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Module 4 Theme 11b
Traditional versus revised Darwinian views of sexual selection
The traditional Darwinian view emphasized the importance of intermale competition (red arrows) and female choice (green arrows) as pre2 0 1 0 copulatory determinants of mating success. The modern Darwinian view acknowledges that [1] intrasexual competition also occurs among females and that females can, in some circumstances, produce extravagant adornments to attract males. [2] sexual selection acts at the copulatory level and post2 0 1 0 copulatory level, via sperm competition and cryptic female choice, to affect reproductive success. [3] sexual conflict (brown arrows) can also occur at any of the three phases.
Trivers’ theory of sexual selection and parental investment: statement, and sex
related limiting factors and reproductive strategies
Bateman’s [1948] three principles:
**- Reproductive variance (MM>HH): reproductive success [RS] is more variable among males than among females;
- Promiscuity (MM>HH): mating success [MS] of males is more variable than that of females;
- Reproductive success [RS] and mating success [MS] are positively related, and this association is stronger in males than females;**
Anisogamy and parental investment
- Costs of gamete production (HH>MM): it is more costly to produce eggs than sperm. Therefore, the number of eggs available is generally far smaller than the number of sperm
Limiting factors:
- Males: number of eggs/females fertilized/monopolized
- Females: quality of sperm/resources received from males, SO…
Strategies selected for:
**- Males: polygyny/promiscuity, in order to maximize the number of sperm that fertilize eggs.
- Females: choosiness, in order to mitigate the penalty for having their eggs fertilized by low quality males or by males disinclined to invest in the care of the mother/offspring.**
Differences between the sexes in sexual behaviour may arise from fundamental differences in parental investment that affect the rate at which individuals can produce offspring. The sex that can potentially leave more descendants gains from high levels of sexual activity, whereas the other sex does not. An inequality in the number of receptive individuals of the two sexes leads to competition for mates within one sex, while the opposite sex can afford to be choosy.
Bateman principles applied to humans
Bateman [1948]
- Reproductive variance (MM>HH): reproductive success [RS] is more variable among males than among females;
- Promiscuity (MM>HH): mating success [MS] of males is more variable than that of females, and is greater for males than for females;
- Reproductive success [RS] and mating success [MS] are positively related, and this association is stronger in males than in females;
The first principle is largely supported. When all populations of human beings are considered together, males exhibit higher mean variation in RS than females. However, despite the expectation that male variance in RS will always exceed female variance, these data reveal large interpopulation variation in the ratio of male to female variance in RS, ranging from 0.79 to 4.75. This variation between populations is inconsistent with the universal sex roles that Bateman envisaged.
HOWEVER, male RS is significantly greater than female RS ONLY when the mating system is polygynous or monogamous serially.
Bateman’s second principle is difficult to test, partly because there is no clear cut relationship between assumed versus actual polygyny. In general, the lack of good datasets on sex differences in actual number of mating partners, and the likelihood that partner numbers vary across populations, means that it is currently difficult to make reliable generalizations about how Bateman’s second principle will apply to human beings.
Bateman’s third principle is difficult to assess in human populations because the relationship between polygyny [MS] and reproductive success [RS] is likely to vary as a function of a number of factors, especially in scenarios in which polygyny is costly, that have not yet been adequately measured.
Intrasexual competition (post-copulatory strategies)
When male Mormon crickets mate, they transfer to their partners a nuptial gift, it is an enormous edible spermatophore. Given that this spermatophore constitutes 25 percent of the male’s body mass, most male Mormon crickets probably cannot mate more than once. In contrast, some females are able to produce several clutches of eggs, but they have to persuade several males to mate with them if they are to have their eggs fertilized. I
Inter-sexual selection (female choice) with behavioural benefits
Song repertoire size as an honest signal of male paternal effort. In the sedge warbler, males with larger repertoires (as measured by the number of different syllable types used to construct the warbler’s songs) help produce chicks that weigh more upon fledging. So, females choosing males on the basis of their repertoire of syllable types gain a reproductive advantage over females who do not use that signal to choose male mating partners.
