What defines males and females? Gamete size.

Males produce many small gametes (sperm). Females produce a few large gametes (eggs).

How did dichotomy come about? In distant ancestor gametes probably were equal sized. Shed into water and joined together to form zygote.

Any small fast gamete would have had a selective advantage in seeking out others. Expect sperm to evolve.

Once sperm evolved, large egg is only other logical strategy. Small or medium-sized eggs provide too little nutrient for zygote When fertilized by a tiny sperm. Hence, get bimodal distribution of very small and very large gametes.

Hamster egg and sperm

Because of the difference in parental investment of two sexes, females (in general) are the choosy sex. Parental investment: any effort (nutrients, time, energy) invested in producing and rearing young.

This fundamental difference dictates basic patterns of male and female reproductive behavior. Females can produce only a few offspring because they must invest heavily in each.

In general, females increase reproductive success by investing heavily in the offspring and by mating with the best quality males. Kiwi and egg.

Female ruff assessing male

Males increase their reproductive success by increasing their number of matings by competing for access to females.

The greater the difference in parental investment by the two sexes, the more intense the competition among members of the sex that invests the least (generally males) for access to the opposite sex.

In many species, males contribute little or nothing to rearing the young. In these cases, females choose best male genes or allow males to fight for access. Male Raggiana Bird of Paradise displaying.

In such high-variance systems, a few males obtain almost all matings and most males get none. Male and female Sage Grouse

When both sexes invest more evenly in rearing offspring, male and female reproductive success is more evenly balanced.

In some cases, males are the choosy sex. In these cases, males invest more or have the lower reproductive potential.

Male pipefish carry eggs and young in a brood pouch and provide nutrients and oxygen for several weeks. In that time female can produce enough eggs for 2 males.

Male brood space is limited and hence males are the choosy sex. Males select females based on their size and the size of their ventral fold.

A male Mormon cricket (a type of katydid) transfers a large edible spermatophore to a female when he mates. Spermatophore is 25% of males body mass. Mormon crickets Hence, males can probably only mate once, but females can mate more often because energy from spermatophores used to make eggs.

Spermatophore Female Mormon cricket eating spermatophore

When Mormon crickets swarm, males have access to multiple females and are choosy. Select heavier females because these can produce more eggs. A 3.2 gram female produces 50% fewer eggs than a 3.5g female.

Mean sizes of rejected and accepted female Mormon crickets

Which sex is the choosier can vary seasonally. In an Australian katydid pollen supplies vary seasonally. When pollen scarce spermatophores are hard to make and males are choosy. When pollen is abundant, females become the choosy sex.

Sexual Selection

The process that determines whose genes get into the next generation is: Natural Selection.

Many factors affect an individuals fitness i.e., the number of copies of its genes. it gets into the next generation. Include: foraging ability, ability to avoid predators, and ability to obtain mates

Darwin proposed that elaborate display structures (e.g. colorful plumage) and weapons (e.g. antlers), exist only because they contribute to this last component of natural selection. They increase an individual’s ability to obtain mates.

Component of natural selection exclusively concerned with obtaining mates is called sexual selection.

Elaborate plumage increases reproductive success, but also increases mortality. There is, therefore, a trade-off between survival and reproductive success.

Two components of sexual selection Male-male Competition (not always male:male, but non-choosy sex : non-choosy sex competition doesn’t trip off the tongue) Female Choice (preference by females for certain physical characteristics in males).

Contests between males to hold harems are common in mammals e.g. deer, lions, antelope, elephant seals. Male-male Competition

Males that dominate other males can secure harems of females and obtain exclusive mating access to them. Bull elk and harem

Southern Elephant Seals Strong relationship between fighting success and reproductive success.

Northern Elephant Seals

Competition between males has led to extreme sexual dimorphism when males can potentially control large harems. Male and female fur seals

In seals there is a strong relationship between harem size and relative sizes of males and females. In harbor seals, harems are small and sexes similar in size. In elephant seals, harems are large and males much larger than females.

