Post on 16-Dec-2015
Further sexual dimorphismOften we see species that have some bizarre looking traits that don’t seem to have any logical function. As a matter of fact, in many cases these traits would even seem to hurt the organisms survival
Peacock Peahen
Typically, these elaborate traits are seen in themales of the species, but not the females
In other words, Sperm is cheap!
Sexual selection is a consequence of anisogamy
Male vs. Female investment in offspring
Female gametes cost more per unit than male gametes
Female reproduction will be limited by access to resources
Male reproduction will be limited by access to mates
Relative Parental Investment (RPI):
Mating Strategies and Reproductive Success
Natural selection will favor males that attempt to mate often
•Males should compete for females
A female’s reproduction is limited by the number of eggs she can make--> a single mating may fertilize all her eggs
•Females should be choosy about which male she mates with
Intrasexual selection… Male-Male Competition
Male bighorn sheep
Humans?
Marine Iguanas (Galapagos Islands)
Males are bigger than survival rates would predict
Male Iguanas compete for territories
N Average size (Snout-vent length)
Males that copulated
253 401
Males that tried to copulate
343 390
Larger males with better territories were more likely to reproduce
In northern elephant seals, successful males have “harems” of females. Fewer than 1/3 of the males mate. Male-male competition is vicious and often lethal.
In species where not so much is at stake less lethal forms of male competition may be selected for.
Intrasexual selection… Sperm Competition
1. Larger ejaculates2. Mate Guarding3. Copulatory Plugs4. Other specialized
structures
Intrasexual selection… Infanticide
When male lions take over a pride of females, they kill any non-weaned cubs fathered by previous males
Advantage for males: females return to breeding condition 8months earlier than they would otherwise
The role of female choice
Darwin recognized that females were typically the choosers, and males competed tremendously with each other to be chosen.
Victorian science was not very PC.Darwin often caricatured his observations by stating that females were coy by nature and males were eager, or aggressive, when it came to mating.
Intersexual selection… Female Choice
Females may select for males with traits that indicate their quality
Male widowbirds with longer tails may advertise “good genes”
Courtship displays by males
Intersexual selection… Female Choice
Male hanging flies providetheir mate with a “nuptial gift” of a dead insect treat
Male redback spiders place themselves in position to be eaten by the female
Females may select males that provide her with a reward
Gender roles depend on relative investments of males vs. females
In birds, about 90% of species show “monogamy”, but only 20-25% not promiscuous
Females who choose males that will stick around will do better, but cheating with a flashy male may get your offspring better genes
Gender roles are not set in stone
Male Jacana birds incubate eggs, guard offspringFemales mate with several males- “harem”