Sexual Selection

Sex and the Marine Environment

What is sexual selection?   Natural Selection: Differential reproductive success due to variation among individuals in survival and reproduction.

What is sexual selection?   Darwin (1871) defined sexual selection as: Differential reproductive success due to variation among individuals in success at getting mates.

Sexual selection is non-random variance in reproductive success. Two forms of sexual selection: Intrasexual selection: direct competition for mates between members of the same sex, usually male-male competition. Intersexual selection: differences in attractiveness to the opposite sex, usually non-random mate choice by females. RANDOM MATING piece of Hardy-Weinberg : hinges on both natural & sexual selection. Some debate – are NS and SS really different things? Intraspecies’ selection versus interspecies / interenvironmental

Mating systems Monogamy One male mates exclusively with one female Polygamy Individuals mate with more than one partner Polygyny Some males mate with more than one female Polyandry Some females mate with more than one male Promiscuity Males mate with more than 1 female and vice versa Monogamy Polygyny Promiscuity

Mating systems Mating systems influence sexual selection Strength of sexual selection Variance in mating success All males have same mating success = 1 mate Some males = 0 mates Some males = 1 mate Some males = 2 mates Most males = 0 mates One male = 8 mates Monogamy Moderate Polygyny Strong Polygyny

Harem polygyny Elephant seals (Mirounga) - breeding females cluster together on beaches - allows males to defend a harem of many females at once Male elephant seals weigh up to 3x more than females! Male reproductive success is highly variable: 8 individual males inseminated 348 females in one study! Bull male elephant seals engage in violent, bloody fights over females – large size confers an advantage in male combat

Polyandry When is polyandry favored? What is the consequence? When males become limiting resource for reproduction This often occurs when low offspring survival requires male parental care, so that males have the greater Reproductive effort and lower reproductive rate What is the consequence? Sexual selection is stronger on females than males Saddleback Tamarins, Spotted Sandpipers, and Red-necked Phalaropes are examples of polyandrous species

The form of Sexual Selection is directly related to the relative investment in offspring production. Bateman’s principle: The sex that invests more in offspring production has fewer reproductive opportunities. Thus, the sex that has a higher investment Should be more discriminating (choosier). Become a limiting resource for the opposite sex. This is usually the female sex

ANISOGAMY FEMALES: Sex that produces relatively few, large well-provisioned gametes (eggs) Theoretical work – proposes disruptive selection on gamete size from ancestral isogamous condition. Initial favoring of more, smaller gametes to increase fertilization chances. Subsequent disruptive favoring of larger (but fewer) egg gametes to increase fertilization success (viability, nutrition, etc.) In humans – sperm and eggs are smallest / largest cells in the body respectively. MALES: Sex that produces many, “cheap” gametes (sperm)

Females are more invested Females also often invest more than males in other forms of care Females bare the burden of pregnancy Care for offspring Only in fish is male care more common than female care Males guard the eggs in their territory so they can also secure multiple mates

Why don’t males care for offspring more often? Females can sometimes mate multiply (polyandry) Therefore, not all offspring may be sired by the male

Why don’t males care for offspring more often? Females can sometimes mate multiply (polyandry) Therefore, not all offspring may be sired by the male (paternity sharing) Opportunity costs A male’s mating success is more driven by how many pairings he can achieve Especially because females are choosy

Why do females invest more in offspring care? If females invest more prior to mating, there is competition amongst males for female investment This causes variability amongst male success As long as females continue to provide care, it is less likely males will contribute (a positive feedback loop) Dominant males with great mating success are not inclined to care after mating

The form of Sexual Selection is directly related to the relative investment in offspring production. Bateman’s principle: The sex that invests more in offspring production has fewer reproductive opportunities. Thus, the sex that has a higher investment Should be more discriminating (choosier). Become a limiting resource for the opposite sex. Evolution of gender and sex resources This is usually the female sex

The asymmetric nature of sexual selection often leads to dramatic sexual dimorphism in characters directly related to male-male competition and/or female choice. Male-Male competition – Intrasexual selection Female choice – Intersexual selection. The peacock example is an example of…? Peacock Peahen