Male-male competition among marine iguanas.  Natural selection acts strongly on body size of male marine iguanas on Galapagos Islands.  Intermediate size males survive better than larger or smaller males.  Reason is that a large body is expensive to maintain and obtaining enough food can be difficult, even though large iguanas can harvest more food.

Male-male competition among marine iguanas.  Maximum male body size consistently exceeds the body weight that can be sustained, but female body weights do not.

Asterisks indicate maximum body sizes that iguanas could maintain successfully in each of two study years.

Male-male competition among marine iguanas.  Why is male body size larger than we would predict based on maximizing survival?

Male-male competition among marine iguanas.  Female iguanas lay one clutch of eggs per year and mate only once.  Females invest as much as 20% of their body mass in a clutch, so they invest much more than males. Males compete to fertilize females.

Male-male competition among marine iguanas.  Male iguanas stake out territories on rocks where females bask between feeding bouts and fight other males to defend their territories.  Territory holding males much more attractive to females.  Male mating success strongly related to his ability to hold and defend a territory that females like to use.

Territories of numbered male marine iguanas.

Male-male competition among marine iguanas.  Territories held by males 65 and 59 were strongly preferred by females for basking.  Male 59 was the largest male in the colony and to claim the territory had to eject 4 other males.

Male-male competition among marine iguanas.  Male 59 had more than four times as many copulations as any other male in the colony.  For the colony as a whole mean body size of males who got to copulate was significantly larger than mean body size of all males who tried to copulate.

Male-male competition among marine iguanas.  Because body size is heritable and confers such a huge advantage in mating, male marine iguanas male marine iguanas have been sexually selected to have large body size despite the survival costs.

Other strategies besides dominance   Not every male can be a dominant male, so frequently males must use other approaches to obtain mates.

Subordinate strategies among baboons   Baboon troops have dominance hierarchy, but subordinate males form friendships with females and protect the female and her offspring.   Females often mate with these males rather than the dominant male.

Low ranking male baboons also may form alliances against higher ranking males to take over oestrus females.

Satellite males   Some males (satellites) attempt to intercept females moving towards males that are calling or defending a resource valuable to females.

Great Plains Toad and Bighorn Sheep satellite males.

Conditional mating strategies in Scorpionflies.

Conditional mating strategies in Scorpionflies Male Scorpionflies use one of three mating strategies. I. I.Defend dead insects as food resources for females. II. Offer saliva as food gift to females. III. Intercept females and force copulations.

Conditional mating strategies in Scorpionflies   Randy Thornhill placed 10 males and 10 female Scorpionflies in a cage with 2 dead crickets.

Conditional mating strategies in Scorpionflies   Two males controlled crickets: averaged 6 copulations.   Saliva gift males got about 2 copulations each.   Forced copulation males (couldn’t make gifts) males achieved about 1 copulation each.

Conditional mating strategies in Scorpionflies   If different strategies are genetically determined, expect less successful to disappear.   Why are they all retained in population?

Conditional mating strategies in Scorpionflies   Because Scorpionflies play strategies conditionally.   They play the role that is the best they can do.

Conditional mating strategies in Scorpionflies   When the two dominant males were removed from the cage, other males switched their strategies.   Two saliva gift males took over crickets.   Two interceptors took over abandoned saliva gifts.

Pure mating strategies in crustaceans .  Paracercis sculpta lives in intertidal sponges.   Three distinct male types exist.   Each has a different mating strategy.

Pure mating strategies in crustaceans   Alpha: Large. Exclude other males from sponge.   Only one alpha per sponge.

Pure mating strategies in crustaceans   Beta: Medium sized. When encountered by alpha pretends to be a female and mates. Sneaks matings from female.   Gamma: Tiny. Alphas throw gammas out of sponge. Sneak matings from females.

