Sexual SelectionSexual Selection YTR21os8gTA

The fundamental difference between the sexes and its implications: - parental investment and sexual selection - sexual conflict: between and among the sexes - the evolution of bizarre morphology and behavior for the purpose of attracting mates - mate choice and why females are the choosier sex - an understanding of what each sex looks for in a mate - signaling theory and handicaps - mating strategies - why some species change their sex in their lifetime - why are males often the first to abandon their mates/young

Sexual Selection – differences in reproductive success caused by competition over mates that is related to the expression of traits for such competition Sexual Selection is not defined in relation to reproductive success – e.g., many traits such as a brood pouch or parental alarm calls increase survival of the young - but rather it is defined by improving reproductive success via competition over mates, whether by aggression, mate choice, or some alternative Rewriting Darwin’s postulates for sexual selection: - struggle for reproductive “existence” (i.e., access to/competition for mates) - variation in traits influences the struggle - that said traits are heritable

Sexual Selection is a subset of Natural Selection – but the evolutionary consequences differ in remarkable ways. Traits that evolve via Sexual Selection may be disfavored by natural selection, that is, selection that ignores competition for mates. By competition we mean the use of a resource that reduces its availability to others

Sexual Selection is based on variance in mating success. To demonstrate sexual selection requires that Variance in the trait leads to Variance in mating success due to competition among rivals, mate choice, or some other mechanism 3 examples: Elephant seals ASsg

Red Deer F M Reproductive lifespan 27 7 Fecundity/mating success 8 32 Offspring survival Percent contributions to LFR

Percent of copulations (105 observations) Sage grouse

These 3 examples illustrate: (1) strong variation in reproductive success – many males in fact may never mate at all (2) Variation is influenced age, which often serves as a correlate for experience or body size/size of fighting appendages (3) That the strength of sexual selection – inferred from the degree of variance in reproductive success – is most intense among males

How did it all begin? sexes often identified via 1° and 2° sexual characters – the fundamental difference is: Sexual reproduction – gamete formation by meiosis and fusion of genetic material from two individuals almost always involving 2 sexes: male and female Anisogamous Reproduction Females = produce large, immobile, food-rich gametes called eggs Males = produce tiny, mobile gametes (sperm) that are little more than self-propelled DNA

Anisogamous Reproduction was probably inevitable... There are 2 selection pressures on gametes: (1)larger zygotes improves zygote survival (2)more gametes increases chances of fertilization As resources for reproduction become limiting, these 2 pressures oppose one another – the compromise solution is the evolution of 2 different sexes, one producing few, large gametes the other many, small gametes. This fundamental asymmetry in gamete size and associated investment in offspring has led to the far-reaching consequences of sexual behavior …

Females invest more resources than males into each offspring, both at the gamete level and often in other forms of care Males can fertilize eggs at a faster rate than they are produced – 5 ml of human semen has enough sperm to fertilize 2x the USA population!! Thus, females are a scare resource for which males compete Males increase their rep. success by finding and fertilizing as many females as possible Whereas females can only increase rep. success by turning food into eggs or offspring at a faster rate Or better yet, increase offspring survival …. males females (after Bateman 1948) Offspring versus number of mates Drosophila melanogaster

Females invest more into parental effort (investment over a lifetime) Males invest more into mating effort Males have a greater maximum potential rate of reproduction Elephant seal1008 Red deer2414 Man88869 Kittiwake gull*2628 Male Female The relative payoff for a successful male is much greater than for a successful female – therefore males are often under tremendous sexual selection for competing for mates Percent of copulations

Sperm competition – fights are risky activities and the benefits may not always justify the costs involved, therefore many males compete with each other using less conspicuous, but effective and bizarre means Dragonflies (Odonates) are one of a numerous number of inverts that store sperm (in the spermatheca) for use at a later date

Sperm displacement in Odonates barbed, whip-like flagellum used to remove sperm in the narrow ducts of the spermatheca Horn-like structure repositions sperm of the previous male into the corners of the spermatheca Coccethemis erythraea Orthetrum cancellatum uninflatedinflated penis

Mate-guarding combats sperm displacement strategies

Other options are to dilute the effect of your neighbor... Testes Mass (g) against Body Mass (kg) multi-male groups single male harems monogamous H human – 20g gorilla – 35g chimpanzee – 120g

Other bizarre forms of sperm competition “Chastity Belts” (copulatory plugs) – Monoliformes dubius (Acanthocephalan worm) – cements shut the female’s genital opening Xylocoris maculipennis – fertilizes the female by simply piercing the body cavity; sperm swim around until they encounter eggs to fertilize. Occasionally males pierce other males – the sperm then swim into the victims testes where they wait to be passed on to a female at the next mating male Heliconius erato butterflies deposit an anti-aphrodisiac on females after mating to discourage other males – an odor males use to repel each other in other contexts

Mechanisms of Sex. Sel. - Scrambles early search and quick location of mate; well-developed sensory and locomotory organs - Endurance ability to remain reproductively active for a longer time - Contest** traits that improve success in fights; alternative mating tactics of inferior competitors to avoid contests - Mate Choice** traits that attract and stimulate mates; offering direct benefits (territories, nutrition) to mate; alternatives such as forced copulation - Sperm Competition** mate guarding, sequestering sperm, mating plugs; production of abundant sperm to dilute out your neighbor