Anthro 101: Human Biological Evolution

Lecture 11: Primate Reproductive Strategies

Office Drop-in Hours AHS 308 Tutoring Lab Hours AHS 232T 5:30 - 6:30 M & T 1 - 4Th 11:15 - 12:15, 1:15 - 3:15 Student ID requiredF 12 - 1:30by appointment

Attendance survey

What are the three benefits of living in a social group?

What is one cost of living in a social group?

Studying Primate Behavior

• An evolutionary approach to behavior

• Female Reproductive Strategies

• Parental Investment• Sexual Selection

• Male Reproductive Strategies

Behaviors are adaptations to particular social environments

• Behavioral strategies• Course of action under certain circumstances• Does not imply conscious reasoning, deliberate

planning, or intent

• How does the behavior effect and individual’s fitness• Costs vs. Benefits of an action

Mammalian females are committed to invest in offspring

• Internal gestation• Lactation

• Maternal investment obligatory

• Paternal care optional

Costs of Maternal Care

• Energy• Lactation, gestation• Calories & nutrients

• Time• Callitrichids - twins 2x per year• Monkeys - one infant every 2 - 4 years• Apes - one infant every 5 - 7 years

0

500

1000

1500

2000

2500

3000

Normal Pregnant Lactating

Calorie Requirements

Primate females invest heavily in each infant and produce few total offspring

• Litter size = 1 (max 2)• Long inter-birth intervals • Lifespan < 20 yrs• Not all infants survive

Each infant represents big fraction of females’ reproductive output

Golden monkey

What strategies do females use to enhance reproductive success?

1. Care

2. Competition

3. Cooperation

1. Female primates provide many types of care for their offspring

• Nourishment• Transportation• Warmth• Protection from

predators• Protection from

harassment QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

How much care should females give offspring?

• More care higher chance of survival

• BUT care for present infant, reduces ability to care for older infants AND future infants

• Females have finite reproductive career

• Care for infants taxes maternal resources

• Mothers must make tradeoff

• Few very high quality offspring

• Many very low quality offspring

• Primate mothers invest heavily in each offspring and have only a few

Golden monkey

2. Female reproduction is limited by access to food:

Silver leaf monkey

• Competition• Scramble• Contest

3. Competition sometimes favors cooperation among females

• Between group competition• Territorial species• Females in the same troop cooperate to confront

females from other groups• Larger groups will get access to more resources

than smaller groups

3. Competition sometimes favors cooperation among females

• Within group competition

• females form alliances in some species

• affect access to resources

• affect rank acquisition

• Alliances often composed of kin

• Social bonds (friends) may also enhance reproductive success

• Not about rank

• Not about fighting for resources

Cooperation: In marmosets and tamarins, female RS depends on helpers

Selection shapes male reproductive strategies of males, too

• Parental investment is costly

• Mammalian females are obligated to invest heavily in offspring

• Mammalian males have more options than females

• Females are limited in the number of offspring they can have

• Females are a limiting resource for males

1. Males can increase RS by investing in offspring

• Expect males to invest when • Finding additional mates

difficult• Females spaced out (time)• Female mate synchronously

• Fitness of kids raised by one parent low• infants are very big• litter size > 1• high risk of

predation/infanticide

2. Males can increase RS by competing

• Sexual Selection favors traits that increase success in competition for mates

• more pronounced in sex with limited access to mates

= MALES (in mammals… usually)

• Intra-sexual selection = male-male competition for access to mates

• Inter-sexual selection = female selection of males with the most attractive traits

Intra-sexual selection in primate males

• Male-male competition favors• Large body size• Large canines• Mate guarding

Inter-sexual selection in primate males

Inter-sexual selection = Female choice favors

• Flashy colors• Energetic displays• Friendly behavior• Paternal care

mandrill

tamarin

Pair-bonded species: Marmosets & Tamarins

• Male RS tied to his mate’s RS• Males invest in offspring

• Carry infants• Share food with infants

• Males guard females vs. rivals• Closely bonded to mate

Dusky titi monkeys

Pair-bonede species: Gibbons and siamangs

Males are attentive to mates

Sing duets in territorial displays

Females have priority of access

Males help care for infants

Competitive males & distant fathers: multi-male groups

• Male RS tied to number of different females he can mate with• Males compete for dominance rank