Intersexual-selection (female choice) with NO material benefits: healthy mate
hypothesis, good genes hypothesis, handicap principle, runaway selection
Females choose males that are healthier. They use song rate as a reliable indicator of the male’s health status, as this signal has been found to predict a male’s ability to mount an adaptive response against an infection. Somehow it indicates the male’s capacity to activate a physiological response (e.g., the immune system) to counteract infection
Mate preferences create sexual selection that maintains and spread the genetic basis for those preferences because the offspring of the preferred males receive good genes from their fathers.
The underlying premise here is that organisms that can handle changing environments may well be those favoured by the process of natural selection. And symmetry may be a cue others may use to assess an individual’s ability to adapt adequately to changing and often stressing and challenging conditions. So, it has been argued that females might use symmetrical characteristics as a way of assessing male genetic quality.
The tail is a costly trait in this species, in fact, it is a handicap , that is, a trait that contributes negatively to the male’s survival (they are more easily detected by predators and they are less able to outmaneuver them in case they have to escape). So, if a male with such a handicap is able to survive, it means that he overally has good genes. So, the tail is a honest/reliable indicator of a male’s overall genetic quality.
(Run-away selection) This approach argues that discriminating females acquire sperm with genes whose primary effect is to influence their daughters to prefer the male traits their mothers found attractive and to endow their sons with attributes that will be prefererred by most females, even if those traits actually reduce the survival chances of individuals that possess them. For example, a preference for the elaborate songs of male canaries could be adaptive for females if their sons inherit the capacity to sing attractive songs, even if they are costly to produce, because these songsters may be especially appealing to females in the next generation.
Intersexual conflict and sexual coerción
Sexual coercion as the “use by a male of force, or threat of force, that functions to increase the chances that a female will mate with him at a time when she is likely to be fertile, and
to decrease the chances that she will mate with other males, at some cost to the female”. There are three types of sexual coercion:
- (^) Direct sexual coercion, (that which “functions to increase the chances that a female will mate” with a male), involves the use of force to overcome female resistance to mating
- Indirect sexual coercion (or coercive mate guarding) is the use of force “to decrease the chances that [a female] will mate with other males.” Coercive mate guarding represents a distinct form of sexual coercion because it functions to constrain female promiscuity rather than to overcome female resistance.
- Infanticide is a final form of sexual coercion involving aggression directed not at a female but at her infant. Some see it as a form of direct sexual coercion, and others see coercive mate guarding as a male’s response to the female counterstrategy of paternity confusion through promiscuity.
There are three categories of direct sexual coercion
- Forced copulation (“rape” in the human literature ) involves violent restraint, resulting in immediate mating.
- Harassment involves repeated attempts to mate that impose costs on females, inducing eventual female submission.
- Intimidation involves physical retribution against female refusals to mate, increasing the likelihood of submission to future advances.
There are three categories of indirect sexual coercion
- Herding involves short2 0 1 0 term aggression directed toward females to induce immediate separation from rival males and to restore proximity to the guarding;
- Punishment involves physical retribution against female association or copulation with other males, decreasing the likelihood of these behaviours in the future.
- Sequestration involves forceful separation of females from a social group, particularly during periods of maximal fecundity, to prevent copulations with rival males.
Intersexual conflict and infanticide
Infanticide by males is the expression of two processes of sexual selection:
1. Intrasexual selection between males , as the infanticidal male increases his reproductive success by eliminating other males’ offspring and their own future offspring’s competitors 2. Intersexual conflict , as it imposes a tremendous fitness cost on the female whose offspring is eliminated.
The life history hypothesis for infanticide by males. Case 1 (presumed ancestral state): short lactation (L) and long gestation (G); no vulnerability to infanticide by males. Case 2: long lactation and short gestation; vulnerability to infanticide. Case 3: despite relatively long lactation, low infanticide risk because of seasonal breeding
Phalluses with a glans/coronal ridge configuration that approximated a human penis resulted in appreciable displacement of simulated semen.
