Chapter 10.docx

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University of Toronto Scarborough
Biological Sciences
Kamini Persaud

BIOC54 Chapter 10 Textbook Notes Tharsini Sivananthajothy February 19, 2012 Sexual Selection I: Males vs. Females, Intra-sex Competition Page 329 359 - Male satin bowerbird builds a bower - Collects blue rubber ands and blue parrot feathers there - When female comes, he chortles and squeaks with a courtship dance while opening and closing his wings with a buzzing trill - He may also bob up and down while imitating the songs of several species of other birds - Female satin bowerbirds visit several bowers but not to mate on these inspections - After the first round, female takes a break of a week to construct a nest before returning to a number of bowers where she observes the full courtship routine of several males - These inspections take several weeks before female settles to one male o She enters the males bower and she is courted again after which is gets copulated - She flies out after and will not have contact with her partner and will incubate and rear young by herself - Mate will stay at his bower and copulate with other females - Both sexes look different and their reproductive tactics are different as well - Usually males do the courting and the females do the choosing The Evolution of Differences in Sex Roles - males build bowers and females do not - what were the evolutionary origins of bower building by males? - why has the bower building of males of this species been maintained by selection following its origin? - First question satin bowerbird is one of the 2- speices in the bowerbird family and of which 17 build bowers - No other bird builds anything like these structures and so the trait appears to have evolved just once although it is possible that avenue bower building and maypole bower building evolved independently - But if we assume that the trait evolved once, a single common ancestor gave rise to it - Two early species derived from this ancestor gave way to avenue and maypole bower building - Among the two groups, species bower building behavior varies greatly and even differences found between closely related species as well - Two geographically separated populations of the same species build different maypole bowers even though the birds in the two areas are genetically similar - Changes have occurred so rapidly that close relatives lack shared features that would help us establish what the first bower might have looked like - What does the male gain by spending so much time constructing his bower o One possibility is that the bower shows info on his quality as a mate to females and was rewarded when discriminating females copulated with him - If this was true, then we expect male mating success to be correlated with some feature of the bower like the skillfulness of the construction and decoration or the number of stolen blue feathers - Even humans can detect differences among the bowers built by different males - Some are neat and symmetrical while others are messier and less professional - Females are likely to notice these because they were less likely to show startled responses when visiting well decorated highly quality bowers BIOC54 Chapter 10 Textbook Notes Tharsini Sivananthajothy February 19, 2012 - Less a male startles a female, the more likely she will mate with him so bower quality may correlated with mating success and mating success directly translates to male genetic success because females rarely mate with several males - Typically uses the sperm of a single partner to fertilize her eggs - If male quality bower hypothesis is correct, then attractive well decorated bowers should be built by males that are superior in some way to those birds that cant erect a top-flight bower - Doucet and Montgomerie prposed that good bower builders might be healthier birds, less likely to infect their mates with parasites and disease microbes and more likely to possess sperm with genes for disease resistance that could be passed on to their offspring - This is true and males that build better bowers have fewer ectoparasites and they are less infected as juveniles as well - Another idea is that the bowers quality is in some sense an indicator of the developmental history of the male o Birds that had plenty of food as they matured should have well constructed braind and so should excel in demanding tasks such as bower building - Madden recognized that if this hypothesis were true then the brains of bower building birds should be larger than non bower building - This is also true see Figure 10.4 - because most female birds mate with just one male, the most popular female, the male reproductive success in any given breeding season is very unequal - why do females do the choosing and why do males do the building - it is common throughout the animal kingdom for males to try to mate with females and females are happy to mate with the males they choose - this pattern is related to the fundamental differences that females produce eggs and males produce sperm - eggs are larger than sperm while sperm are only big enough to contain the males DNA and enough power to fuel the journey to an egg - egg is usually 15 20% of the females body mass - males may have 100 billion sperm while only 3500 eggs coho salmon - critical point is that sperm usually outnumber the many fewer large eggs available for fertilization in the population - this sets a stage for competition among males to fertilize those eggs - males contribution of genes to the next generation depends directly on how many sexual partners he has the more mates means the more eggs fertilized which means more descendants and greater the males fitness relative to less successful individuals - beneftis to males of maximizing the number of inseminated parnters have cuase for favour males that can discriminate between familiar mates and unfamiliar new females o Anolis lizard shows ten times as many courtship displays toward new females that have shown up in their territory than to old mates - Females try not to have as many sexual partners because their reproductive success is limited by the number of eggs they can manufacture - Eggs are costly to produce because they are large which means females have to secure the resources to make them - After one batch of eggs are fertilized, females has to spend more time and energy carring for the resultant offspringBIOC54 Chapter 10 Textbook Notes Tharsini Sivananthajothy February 19, 2012 - During the breeding season, satin bowerbird females are likelty to forage, build a nest or care for young than look for a mate while young male bowerbirds are on duty at their bower everyday - Parent investment: expenditures of time and energy and risks taken by parent to help one offspring, if they redue the chance that the parent will reproduce successfully in the future - Trivers developed parent investment to look at the tradeoffs for parents that make contributions to their young o Parental investment may increase the probability that an existing offspring will survive to reproduce o This fitness benefit comes at the cost of the parents ability to generate additional offspring down the road o Females are more likely than males to derive a net benefit from taking care of their existing offspring Offspring they care for are extremely likely to carry their genes Males paternity is often uncertain since females accept sperm from more than one individual Males have less incentive to be parental in those species in which parental males would lose fertilization opportunities If a male can mate with several females, it pays him to do so especially if this attributes will give him an edge in the race to fertilize eggs Unsuccessful males are trapped as a result of having inherited the nonparental attributes that work well only for more competitive rivals of the same sex
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