Andersson Widowbirds

Charlies Darwin postulated that males’ ornamentation will bear some cost on survival, attracting predators or reduce mobility, but this loss of fitness will be regained by increased mate success. These traits are used in direct male-male conflict or are preferred by females. Andersson Widowbird (Euplectes progne) is classic case demonstrating a trait that reduces survival, but increase mate success in 

Male Widow bird in flight
sourced:www.biodiversityexplorer.org

Andersson figure
 showing results of experimental alteration

             Widowbirds are highly sexually dimorphic females are a plain brown camouflage; whereas male are black with red marking and have extremely long trails. The tail of male Widowbird is so long that it disrupts its ability to fly making the easy targets for predation. In Andersson experiment, looked male-male interactions and females mate choice. He found that a male, regardless of trail size, could hold equal amount of territory; this meant female selection must be driving the process. By experimentally altering male tail lengths, adding extensions or cut feathers, he showed that females were selecting for longer tails and therefore maintaining the trait. The is a classic paper of evidence for sexual selection and mate choice

References  

Andersson, M. 1982, "Female choice selects for extreme tail length in a widowbird", Nature, vol. 299, no. 5886, pp. 818-820.

Predation risk and mate choice

The essence of mate choice is to select a partner that will give you the most offspring, and that will in turn be able to pass their genes on to the next generation. Females’ preference is often driven to the fittest males do this. Males often display their fitness by having colourful visual marking, advertising their resource acquisition ability.  However, a colourful male will be more likely do to attract attention from predators as well. With these to pressures, a trad-off between pretty or eaten is established. However, female guppies (Poecilia reticulata) have shown their ability to select correctly.

Three morphs of male guppies

This was demonstrated when, females from two populations high and low predation , were tested to see what effect having a predator had on mate choice. Researchers found that if a predator was could not be seen, females preferred males with colourful markings. However when tested and the female could see a predator, the one from a high predation environment demonstrate a change in mate preference selecting less ornate males. This seems show that, female guppies can account for this trade-off and select the males that will give the most offspring for the environmental pressures.

Godin, J.J. & Briggs, S.E. 1996, "Female mate choice under predation risk in the guppy", Animal Behaviour, vol. 51, no. 1, pp. 117-130.

Human sexual selection: male voice and face shape Part 1

Continuing from my last post, I want to look at the other factor thought to influence male features (face and voice): female-male interactions, where females select males, favouring traits that females find attractive. A man’s face and voice works as advertising for their mate value and health. This was shown by a study that asked women to rate attractiveness of digital altered males faces and voices, women would often rank more robust faces and deeper voices more attractive. These results shows that females will have an effect on male feature. It was concluded from this that an “average” male face and voice was the most attractive. However, the case in not so simple, there are trade-offs with mating with over masculine men.

Images showing a digital alter male face to look more or less masculine.

Having more masculine features is generally linked with higher levels of testosterone, however heighten testosterone may have other consequences for a man’s health and behaviour. Higher levels of testosterone can act as an immunosuppressant, affecting a man’s general health and so his long-term mate value. Though, there has been studies showing that the level of suppression maybe condition dependent and therefore have greater impact on males in poor-condition. This would mean a masculine male would be strong and dominate when in good- condition, but once his health declines he would become a poor mate choice. And so, this would allow less dominate males to have equal mate success, because they would be around for a longer time, and would be a better as a long-term mate selection for a females.

Another disadvantage of selecting masculine mates is the difference in behaviour. Evidence has shown a positive correlation between testosterone levels and male infidelity, violence, divorce rate and commitment to offspring. More masculine males are perceived to possess other undesirable traits such as emotionally cold and uncommunicative. These undesirable behaviours result in women favouring men with less dominate traits as long-term partners. It is therefore not as easy as being the most masculine to win mates, it is also depends on one the long term befits associated as a partner. In addition to this, female preferences maybe also be dependent other factors, such as her own attractiveness; hormonal levels during different stages of menstrual cycle; as well as past experiences with men.