Sexual selection in humans: mate choice
Hypothesis 1: short-term mating is more important for men than women
Hypothesis 2: Men seeking a short-term mate will solve the problem of identifying women who are sexually accessible.
Hypothesis 3: Men seeking a short-term mate will minimize commitment and investment.
Hypothesis 4: Men seeking a short-term mate will solve the problem of identifying fertile women.
Hypothesis 5: Men seeking a long-term mate will solve the problem of indentifying reproductively valuable women
Preferences for an age difference between oneself and one’s spouse differ for men and women, and this difference is exacerbated with age. These findings are interpreted as indicators of the importance of reproductive value when men make preferences for women, which remains stable over time.
Hypothesis 6: Men seeking a long-term mate will solve the problem of paternity confidence
Hypothesis 7: Women seeking a short-term mate will prefer men willing to impart immediate resources
Hypothesis 8: Women will be more selective than men in choosing a short-term mate
Hypothesis 9: Women seeking a long-term mate will prefer men who can provide resources for their offspring
Two general hypotheses:
addition to its antibiotic function, non 2 0 1 0 copulatory orgasms may act eliminating the last copulation’s surviving sperm that still remains in the woman’s sexual tract.
What accounts for species differences in parental care?
Paternity certainty : parental care is more likely when paternity certainty is higher, that is, when fertilization is external.
Order of gamete release : in external fertilization, if gametes by each sex are released sequentially, then the first to release them has a chance to desert first (this has been rejected).
Association: in internal fertilization, the female is most closely associated with the embryo, and this mat set the stage for embryo retention and live birth, followed by care of the young. With external fertilization, eggs tend to be laid in a male’s territory and it is the male who is most closely associated with the embryos. This hypothesis is the best predictor of data in the table: territoriality is more common with external fertilization.
A. Key (not only) factor: Mode of Reproduction
A. Fertilization: external versus internal B. Foetus development: external versus internal C. Postnatal feeding: absent versus obligate D. Reproductive potential: gametes/reproductive ‘projects’
B. Result
A. Females are more likely to be the sex to invest parentally (that is, there is maternal care) when fertilization is internal, embryo development is internal (viviparity), there is obligate postnatal feeding and the number of gametes/reproductive projects are reduced.
Sex allocation and sex ratio: Fisher and Trivers and Willard
Sex allocation is a measure of how parental resources are allocated (partitioned) between male and female offspring. Sex ratio refers to the number of males and females produced.
Fisher made clear that it is the total amount of parental investment in each sex that
has to be equal (i.e., sex allocation) at the population level, rather than the number of males and females (i.e., the sex ratio). When the number of males and females is equal, both sexes are equally efficient at transmitting genes to the next generation. As long as the population sex ratio sticks to 1:1, natural selection has no effect on the tendency to produce one sex or the other and parents become indifferent to the sex of their offspring. However, as soon as the population sex ratio deviates from 1:1, frequency 2 0 1 0 dependent selection favours the parents that produce the
Assumption 2 was met [B]: The original fortune2 0 1 0 maker left more grandchildren through his sons than through his daughters. Furthermore, sons were, on average, richer than daughters within the same family , suggesting that sons received greater parental allocation
Sex allocation and sex ratio: resource competition and dispersal
Fisher’s theory assumes that related individuals do not interact, either cooperatively or competitively, if such interactions occur, then individuals can be favoured to bias their offspring sex ratio to reduce competition between relatives, or increase cooperation.
- LOCAL RESOURCE COMPETITION (LRC): Birth sex ratios are biased towards females in species in which females are the primary dispersers, unbiased in species in which both sexes disperse and biased in favour of males in species in which males disperse.
- LOCAL RESOURCE ENHANCEMENT (LRE): Birth sex ratios are biased towards males in cooperatively breeding primate species. This is because males are more active and effective helpers than females in these species.
- CONCLUSION: Birth sex ratios are NOT linked to sex differences in investment costs (i.e., costs of rearing male or female offspring), as predicted by Trivers & Willard’s hypothesis, as birth sex ratios do not correlate with sexual dimorphism.
