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77 terms

Sexual Selection

What are the two types of sexual selection
Intrasexual selection and Intersexual Selection
Intrasexual selection
individuals of one sex (typically males) evolve traits that enable them to compete with other individuals of the same sex to win mating opportunities
Intersexual Selection
Individuals of one sex evolve traits (physical and behavioral) that are preferred by members of the opposite sex (typically females)
Mate Choice
Individuals of a sex preferring certain traits and behaviors on other individuals
Does sexual selection lead to sexual dimorphism?
Sexual dimorphism
Trait differences between males and females of a species, most of the time exaggerated.
Runaway selection
When the female likes big horns, her sons will most likely like big horns, more big horned males are produced into the populations. The females are then going to mate with big horns. Which came first the preference or the choice
(differences in gamete size and
degree of investment . Explains why behavioral sex difference exist
Female Gametes
Eggs are large, nutrient-filled, expensive to produce,
limited in number, and produced infrequently. If fertilized this
will lead to high costs to the female.
Male gametes
the sperm is small and contains no nutrients. It is cheap to produce and is constantly made throughout life
Reproductive capability
Females are classified as the 'slow sex" and males are classified as the "fast sex"
Female reproductive strategy (criteria)
Physical features- size, and strength which confer dominance and so preferential access to resources Behavioral features- may indicate willingness to invest or good parenting skills
Do females gain from Multiple mating?
Not really. They gain very little from multiple mating, therefore they seek quality not quantity. Almost every reproductive female will be able to find a mate of some sort
Male Reproductive Strategy (Criteria)
Males are less choosy as they have very little to lose and everything to gain if they mate as much as possible.
Do males seek multiple mating?
Yes, males are not tied to offspring rearing and so they seek quantity as opposed to quality, but they would prefer a superior female.
Males reproductive success is variable
a small number of males will achieve many matings, while many males may never mate
Bateman's Gradient
the number of offspring fathered by males increased in proportion to the number of females with which the
male had mated.
Female reproductive success did not increase as her number of
partners increased. the steeper the gradient, the stronger sexual selection
What is the driving force behind sexual selection?
the degree of parental investment each sex devotes to their offspring.
Parental Investment (PI)
any investment by the parent in an individual offspring that increases the offspring;s chances of survival (and hence reproducing) at the cost of the parent's ability to invest in other offspring
Male's involvement (precocial)
the opportunities for male investments are low. The males compete for multiple mates much more frequently.
Male's involvement (altricial)
the male can share more evenly with the female
Mating in Northern elephant seals
Mating in northern elephant seals takes place on land in dense colonies; males establish dominance hierarchies and only the dominant males breed.
Typically less than 33% of all males copulate at all, and the top 5
males do at least 50% of all copulations.
Northern elephant seal male involvement
The males make no investment in offspring beyond the sperm and have been known to squash pups as they strive to inseminate females.
Male silver backed jackals
both parents defend the territory and hunt cooperatively. The male plays a crucial role because if the father disappears, mother and pups usually die
What are the proximate causes of male care of young?
Hormonal changes such as increased prolactin and decreased
testosterone can lead to paternal care.
In the California mouse, exposure to the pregnant female's urine,
as well as other cues from her enhances paternal behavior and
reduces infanticide.
Predictions arising from PI theory
1. The sex that invests most heavily in an offspring will be more
choosy about who they mate with (i.e. females).
2. The sex that invests least in an offspring should compete more
vigorously for access to the higher-investing sex (i.e. males).
3. Male competition will lead to physical dimorphism, and will be
most marked in animals where there is little male parental
investment (e.g. elephant seals).
4. Where male investment approaches that of females, competition will be reduced as will sexual dimorphism
5. Where male parental investment is higher, females will possess
typically male characteristics (not true in any mammals).
To avoid cuckoldry?
To avoid 'cuckoldry' (unknowingly rearing another males' offspring) the male must continually provide resources and protection, and will generally devote an almost equal amount of effort to child-rearing.
What are the 3 problems to solve when choosing a mate?
1. Perception
2. Integration
3. Decision
Perceiving certain sexual cues that provide reliable information about a potential mate
Information from various cues are integrated to gain information
about health, fertility, parenting skills, and social status.
In many species the male has to carry out an intensive and time consuming
task before the female will consent to mate.
Where the male does not provide an initial investment, the
female has to choose a mate on the basis of how good his genes
The Handicap Principle
females only assess those traits that are 'honest' indicators of
male fitness and are 'costly' to produce and maintain (e.g.antlers).
Such traits would only be associated with the most healthy
males who could afford to produce and maintain such features
secondary male sexual characteristics (antler size etc)
facilitate female appraisal of a males ability to withstand the
detrimental effect of parasitism.
This theory is a modification of the 'handicap principle' as the
development and maintenance of secondary sexual characters
may be a considerable handicap
Testosterone produces aggressive, sexual, and competitive
behaviours at puberty. Higher levels confer more impressive
High level testostrone
high levels of testosterone impair the functioning of the immune system making the individual more prone to disease and parasitism.
Mature males with a superior immune system can produce and
maintain secondary characteristics but are not prone to disease
and parasitism. Those with inferior immune systems may develop the characteristics, but cannot maintain them.
Symmetrical antlers
Male reindeer with more symmetrical antlers had a more
efficient immune system and were better able to cope with parasitism.
