Sexual selection Flashcards
The Sexual Struggle
- Darwin puzzled over extravagant traits that were usually found in only one sex (and most often in males)
- If these structures were necessary for survival, females would have them too (and in many cases, they actually seem bad for survival)
- Instead, these traits have evolved because they were of advantage in competition for mates, a process called ‘sexual selection’
- Can work with OR against natural selection
The sexual struggle is two parts:
- Males versus males
- Males enticing females
What are the two parts of the sexual struggle?
- Males vs. males
- Males enticing females
Why is it (usually) males who compete for females?
- Sexual reproduction: formation of gametes by meiosis, and then the fusion of genetic material from two individuals
- Almost always involves two sexes (male and female)
- For behavioral ecology purposes, these sexes are defined by differences in gametes
- Females make eggs: large, immobile, food-rich gametes
- Males make sperm: tiny, self-propelled
Why is anisogamous reproduction (fusion of two same-sized gametes) so common?
- This can be modelled as an evolutionary ‘game’
- Assume zygote survival depends on size (larger=better)
- Smaller gametes selected to seek out & fuse with larger gametes (parasitism)
- However, gametes selected to resist smaller gametes (would prefer to fuse with larger gametes)
- However, fusion between large and small gametes will predominate because there are so many small gametes
- Smaller gametes are made in larger numbers/greater number of genotypes –> faster evolution (outwit defenses)
Parker et al. showed that:
- The result of this arms race will be individuals that specialize in producing one gamete type
- Producers of medium-sized gametes will die out
The result of the anisogamy evolutionary arms race is that individuals that ___
Specialize in producing one gamete type
Parental investment vs. sexual reproduction
- Where one sex invests considerably more than the other, members of the latter will compete among themselves to mate with members of the former
- Leads to fundamental patterns of behavior and resource investment in males and females
- While a female can usually best increase her reproductive success by giving more resources to individual eggs and offspring, a male can best increase his success by finding and fertilizing as many females as possible
Comparison of male and female number of offspring during a lifetime
- Males can nearly always achieve a greater number of offspring during a lifetime than females
- When females invest more in each offspring than males, male courtship and male behavior is mostly directed towards competing for female interest and investment
Why do females care more even after mating: Polyandry
- For a given male, some of the offspring in a brood are not his
- By helping, he’d also help raise another individual’s offspring
- Benefit to his few offspring < benefits he gains by just going off and mating again
Why do females care more even after mating: skewed mating success
- In many systems, some males obtain an above-average share of the mating
- Say we have 20 displaying male grouse, only 1 mate
- Equal numbers of females and males in the population
- The single best male grouse has 20x the reproductive success of any female
- Lack of parental care detracts more from female’s future RS than the male’s
Operational sex ratio
- The operational sex ratio (OSR) is the ratio of sexually active males to sexually active females in a population
- An OSR of 100 indicates a balanced sex ratio
- An OSR below 100 indicates a female-biased population (e.g. 81 males for every 100 females in a population)
- An OSR above 100 indicates a male-biased population
- OSR is used to predict the intensity of mating competition and to predict which sex is competing for which
- If there are more females than males, females will compete for males
- Given that every fertilization removes a female (but not a male) from the mating pool, female-biased OSR is rare
- OSR can be influenced by differential mortality during pre-reproductive stages, the sex ratio at birth, differential migration, and differential mortality in senescence
- OSR predicts the direction of and opportunity for sexual selection
An OSR below 100 indicates a ___-biased population
Female
Why is female-biased OSR rare?
Because every fertilization removes a female from the mating pool, but not a male
Evidence for sexual selection: combat traits
-Abundant evidence that males with greatest size, strength, or most developed weapons achieves the highest mating success
-Northern Elephant Seal:
- Females haul up on beaches, are grouped dependable resources
- Males fight to monopolize harems
- The largest/strongest males win the largest harems
Female choice: Darwin’s second idea (and the one science left behind for longer)
- Darwin proposed female choice as a mechanism for sexual selection
- Andersson: Long-winded widowbirds
- Males have remarkably long tails (>0.5m)
- Females have short tails
- Males defend territories in grassland to which they attract females to nest
- Tails are not used in contests
- When females fly by, males perform a slow, cruising flight to display tail
- The possibility that intrasexual compe·
tition among males maintains the long tail was not supported: males with shortened tails held their territories as long as did other males. These results sagest that the extreme tail length in male long-tailed widowbirds is maintained by female mating preferences.