Sexual dimorphism

Selection for combat in males Natural selection selects for male morphologies that make them more competitive Larger body size Greater strength Weaponry

Selection in Males: Body Size Larger male copepods can carry larger spermatophores Mating with large males allows females to fertilize more eggs and reduces the need for frequent matings

Weaponry Fiddlers! h4ales compete for the possession of burrows that are more vlslted females, thus taking over burrows is an alternative strategy to burrow digging Male waving activity, mudball distance and claw size can beconsidered as multiple visual signals available to females for mate choice. Females' first choice cn- tenon would seem to be burrow quallty

Weaponry Narwhals! Biology traditionally considers the tusk a secondary sexual characteristic, similar to the mane of a lion or the tail feathers of a peacock. It may help determine social rank, maintain dominance hierarchies, or help young males develop skills necessary for performance in adult sexual roles. Narwhals have rarely been observed using their tusk for fighting, other aggressive behaviors, or for breaking sea ice in their Arctic habitat. The tusk is an innervated sensory organ with millions of patent nerve endings connecting the external ocean environment with the brain.[19][20][21][22] There is at least one recorded case of a tusk being used against another species. A broken tusk was found embedded in the melon of a beluga, suggesting a figh Females sometimes possess tusks but it is rare

And in terrestrial birds Some really awesome courtship behaviors Marine birds

(Megaloceros giganteus) Sexual selection can be very strong and often opposes natural selection. This can lead to exaggerated and sometimes maladaptive development of male traits. Irish Elk! Most recent carbon dating of fossils – extinct ~7700 years ago. Lot of debate as to why extinct. Combination of human hunting, climate change. Antlers – nutritive stress during unstable food periods wrought by climate change. Other deer w/ less ridiculous ornamentation seemed to do better. Irish Elk (Megaloceros giganteus)

Sexual vs natural selection Guppies (Poecilia) - sexual selection can favor traits that reduce survival - laboratory selection studies by John Endler Natural selection (predation) favors spot patterns that match background Sexual selection (mate attraction) favors male patterns that contrast background

Intra- and inter-sexual selection Intrasexual selection All of the examples thus far have involved interactions within a sex - Darwin’s theory of intra-sexual selection was readily accepted, even in his time Intersexual selection Intersexual selection has always been controversial We know mate choice occurs because we can observe it directly However, it is debated if and how some sexually dimorphic traits evolve in response to mate choice

Are females really choosy? Females can prefer elaborate ornaments Long tailed widowbirds Elaborate ornaments are often costly

Why are females choosy? Two broad benefits: Good resources Good genes Competitive males often secure the best territories Good genes Body size and other competitive characteristics will be inherited by the females offspring Humback whale size and mating calls But not all mating systems involve competition for mate territories

But what are the genetic benefits? 1) Fisher’s hypothesis: gaining attractive sons Females want sexy sons to attract more mates in the future Often produces runaway sexual selection Usually begins with a trait that confers some adaptive benefit and then is further exaggerated because it is selected for by females

Fisher’s runaway model Fitness Tail length Total male fitness (survival + mating) Fitness due to survival Survival Selection Sexual Selection Female choice adaptive for survival

But what are the genetic benefits? 1) Fisher’s hypothesis: gaining attractive sons Females want sexy sons to attract more mates in the future Often produces runaway sexual selection Usually begins with a trait that confers some adaptive benefit and then is further exaggerated because it is selected for by females 2) Handicap hypothesis

GOOD GENES MODEL The Handicap Principle (Zahavi 1975) ELABORATED MALE TRAITS MAY BE INDICATORS OF HERITABLE GENETIC QUALITY (I.E. FITNESS). The Handicap Principle (Zahavi 1975) Some males may have a heritable trait that reduces viability. Only males with “Good Genes” can survive despite the handicap. Females that mate with these males will have offspring with higher fitness. Female offspring particularly fit because they inherit good genes without the heritable handicap

GOOD GENES MODEL Hamilton and Zuk added to this hypothesis: ELABORATED MALE TRAITS MAY BE INDICATORS OF HERITABLE GENETIC QUALITY (I.E. FITNESS). Hamilton and Zuk added to this hypothesis: Sexual displays are reliable indicators for genetic resistance to disease or parasites Advertisements of healthy animals