Paracercis sculpta males

Pure mating strategies in crustaceans   Are strategies pure or conditional?   If pure: should have (i) genetic basis and (ii) mean reproductive success of strategies should be equal.

Pure mating strategies in crustaceans   If strategies conditional: (i) strategies should be environmentally induced.   (ii) mean reproductive success of strategies need not be equal.

Pure mating strategies in crustaceans  Payoffs to the different male types appear to be equal which suggests the strategies are pure rather than conditional.

Sperm Competition Females commonly mate with more then one male. Males attempt to maximize their chances of fertilizing eggs through sperm competition.

Sperm competition in damselflies.

Male damselflies defend territories in which females lay eggs. Female lays eggs in several territories and mates with each resident male.

Mating damselflies Female Male

Female swings abdomen under male and male copulates. Male has highly specialized penis that he uses like a scrubbing brush to remove sperm from previous males.

Spines on penis and lateral horns that are covered with spiny-like hairs snag sperm and remove them. When previous males sperm removed (up to %) male inserts own sperm.

Damselfly penis showing lateral horn Spines

Lateral horn close up Sperm caught on spiny hairs.

Other sperm competition strategies are widespread. A shark’s penis has two tubes, one of which can spray seawater and may flush sperm out of female.

Male Dunnocks peck a females cloaca before mating to make her release sperm

Mate Guarding Males also attempt to ensure their paternity by preventing other males from mating with a female.

  Mate Guarding in dunnocks

Benefits of guarding must be weighed against cost of lost mating opportunities for the male. Under what conditions should you expect guarding behavior to pay off?

Expect mate guarding when: Female is likely to mate again and use sperm of most recent partner to fertilize eggs. When females are scarce, so chance of finding another female is low.

Mate guarding should be most intense when female can be fertilized. In white-fronted bee-eaters females can be fertilized only in 4-5 day period during egg laying.

During this period males mate frequently With their females and guard them almost continuously. Other males also harass females and try to copulate with them.

Timing of bee-eater mate guarding.

Female choice

Female choice based on (i) Material benefits obtained from male (ii) Male attractiveness and display

Material Benefits Many females require a nuptial gift from males before they will mate. The nuptial gift enhances the females reproductive success by e.g. enhancing her ability to make eggs.

Spermatophore benefits to female katydids. Upper line with spermatophore.

Male Gluphisia moths provide Spermatophores that are high in sodium ions. Sodium stored in eggs. Sodium is valuable to larvae which eat low sodium food.

In many species females allow males to copulate only while she is still consuming the gift. Gift size is, thus, very important to male reproductive success

Ultimate nuptial gift some males provide is allowing themselves to be eaten by female. Occurs in praying mantises and spiders.

Female choice based on male attractiveness

Long-tailed widowbirds are African savannah birds have tails that are up to 0.5 m long, which they display to females.

Malte Andersson: Tail length crucial factor in attractiveness of male widowbirds to females. Altered tail lengths of males. Observed effects on mating success.

Four treatments: Tail shortened Tail lengthened Controls Tail cut and re-glued same length Tail not cut.

Success in attracting mates before treatments

Success in attracting mates after treatments

Extrinsic displays of bowerbirds. Bowerbirds are large songbirds that occur in New Guinea and Australia. Males construct elaborate display structures called bowers.

McGregor’s Bowerbird

Bowers are made of grass or sticks and decorated with brightly colored objects. Two major styles of bower: Maypole bowers and Avenue bowers. Builders of each bower type more closely related to each other than to builders of other type.

McGregor’s bowerbird and its simple maypole bower

McGregor’s Bowerbird maypole bower

Satin bowerbird at its avenue bower

In bowerbirds the stimulus value of bowers appears to have somehow replaced the stimulus of bright plumage.

In general, bowerbirds with most elaborate plumage build least elaborate bowers.

Males maintain their bowers and continually replace wilted or damaged material. Males also frequently steal items from each other. The bower is used only for mating. It is not used as a nest.