As one may imagine, it is not all so simple.  Human attractiveness is based on far more than the initial appearance, of which there is multitude of variables. The research conducted often asks basic question- attractiveness 1-10-  using portrait images, this approach, which is good for standardisation, fails to take into account gait, fashion, personal, pheromones etc., though it does provide and interesting perspective from which to look.

References

Puts, D.A., Jones, B.C. & DeBruine, L.M. 2012, "Human sexual selection ", Journal of sex research, vol. 49, no. 2-3, pp. 227.

Human sexual selection: male voice and face shape Part 1

Human sexual selection is a complex system of interactions, even with our modern social constructs and laws in the mix nature still plays its role. Humans are highly sexual dimorphic primate with differing body form and behaviours for males and females. Here I wanted to look at what selection pressures determine male features, in particular the traits of male voice and face shape. I did not look at female features here, as some research suggests that female face shape and voices are only somewhat influents by sexual selection, and that selection acts mainly on males.

Male features are thought to be influence by two selection factors: male-male interaction, where males are in contest with each other, favouring traits for excluding other males; and female-male interactions, where females select males, favouring traits that females find attractive. In male-male interaction, a male fends off other males in two ways by actual threat or by potential threat – display of dominance.  Although, actual threats may be less important in our modern society with law preventing them, they would have been in an evolutionary context a key factor. A perceived threat, on the other hand, may still affect mate success, as it would exclude other males. Being able to exclude other males would give a greater chance of mate success. And so, being/showing dominance by your outward appearance, from a male-male perspective, can be advantageous.

Studies have shown that there is a relationship between masculine vocal and facial features, and dominance. If then dominance led to better success, natural selection would act on these two traits. There was found to be a correlation between males having a lower more masculine voice and the perception of dominance by other males. In a study, it was shown that men would alter the pitch of their voice in accordance with their perceived dominance. It was seen that, a man would lower his pitch when talking to a male he thinks to be weaker than himself, and raise the pitch if he thinks he is weaker.

 By a similar token, men with stronger jaw line are perceived as being more masculine and more dominant. A stronger jaw line is thought be dominant in both appearance and practicality. Unlike voice, having a strong jaw would help in an actual fight; strong jaws are less likely to be broken. It is also thought that, facial hair may have evolved to increase the apparent size of the jaw. This could explain why some studies had found that: a male’s face with a beard was rated more dominate than clean-shaven one. Therefore, it can be seen that male-male interactions are controlled by the traits that intimidate/exclude competitors by a show of dominance.

to be continued 

References

Puts, D.A., Jones, B.C. & DeBruine, L.M. 2012, "Sexual selection on human faces and voices", Journal of sex research, vol. 49, no. 2-3, pp. 227.
C.L Apicella, D.R Feinberg & F.W Marlowe 2007, "Voice pitch predicts reproductive success in male hunter-gatherers", Biology Letters, vol. 3, no. 6, pp. 682-684.
Boothroyd, L.G., Cross, C.P., Gray, A.W., Coombes, C. & Gregson-Curtis, K. 2011, "Perceiving the facial correlates of sociosexuality: Further evidence", Personality and Individual Differences, vol. 50, no. 3, pp. 422-425.

Bearded ladies

In sexual dimorphic species, it happens that females sometimes have rudimentary male structures, such as ornate markings. These structures are often code for by the same genes in male and females, however in females they incur both social and reproductive costs.  A study looking at Eastern Fence Lizards (Sceloporus undulates), examined the reproductive success of females with and without these structure.

a) males b) ornate female- 'brearded lady' c) standard female

The male Eastern Fence Lizards have bright blue throat marking, which they use when courting females and warding off other males. The females tend to have white throats and bellies, and a general brow colouration, though some females have a blue tinge – ‘bearded ladies’.

