II. SEXUAL SELECTION

D. Intersexual Selection

2. Evolutionary Models of Female Mate Choice

There are four models that are relevant - "direct benefits," "good genes," "runaway selection," and "sensory bias." We shall review the parameters of each and provide examples when relevant.

c. Runaway Selection Model

1. The assumption is that two genes exist - a gene that governs a particular trait in males and a gene that regulates mating preference in females. Over time, these two traits become linked.

2. Imagine a hypothetical species in which

a. some of the females have a heritable preference for bright colored males whereas other females have no preference

b. some males are more brightly colored than other males based on genetic differences

c. We will also assume that both genes (color preference and bright coloration) are present in both sexes but only one is turned "on" based on sex (color preference for females and coloration for males).

d. Males with the bright coloration will have higher reproductive success (can mate with all females) whereas males with less bright coloration can only mate with the females that have no preference.

d. Eventually these traits of preference and coloration will be linked such that as the frequency of one changes so does the other.

e. Then the process can run away; meaning that selection will produce increasingly exaggerated color in males and stronger and stronger preferences in females

d. Sensory exploitation model

1. Females prefer males that maximally stimulate their sensory receptors.

2. Existing sensory biases in females are tapped into by males

3. Imagine a brown colored bird species which feeds on red berries. Thus, the sensory system has already been selected to prefer red color and the birds have a strong preference for that color. Now imagine that a mutation, a male with a red tail, appears in the population. Females may very strongly prefer males with red tails because of their pre-existing sensory bias.

4. Example - female primates of many species prefer red coloration in males. How did this evolve?

a. primates have trichromatic vision enabling them to detect red-orange fruits; and primates with trichromatic vision appear to prefer these colored fruits

b. Did this preference for fruit color lead to a preference for red coloration in males?

c. One possible test is to determine whether trichromatic vision preceded the appearance of red coloration in males.

d. The result of a phylogenetic analysis was:

 

E. Putting it all together - Satin Bower Bird

We have discussed male competition (intrasexual selection) and female choice (intersexual selection) as if they are entirely separate processes, but these two processes actually interact and are frequently present together in various species. We shall look at both processes in the satin bower bird. As we do, think both of the male behavior that is relevant as well as the models of female choice.

1. What are the elements of satin bowerbird behavior

a. As part of their mating behavior, males:

 

 

 

b. As part of their mating behavior, females:

 

 

 

c. Description of Bowerbird mating behavior (see videoclip)

What is bower trashing?

 

 

d. Objects

1. Are blue objects preferred? How was this tested?

 

2. Is the number of blue objects related to reproductive success? What were the conditions in which the above statement was true?

 

 

e. female response varies by age

1. How do young females behave?

 

 

 

 

2. How do older females behave?

 

 

 

 

3. Using robots, Patricelli show that older females prefer sensitive males?

What is a fembot? What did it do?

 

 

 

4. When fembots were placed in the bowers of successful and unsuccessful males, what happened? (Draw the bar graph)

 

 

 

 

What does Patricelli mean by the phrase "sensitive males?"

 

 

 

III. MATING SYSTEMS

A. Mating Systems

1. represent the interaction between mating strategies and the environment

2. complexities (how do we develop an accurate classification system)

a. is it based on the number of mates? (if so in a breeding season, lifetime?)

b. is it based on the presence of a pair bond

c. how do we include both the male and female perspective?

d. how do we deal with extra pair copulations?

e. what happens when different mating systems exist within the same species?

3. No system can adequately deal with all these issues. For the discussion below, our focus will be on the number of mates

B. Types of Mating Systems - Monogamy

1. definition

2. an excellent example - Albatrosses who mate for life - describe features and explain why albatrosses are monogamous (see video)

3. Mongamy would seem to constrain the male mating strategy, so why should the male stay? - possible hypotheses below

a. mate-guarding (extension of mate guarding so that the only the male's sperm are used)

 

b. __________________ (male care greatly increases the survival of young)

 

c. female enforced (females actively prevent males from mating again)

4. Monogamous species often engage in extra-pair copulations (Why?)

a. for the male, the benefit is in increased reproductive success

 

b. why for female (advantages of EPCs)

1. _______________________________________

 

2. better genes

 

 

3. Genetic compatibility hypothesis

 

 

 

4. material benefits

 

 

 

c. What are some of the disadvantages?

1. Time spent finding and breeding with another male

2.

 

3. Getting a sexually transmitted disease

C. Types of Mating Systems - Polygyny

1. definition

2. types

a. female defense (access to females not a territory; langurs; mallard ducks)

 b. resource defense (defends resources for females such as a territory as in redwing blackbirds; but not all redwings are polygynous; WHY?)

1. The finding of both monogamy and polygyny in this situation is explained by the POLYGYNY THRESHOLD

2. Females may gain higher fitness by mating with an already mated male (creating a polygynous situation) than mating with an unmated male

3. The polygyny threshold refers to the difference in territory quality for a female to have higher fitness as a second mate as compared to fitness gained with mating with an unmated male

4. draw the graph below (be sure to place the letters in the appropriate spot and determine the outcome)

 

 

 

 

 

 

When given a choice between A and B, which yields higher fitness?

When given a choice between A and C, which yields higher fitness?

When given a choice between A and D, which yields higher fitness?

c. Lek polygyny

1. Leks are ancestral grounds used for mating

2. males defend small mating territories on the lek and only a few males mate with many females

3. What do leks look like from above? Draw scheme

 

 

 

4. Only the centermost males (1-3) breed with about 90% of the females.

5. Young males start at the periphery and take years to work their way to the center.

6. Why do males congregate in such groupings as leks.

a. hotspot hypothesis

 

 

b. hotshot hypothesis

 

  

c. female preference hypothesis

  

 

7. An interesting case of the ruff. Meets all of the above requirements for lek mating described above but also has two types of males

dark territorial males actively defend a small 3 square meter territory for all daylight hours.

satellite (pale) males can enter territories and are actively wooed by dark territorial males.

Because both males can fertilize offspring, why should a territory holder permit a satellite male to enter his territory?

The answer is

 

 

D. Types of Mating Systems - Polyandry

1. definition - a female has more than one partner during a breeding period

2. It is usually associated with sex-role reversal

3. The jacana is a good example (see video clip)

a. the female defends a large territory that includes nest sites for several males

b. the males care for offspring

c. males are considerably smaller than the female