Satin Bowerbird Studies by Gerald Borgia. Male Satin Bowerbird, which has bright blue eyes decorates its bower with anything blue it can find. Items include: trash, glass, paper, feathers, pottery, etc.

Male Satin bowerbird at bower.

Male Satin Bowerbirds destroy other males bowers and steal items whenever they can. Dominant males are better able to defend their bowers and have more time to destroy other males bowers.

Ability and status of male directly related to quality of his bower. Females prefer males with well-made, well-decorated bowers. Females visit an average of 3.6 bowers before mating.

Satin Bowerbird

In Borgia’s studies 5 of 22 males accounted for 56% of 21 copulations. These males had the most blue parrot feathers, shells, and leaves as decorations. Also had best built bowers in terms of symmetry, stick size, and construction.

In experiments, males whose decorations were removed obtained fewer matings.

Explanations for female choice Good Genes versus Fashion Icons

Good genes ideas propose that exaggerated male plumage signals genetic or physiological superiority.

Zahavi’s handicap principle: Elaborate plumage handicaps a male, for example by being expensive to maintain or by making him more obvious to predators. A males’ ability to tolerate such a handicap demonstrates his quality to potential mates.

An alternative good genes idea is the Hamilton and Zuk hypothesis that ornamental plumage signals resistance to diseases and parasites. Females are able to evaluate resistance by assessing a male’s coloration and display.

W.D. Hamilton and Marlene Zuk carried out experiments with red junglefowl. Hens prefer males with large fleshy combs. Combs affected by testosterone which is related to physical condition. Males artificially infected with parasitic nematodes have smaller combs and are less favored by hens.

Testing the Hamilton-Zuk hypothesis It is not enough to show only that females prefer males with lower parasites burdens. There are 4 key assumptions of the hypothesis.

Assumptions of the Hamilton-Zuk hypothesis (i) Parasites reduce host fitness. (ii) Parasite resistance is genetic. (iii) Parasite resistance is signalled by elaboration of sexual ornaments. (iv) Females prefer males with the most elaborate signals.

Anders Pape Møller tested the Hamilton-Zuk hypothesis with experiments on Barn Swallows.

Barn Swallows are monogamous. Males have elongated outer tail feathers which they display to females

Møller first showed that females prefer males with longer tails. Males with artificially lengthened tails were more attractive to females and obtained more EPC’s and were able to obtain mates quicker.

Bloodsucking mites are common nest parasites of swallows. Nestlings in nests with high parasite loads are lighter and have lower survival rates. There is a large amount of variation in the level of resistance that chicks show to mites.

Møller switched chicks between nests. Nestling resistance to parasites was correlated with that of its parents even when the chick was reared in another nest. Therefore, parasite resistance is partially genetically determined.

Parents with longer tails had young with lower parasite burdens regardless of the nest the chicks were reared in. Strongest relationship between male tail length and offspring parasite load. Conclusion: tail length indicates resistance to parasites and this resistance is heritable.

Fashion Icon Hypotheses These hypotheses suggest that the elaborate plumage of male birds is a result of runaway preferences by females for particular colors or plumage patterns.

The idea behind runaway preferences is that females may have innate preferences for certain colors or plumages. There need be no “reason” for this.

If such preferences are genetically based they can be selected for. A preference may spread at first passively in the population, but then spread more rapidly as it becomes more common and if there is a linkage between the genes for the female preference and the male character being selected for.

Nancy Burley has shown that Zebra Finches have innate preferences for certain colors.

Male Zebra finches given red leg bands are more attractive to females than males with green leg bands. Also, males given artificial “hats” that are red or white become more attractive, but males given yellow, blue, or green hats become less attractive.

Male Zebra finches also make choices and prefer females with black leg bands and reject females with red bands. Thus, mating is assortative because the “best” males mate with the “best” females.

The good genes and fashion icons hypotheses may both be relevant to the evolution of brightly colored male plumage and the effects may reinforce each other.