Evolution of Reproductive Behavior Continued

Sexual Selection Darwin: “the advantage which certain individuals have over others of the same sex and species, in exclusive relation to reproduction” Accounts for costly courtship rituals and elaborate morphology Time, energy, developmental trade offs, visibility to enemies Slightly different from regular natural selection which emphasizes enhancing survival (a special case) Pipefish. Why do they demonstrate sexual selection vs just male female roles? Importance of these examples of role reversals.

Developmental Cost Dung beetle: horns or eyes? Horns win fights, but probably not all because the others can see better.

When males compete Dismantle bowers Defense Outright fighting Body size is selected Body weapons are selected Speed Horns, antlers, tusks Clubbed tails or spiny legs Sexual selection for the trait of watching and guarding the bower, willingness to fight to defend territory because loss of territory leads to loss of opportunity to mate. In fights, winners tend to mate more often so traits that enhance winning are sexually selected.

Speed is sexually selected

Convergent Evolution of body weapons

Dominance Battle for a short time early on to establish a dominance hierarchy Alpha-male reproduces more Fights less: threatening displays discourage would-be battles Savanna baboons Subordinate males will challenge alpha Males get bitten and infected > females Is it worth it? Initial studies found that males all mated about the same amount. Explain?!

Mating success Effects on mating success measured in many troups over many breeding seasons in a Kenya preserve.

And genetic success Gentoyping by PCR Key feature is to mate when the female is in estrous. And guard her from other males during her fertile period.

Advantage, but limited Reasons: subordinates show willingness and ability to care for a female’s offspring—she seeks him out next time Subordinates form male-male friendships and gang up on the alpha, forces him to surrender the female he has been guarding (18/28 cases observed in a pack of 8 males). Alternative tactics. Compensation.

Conditional Mating Strategies Evolve when circumstances are such that individual variation leads to differential success Evolution favors flexibility, so that when social or physiological constraints become apparent, an individual can select a behavioral tactic that gives them the best chance for success It is conditional because the same animals can adopt multiple different strategies over its lifespan depending on the conditions, socially or environmentally.

Conditional Strategies Even the playing field for disadvantaged members Why would this be selected for? Male horseshoe crabs swim and grab onto females (attached) or are solo, satellite males that crowd around paired couples hoping to mate with the female after the attached male releases her. Expt: wrap claws in plastic, put males at sea. Make note of the condition of the male, health of eyes, shell, etc. Poor condition males more likely to be satellite maters—have higher success than poor condition males that try to attach to females and fail. Main point: lesser males have greater success adopting a different strategy than using the strategy of higher males.

Alternate Mating Strategies Example: Faster sperm transfer Small male iguanas “pre-ejaculate” so that when they mount a female, on the fly, ‘old’ sperm can be immediately transferred before a larger male pulls him off.

Example: Body parts trade-off Uses small body size to sneak up and inseminate a female with more sperm than a larger horned male could.

DQ 7.8 Roosters exhibit male dominance hierarchies, with dominant males having greater copulatory success. Dominant males transfer more and faster sperm to females with red combs on head Subordinate males transfer same # and quality sperm to all mates How does sexual selection explain these differences? What would happen if two dominant males were placed together until one became subordinate? (Use conditional strategy theory)

Panorpa Scorpionfly: 3 strategies Secrete saliva on a leaf, wait for female to come by and eat it Defend a dead insect, a very delectable food gift for a female Grab and force to mate Who is most successful? Most copulations? Why? How would you test it?

Each tries to maximize fitness—dominant, more mates, give best female best sperm—assume red comb is better female. Subordinate, fewer mates, maximize each mating by giving equal quality sperm Newly subordinate male will start to produce fewer sperm and give equal quality to all mates 2 dead crickets, 10 males, 10 females in a cage. Different sized males. Test: remove largest males, observe behavior of remaining males