In experiments, done by Pennsylvania State University, the effect of being a ‘bearded lady’ was assessed. The team captured and raised lizards in the lab, and then observed mating behaviour. Researches presented males with two females, one with ornate marking, and one without. They found that males preferred female without makings, spending more time on court ship. Ornate females also had a lower reproductive output, and eggs that were laid hatched later then non-ornate females. This showed that ‘Bearded lady’ lizards suffer a cost from their features.

Where there is a cost there must be some benefit, and the exact benefits are not resolved. There a a few postulated benefits, include more aggressive females-due to higher testosterone- which serves which may serve in female-female completion or in predator defence. The trait could be a “snapshot” of a phenotype that is about to be lost from the gene pool.  The reasoning for this trait still waits to be revealed.

Reference: 

Swierk, L, Langkilde, T 2013, ‘Bearded ladies: females suffer fitness consequences when bearing male traits’, The Royal Society, vol.9 no.6.

Isopod morph α, β, ϒ

In the Gulf of California, living on intertidal sponges is a species of isopod Paracerceis sculpta, which displays another type of male polymorphism. Males have ,again, three morphs body types that each has variation in reproductive successes depending on what other type of males are present. Living on or in the sponge, three male morphs maybe present: 

ϒ                             β                                  α
The tree male morph types

Large α males that defend harems, smaller β males that mimic females in appearance and behaviour, and minuscule ϒ males that hide themselves within the sponge with large harems. To understand how male-male-male interactions functions, researches observed and alter male densities and recorded their relative reproductive success. The reproductive capability of each male morph was found to be equal, if isolated with females.  However, depending on what male morphs, as well as the number of females, were present the reproductive success changed.  The researches devised a set of rules for matting success (Table 1).

The researchers, after two years of testing and observation, using genetic marks to trace paternity, found these rules to be effective way to explain the interaction. 

Reference 

Shuster, S & Wade, M 1991, 'Equal mating success among males reproductive strategies in marine isopod', Nature, vol 350, pp.608-610.

An evolutionary game of Rock Papers Scissors.

 Introducing the common side blotched lizard (Uta stansburiana) from the coast range of California. This species has a very cool polymorphism among males. Polymorphism means that different males will have a different colours or body shape/size. In the population of side blotched lizards there are three types of male morphs:

Orange throat males:

 Larger and more aggressive than other male morphs. They can defend large territories and in direct male-male interaction will win. However, Orange males are not very observant of what its females are doing.

Blues throat males:

 Moderate size not as aggressive as orange males. They defend a moderate to small territory. Blue males are well connected and “know” each female in his group.
 Yellow throat males:
 Small, unaggressive and avoiding direct conflict. They do not defend any territory. Yellow males look like females and use this to sneak mates.

 Each one of these male morphs is heritable. The three types of males have their pros and cons, and the interaction of all three plays out in to a dynamic yet stable game of “rock, paper, scissors”. An orange male (rock) by using its superior strength can steals mates from blue males (scissors). However, an orange male will lose to a yellow “sneaker” male (paper), who thanks to the orange males inattention to his female. Yellow males infiltrate orange male’s territory, disguised as a female, and mates with the orange’s females. Yellow (paper) male’s strategy will fail when met by a blue (scissor) male. The blue males, with his smaller group of females which he “knows” individually, is attuned to when a yellow males tries to sneak to mate with his females and is able to scare off the smaller yellow male.

 

 This system was study for six years where researchers observed a fluctuating in the numbers of orange, blue, and yellow males in the population. The population of these lizards neatly demonstrate frequency-dependant selection. That is, as one morph becomes more prevalent it greatly increases the fitness of its counter male, and the counter male will increase its frequency. This new morph prevalence will favour the new morph’s counter male, and so the third morph will become dominate, which in turn will lose to the first morph.
 Even with a game like “rock, paper, scissors”, again we can see that nature thinks of things first.

Reference:
Sinervo, B. & Lively, C.M. 1996, "The rock-paper-scissors game and the evolution of alternative male strategies", Nature, vol. 380, no. 6571, pp. 240-243.