11/14/2012. Darwin attributed sexual differences to sexual selection.

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1 Darwin: Sexual size dimorphism is common. male elephant seals Many mammals males bigger than females female Sexual differences include differences in morphology, behavior and physiology. Morphology male Many spiders males smaller than females Behavior rainforest spider female Physiology differences in hormones, brain structure, and gonads. Darwin attributed sexual differences to sexual selection. Drawing of tufted coquette from The Descent of Man and Selection in Relation to Sex by Charles Darwin sexual selection results from advantages that certain individuals have over others of the same sex and species, in exclusive relation to reproduction " Quote from Descent of Man by C. Darwin 1

2 Sexual selection is a special form of natural selection. Involves two mechanisms: A. Mate competition Mate competition members of one sex compete with members of the same sex for access to mates B. Mate choice male marine iguanas bull elephant males Mate choice members of one sex choose mates among members of the opposite sex Mate Competition! 2

3 1. Male-Male Contests Types of Mate Competition 1. male-male contests 2. sperm competition 3. mate guarding 4. infanticide 5. alternative mating strategies Contests typically reflect assessment of size or strength. Sometimes physical combat Sometimes ritualized display % copulations Dominant male Mountain gorillas live in groups In elephant seals too, fighting establishes dominance status, Some groups have multiple males. which determines male mating success. Male dominance established through aggression, Which determines male mating success. 3

4 Video!! Among seals and related marine mammal species, sexual size dimorphism is correlated with # females monopolized by males. 4

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6 2. Sperm Competition = male-male competition at the level of sperm. Common wherever females mate with more than one male. Example. Sperm wars in Drosophila In many animal species, the last male to mate with a female fertilizes the next eggs out. This pattern is known as last male precedence. Last in, first out. Why? In Drosophila, the mechanism was evaluated using males with genetically-engineered sperm. Procedure: mate female first to male with one kind of sperm, then to male with other kind of sperm. Drosophila sperm which express a fluorescing protein (green tails) and normal sperm (grey tails). Possible outcomes: 1. Displacement of first sperm from storage sites. 2. Stratification in storage sites with first sperm buried deep in storage sites. Photo courtesy of Kelly Dyer 6

7 Results: Last male precedence due to: stratification displacement a. displacement of first male s sperm from female storage organs. BUT ALSO Colored areas are sites of sperm storage (Male #1 in green; Male #2 in red) b. incapacitation of first male s sperm! Optimal Behavior Under Sperm Competition Copulation duration in dung fly, Scatophaga stercoraria Copulation duration in dung flies % Eggs fertilized predicted Luc Viatour Males defend dung patties as territories Females lay eggs and larvae develop in patties Female is patch Male s decision variable is copulation duration Currency of fitness is % eggs fertilized Search & guard time ( min) observed Time in copula (min) Tradeoff between sperm transferred to one female and sperm available for next female 7

8 Increasing ratio of males to females should increase search time for next female Effect of Sex Ratio on Mating Duration which should increase duration of copulation with current female. We tested prediction in our lab using walnut flies. Copulation Duration (sec) n = 2 <1 n = 31 1 n = 16 n = 7 n = 25 >1 >2 >3 C. Hedgcock Local Sex Ratio (male:female) Alonso-Pimentel and Papaj (1996) Strategies for Sperm Competition a. Increased sperm production -- some primate species b. Retaliatory copulation -- bighorn sheep c. Copulatory plugs -- butterflies d. Traumatic insemination -- bedbugs e. Sperm trains -- murine rodents f. Sperm removal devices -- dragonflies & damselflies Strategies for Sperm Competition a. Increased sperm production -- some primate species b. Retaliatory copulation -- bighorn sheep c. Copulatory plugs -- butterflies d. Traumatic insemination -- bedbugs e. Sperm trains -- murine rodents f. Sperm removal devices -- dragonflies & damselflies 8

9 Sperm competition expected to be more intense when females mate multiply. In primates, frequency of mating varies: monogamous: males and females mate just once polygynous: males mate multiply multi-male/multi-female: both males and females mate multiply Sperm competition expected to be most intense in this last mating system. Testes size for species with multi-male/multi-female mating system tend to fall above the line fit to all the data. Testes Size (controlled for Body Size) Number of Males Male flies held in more malebiased sex ratios grow larger testes! Laura Conner and D. Papaj, unpubl. Strategies for Sperm Competition a. Increased sperm production -- some primate species b. Retaliatory copulation -- bighorn sheep c. Copulatory plugs -- butterflies d. Traumatic insemination -- bedbugs e. Sperm trains -- murine rodents f. Sperm removal devices -- dragonflies & damselflies 9

10 Murine rodents have hooked sperm. Sperm hook together in female tract. Sperm trains move faster than individual sperm. Sperm are more hooked in species with larger testes, indicative of stronger sperm competition. Fig Sperm removal in damselflies Mating is weird!!! Sperm morphology is highly variable! Males have evolved: claspers to clutch female! a new penis! with a scrub brush!! 10

11 Female places genitalia over new penis. Male scrubs old sperm from female s sperm storage organ and deposits sperm. Ischnura damselfly sperm removal devices 3. Mate Guarding 3. Mate Guarding (cont d) a. post-copulatory mate guarding Examples. Damselflies and many birds. Males copulate and then guard until eggs laid. b. pre-copulatory mate guarding Example. Gammarus amphipods Male guards immature female until she molts and is receptive. 11