Females can also choose based on non-visual traits
Catchpole:
- Studied stongs of the European sedge warbler
- Songs are sung by the male after arrival on the breeding territory
- Once a male is paired, it stops singing
- Males with the most elaborate songs were the first to acquire mates
- When females were brought into the lab and treated with oestradiol to make them sexually active, they were more responsive to elaborate playbacks
Females prefer elaborate traits … so why don’t all males have them?
- Trade-offs
- Barn swallows: males with experimentally-elongated tails paired up more quickly and were more preferred
- BUT, those males foraged less well
- Caught smaller prey, grew poorer quality feathers for the next year
Female benefits of being choosy: good resources
Territories:
- Male North American Bullfrogs defend territories where females lay eggs
- Some territories are better than others
- Females prefer good sites, held by the largest, strongest males
- By choosing a certain male, females are ‘choosing’ access to high-quality resources
Food:
- Female hanging flies will mate with a male only if he provides a large insect for her to eat during copulation
- Larger insect –> more time for copulation –> more fertilized eggs
Other female benefits of being choosy
- Sometimes, all a female gets from a male is sperm to fertilize her eggs
- Even in such cases, females can be very choosy
Could females be getting genetic benefits from choosing a specific mate? Two hypotheses for how females might select mates based on good genes have been put forward
What are the two hypotheses?
- Fisher’s hypothesis
- Zahavi’s hypothesis
Fisher’s hypothesis of genetic benefits
- Male displays may be sexually selected because they are attractive to females
- This leads to what’s called ‘Fisherian Runaway’
Fisherian runaway
-As preference and traits spread:
1) Long-tailed males gain a double advantage:
- They’re still more attractive AND now more likely to get a mate (because the preference is spreading among females)
2) Females get advantages too:
- Her sons are very attractive
Fisherian runaway: sexy sons
Testing the Fisherian hypothesis
-Need to show 1) genetic variation in BOTH female preference and male trait, and 2) genes for preferences and traits covary
Zahavi’s Hypothesis of genetic benefits
- Long tails are a “reliable signal”
- Low-quality males CAN’T cheat and display
long tail feathers because the tails are a
handicap - If the ability to survive the handicap is
heritable, this will be passed to offspring - Females select for good genes by selecting
only mates who honestly display their quality
Testing for Zahavi’s ‘good genes”
-The Hamilton-Zuk Hypothesis:
- Good genes are related to genetic resistance to disease
- Hosts and parasites are in a never-ending arms race, so the ‘good genes’ are changing all the time –> females benefit from being chosy
-To test this hypothesis, we need to show:
1) Parasites reduce host fitness
2) Parasite resistance is genetic
3) Parasite resistance is signaled by sexual ornaments
4) Females prefer to mate with males with more elaborate ornaments/signals
Testing Zahavi’s hypothesis: three-spined sticklebacks
Sometimes, females are ornamented
- Sometimes, males make a large contribution to parental investment
- In those cases, males can be choosy
Mutual mate choice:
- When both sexes invest heavily in parental care, it benefits each sex to choose a high-quality partner
- Both males and females can develop similar ornaments
Female advertisement:
- Selected for when females live in multimale groups
- Females under selection to access the highest quality available male
Sex role reversal
-In some cases, female competition for males is so strong there is reversal of the usual sex roles
-Pipefishes and seahorses:
- The male becomes pregnant
- Females produce eggs faster than males incubate and birth young
- So males become a limiting resource for females
- Females compete for males
- Males prefer larger, more ornamented females who produce larger clutches
- Can be due to seasonal variation
Sex role reversal due to seasonal variation
- In katydids in Australia, when food is scarce, the male’s protein-rich spermatophore is costly to produce and very valuable to females
- Females compete for males and males are choosy
- When pollen-rich grasses come into flower, males can produce spermatophores more easily
- At this point, access to repetitive females limits male success and males compete for females
Sexual selection beyond mating: sperm competition
Parker’s novel insight:
- In many species, females mate with multiple males and can store sperm from multiple males at once
- Thus, sexual selection can continue after the act of mating
- Sperm can compete with each other to fertilize an egg
- There are two types of sperm competition
What are the two types of sperm competition?
- Competition between sperm from rival males
- Choice of sperm from a certain male by a female: ‘cryptic female choice’
Why do females copulate with more than one male?
- Cost of resisting exceeds cost of acquiescence (e.g. male harassment)
- Material/direct benefits from multiple mating
- Genetic/indirect benefits for offspring
