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sexual selection.docx

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Brett Beston

April 1 , 2013 Psych 2TT3: Animal Behaviour Sexual Selection Which one is the Male? - females tend to be less colourful, more drab - deer males tend to have antlers and females don’t - differences between sexes is driven by some sort of selective force Darwin - we are, however, here concerned only with that kind of selection, which I have called sexual selection. This depends on the advantage which certain individuals have over other individuals of the same species - intrasexual selection: members of one sex - intersexual selection: one individual chooses another individual of the opposite sex Natural Selection Vs. Sexual Selection - natural selection: change in frequency - sexual selection: variance in reproductive success due to some ability to acquire mates - natural selection:  utilitarian, functional  solves a problem  sensible  economical  fixed  constructive  dull - sexual selection:  showy, elaborate  impresses an audience  whimsical  wasteful  changeable  destructive  exciting  flexibility  tend to be seen in males Why do Males and Females Look Different? - because of sexual selection (which is a type of natural selection) - natural selection: differential survival and/or reproduction of individuals differing in one or more heritable traits - sexual selection: differential reproduction owing to variation in the ability to obtain mates by individuals differing in one or more heritable traits Optimal Gamete Size - gamete = egg or sperm - different type and number of gametes produced by males and females - small and quick – must find other gametes - large, slow, healthy gametes - intermediate games tend to be unsuccessful, cannot support growth of the gamete Divergent Selection on Gamete Size - females produce fewer but larger gamete - each egg is more valuable in terms of relative size and relative rarity - sperm is cheap in terms of a currency Investment per Gamete - large in females - small in males - females are limited by resources to gametes – they can invest in relatively few gametes - males can produce many gametes – they are limited by access to females - causes competition in males for scarce availability - females need to be choosy as they have limited amount of eggs Anisogamy - difference between sizes of gametes between males and females - the evolution of anisogamy:  females and males - consequences of anusogamy  unequal parental investment – beginnings at the period of gestation  unequal variance in fitness – males, as they can produce many sperm are able to potential have a very high fitness Two Outcomes - males compete with other males for access to females - largely thought that intrasexual selection was the only mechanism of sexual selection Red Deer - only the strongest males hold harems - 23% of harem holders have wounds - 6% of all harem holders are permanently injured - fighting for mates is dangerous - favour a mechanism that avoids possible fatal consequences - males us an honest indicator of their fighting abilities: how loud they can roar - largest males can produce the deepest sounds - hard to fake when you have shorter vocal cords Roaring in Red Deer - don’t see many roaring in pre-rut or post-rut during the breading season - holders roar the most - series of mutual assessments with other males - assess based on size, close in size enter roar contest - maximum roar rate is correlated with likelihood that male will win a mate - more mature males that have equal experience - roaring escalades to parallel walk then to fighting or walking away Male-Male Competition via Cukoldry - cuckoldry: the practice of making cuckolds; sexual conquests of married women - in the animal world:  extra pair copulations  sneaking  satellite Extra Pair Copulation - monogamy used to be considered the norm - birds: monogamous during a breading season - eggs in nest of male and female were not genetically related to males - left: amount of copulation per hour with their mate – frequency increases 4 days before eggs are going to be laid when female is more fertile - EPCs: frequency of mating is around 2-5 times, females are seeking more epc at the peak of their fertility - Spend more time with the mate - Difference between socially and genetically monogamous - Socially monogamous: best parental skills - Genetically monogamous: provide the best genes - A male attempts to keep b male away - Female seeks mating with B male - Female gains from cheating on A male - B male also gains, because he incurs almost no cost of raising offspring - In fish we see different tactics - No benefit towards the alpha male Sneaking - occurs in many fish (with external fertilization) - the sneaker male is smaller than the parental male and sneaks to fertilize eggs during spawning - physically and behaviourally distinct - use hit and run strategy - A: creating nest to attract females - B: sneaker males, much smaller in size, hand around in the bushese  Physically and behaviourally distinct  Attract females to nesting site  Sneakers adapt hit and run, come between when spawning  Females are not bothered as they only have 80% of eggs fertilized, sneaker increases percentage of eggs fertilized  We need the dominant male as we need a nest and someone to take care of the eggs Satellite Males - bluegill: parental male, female, and satellite male - satellite males take on appearance on females - male welcomes satellite males as they wish to fertilize both females - satellite will then release it’s sperm to fertilize the real females egg Two Outcomes - females are choosy, males are typically more promiscuous - both occur in nature - handicap hypothesis: males that are able to escape due to trait characteristics must have good genes Lek Polygyny - no parental care - males aggregate at traditional sites (leks) and display communally - males get together to display to females, puff up chest - females looking for specific traits - males tend to display traits that can be costly Male Ornaments Serve in Female Choice - epi: outside characteristics, plumage variation - trump towers Mate Choice - the focus is on females choosing males (do males choose females?) How do Female Flies Choose? - typical copulation pattern - females are being discriminate about which male they will actually mate with - assessing characteristics - sometimes not the best decision to mate with dominant male as they are more aggressive during sex What do Female Flies Consider while Choosing Males? - smell = pheromonal composition: compatibility and quality - song: compatibility and quality - body size: larger is better (below a species threshold) - recent matins (discriminate against more than 2 recent matings) Mate Choice in Pied Flycatchers - females prefer males who sing as it attracts them to a particular area Repertoire Size and Pairing Date in Sedge Warblers - females prefer males able to sing more than one song - males with greatest number of songs in their repertoire must go through fewest number of pairing dates Why do Females Choose? - what are females getting out of being so selective - direct: what resources they receive - good genes: genetic benefits their offspring will get - runaway selection - sensory exploutation Direct Benefits - natural selection favours females who prefer males with the highest positive effect on females’ fitness (survival and/or reproduction): increase child rearing potential - do direct benefits explain female mate choice in humans? Women’s Mate Preference - women have higher expectation of earning capacity in males and increases as the severity of the relationship increases as well - women always rate financial resources twice as important as a male does Nuptial Gifts in Scorpion Flies - females reject males that do not offer them prey (dead insects) - females prefer males that offer large prey - reject males that do not offer an insect - given density and dangerous environment it makes sense as females are at risk when foraging - as the population is so dense, food is dense - females are so selective that only about 10% of prey items meet the criteria that they deem worthy enough The Females Mate Longer with Males that Offer Large Prey Males that Mate Longer Transfer more Sperm to Females - anything that last at least 20 minutes results in maximum sperm transfer - anything less than 8 results in no sperm - males develop courtship to steal prey Direct Fitness Benefit of Nuptial Gifts in Scorpion Flies - females that receive larger nuptial gifts produce more eggs and perhaps have longer lifespan because of the nutrition and the fact that there is less amount of time she’s spending trying to forage on her own Direct Benefits - data from many species support the direct benefit model - that is, in many species, female choice results in increased female reproduction and/or survival Why do Females Choose? - assumes females aren’t making a choice based on direct benefits alone - based of what their offspring will receive - males that have the best genes often give best resources Mate Choice for Compatibility - good gene selec
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