Fluctuating symmetry
refers to random deviations from symmetry in paired traits. For example, when a paired trait, such as horns or canines, is measured for thickness, length, or some other attribute - the two sides might differ. These deviations relate
to the inability of the male to maintain developmental
homeostasis in the presence of
environmental variation and
Greater asymmetry
associated with low food quality and quantity, habitat disturbance, pollution, disease and genetic factors associated with inbreeding, hybridization and mutation.
Major Histocompatability Complex (MHC)
The MHC is a large chromosomal region containing genes that play a role in immunological self/non-self recognition.
Female rodents learn the MHC identity of their relatives during
development via pheromones, and prefer to mate with males
who carry dissimilar genes.
Sex ratio
The sex ratios of populations of most species tend to be 1:1 at
birth but may deviate significantly from this ratio by the time
they are adults
Operational sex ratio
only considers reproductively active members of the population. Deviations in this ratio can have significant influences on the mating system as members of the abundant sex compete for access to members of the scarcer one.
Healthy Mothers
There is a suggestion that mothers in the best physical condition produce healthier offspring that therefore better able to compete for resources and ultimately mates.
Variables for competing males
the reproductive success of males is more highly variable (i.e., Bateman Gradient) and males should require greater maternal investment to be successful.
Data from mink, seals, deer, sheep, pigs
females in good body condition produce more males, and
females in poor condition produce females.
Investment difference in European roe deer
High quality mothers invest more in
daughters than in sons suggesting that mating success in males
is not that much more variable than females. More fertile males produce more sons
Hamadryas Baboons
produce more daughters than sons but in this species the daughters (who remain in the natal group for life) share the social rank of their mothers and can benefit from their mothers.ranks Baboon sons leave
the natal group and thus do not benefit from their mothers rank.
How do adjustments in sex ratio occur?
Some evidence points to the timing of insemination within the estrus cycle with more of one sex being conceived early and the other being
conceived later. This is true of white-tailed deer, rats, hamsters,and humans
Death rates interuterine
Are typically higher for males than females and are higher still in females who are in poor body conditions
Females incite competition
For example northern
elephant seals vocalize loudly whenever a male attempts to
copulate. This attracts attention and tests the dominance of the
dominant harem master
Sperm competition
Competition among males doesn't cease with the act of copulation.
Females of some species may mate with several males during a single estrus period
First male advantage
reducing the chances of a second male being succesful
Second male advantage
reducing the chances that the first male's sperm will be successful
Adaptation of first males to ensure offspring
include mate guarding behavior and
the deposition of copulatory plugs, both of which reduce the
chance of sperm displacement by a second male.
Adaptation of second males to ensure pregnancy
restricted to dilution of the
first male's sperm by ejaculation of large volumes of sperm from the second male.
Copulatory plugs
Copulatory plugs appear in rodents, bats, and some primates.
Although plugs in some species do appear to block subsequent inseminations, in some they appear ineffective and may be retain sperm in the female's reproductive tract.
Testes size
Among mammals, species in which only one male has access to
one or more females have smaller testes than do species in which
more than one male has access to females.
Exceptions to testes size theory
the southern grasshopper mouse has testes that are 5 times larger
than predicted even though it is thought to be monogamous.
multiple matings Not all species are polyggnous Polygyny results in greater variation in reproductive success for males than for females and so selective pressure tends to act more strongly on males
Which environment fosters monogamy
densely forested area where communication over long distance is difficult, small family units and monogamy are mire common. Also the same is true when food is evenly distributed.
Which environment fosters polygyny
Clumped distribution of resources
one male- one female in an exclusive mating relationship
includes all multiple mating, non-monogamous mating systems
Subsets of Polygamy
Polyandry, Polygyny, Promiscuity
one female with two or more males at a time
one male with two or more females at a time
the absence of any prolonged association and multiple matings by at least one sex
Monogamy regular strength
All males have same mating success=1 mate
Moderate Polygyny
Some males= 0 mates Some males= 1 mate Some males= 2 mates
Strong Polygyny
Most males= 0 mates One male= 8 mates
Example of animals with extensive male care
Among Primates, the forest dwelling marmosets and tamarins are all
monogamous with extensive male care of young.
The larger carnivores, especially canids, are nearly all monogamous
Several forms of Polygyny
• Resource defense polygyny
• Female defense polygyny
• Male dominance polygyny
• Scramble polygyny
Resource defense Polygyny
males defend areas containing the
feeding or nesting sites critical for reproduction, and a females
choice of mate is related to the quality of the male and his territory.
Polygyny threshold
the point at which a female would do better to join an already mated male with a good territory rather than an unmated male with a poor territory
Female defense polygyny
may occur when females are gregarious for reasons unrelated to reproduction, as when females herd for protection against predators, or gather around resources such as
food or nesting sites. Some males monopolize females and exclude
other males from their harems.
Male Dominance Polygyby
If female movements or concentration areas are predictable, the males may concentrate in those areas and pool their advertising and
courtship signals. Females then select a mate from a pool of males.
Scramble Polygyny
may operate as males try to obtain copulations. Where females are highly dispersed, males become highly mobile during the breeding season. Rather than competing
with other males the most successful
breeders are those that cover the most area in search of females
This is extremely rare amongst mammals and in most cases
where females mate with more than one male it is probably more
appropriately referred to as promiscuity.
True Polyandry
larger canids do show true polyandry - the African wild dog, although usually monogamous, sometimes
exhibits polyandry, where females are mated by several males
and they all provide care to the young.