Sex and Darwin: How to Choose a Mate

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Sex and Darwin How to Choose a
Mate ANNE MOORE SHENENDEHOWA HIGH SCHOOL
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If you cant eat it or run away from
it, MATE with it! --Anonymous (really!)
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When Darwin published The Origin of Species in
1859, he created a big controversy by naming
Natural Selection the driving force behind
evolution. Natural Selection in its most basic
form was supposed to support adaptations (both
morphological and behavioral) that increased an
organisms opportunities to survive and mate.
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Darwin noticed, however, behaviors and
adaptations that seemed to do just the
opposite. These all seem to say, Pay
attention to me.
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Since these behaviors and adaptations were not
recent in origin, Darwin concluded that they must
be favored by evolution and were probably not
going to change. These Evolutionary Stable
Strategies (ESS) must confer some advantage to
the individuals that possess them. Perhaps,
these individuals actually produced more
offspring by having these adaptations than
not. Perhaps, there was selection specifically
related to sexSexual Selection
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So, in 1871, Darwin published his explanation for
the apparent discrepancy between Natural
Selection and what he was seeing in the world
around him.
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The Theory of Sexual Selection attempts to
explain why organisms have morphological
structures or behaviors that seem to shorten
their life spans by making them more obvious to
predators or competitors.

Red kinda

stands out!
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So what well be looking at are the interactions
between members of the same species to see how
they influence the behavior and structural
modifications of others in relation to attracting
mates and defending territories.
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How do organisms mate? What are their
strategies? Can we explain them?
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• How do Organisms Mate?
• There are 5 recognized animal mating systems
• Perennial Monogamy
• Seasonal Monogamy
• Polygyny
• Polyandry
• PromiscuityFor some reason I never have to
  explain this one!

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• Perennial Monogamy
• 1 mate/mating season/life
• Extensive parental care
• Swans and Geese

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• Seasonal Monogamy
• One mate/mating season
• One male and one female
• May have evolved as a hedge against mates dying
  in the off-season
• Kittiwakes

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POLYGYNY A type of polygamy One male mates
with two or more females Most mammals and about
2 of the worlds birds
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• POLYANDRY
• One Female mates with two or more males (well
  its about time!)
• Found in jacanas where the females is more
  colorful and territorial
• The male incubates the eggs!

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• Promiscuity
• There are no pair bonds
• Males and females mate with one to many of the
  opposite sex
• No one individual has rights over any of the
  opposite sex
• 6 of bird species

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The lines represent mating bonds (polygyny,
polyandry or monogamy) or copulations
(promiscuity).
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Very often, there is a mixture of systems. The
type of system used is very closely linked to the
amount of parental care the offspring require.
See Trivers 1972.
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• Types of sexual selection (strategies)
• 1. Male Dominance (male-male competition)

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2. Female choice
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• 3. Combinations of the two

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What is Male Competition? Male competition refers
to the evolution of special weapons or behaviors,
in males, that permit them to compete for mates.
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• SO WHAT KINDS OF SPECIAL WEAPONS?
• We can break these into two groups
• Obvious and
• Subtle
• Arent biologists clever?

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Obvious structures include things like
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While subtle things include
Special Plumage
Color
Erectile Hair
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Behaviors can also be divided into obvious and
subtle. I guess you might as well stick with
something if it works!
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• Obvious Behaviors are things like
• Drumming
• Calling or Singing
• Overt Displays
• Lets see what some of these look like.

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Drumming
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Calling or singing
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Overt Displays
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SO, IF YOU ARE OBSERVING ANIMAL BEHAVIOR FOR A
LONG TIME, HOW MIGHT YOU BE ABLE TO TELL IF THAT
ANIMALS MATING SYSTEM IS ONE OF MALE-MALE
COMPETITION?
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• CHARACTERISTICS OF MALE-MALE COMPETITION
• Females are the clear object of male aggression.
• Few males win many females while there are many
  males with none.
• Success in acquiring females is correlated with
  dominance.

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• Pair bonding is weak or absent.
• Sexual dimorphism in structure and behavior is
  extreme.
• Full physical maturation comes a year or more
  later in males than in females.
• Mortality rate is higher in males than in females.

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• There are likely to be more adult females than
  males.
• Little or no paternal care.
• These are generalizations and exceptions can be
  found, especially in bullet 3 of the first slide.

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Male competition can also extend to the cellular
level. Sperm Competition, competition between
sperm from different males, also exists
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So, how do sperm compete?
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• There seem to be two ways sexual selection has
  addressed this problem
• Sperm number
• Sperm size

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The Splendid Fairy Wren male, ejaculates 8
billion sperm with every mating! What a guy!
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Females are highly promiscuous. By increasing
the number of sperm released in each mating,
males in promiscuous species hope to increase the
chances that their sperm will fertilize an egg.
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On the opposite end of the spectrum is the
Seaweed Pipefish. In this species and related
Sea Horses, males have pregnancies where they
incubate the fertilized eggs.
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These species and others that are not
promiscuous, have relatively low sperm counts.
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So, if sperm production is genetically
controlled, successful males with large sperm
counts in promiscuous species, will increase in
the population along with teste size.
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• The complicating factor is the female
  reproductive tract which is often hostile to
  sperm.
• Acidic environment can kill 90 of sperm
• Cervical mucus plug prevents 90 of the remaining
  sperm from entering the cervix.
• White blood cells attack the remaining sperm.