• Male dominance rank is function of size & strength• Rank orders change frequently• High ranking males monopolize conceiving females• Male rank is correlated with reproductive success

Multi-male groups• Baboons• Langurs• Chimpanzees

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

The Bone Room 2010

More intense male-male competition leads to greater sexual dimorphism

Baboon males are twice as big as females

Males are well-armed for conflict

Males fight over access to receptive females

These fights can be very costly to males

High rank enhances male reproductive success

0

0.10.2

0.30.4

0.50.6

0.70.8

0.9

1 2 3 4 5

Male Dominance Rank

Proportion of Infants Sired

Baboons

Monogamous 1-♂, Multi-♀ Multi-♂, Multi-♀

Obs

erve

d/E

xpec

ted

Siz

e

0.5

1.0

1.5

2.0

Intense male-male competition can also lead to sperm competition….and larger testes…in species where females mate promiscuously

In multi-male groups, some males provide low cost care

• Males usually tolerant of juveniles • Males support juveniles in aggressive

interactions• Some evidence that males selectively help

own offspring

One-male groups: Competition to gain access to females is intensified

• Males compete for access to groups of females• Outsiders exert constant pressure on resident males• Tenure of resident males often short

Infanticide is a sexually-selected male reproductive strategy

• Females nurse infants for many months

• If unweaned infant dies, female resumes cycling immediately

• Death of infant makes females available for mating sooner

• Infanticidal males gain immediate mating opportunities

• If male tenure is short, infanticide enhances male mating opportunities

If infanticide is a sexually-selected male reproductive strategy, we predict:

1. Infanticide will be linked to changes in male residence or status

2. Males will kill unweaned infants

3. Males won’t kill their own infants

4. Infanticidal males will gain reproductive benefits

Evidence supports all four predictions

Takeover

Rise in rank

Resident

disappears

All others

Immigration

1. Infanticide is associated with changes in male status:

Males don’t kill unless they GAIN reproductive access they did not have before

1. Males begin to kill infants soon after they join group

Number of Infanticidal Attacks

02468

1012141618

< 1 1 2 3 4 5 6 7 8 9

Male Tenure Length (mos)

Hanuman langurs, Borries & Koenig 2000

0

20

40

60

80

100

0-3 3-6 6-9 > 9

Age (mos)

2. Males kill unweaned infantsProbability of surviving presence of infanticidal male

Langurs

Howlers

3. Males don’t kill own infants

Not related

Unlikely

Possible

Likely

No Data

4. Infanticidal males gain reproductive benefits

• Infanticide brings females back into estrus• Infanticidal males often mate with mother of dead infant

0 5 10 15 20 25 30

Yes

Probably

Possibly

No, infanticidal male ousted by new male

No, infanticidal male ousted by females

No, infant was orphan

No, mother disappeared soon after

No, alpha male recovered from illness

No, mother member of different group

0

10

20

30

40

50

Sifaka Howler Langur BlueMonkey

Baboon Gorilla Chimp

High

Low

% d

eat

hs

du

e to

infa

ntic

ide

Infanticide is a major cause of mortality

Counterstrategies to thwart infanticide

• Defend victims of attack

• Mothers

• Female kin

• Males present at conception

• Fathers

• Confuse paternity

• Estrus swellings

• Mate with many males

• Mate with newcomers

In baboons, male-female ties may be response to infanticide

• In some populations, infanticide is common when new males join group or males rise in status

• New mothers form associations with particular males

• possible father of current infant

• Males protect females’ infants

Sexually-selected infanticide has now been documented in a number of taxa

• All the major groups of primates• Prosimians• New World monkeys• Old World monkeys• Apes

• Lions• Rodents• Birds

Many still think its pathological and not adaptive

Controversy persists because people confuse “is” and “ought”• This is called the “naturalistic fallacy”

• assume that natural phenomena are right, just, unchangeable, good

• Worry that if infanticide is adaptive for langurs or lions, it would be justified in humans

• But this reasoning is wrong

• we can’t extract moral meaning from behavior of other animals or what is natural

In-Class Activity #5

• Write a personal ad seeking a mate from the perspective of a male or female of any primate species you choose.

• Include details that are specific to that species and sex• Do NOT say what species you are writing the ad for

• May work in small groups - write everyone’s name down