12 4. Infanticide A male lion kills cubs of rival when it takes over a pride. WHY? 1. Fewer resources to someone else s cub 2. Females come into estrus sooner. 5. Alternative Mating Strategies Example. Johnny-come-lately behavior in horseshoe crabs Example. Satellite males in frogs Males hang out in the vicinity of a holder of a high-quality territory, and mate with females as they move to the territory. Video Males crowd around a paired couple as they move onto the beach, and release sperm in the vicinity of the couple. 12

13 Tursiopsspecies in Shark Bay, Western Australia BUT ARE COALITIONS MADE UP OF LOW-RANKING MALES? Kenneth M. Gale, Bugwood.org WE DON T KNOW Kenneth M. Gale, Bugwood.org Example. Alliance formation in savanna baboons. Low-ranking males form coalitions and wrest females from a high-ranking male. Example. Alliance formation in bottle-nosed dolphins Coalitions of males: 1. herd females, preferring those in estrus 2. defend females from other males 3. wrest females from other coalitions Example. 3 strategies in bluegill sunfish Territorial males Sneaker males Female mimics Example. Female mimicry in bluegill sunfish Males similar in size and coloration to females. Mimicry allows male to stay close to another s territory, and release sperm when territory-holder is pairing with a female. territorial male sneaker male female mimic 13

14 Example. Female mimicry in marine isopods Males come in three genetically distinct forms. female mimic Three alternative strategies in ruffs territorial male sneaker male At equilibrium frequencies, all morphs have the same fitness. rufous male whitemale dark male Territorial Resident Satellite Marginal male 1. Territorial Residents defend territories from Marginal Males and other TR s. 2. Marginal Males are floaters that may become TR s. 3. Satellites form couples with TR s, are tolerated on their territories, and may steal copulations from the TR. Are these genetically different? Satellites Independents: - Territorial Residents - Marginal Males Independents and Satellites are genetically different (= genetic polymorphism) But MM s and TR s can change into each other (= developmental polymorphism) 14

15 The rock-paper-scissors game in side-blotched lizards There are 3 male morphs in side-blotched lizards. Differences are genetically-based. orange blue yellow Ultra-dominant (v. aggressive) Guard mates Mimic female & sneak matings Rock blunts scissors, paper covers rock, and scissors cuts paper Ultra-dominant orange beats mate-guarding blue; Mate-guarding blue beats yellowthroated sneaker; orange blue yellow Yellow sneaker beats orange. Ultra-dominant (v. aggressive) Guard mates Mimic female & sneak matings Net result: No one strategy beats all. Thus, all morphs persist! Large territories Several females Small territory One female No territory 15

16 r/video_only.html Do the sexes differ with respect to mate competition and mate choice? Yes! Yes!! YES!!! Generally speaking, Males compete more intensely for mates than do females. and Females are choosier about their mates than are males. Why? Answer: Anisogamy Anisogamy is the occurrence of gametes of different sizes Males make many small gametes, termed sperm. Females make few, large gametes, termed eggs. Hamster egg and sperm 16

17 NO! There is a TRADEOFF!! Why two gamete sizes? Selection should favor: 1. gametes effective at fertilizing other gametes. 2. gametes, once fertilized, better at developing to adulthood. A gamete that is good at #1 is probably not good at #2, and vice-versa. So gametes end up being a. large and full of nutrients, OR b. small and highly mobile disruptive selection Can the same type of gamete satisfy both #1 and #2? small large small large small large gamete size Evidence: In 1940 s, Bateman confirmed this prediction, using Drosophila fruit flies. 100 males Anisogamy leads to three predictions. Fitness 50 females Prediction #1: male fitness will be limited by number of mates, whereas female fitness will not be limited by number of mates No. of Mates Bateman s principle male fitness improves with # mates, female fitness does not. 17

18 Prediction #2. Because females invest more into each gamete, they should invest more in offspring care. Pattern: Females generally do invest more in parental care. Prediction #3. Given greater investment in gametes and in offspring care, females should be choosier about who they mate with, and males should compete more for mates than do females. Patterns: We generally see both patterns. Nice story is it true? Role Reversal in Mormon crickets If true, exceptions should prove the rule: Where males invest more in parental care, then males should be choosy and females should compete for mates. In other words role reversal on all counts! During mating, males transfer sperm plus a nutritious packet of protein, This spermatophylax may be 25% of male s weight. Large male investment generates role reversal. Females compete for males. 18

19 Role reversal in jacanas Role Reversal in Pipefish Also in rheas and red phalaropes (VIDEO) jacana Female jacana mates & lays eggs, female leave, male tends nest. Female finds new mate & lays eggs in new nest, then leaves. Etc. Female defends territories containing the nests from other females. Females try to steal males by killing eggs of males in other territories (= ovicide). Male pipefish do the caring Last bit is evidence of females competing for males. Male brood pouch Male giving birth Father tending to his young Summary In several species of pipefish, females have ornaments and males exercise mate choice. Also a reversal of Bateman s principle: Female fitness increases with # s of mates; male fitness changes little with # s of mates. Males compete more intensely for mates than do females. and Females are choosier about their mates than are males. However some scientists believe this view is incorrect. 19