Mating

for quiz 5

Sexual Dimorphism

• The first was competition among males for
access to females.
• The second component of sexual selection was
female choice.
• Females of many animal species compare the
displays or other traits of males and choose to mate
with those that are judged to be the most attractive
or strongest individuals.
• The more controversial idea at the time.

Mating Systems and Sexual Selection

-Sexual selection is a form of directional selection
that acts on genetically variable phenotypic traits
that affect the reproductive success of individuals
of one sex.

-Traits that contribute to reproductive success can
have great costs (e.g., depleting energetic
reserves).

Sexual Selection on Males

• Males are usually subject to more intense
sexual selection than females because
individual variation in mating success is
greater for males than females.
• Different investment in making gametes
(sperm vs. ova).
• Females provision eggs with yolk
reserves.
• Retaining and provisioning embryos and
female parental care are expensive.

Scramble Competition

• When receptive females are available to males for relatively
short periods of time and are spatially aggregated, the most
efficient way for males to obtain mates is some form of
scramble competition in which males compete to locate
females as quickly as possible, invest relatively little time in
mating with each female, and attempt to mate with as many
females as possible.

• Females don’t get much choice among males, so male-male
competition tends to be the most important determinant of
variation in male mating success.

Mate Searching

• For animals that are widely dispersed in their habitats, males and females may have some difficulty finding one another.
• If males do not produce long-distance signals to attract females, then they may have little choice but to find mates by moving.

Mate Guarding

• When searching for mates is costly, a male may enhance his reproductive success by investing time in guarding individual females and preventing other males from mating with them.

→When male competitors are abundant and females are relatively scarce, selection will tend to favor prolonged mate guarding.
→When male competitors are scarce, but females are relatively abundant, males may guard individual females only briefly and invest more time in searching for mates.

Multiple Mate Guarding

• In many species in which males guard individual females, males tend to be sequentially polygynous, guarding and mating with several females in succession.
• If females are aggregated, however, there may be opportunities for males to guard more than one female simultaneously.
• There are many examples of male lizards guarding relatively sedentary aggregations of females occupying small home ranges, although it is not always clear whether males are defending groups of females or resources that attract females.

Leks and Choruses

• In some amphibians and reptiles with prolonged breeding seasons, males form aggregations at traditional mating areas and defend small territories where they advertise themselves to prospective mates.
• In many cases these small territories do not contain resources that are attractive to females, nor are they large enough to encompass the home ranges of several females.
• Males display to attract mates, and females may base their choice of mates on characteristics of the males themselves (size, color, the vigor of their displays, etc.).

Resource Defense

• If males can monopolize resources that are attractive to females, then the most efficient mating system often is defense of those resources as exclusive territories.
• Uncommon in amphibians and reptiles, probably because the low energy requirements of these animals make territorial defense of food resources inefficient, but it may occur in some species that feed on spatially aggregated prey.
• Most common in amphibians with external fertilization, because males must be present when eggs are laid and therefore are able to defend oviposition sites against other males.

Male Reproductive Success

• When males have many opportunities to mate in a single season, some males are likely to mate many times, but many others will not mate at all.

• If most males can obtain only one mate in a season, variation in male mating success will be smaller, and sexual selection on male traits will be weaker.
• Complicated by differences in sex ratios in populations.

Male Persistence and Allocation of Resources

• The ability of a male to continue breeding
activities for a long period of time.
• E.g., a male frog singing in a chorus.

Male Competitive Ability

• Most frequently measured as body size, because larger males
almost always win more fights than smaller males.
• Other traits may be important, e.g., large tail fins in red-spotted
newts, massive jaw muscles for biting in male plethodontid salamanders.

Female Choice: Hypothesis 1

-once a preference for some type of male trait arises, it can be maintained, and even increased, through a process of runaway selection

-This phenomenon would occur if there were a genetic correlation between the expression of a trait in males and the preference for that trait in females, so that females choosing males with the trait would pass along genes for the trait to their male offspring.

Female Choice: Hypothesis 2 & 3

• Another hypothesis is that male traits chosen by females are
indicators of male genetic quality, so females that choose
males with these traits will pass along good genes to their
offspring

• A third hypothesis is that females choose males with traits that
elicit responses from the female’s sensory system.
→ E.g., if females are particularly good at perceiving red color,
then males that evolve displays of red structures might be more
successful in obtaining mates than those with display structures
of some other color.

Alternative Mating Tactics

• In mating systems in which a few males monopolize most of the available females, less competitive males often adopt alternative mating tactics that enable them to obtain some matings, although usually not as many as those obtained by dominant males.
• Sneak matings, male mimicry of female behavior, sexual interference with the mating attempts of other males.

Sperm Competition

• When females mate with several different males, either simultaneously in species with external fertilization or sequentially in
species with internal fertilization, there is a potential for sperm of
different males to compete to fertilize eggs.
• So counting the number of mates obtained by each male may yield
an inaccurate estimate of mating success.
• Males may defend territories that encompass the home ranges of
females, guard individual females, or mate frequently with the same
female.

• Multiple paternity is common among lizards, snakes, and turtles,
even when males closely guard individual females.

Pattern of Sexual Dimorphism

• Although large male body size is correlated with mating success in many amphibians and reptiles, the typical pattern in many species is
for females to be larger (sometimes much larger) than males.
• Differences in adult body size sometimes reflect differences in life history strategies of males and females

• For most amphibians and reptiles, number of offspring increases with female body size, because a larger female can pack more eggs
into her body than a small female.

• In some cases, sexual size dimorphism may result from natural selection favoring different food habits of males and females, or other types of ecological divergence