We shall now explore the mysteries of mating behavior. We begin with with a discussion of the sex-bias in the general mating strategy which underscores all subsequent mating decisions and behavior; move on to an understanding of the process of sexual selection; and then proceed to discuss various kinds of mating systems.

I. STRATEGIES

A. Mating Strategies vary by sex

1. males should ___________________________________

 2. females should __________________________________

B. Why this difference? 

1. females

a. produce fewer but larger gametes

b. gametes are valuable both because of size and rarity

c. resulting in severe competition for this limited resource

2. males produce a prolific number of smaller gametes

3. Bateman's principle

a. females should be choosier than males because eggs are costly and a female's ability to reproduce is considerably more limited than a male's ability to reproduce

b. there should be greater variance in the reproductive success of males

II. SEXUAL SELECTION

A. Definition

SS is a component of individual selection that is created by the pressures that males and females exert on conspecifics as they compete for mates and choose among potential partners.

B. Types

1. intrasexual selection which is characterized as involving ________________

2. intersexual selection which is characterized as involving ________________

C. Intrasexual Selection

1. male success depends on what?

2. correlated with a constellation of traits. Although, we often think of male-male competition as involving fights (antelope locking horns, giraffes wrapping their necks around one another, birds fighting at territorial boundaries); in fact fighting is only one of many ways in which males can compete with each other for access to females.

a. TRAITS THAT HELP A MALE COPULATE WITH MANY FEMALES

1. low _______________________________ (Chinchilla video)

 

2. skills in __________________________________ (ruff video)

 

3. if not # 2, then skills in ________________________

 

b. TRAITS THAT INSURE THAT BREEDING IS SUCCESSFUL (be sure to list examples and what these effects are)

1. Mate guarding: a male mates with a female and then remains with her to keep other males away until she lays her eggs. This strategy insures that this male was the last to mate with the female and that no other male subsequently mates with the female. Example: damsel flies 

2. sealing: a male mates with a female, and during the process of mating, his secretions act to block the vaginal canal making it difficult for other males to mate. Example: sperm plugs in rats

3. scooping

 

 

4. swamping

 

 

c. TRAITS THAT REDUCE THE FITNESS OF OTHER MALES (be sure to list examples).

You will now see the sunfish video in which there are 3 strategies that males use to compete with other males for access to females. Two of these we have already discussed (dominance and sneakiness).

1. female impersonation (see sunfish video)

 

2. ____________________

 

3. Infanticide (example, Langurs)

Langurs are old world monkeys that are found in southeast Asia. Unlike other species of old world monkeys, the langur has an unusual social organization consisting of one-male units (harem groups). A single male defends access to 10-14 adult females.

This organization is not the result of a skewed sex ratio. There are just as many males as females in the population. The extra males hang out in bands and wait for their opportunity. Harem males must fight off rivals from these bands, and eventually harem males are ousted. Males retain control for about 1-3 yrs.

When new males take over these units by ousting the previous male, a large number of infants less than 6 months of age disappear.

The assumption is that the new male is killing these infants (some observations also). How can we explain this behavior?

Hypotheses

1. ____________________________________

removes competitors for your offspring and:

 

 

benefits male but not necessarily the female

 

2. __________________________________

indicates that males might be killing infants to regain lost energy

 

3. ___________________________________

represents a response to abnormal conditions; not an evolutionary hypothesis

4. Chart of Predictions

Predictions	Sexual Competition	Cannibalism	Social Pathology
kills other male's infants	-	-	-
consume infant	-	-	-
abnormal conditions	-	-	-
abnormal conds. other species	-	-	-

 

5. Data support which hypothesis ___________________________

6. What about the females (why tolerate it?)

D. Intersexual Selection

1. Does female choice exist? For much of the last century, the focus was on intrasexual competition and the lengths that males went to mate with females. In part, the limited focus may have been due to the difficulty of trying to examine female choice. However, it is interesting to note that virtually all of the prominent animal behaviorists at the time were men. Today, the tables have turned. Studies of how females choose mates has become a hot area in animal behavior. It is also the case that women now outnumber men in this field by a ratio of 2:1. This just reminds us that we bring biases to our study of animal behavior and our gender is possibly one example.

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 an example.

a. Direct Benefits Model

1. Females choose males that provide them with resources above and beyond sperm. These resources can include gifts, quality of territory etc.

2. Example: nuptial gifts in the hanging fly (see video)

Female hanging flies reject males without gifts but they go even further in making a choice. Males generally bring gifts of insects for the female to consume. The size and kind of gift determines how long the female will mate with the male. Males need about 20 minutes to complete the transfer of sperm. Thus, the gift needs to be of high quality. As noted in the video, if the male finishes before the female fully ingests the gift, he will try to take it back from her.

b. Good Genes Model

1. Here the focus is on what is in the sperm, the argument being that natural selection favors females who select males with "good genes."

2. Females who select males with good genes do not receive direct benefits like a gift but benefit indirectly because their offspring get some of these good genes and therefore will be more likely to have higher reproductive success.

3. The challenges to this model are twofold

a. How can females determine which male has good genes?

 

b. ___________________________________________

There will be selective pressure on females to use only honest indicators of good genes

4. What might females choose as indicators of good genes?

a. parasite resistance - What is proposed as the marker for this?

 

Study of Plumage Choice in Pied Flycatchers

---Take 2 drab males and make one have bright coloration

---Females prefer to mate with the shiny male

b. body symmetry (zebra finch leg bands): Nancy Burley discovered quite accidentally that different colored leg bands (which she used to identify individuals) actually influenced mate choice.

Females chose to mate with males wearing _________________

Because males can wear more than one leg band on each leg, we can examine the role of color symmetry in mate choice.

The 3 types of leg band combinations were:

 

 

The outcome was:

 

Why? Symmetry may help identify those individuals who have been successful at dealing with changing conditions during development. Retained their symmetry.