Sex and Darwin: How to Choose a Mate

ANNE MOORE

SHENENDEHOWA HIGH SCHOOL

If you can’t eat it or run away from it,

MATE

with it!

--Anonymous (really!)

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 it’s most basic form was supposed to support adaptations (both morphological and behavioral) that increased an organism’s opportunities to survive and mate.

Darwin noticed, however, behaviors and adaptations that seemed to do just the opposite.

These all seem to say,

“Pay attention to me”.

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 sex—Sexual Selection

So, in 1871, Darwin

published his explanation for the apparent discrepancy

between Natural

Selection and what he was seeing in the world around

him.

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!

So what we’ll 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.

How do organisms mate?

What are their strategies?

Can we explain them?

How do Organisms Mate?

There are 5 recognized animal mating systems:

•Perennial Monogamy

•Seasonal Monogamy

•Polygyny

•Polyandry

•Promiscuity—For some reason I never have to explain this one!

Perennial Monogamy

•1 mate/mating season/life

•Extensive parental care

•Swans and Geese

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

POLYGYNY*A type of polygamy

*One male mates with two or more

females

*Most mammals and about 2% of the world’s birds

POLYANDRY

•One Female mates with two or more

males (well it’s about time!)

•Found in jacanas where the females is

more colorful and territorial

•The male incubates the eggs!

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

The lines represent mating bonds (polygyny, polyandry or monogamy)

or copulations (promiscuity).

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.

Types of sexual selection (strategies)

1. Male Dominance (male-male competition)

2. Femal

e choice

3. Combinations of the two

What is Male Competition?

Male competition refers to the evolution of special

weapons or behaviors, in males, that permit them to

compete for mates.

SO WHAT KINDS OF “SPECIAL WEAPONS”?

We can break these into two groups:

•Obvious and

•Subtle

Aren’t biologists clever?

Obvious structures include things like:

While subtle things include:

Color

Erectile Hair

Special Plumage

Behaviors can also be divided into obvious and subtle. I

guess you might as well stick with something if it works!

Obvious Behaviors are things like:

•Drumming

•Calling or Singing

•Overt Displays

•Let’s see what some of these look like.

Drumming

Calling or singing

Overt Displays

SO, IF YOU ARE OBSERVING ANIMAL BEHAVIOR FOR A

LONG TIME, HOW MIGHT YOU BE ABLE TO TELL IF THAT

ANIMAL’S MATING SYSTEM IS ONE OF MALE-MALE

COMPETITION?

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.

•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.

•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.

Male competition can also extend to the cellular level.

Sperm Competition, competition between sperm from different males, also

exists

So, how do sperm

compete?

There seem to be two ways sexual selection has

addressed this problem:

1. Sperm number

2. Sperm size

The Splendid Fairy Wren male,

ejaculates 8 billion sperm with

every mating! What a guy!

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.

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.

These species and others that

are not promiscuous,

have relatively low sperm

counts.

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.

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.

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 sperm=more hostile=more sperm!

Sperm size is another way males

may try to out compete each

other.

However, increased size may preclude increased number.

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)?

HOW MIGHT FEMALES CHOOSE MATES?

•Male vocalizations

•Male displays and plumage

•Nests and Bowers

•Territory size or quality

•Habitat or Diet

Male Vocalizations

These recordings come from a CD called “Voices of the Night”, produced by the Cornell Lab of Ornithology.

Hyla versicolor Hyla chrysocelis

Other examples:

Fisher (1958), introduced the idea that selection also works on the female’s ability to differentiate among males.

In Type I selection, females prefer males with generally advantageous traits.

In Type II selection, females prefer males with traits advantageous for attracting females only.

Some traits might be eliminated by selection if they didn’t help to attract females.

Woolbright, 1983 attempted to quantify how calling in male amphibians affected

their size and created the size dimorphism seen in many

anuran amphibians.

In this picture, the male is on the right and the female on the left.

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.

How might a females find a male in all that nature out

there?

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…

•All Purpose Territories (APT)

What are the characteristics of these different territories and what might they tell us about female strategies?

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.

•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.

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.

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

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.

Male Red-Winged Black Bird

•Females may choose mates based on some characteristic of the territory or the male’s 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.

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.

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?

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

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.

How can this be???

It appears that there is no cost to an animal’s survival that cannot

be compensated for by an advantage in attracting more

mates—as long as the male lives long enough to reproduce.

“Live fast…die young!”

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.

Each parent has features it’s 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.

Orians attempted to model how environmental factors determine the mating system of organisms.

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.

Monogamy is most likely driven by higher childcare needs than bigamy.

Our original questions were:

•How do organisms mate?

•What are their strategies?

•Can we explain them?

Let’s 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.

•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 species—maintaining species boundaries and selecting good mates.

The Strategies Appear Simple:

Be as obvious as possible without dying before mating as much as possible.

No hiding these Scarlett Ibis!

Finally, how do organisms mate?

Just about any way they can!!

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!