Dancing with the Stars! Sexual selection Intrasexual selection: most common is male-male competition Intersexual competition: most common is female choice What does the fancy stuff say about the male? Are there any cases where sexual selection acts more strongly on females than males? GOAL Why does sexual selection act mostly on males? Predict when would it act on females Investigate secondary hypotheses since Darwinian sexual selection Bachelor/ette ABachelor/ette B Averaged faces made symmetrical. Averaged faces. Natural faces Artificially symmetrical faces Renee Firman and Leigh Simmons University of W. Australia a relationship between body symmetry and semen quality and quantity in men Across diverse taxa, increased asymmetry is associated with: Increased morbidity Mortality Poor fecundity Developmental Stability and Evolution Moller and Swaddle Part of the Jamaican Symmetry study In humans fluctuating asymmetry has been shown to be related to age, body weight, metabolic rate, running speed, intellectual performance, predisposition to breast cancer, and other harmful conditions Assumption = symmetry implies underlying good genes Batemans principle In species where mothers invest more in the offspring than do fathers, females are expected to be more selective in mate choice And so males are expected to invest more in courtship display Option 1 Be fabulous Option 2 Build or do something fabulous Option 3 Make more sperm A highly polygamous species of woodmouse Females mate with a single male. Females mate with multiple males. ? ? Option 4 Make better sperm Drosophila bifurca One D. bifurca sperm (6 cm long, about 20x that of the fly) The idea for today If you look fabulous, or are doing something fabulous, a female needs to correlate the fabulousness with underlying good genes Darwin: The females are most excited by, or prefer pairing with, the more ornamented males, or those which are the best songsters, or play the best antics; but it is obviously probable that they would at the same time prefer the more vigorous and lively males, and this has in some cases been confirmed by actual observation. Origin of the Species If so, then Courtship rituals should reveal genetic or phenotypic quality (and therefore, symmetry) In Jamaica Dance is experienced as an important courtship ritual Nature 438: (Dec ) Predictions the authors made: 1.Symmetrical people will be perceived as better dancers. 2. Symmetrys effect will be stronger in males. 3. Females will be better at seeing the effect of symmetry on dancing ability. Thus, the degree of symmetry should more strongly correlate with male dancing and females should discriminate better Dancing was recorded using reflectors on the dancers bodies. OK, ladies! Which dancer is more symmetrical? Bachelor number one or Bachelor number two Symmetrical people are perceived as being better dancers Results Females are better discriminators A jarcana, a species where females compete for males Zahavis handicap principle Female thinking Wow- for you to survive, with that big of a target on your back, you must be pretty robust Fishers Runaway Selection Hypothesis The trait being selected for is driven to such an extreme that fitness is reduced. Fishers Sexy Son hypothesis Female choice: Small antlers = son with higher chance of survival Big antlers = son with higher chance of having sex Zahavis honest advertising (assume same scenario as previous slide- this Is a lead-off of the handicap principle) In version A, the male has a long tail that he grew and with which he survived the slings and arrows of multiple negative selective forces. In version B the male does not have an excessively long tail, but whenever he senses that a female may be looking, he hangs a long branch of a certain tree from his butt so he looks like he has a long tail. Females that cant choose have lower fitness. Females are selected to detect honesty. Here, females drive males into a state of fragility by selecting for traits that reduce survivability and longevity. This allows the female to get what she needs from the males (a mix of genes) with a greater chance that the male will fall out of the picture sooner than later. Linden Fragile Male hypothesis A very fit, asymetrical male!