Mating systems encompass the collective behavioral and morphological traits associated with interactions between the sexes for the purposes of sexual reproduction.

Given that reproduction represents a critical feature of overall fitness, there is often strong selection for traits that provide even small advantages over competition for resources (females) or mates (males).

For each sex, consider how fitness is potentially influenced by various factors...

Numerous different behavioral strategies (5-10) may evolve to improve each of these components.

Not surprisingly, an enormous number of behavioral combinations are possible.

Which combinations are ultimately selected depends on:

Environmental constraints

Resource availability, disturbance, temperature, etc..... an optimality problem

Intrasexual interactions

The optimal strategy within a sex depends upon what other members of the same sex are doing... Male/male competition may be especially important

Intersexual interactions

The optimal strategy for one sex may depend upon the strategy adopted by the opposite sex.... This can lead to patterns of parental care, mate searching strategies, female choice, etc...

Specific mating systems evolve because certain combinations of these three variables influence individual fitness in ways that produce ESS conditions.

Despite the myriad possibilities, some basic patterns emerge

Basic categorizations of matings systems are: Also: see Figure 9.1 in your text

Monogamy: one male with one female, extended pair bond, often shared parental care

Polygyny: one male with several females, often with extended pair bonds, usually female parental care or no care

Resource defense vs. female defense... models to polygyny threshold

Polyandry: one female with several males, often with extended pair bonds, usually male parental care

Jacanas

Spotted Sandpipers

Black Coucal

Polygynandry: females mate sequentially with several males, males mate simultaneously with several females, short-lived pair bonds, usually male parental care

Promiscuity: both sexes may have multiple mates, no pair bonds and either female or no parental care.

Some animals may show a range of mating systems based on habitat, resources, predation risk, and demography... read about Dunnocks in your text!

 

The evolution of these different mating strategies depends on patterns of resource availability, predation risk, environmental stability, and levels of parental care.

Careful...

The same basic pattern of mating behavior may evolve for several different reasons.

The components used to describe the system are not necessarily the causal factors.

Most male strategies are geared towards improving female encounter rates and fertilization rates.

Most female strategies are geared towards improving access to resource and ensuring offspring survival.

Competition for mating opportunities and resultant differences in mating success among individuals of the same sex can create variance in fitness.

This variance is called Sexual Selection.... a unique component of fitness that stands apart from other aspects of fitness that are influenced by Natural Selection.

Traits that confer fitness via Sexual Selection may run counter to the pressures of Natural Selection

Darwin recognized the special nature of sexual selection and identified two types of sexual selection

Both can lead to "runaway" selective pressures that favor the development of elaborate or exaggerated traits so often associated with mating behaviors and displays

Intrasexual selection: interactions between members of the same sex that cause different access to mates (usually male/male competition)

Traits that confer competitive superiority should be strongly selected for, leading to "arms races" for the elaboration of such traits.... this creates strong sexual dimorphism.

Obvious examples include the antlers of deer, horns in beetles, large body size in elephant seals, etc.

 

Intersexual selection: interactions between members of the opposite sex that cause differences in mating success for at least one sex (usually female choice of certain males).

Here, two traits are needed... the male trait that is chosen by females and the female trait of preference for a certain type of the male trait.

Once the link between preference and trait is established, the trait may quickly "run away" to a value that either causes extinction or is balanced by increased mortality (natural selection).

Choice by females for a specific trait presumably carries some cost, but there can still be selection for choice:

a) no benefit: traits selected for will be those that minimize female costs (little or no trait elaboration).

b) direct benefit: selected traits should correlate with benefit (e.g. indicate level of parental care, territory quality, low parasite or disease load, etc... leading to elaboration of trait).

c) indirect benefit: selected traits indicate to females which males have the best genetic make-up for offspring fitness (e.g., good foraging efficiency, disease resistance, high predator avoidance, etc.).

Handicap models of female choice center on "good genes" arguments and the idea that males with better genes will be able to either a) accept a greater handicap or b) survive better for a given handicap.

The potential strength of sexual selection can be assessed from the variance in mating success among all members of the same sex...

 

i.e., the greater the difference between "winners" and "losers" the stronger the potential selection

Careful: differences in winners and losers may have no genetic component (e.g. differences due to environment)... thus, no selection in this case.

 

Extreme variance in male reproductive success due to sexual selection can lead to alternative mating tactics.

 

Often linked to life history (make the best of a bad situation when young and small)

 

"Sneaky mating strategies", "jacks", etc...

And then... there is always sex-change...

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