3/26/ Sexual dimorphism is differences between males and females of a species. 2. Anisogamy. 1. Fecundity

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1 Sexual Dimorphism 1. Sexual dimorphism is differences between males and females of a species. 2. Anisogamy A. sexual reproduction involving the fusion of two dissimilar gametes;individuals producing the larger gamete are defined as female, and smaller gamete male B. Anisogamy results in differential investment in reproduction (Fig. 11.8) 1. Sexual selection is the differences among individuals at getting mates A. In general males and females may not employ the same strategy for maximum fitness B. Males competitive for mates, many mates C. Females choosing mates, high quality mates D. Eggs (or pregnancy) are more expensive than ejaculates. Cervus elaphus, red deer Poecilia reticulata, guppies 1 2 Unequal Investment Results in Sexual Selection 1. Fecundity A. Reproductive capacity (males usually produce more) 2. Uncertain paternity may explain why male parental care is rare A. Females have certain paternity B. Males have uncertain paternity C. Females typically make a larger parental investment in each offspring than males Asymmetries in Sexual Reproduction, cont. 1. Females provide substantial parental care and males provide none whatsoever in >90% of mammals. 2. Even when females do not care for the young (i.e., many reptiles, and insects), females make a larger investment in each offspring than males. 3. Female s reproductive success is limited more by the number of eggs she can make (or pregnancies she can carry) than by the number of males she can mate with. 4. Investment differences can extend past fertilization (Fig ) 1. Operational sex ratio: ratio of male to female individuals available for reproducing at a given time 2. Slower rate of reproduction by females leads to male biased OSR 3. Differential reproductive success resulting from competition for mates A. Intrasexual selection B. Intersexual selection 4. Sexual selection typically stronger on males A. Maximize fitness by mating with multiple females Limitations on reproductive success differ for the sexes (Fig. 11.9) 1. Females are limited by fecundity 2. Males are limited by the number of mates they can obtain 3. Male reed warblers guard harems 1

2 Behavioral Consequences of Asymmetric Limits on Fitness 1. Intrasexual selection A. Individuals are subject to strong sexual selection a. Usually male-male competition b. the fitness of males is limited by access to females c. The females then mate with the winners. 1. where the operational sex ratio is skewed, the abundant sex can try to monopolize females or resources using powerful weapons and displays. 2. Sexually dimorphic result from sexual selection (Fig ) A. Ornaments: attractive traits that increase mating success B. Armaments: weaponry used to outcompete other individuals 7 8 Male-male competition leads to extreme variance in reproductive success (Fig ) Competing for mates is costly (Fig ) Males may compete for territory (Fig ) 1. The forelegs of male harlequin beetles are used in grappling contests over sap oozes where females feed, mate, and lay eggs. 2. The harlequin beetle-riding pseudoscorpion, Cordylochernes scorpioides (Fig B) disperse on the backs of harlequin beetles 11 Behavioral Consequences of Asymmetric Limits on Fitness 1. Intersexual Selection 1. Individuals are subject to weak sexual selection 1. Usually female choice 2. The fitness of female is not limited by opportunities to mate 3. males can advertise for mates 4. The females then choose among the advertisers. 2. Male Choice: the reverse occurs (Figs ) 12 2

