**SEXUAL REPRO TEXT

Sexual reproduction is much more common in the animal kingdom than asexual reproduction.

**SEX PENALTY TEXT **PARTHENOGENETIC LIZARDS

 How do we define the sexes, and identify the sex of a given animal?

**GAMETE IMAGE

-Males = sex with smaller gametes (sex cells). Sperm are among the smallest cells in any animal's body.

-Females = sex with larger gametes. Eggs are among the largest cells.

 An important concept in understanding the evolutionary outcome of the 'battle of the sexes' is Robert Trivers' definition of Parental Investment (PI) = anything a parent does to increase the RS of a given offspring that incurs cost in the parent's ability to invest in other offspring.

**TRIVERS PI TEXT

 There is an inherent bias in differential Parental Investment (PI) between the sexes because of the sex cell difference: Gametes are often a much larger proportion of female's body weight than male's

Example- Birds: each egg weights 15-20% female's body; male's total sperm production for season = 5% body weight.

**KIWI X-RAY

In addition to gamete investment, other forms of biases among sexes in PI often occur. We'll come back to this later when we cover sexual selection.

 Animal behaviorists make an important distinction between the mating behavior we observe and genetic success, because the two are not always equal ("Your cheatin' heart...").

**MATING AND BREEDING TEXT

Mating system = description of social interactions related to male/female pairing in a species. Who spends time with whom? Social pair bonds.

Breeding system = description of gene exchange. Who fathers whom? The breeding system is more evolutionarily relevant than the mating system.

 Some important types of mating systems: 

Monogamy: Male and female partners stay together, and maintain a social bond, (apparently) exclusive of bonds with other partners. Often associated with biparental care of the offspring.

 **ALBATROSS PAIR **GIBBONS **HORNBILLS 1 **HORNBILLS 2

Serial monogamy: Social male/female pairs last one breeding season, then the individuals switch to new partners.

 **BLUE FOOTED BOOBIES

 Polygyny: One male mates with many females. This is the most common mating system across the animal kingdom. Usually associated with female parental care.

**DEER HAREM

 Polyandry: One female, multiple males. This is the rarest mating system, but it has evolved in a number of types of animals. It is often associated with male parental care.

**JACANAS

Scramble competition for mates can sometimes lead to polyandry.

**ANT AND TOAD MATING BALLS

 Some ecological factors and social are thought to favor the evolution of monogamy and shared parental investment:

-When access to receptive females is very limited (for example, when females are widely spaced or rare).

-When males have the opportunity for mate guarding- males can assure their paternity by staying with one female, and fending off other males.

-When male parental care has very great payoffs (for example, in harsh environments).

**SEAHORSES

 Extra-pair copulations or EPC have recently been documented in many species with monogamous mating systems. This is part of why the mating system does not always equal the breeding system.

-Both males and females of many socially monogamous species leave their partners to seek copulations with other mates. This often involves "sneaky" behavior, apparently to avoid detection and retribution by one's social mate. The sneaky nature of this behavior makes it hard for scientists to detect.

-New evidence of genetic relatedness patterns obtained using DNA markers shows extra-pair breeding to be common in many species, even when the EPC is not observed.

-In some birds, "attractive" males sire many offspring in their male neighbors' territories. Why do the less attractive males stay put, and care for the offspring in their nests?

-In humans, evidence from blood typing showed EPC commonly led to pregnancy. Genetic studies have shown that, as a conservative estimate, between 5% and 30% (typically, about 10%) of American and British babies are sired by fathers not married to their mothers.

**SEXUAL SELECTION TEXT

PI differences among the sexes lead to differences in sexual selection: Sexual selection is a concept proposed by Charles Darwin, as a special category of natural selection.

**PEACOCKS-DRAMATIC TRAITS **RUFF AND REEVE

-Evolution in response to sexual selection results if individuals within a sex have heritable differences that affect their reproductive output, either as a result of competition with members of their own sex for access to mates, or as a result of mate choice by the other sex.

**HUIA BIRD CAUTION

By definition, mean (average) RS among sexes is equal. Every time a female reproduces, a male also reproduces. But the amount of variation among individuals (within sexes) can differ between sexes.

-In general, the sex that exhibits higher PI becomes a limiting resource for the sex with lower PI. The sex with lower PI then benefits more by seeking more mates, rather than by trying to make parental investment. Most members of the high-PI sex get mated, and their RS depends on making lots of investment in their young. In contrast, many members of the low PI sex get little or no reproduction, and a few get most of the mates.

**SAGE GROUSE COPULATION DATA **PHALAROPE- FEMALES COMPETE **GRASSHOPPERS- MALES CHOOSE

-This leads to sexual selection pressure: the outcome is often competition among member of the low-PI sex (usually males), and choosiness about who to mate with by the high-PI sex.

One outcome of PI differences among sexes- Intrasexual (usually male/male) competition

-Within-sex competition favors the evolution of fighting structures

-It is often associated with increased male body size, and with sexual size dimorphism

Sometimes, males compete with each other for social access to females. This can result in the evolution of 'harems.'

**MALE & FEMALE ELEPHANT SEALS **MALE ELEPHANT SEAL FIGHT ** REINDEER ANTLERS SLIDE **IRISH ELK SLIDE

 Males can also compete with each other for control of resources used by females.

**RUFFED GROUSE WITH SOUND (TERRITORY) **RHINO BEETLES (OVIPOSITION SITE)**

Sperm competition- more subtle- can also occur.

Possible adaptations for sperm competition include removal of rivals' sperm, increased sperm number (remember our earlier primate example), and mating plugs to prevent rivals' sperm transfer.

**DAMSELFLY PENIS SLIDE** 

Cryptic female choice- female reproductive tracts themselves can also select against certain males, even after copulation.

**INSECT REPROD TRACT DRAWINGS

The other main outcome of PI differences among the sexes is intersexual mate choice (usually, females choosing among males).

Females can use a number of criteria for making their choices:

-Resources offered by males, including parental care, or territory/food that the males control.

-Male characteristics: this type of choosiness is indicated by the evolution of sexual dimorphism that is not related to ecology.

**TANAGER DIMORPHISM SLIDE**

The male characteristics that females find attractive can be arbitrary characters that don't directly indicate the male's quality. This can result in runaway sexual selection, where male features that are NOT otherwise adaptive can evolve!

**TUNGARA FROG SING AND NEST **FROG-EATING BAT **GUPPIES

Alternatively, females may choose males based on characteristics that indicate their genetic quality: these are called Good genes models of mate choice.

**NURSE SHARK CONTEST **SHRIKE DISPLAY **HANGFLY GIFT **ROADRUNNER GIFT  **HUMAN GIFTS

Handicap principle- unusual male characters impose costs on their bearers. Females may prefer males that can survive despite these handicaps.

**WIDOWBIRDS

A special case of female choice is present in mating behavior called "leks": Leks are male mating aggregations with no associated resource for females.

**BLACK GROUSE **FROGS AT POND **BOWERBIRD 1 **BOWERBIRD 2

**END**