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With this system, hopefully, only the most
superior sperm will be able to fertilize an
egg. However, this could become a never ending
cycle because more spermmore hostilemore sperm!
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Sperm size is another way males may try to out
compete each other. However, increased size may
preclude increased number.
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So, you are a male in a promiscuous species and
you want to mate with a female who has already
mated. What do you do to make sure your sperm
fertilize her egg(s)?
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• HOW MIGHT FEMALES CHOOSE MATES?
• Male vocalizations
• Male displays and plumage
• Nests and Bowers
• Territory size or quality
• Habitat or Diet

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Male Vocalizations These recordings come from a
CD called Voices of the Night, produced by the
Cornell Lab of Ornithology.
Hyla versicolor
Hyla chrysocelis
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Other examples
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Fisher (1958), introduced the idea that selection
also works on the females ability to
differentiate among males. In Type I selection,
females prefer males with generally advantageous
traits.
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In Type II selection, females prefer males with
traits advantageous for attracting females
only. Some traits might be eliminated by
selection if they didnt help to attract females.
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Woolbright, 1983 attempted to quantify how
calling in male amphibians affected their size
and created the size dimorphism seen in many
anuran amphibians.
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In this picture, the male is on the right and the
female on the left.
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Woolbright was able to quantify the loss of
weight due to the energentics of calling and
defending a territory. Females might choose
large males because they are able to survive the
loss of weight and are, therefore, more fit.
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How might a females find a male in all that
nature out there?
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• It appears that the type of territory a male
  defends, may help females make choices. There
  seem to be three major types
• Isolated mating territories (IMT)
• Communal Mating Grounds (CMG) and

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• All Purpose Territories (APT)
• What are the characteristics of these different
  territories and what might they tell us about
  female strategies?

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• IMT
• Mating stations are isolated so inter-male
  aggression does not prevent females from making a
  choice among males.
• This system is a strong case for females
  selection of displays, plumage and coloration.

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• Female decisions may be based on appearance and
  behavior of males toward females.
• Females visit many males prior or making a
  decision.
• Male competition probably plays a part in
  determining territories, initially.

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• CMG
• These are also referred to as leks.
• Traditional gathering spots where males come
  during the breeding season.
• Both sexes come from wide areas and are often in
  prominent spots.

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• With many males present, the mating site is more
  prominent than with a single male.

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• APT
• Male competition may be used to set up the
  territory initially.
• Typically, migratory birds use this method where
  males return to territories prior to females.

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Male Red-Winged Black Bird
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• Females may choose mates based on some
  characteristic of the territory or the males
  ability to defend a suitable territory.
• If the species is polygynous, the number of
  females a male can attract is dependent on the
  suitable number of nesting sites.

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• GENERAL CHARACTERISTICS
• Many are the same as male competition.
• Frequency of violence and male-male encounters
  differs.
• Structural adaptations are designed to attract
  females not defend against males.

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Things can, however, get a little out of
hand. Run-away Selection can occur when a slight
adaptive dimorphism confers a survival
advantage. Females should evolve genes that
should cause them to prefer these male genes.
Why?
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• Well, perhaps a threefold advantage
• Sons are more attractive to females
• Daughters are produced who can select good
  males
• The adaptiveness of the dimorphism itself

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As individuals with the dimorphism and the
preference for the dimorphism in the population
increase, both the dimorphism and the preference
for the dimorphism should also increase. However,
this can increase until the dimorphism is
exaggerated and becomes a burden.
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How can this be??? It appears that there is no
cost to an animals survival that cannot be
compensated for by an advantage in attracting
more matesas long as the male lives long enough
to reproduce. Live fastdie young!
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Sexual Imprinting may start the entire
process. During a critical period in their
development, the young of some species learn to
recognize the opposite sex of their species.
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Each parent has features its mate learned to
select in infancy. The young are likely to
inherit this feature and, through imprinting, a
preference for it. The exaggeration of that
feature, permits the nervous system to pick it
out from the irrelevant stimuli of the world.
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Orians attempted to model how environmental
factors determine the mating system of organisms.
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Like Trivers, he determined that parental care is
critical in the evolution of mating systems. If
a male does not help with childcare, there may be
no advantage for the female staying.
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Monogamy is most likely driven by higher
childcare needs than bigamy.
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• Our original questions were
• How do organisms mate?
• What are their strategies?
• Can we explain them?

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Lets start with the third bullet. With the help
of genetics, we can partially explain what we
see. We know the drive is to get your genes into
the next generation combined with the best genes
you can find.
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• Therefore, there has to be some attribute,
  physical or environmental, that separates the
  good from the bad.
• The prevailing focus is on behavior, territory or
  adornment that is attractive to that
  speciesmaintaining species boundaries and
  selecting good mates.

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The Strategies Appear Simple Be as obvious as
possible without dying before mating as much as
possible.
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No hiding these Scarlett Ibis!
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Finally, how do organisms mate? Just about any
way they can!!
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I would like to thank the following people for
their help in preparing this presentation My
colleague, Steve Davidson for his technical and
literary support. Gabriella August for her
support and the loan of many resources. Jason
Cryan for reviewing too many versions of this
power point!