’ that males have adapted?

Below are some explanations, some of which are still in debate in the scientific literature.

Females that are picky might get better genes for their offspring

In this case, females have preferences for males who indicate their superior genes through more attractive displays. An example of this is the Gray Tree Frog (Hyla versicolor), a species of arboreal frog which live in the woodlands of the United States and South-eastern Canada. In the breeding season, these males give either short calls or long calls to attract mates. Researchers observed that a large amount of the time female Gray Tree Frogs purposefully ignored short-calling males and travelled greater distances in order to find a longer calling male in the distance. Welch et al (1998) studied whether males giving long calls were genetically superior to those with small calls by comparing the fitness of the tadpoles.  The conclusion was that offspring of the long-calling males had a higher fitness than those of the short-calling males, which was particularly evident in their ability to feed better as tadpoles.

In this hypothesis, good genes evolve under direct natural selection, and as a result, the preferences of the opposite sex evolve due to indirect selection (Ryan, M.J. 1998).

Gray Tree Frog - Image by Marc Behrendt

Females that are picky might benefit through gaining resources

In this case certain males of the species may provide better resources (such as food, superior territory and parental care) compared to other males, and consequentially, they appear more attractive to the female (Freeman 2014). Over the generations, females that distinguish good providers from bad ones will chose the one that is beneficial to either her or her young (or both) and males with better provision of resources survive and reproduce better than others, even if their trait appears to contradict natural selection. 

An example is the beautiful Blue Tit (Parus caeruleus), whose carotene colouration is directly related to their ability to find caterpillars which are a good sense of nutrition for young birds. In an experiment to distinguish between the good-parent hypothesis (direct signal) and the good-gene hypothesis (indirect signal), Senar et al (2002) concluded that yellow plumage brightness of father Tits is directly correlated with their offspring investment, and therefore females sexually select for these males.

 

Blue Tit Male - Image by Silvestr Szabo

Introducing the Fisherian Runaway/Sexy sons hypothesis

Runaway Selection - Image by Mathilde Gordon 

Ok, so I promised I would explain the Austin Powers photo from my last post. Well, here goes.

Just like the good genes female preference hypothesis, the hypothesis for runaway selection states that preferences evolve because they are genetically correlated with male traits which are under direct selection (Ryan, M.J. 1998).

...Say what?

Basically, female preferences and male traits co-evolve through genetic correlation. In the good genes selection, male display signifies to the female a superior genotype, whereas in runaway sexual selection, females prefer the attractiveness of “sexy sons,” (a.k.a. Austin Powers) which are males that have a greater mating success, but aren’t necessarily superior in terms of other aspects of fitness.

It starts with a trait arising in a population through natural selection. A majority of females favour this trait, based solely on the fact that the male possesses it. It is an arbitrary preference, based on a completely random whim. Even when this trait is no longer advantageous, it remains to be sexually selected for. Over the generations this leads to a runaway selection as a result of a positive feedback mechanism. Certain males with the right trait are selected for à they reproduce à their offspring are more likely to possess the trait à females become more likely to have a preference for that trait à more males with the right trait are selected for, etc.

There is selection on the females to maintain the preference and selection on males (by the females because of the preference), and this forms a continuing loop, as shown in the image above!

In previous blogs I have talked about female choice being the reason why males evolve such elaborate traits. Although these traits don’t seem to “follow the rules” of natural selection, the consequential increased mating success of the males more than compensates for threats to their survivorship. I hope this last blog has demonstrated a few reasons why females care about these traits in the first place, and how they come about evolving preferences.

References:

• Freeman, S., Herron, J.C., 2014, Evolutionary Analysis, Pearson Education Limited, Edinburgh
• Ryan, M.J. 1998, ‘Sexual Selection, Receiver Biases, and the Evolution of Sex Differences,’ Science, Vol. 281, No. 5385, pp. 1999-2003
• Senar, J.C., Figuerola, J. and Pascual, J. 2002, ‘Brighter Yellow Blue Tits Make Better Parents,’ Biological Sciences, Vol. 269, No. 1488, pp. 257-261
• Welch, A.M., Semlitsch, R.D. and Gerhard, H.C. 1998, ‘Call Duration as an Indicator of Genetic Quality in Male Gray Tree Frogs,’ Science, Vol. 280, No. 5371, pp. 1928-1930
• http://www.paleoprep.com/Photography%20page/Frogs/01_image.htm, (29^th March 2014)
• http://www.photoextract.com/photo/274073.html, (30^th March 2014)