Notes on the Web - Unit Five - Part 5

Sexual Selection and Explanations of some Bizarre Animal Behaviors
Bruce G. Stewart

 

General Objectives and Study Guide

 

Your objectives for these Notes on the Web and associated readings and exercises are:

 

Related Textbook Readings:

Sexual Selection

Sexual selection is an interesting subset of natural selection that is caused by the fact that traits that may help males reproduce can be different than those that help females reproduce.  Examples of traits affected and can be quite different between males and females of a species include:

The intensity of sexual selection is increased by:

The degree of sexual dimorphism increases with intensity of sexual selection;  however opposing forces of natural selection can cause limits on how far this can go.   For example, how big could a male get before the energy cost of feeding and metabolism, would outstretch his ability to use the same resources available?

Illustrations of Sexual Selection (Including Comments on Why There are "Transvestite" Beetles!)

Costa Rican Rove Beetles (Leistotrophus versicolor) [from research of Forsyth and Alcock (1990), and Alcock and Forsyth (1988)]; (The taxonomy of these organisms for your information is: Phylum Arthropoda, Class Insecta, Order Coleoptera, and Family Staphylinidae.)

Feeding strategies used by the Costa Rican Rove Beetle has a bearing on the intensity of sexual selection.  There are two main strategies:

Costa Rican Rove Beetle Mating Strategies and Sexual Selection

Strategy #1 = males gain access to females that visit feeding sites.  However, males fight over these sites, and it is not just a random subset of the male population that wins the right to the sites.  Combat is won by males with larger body size and mandibles [large morph (LM) males or major males].  I will present data from Alcock and Forsyth that support this observation.  Thus, in Costa Rican Rove Beetles,   sexual selection can favor increased: aggression, body size and mandibles

Strategy #2 = males gain access to females by female mimicry!  (some males have small body morph (SM) similar to small females)  "sneaky males."  These males mimic courtship behavior of female (abdominal tapping, etc.) and allow the major males to court them!  This increases the SM's time at dung site because LM males attempt to court and mate with SM males instead of engaging them in combat.  SM males court and mate with females if possible while being courted by LM males.  This type of sexual selection maintains males with smaller bodies and female-like mandibles and explains the strange behavior.  More details will be given in lecture.

The Red-winged Blackbird (Agelaius phoeniceus) - or Why Do Male Birds Sing, Why are They Often Bigger and More Colorful, and Why are There Other Sexual Differences

Basically, the story is this.  Sexual dimorphic characters of mailes (e.g. larger body size, jet black bodies with bright red epaulets, varied and well developed song, higher aggressiveness) are related to defending territories from other males.  Females choose the best nesting sites.  Males who can defend better sites mate with more females (polygamy), and exhibit greater fitness.

Bluegill Sneakers

Sneakers are small males that become sexually mature very early in age when they are small enough to hide behind females spawning in the nest of a large colorful male.  The sneakers release sperm on the opposite side of the female from the large male, thus achieving fitness!

Other Examples

All of these examples and several others will be explained and illustrated in class lecture.

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