SEXUAL SELECTION

Chapter 11 in the 4th edition, Chapter 10 in the 3rd.

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INTRODUCTION

1. Sexual dimorphism

   1. Males and females differ markedly in size, appearance, and behavior.

   2. Examples

      1. Marine iguanas, etc. (Fig. 9.1)

      2. Humans (Fig. 9.2)

   3. Charles Darwin (1871)

   4. Differences between sexes can decrease fitness.

      1. Long-tailed widow birds (Fig. 9.3)

   5. Sexual selection

10.1 – ASYMMETRIES IN SEXUAL REPRODUCTION

1. Selection pressures for females versus males.

2. Orangutans as an example

3. Female versus male mammals.

4. Females versus male reptiles, insects, etc. 

5. Female’s reproductive success  

6. Male's reproductive success  

7. This is predicted to produce a fundamental asymmetry:

   1. Access to females will be a limiting resource for males,

   2. but access to males will not be a limiting resource for females.

Asymmetric Limits on Reproductive Success in Fruit Flies

1. Bateman’s experiments with fruit flies, Drosophila melanogaster (Fig. 9.4)

Behavioral Consequences of Asymmetric Limits on Fitness

1. Mating behavior reflects differences in limiting factors for reproductive success.

   1. Intrasexual selection 

   2. Intersexual Selection  

2. This treatment in sexual selection is full of generalizations.

10.2 – MALE-MALE COMPETITION: INTRASEXUAL SELECTION

Combat

1. Intrasexual selection involving male-male combat 

2. Galápagos marine iguanas (Fig. 9.5) 

   1. sexual size dimorphism: Males are larger than the females. 

   2. Natural selection on body size.  (Fig. 9.6) 

   3. Mating Systems of Galápagos marine iguanas (Wikelski, Trillmich and colleagues)

      1. Combat for access to mates favors large body size in males (Fig. 9.7)

         1. Males attempt to copulate with many different females

         2. huge variation in male mating success (fig. 9.8)

         3. Before the breeding season, males compete in direct combat (Fig. 9.7) with each other for territories on basking sites (Fig. 9.8)

         4.  In general, calculated selection differentials of 0.42 and 0.77 (Table 9.1) favoring larger males on two different islands.

Sperm competition

1. Definition 
2. Sperm competition can favor several different kinds of traits:
   1. large ejaculates
   2. mechanisms to prevent other males from inseminating female
      1. Prolonged copulation and/or other mate guarding
      2. copulatory plugs
      3. Males may apply pheromones that reduce female's attractiveness.
      4. “sperm scoopers”  (Fig. 9.9)

Infanticide

1.  Infanticide occurs in a variety of mammals, notably lions and some primates.

  A.  Conditions favoring infanticide

 2.  Case study: infanticide in African lions (Fig. 9.12)

 Alternate male strategies

10.3 – FEMALE CHOICE

Female Choice in Barn Swallows

1.  Barn swallows (Hirundo rustica) are sexually dimorphic (Fig. 9.13)

2.  Do females select males with longer tail feathers?

  A.  Experiment by Anders Moller (Fig. 9.14)

  B.  Are female barn swallows monogamous?

 C.  Why choose long tails?  parasite load hypothesis

Female Choice in Gray Tree Frogs

A.   During the breeding season, male gray tree frogs produce four kinds of mating calls

B.   Females preferred long calls to short calls and fast calls to slow calls (Fig. 9.16).

C.   Two potential benefits to a choosy female include:

  1.  The acquisition of good genes for her offspring.

  2.  The acquisition of resources offered by males.

Choosy females may get better genes for their offspring

A.   Gray tree frogs giving long calls are genetically superior to males giving short calls (Fig 9.17)

1.   Five aspects of offspring performance related to fitness in offspring of long-calling versus short-calling frogs (Table 9.3).

Choosy females may benefit directly through the acquisition of resources

A.   Males may provide food, parental care, or some other

B.  Example: the hangingfly (Bittacus apicalis)  (Fig. 9.18).  

Choosy Females May Have Preexisting Sensory Biases

A.  Discussion

B.   Example Water mite (Neumania papillator) [Figure. 9.19]

1.   If there is a  preexisting sensory bias hypothesis, net-stance evolved before male trembling. [Figure 9.20]

 Other Explanations of Female Choice

A.   Female choice may be arbitrary. 

  1.  Runaway Selection [Box 9.3]

    a.  Malaysian stalk-eyed flies (Cyrtodiopsis whitei) [Figs. 9.21-9.22]

10.4 – DIVERSITY OF SEX ROLES

A.   As in seahorses, male pipefishes provide all the parental care.

B.   Female pipefish compete with each other over access to males. 

1.   In Nerophis ophidion, the females are larger than the males and have dark blue strips and skin folds on their bellies. 

C.   Males choose larger females.  [Fig. 9.24]

D.   Females appear to be less choosy.

10.5 – SEXUAL SELECTION IN PLANTS

10.6 – SEXUAL DIMORPHISM IN BODY SIZE IN HUMANS 

1.  Humans are difficult to study

A.  Human behavior is driven by a complex combination of culture and biology.

B.  Ethical, practical, and politically sensitive considerations prohibit or hinder most kinds of experimental or even observational approaches in humans

 2.   Males are cross-culturally on average 10% taller than females. Is it possible this is due to sexual selection?

   A.   Variation in lifetime reproductive success in Kipsigis men and women (Fig. 9.28)

B.  Mating and reproductive success of unokais (killers) versus non-unokais among Yanomamo men. (Fig. 9.29)

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