Evolutionary Analysis

Fourth Edition

Chapter 11

Sexual Selection

Copyright © 2007 Pearson Prentice Hall, Inc.

Scott Freeman • Jon C. Herron

Male collared lizards (Crotaphytus collaris, above) wear brighter colors than females (below). They also attract more attention from predators

11.1 Sexual Dimorphism & Sex

• Sexual dimorphism: A difference between the males and females of a species.

Women and men differ in height

The sexual dimorphism in long-tailed widowbirds (Euplectes progne)

Sexual Dimorphism

• Sexual dimorphism is often a puzzle, because natural selection cannot always explain it.

Sexual Selection

• Sexual selection: Differential reproductive success due to variation among individuals in success at getting mates.

Asymmetries in Sexual Reproduction

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• Parental investment

Orangutan mothers invest considerably more time and energy in each offspring than orangutan fathers

Sexual Selection

• The key to explaining sexual dimorphism is in recognizing that sexual reproduction impose different selection pressures on females vs. males.

Sexual Selection

• When one parent invest more than the other in each offspring, the reproductive success of the heavily investing parent is often limited by resources and time.
• In contrast, the reproductive success of the lightly investing parent is limited by number of mates.

Asymmetric Limits on Reproductive Success in Newts & Pipefish

male (blue)

female (red)

The slope of the best-fit line, or Bateman gradient, is steeper for males than for females (P < 0.001)

male (blue)

female (red)

The slope of the best-fit line is steeper for females than for males (P < 0.004)

Behavioral Consequences of Asymmetric Limits on Fitness

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• 1. Members of the sex subject to strong sexual selection will be competitive – Intra-sexual selection

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• 2. Members of the sex subject to weak sexual selection will be choosy – Inter-sexual selection

A pregnant male pipefish, Syngnathus typhle, gives birth

11.2 Sexual Selection on Males: Competition

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• Combat: Male-male competition can take the form of combat over access to females.

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• Often occurs when individual males can monopolize access to females.

A Galápagos marine iguana

These unusual lizards make their living foraging on algae in the intertidal zone.

Natural selection on body size in marine iguanas

Asterisks mark the maximum sizes at which iguanas were able to maintain their weight in two different years (1991-1992 and 1992-1993).

Male marine iguanas in combat

Note the number painted on the individual on the right; he is participating in a study.

Mating success in male marine iguanas

20 large territorial males

Alternative Male Mating Strategies

24 small nonterritorial males

20 large territorial males

Alternative mating strategies in coho salmon

fighting strategy

sneaky strategy

sneaky strategy

Sperm Competition

• Male-male competition can take the form of sperm competition.
• If a female mates with two or more males, the male whose sperm win the race to the eggs has higher reproductive success.

A Mediterranean fruit fly, Ceratitis capitata

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Production of large ejaculates containing many sperm.

Sperm competition in damselflies

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During copulation (top), the male uses the barbed horns on his penis (bottom) to remove sperm left by the female's previous mates

Infanticide

Lion infanticide

a male lion has just killed another male's cub, which it now carries in its mouth

11.3 Sexual Selection on Males: Female Choice

• When males cannot monopolize access to females, they often compete by advertising for mates.
• Although biologists were long skeptical that females discriminate among the advertising males, female choice is now well established.

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Female Choice in Red-Collared Widowbirds

A male red-collared widowbird

During the breeding season, adult males wear jet black plumage, with long tail feathers and a red collar

Long tail feathers are a ball-and-chain for male red-collared widowbirds

This graph shows the decline in body condition (weight relative to linear size) throughout the breeding season for males with experimentally shortened tail feathers versus controls. The control males lost weight much more quickly

Female red-collared widowbirds prefer long-tailed males

Female Choice in Gray Tree Frogs

A male gray tree frog (Hyla versicolor) singing to attract a mate

Gerhardt et al.'s data on the preferences of female gray tree frogs

Why should females care about which males?

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• A variety of factors has been suggested to explain female preferences.

Choosy Females May Get Better Genes for Their Offspring

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Welch et al.'s experiment to determine whether male gray tree frogs that give long calls are genetically superior to males that give short calls

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Overall, the experiment included batches of eggs from 20 different females fertilized with sperm from 25 different pairs of males

Choosy Females May Benefit Directly through the Acquisition of Resources

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• Choosy females sometimes get food from their mates.

Courtship and mating in hangingflies

A female (right) copulates with a male while eating a blowfly he has captured and presented to her.

Courtship and mating in hangingflies

Choosy Females May Have Pre-existing Sensory Biases

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• Sometimes choosy females may simply be responding to courting males as though the males were prey.

Courtship in the water mite, Neumania papillator

(a) The female (on the left) is in net-stance, waiting to ambush a copepod; the male has found her and is now trembling his legs. (b) The female has turned toward the male in response to the trembling. The male has deposited spermatophores and is now fanning water across them. The sausage-shaped objects on top of the spermatophores are sperm packets.

A phylogeny of the water mite Neumania papillator and several related species

A male fiddler crab ready to attract a mate. This male has constructed a sand hood over his burrow, which will help entice females to visit (as shelter from predators).

Other Explanation for Female Choice

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• Sexy-son hypothesis.

Spotted cucumber beetles in love

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The male is on the left. The male uses his antennae to stroke the female. The faster his movement, the more likely she is to relax the muscle that allows him to deposit his sperm in her reproductive tract.

Runaway Selection

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• Sexual selection by mate choice can reinforce arbitrary preferences - runaway selection.

A group of Malaysian stalk-eyed flies (Cyrtodiopsis whitei) gathered on a root hair to spend the night

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The largest fly is a male; the others are females.

How runaway selection works in theory

The results of Wilkinson and Reillo's paired choice tests for female preference

11.4 Sexual Selection on Females

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• Sexual selection acts on females too.

Polyandry: Multiple Mating by Females

Genetic analyses demonstrating extra-pair copulations in red-winged blackbirds

(a) A paternity analysis using a traditional restriction-fragment length polymorphism. (b) A paternity analysis of the same families using DNA fingerprints.

Female Gunnison's prairie dogs (Cynomys gunnisoni) increase their reproductive success by mating with more than one male

Male Choice - Pipefish

Male choice in pipefish

What Female Traits Male Pipefish Prefer?

End