3 Benefits of female choice 1. Direct benefits: benefit the female directly A. e.g. food, nest sites, protection 2. Indirect benefits: benefits that affect the genetic quality of the female s offspring Males Bearing Gifts p Food rewards for mating are often called nuptial gifts; 2. females can increase the resources available to their young, improving their fecundity Direct benefits (Fig ) Extreme food offerings A. Praying mantis B. Voluntary self-sacrifice in redback spiders (Fig ) 2. Protection A. preferential mating with large (dominant) males that guard her from harassment. a. "last male" fertilization advantage that selects for mate-guarding behavior on the part of the males. B. Male gives female toxic chemicals that she uses to protect her eggs 15 Other direct Benefits (Table 11.2) 3. Territories and/or Nests A. Access to good egg-laying sites: Dragonflies; frogs B. Suitable vegetation for nesting: red-winged blackbirds C. Nests the male has manufactured: weaverbirds D. Territories with good forage: antelope 4. Help Raising Young A. Females choose males who are better egg guardians: sculpins B. Females choose males better at fanning eggs: sticklebacks C. Females choose males better at feeding nestlings: pied flycatchers 16 Other direct Benefits (Table 11.2) 5. Reduced Risk A. By choosing males with bright ornaments or vigorous displays, females pick healthy males who are less likely to transmit mites, fleas, lice, or sexually transmitted diseases to the female. B. Females choose males with large, bright red combs a. junglefowl; these males have healthier immune systems 1. Males may display elaborate ornaments (Fig ) 2. Female preferences are often consistent (Fig ) A. Female golden-collared manakins (C) choose males with the brightest and most conspicuous plumage (D) 17 3

4 TABLE 11.3 Indirect Benefits of Female Choice Good Genes" Process 1. Male ornaments signal underlying genetic quality (good genes) such as efficient metabolism, body condition, or resistance to parasites or disease. 2. among-male variation in the expression of ornaments reliably signals individual differences in overall quality of the males. 3. It predicts that choosy females will produce offspring with higher survivorship or in better condition than less choosy females. Female preferences may arise from preexisting sensory bias (Fig ) 1. Female guppies prefer males with large, bright orange spots. 2. This preference may provide information about the quality of potential mates (e.g. parasite load, sperm number). 3. OR, this preference may have arisen originally as a sensory bias on orangecolored foods (e.g. fruit) and therefoe females are attracted to orange 19 Fisher: Runaway Selection 1. female preference for a male trait will be favored because female with that preference will have sexy successful sons 2. Female preferences are often consistent (Fig ) A. Female stalk-eyed flies prefer longer stalks B. The process is termed "runaway", because over time, it would facilitate the development of greater preference and more pronounced traits, until the costs of producing the trait balance the reproductive benefit of possessing it. 21 Fisher s runaway: preference and trait genetically correlated (Fig ) Ornaments can serve as handicaps (Fig ) Types of mating systems Monogamy: one male pairs with one female Sexual: partners mate with each other exclusively Social: partner pair but may cheat Polygyny: males mate with multiple females Polyandry: females mate with multiple males Polyandry leads to sperm competition among males 4

5 Polyandry selects for male traits that increase paternity (Fig ) Sperm competition drives evolution of larger testes (Fig ) Altruistic Sperm 1. Female wood mice are highly promiscuous. 2. Male sperm are in a race to the egg. 3. Male wood mice have large testes, which suggests that sperm competition is common. 4. Sperm hook up into fast trains. 5. Trains of sperm travel about twice as fast as lone sperm 6. Many of the train's members self-sacrifice so brother can fertilize egg. 26 Sexual conflict results in antagonistic coevolution 1. Sexual conflict: traits that confer a fitness benefit on one sex but cost to the other A. Traits coevolve antagonistically 1. Brennan wanted to understand why some ducks have such extravagant penises. 2. Female ducks have equally weird reproductive tracts. In many species, they are ornamented with lots of outpockets. And like duck penises, duck oviducts are corkscrew-shaped. But while male duck penises twist clockwise, the female oviduct twists counterclockwise. 3. Brennan speculated that all this bizarre anatomy is the result of a peculiar form of evolution known as sexual conflict. 4. A strategy that allows females to reproduce the most offspring may not be so good for males, and vice versa.. 28 Evidence for antagonistic coevolution in Drosophila (Fig ) Sexual conflict in ducks (Fig ) 1. Correlations between penis length and number of pouches (to shunt off sperm from unwanted males) and number of spirals in female reproductive tract. 1. Male fruit flies inject their mates with lots of chemicals during sex, and those chemicals make her less receptive to other males, thereby boosting his chances of fathering her eggs. But those chemicals are harsh and will make female flies sick. 2. Females, in turn, have evolved defenses against those chemicals, blunting their effects 3. After 40 generations of no sperm competition, females lose defenses against these chemicals and die much sooner 5