Evidence of females choosing for sexy sons Females prefer brighter red colors And the intensity of red in sons is correlated with the red in his father But is the only benefit of this trait increased matings? Males with bright red color should not show increased viability, just increased mating success

Evidence of females choosing for good genes Reddish color is derived from carotenoids in a fish diet Better color, better foraging, Even subordinate males express this color early in the mating season, but only more fit males are able to maintain it though the breeding season They guessed that males in poor condition would go all out to produce the reddest signal they could early in the mating season, because they might not have the chance to mate for long before expiring. Healthier males, on the other hand, would keep some of their strength in reserve, because there would be more opportunities for them to mate in the future. This assumption was borne out when the researchers tested their hypothesis. At the start of the breeding season, all of the males had a good red throat, and females paid little attention to the degree of redness when choosing a mate. They didn't trust it as a signal of reproductive fitness. As the summer wore on, however, only the fittest males could maintain the colouration, and only at this late stage did females develop a strong preference for redder throats.

Healthier male fish are more red Red coloration declines in males infected with parasites Healthy red males were also likely to have a strong MHC profile

Genetic Benefits 1) Sexy sons and daughters Only the sexy are selected 2) Good genes as fitness indicators Only good quality males that can afford elaborate ornaments can survive But then how is variation maintained in a population? Mutations Traits are quantitative (many genes are responsible) Variation in choice over time

Female behavior can also help to maintain genetic variation Female copying- females imitate the choices of other females 3 morphed isopod Female isopods prefer to mate with other females and can copy matings of other females This may account for the maintenance of the 3 male morphotypes Female isopods prefer to breed with other females in a spongocoel and while this increases the harem size for alpha males it also gives beta and gamma males more of a chance for mating success

Sex Role Reversal When males make large contributions to parental investment, males may become the choosy ones Seahorse mating In pipefish and seahorses males have a brood pouch in which heholds and protects the fertilized eggs Females can produce several clutches of eggs, so the limiting resource becomes males

Sex Role Reversal When males make large contributions to parental investment, males may become the choosy ones This produces a similar sexual dimorphism in females that have larger body sizes to compete for males

Sexual Conflict Over Mating If males need to maximize matings and females need to choose the best partner Then females need to resist mating and males need to be aggressive Results in a sexual selection arms race Males competing for a female sea turtle

Sexual Conflict Over Mating Male Traits Female Traits Enforced Copulation Resistance Intromittant organs which enhance mating success Elaborate reproductive tracts that are obstacle courses for sperm Mate guarding, frequent copulation Seek extra pair copulation Remove or displace sperm of rival males Sperm Ejection Copulatory plugs and ant—aphrodisiacs Sperm choice Accessory glands to manipulate females Chemical defense

Sexual Conflict Over Mating The male guppy grows claws on its gonopodium (anal fin) to counteract sexually-selective females.

Chase Away Sexual Selection This sexual conflict can drive antagonistic co-evolution between the sexes Females will evolve resistance to male ploys because acquiescence is costly Female swordtail prefer mlaes wit longer swords, and so do swordless platyfish, suggests a pre-existing female preference/bias for tails But the preference and the trait should evolve together according to fisher, but female preference is stronger is species that don’t have swords, suggesting female attraction to swords has declined as swords evolveds

Sexual Conflict Selection has favored Littorina saxatilis female snails that do not signal their gender in their slime trails The females of most species of snail excrete a substance in their mucous trails that enables males to find them more easily, since they can distinguish between trails from females and those from other males. The males follow the mucous trails laid down by females in order to find a partner for mating. However, the females of one of the species studied (Littorina saxatilis) have stopped labelling their mucous trails. The consequence of this for the females is that they copulate less frequently, since the males often follow the trails of other males and must therefore spend twice as long looking for a female. This may appear strange at first sight, since we expect it to be in the females' interests to mate. But we show that copulation is costly for the females and that they already achieve more copulation than is required to fertilise all of their eggs," says Kerstin Johannesson, professor of marine ecology.