Family groups – Conflicts and Interests among the Family

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Family groups – Conflicts and Interests among the Family . Before birth: Optimal sex ratios and adjusting for the circumstances Conflicts between parents and offspring Conflicts among siblings . Male Female. 50. Elephant seal 100 8 Red deer 24 14 Man 888 69.

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Family groups – Conflicts and Interests among the Family

• Before birth: Optimal sex ratios and adjusting for the
• circumstances
• Conflicts between parents and offspring
• Conflicts among siblings

Male Female

50

Elephant seal 100 8

Red deer 24 14

Man 888 69

Percent of

copulations

0

1 2 3 4 5 6 7 8.............14

If its true, why not produce a sex ratio of say 20 females for every male?

and do most organisms deviate from 1:1?

Males produce lots of sperm and a few males monopolize mating

Male

Female

0

1

Male

Female

0

1

If p= frequency of males

(b-d)

= 1/2

p =

(a-b-c+d)

Suppose there are 20 females for

every male...

...then every male has 20 times the

reproductive success of every female

(on average)

Therefore, a parent whose children

are exclusively sons enjoys ~20 times

the number of grandchildren

 female bias sex ratio is unstable

Suppose males are 20 times as

common as females...

Since one sperm fertilizes an egg only

1 in 20 males contribute genes to any

offspring and females have 20 times the

reproductive success of males (on average)

 male bias sex ratio is unstable

The rarer sex has the advantage and only

when the ratio is 1:1 will the expected success

of males and females be equal and the sex ratio

stable

However, the argument should really be stated in terms of parental investment and

not reproductive success...b/c what if sons are 2x as costly to produce?

Why might there be gender differences in the cost of offspring?

FL scrub Jay

In cooperative breeding birds,

it is often males who stay behind

to help and sex ratios are male-

biased

• Metcalf’s study of two wasps,
• Polistes merticus and P. variatus.
• The former has smaller males than
• females and a sex ratio biased
• toward males, whereas the latter is 1:1
• Investment ratio is 1:1 in both

Local Mate Competition – the “Wasted Son”

Suppose 2 sons compete to mate with the same female. An extreme case,

but it illustrates that when brothers compete for the same females one male is

wasted from the mother’s point of view

An extreme example is a totally inbred population because all daughters

are fertilized by the sons.

What to do?

Just enough sons to mate with all females

• Acaronphenox mite – male are never born, rather they mate with sisters
• while still in the mother and dies.
• There broods consist of about 20 females and one male

if very few,

mainly sons

Proportion of sons

2nd Female

if many,

mainly daughters

• Parasitioid wasp, Nasonia vitrispennis
• Werren suggested that the sex bias depends on the extent of local mate competition

- If one wasp parasitizes a pupae the sex ratio is heavily biased towards females (91.3%)

- For the second female it depends on how many eggs she lays

Ratio of offspring

2nd/1st female

Trivers-Willard Effect

An ESS 1:1 sex ratio is at the population level.

Individuals may specialize (1:5 sex ratio) if an an equal number of other

individuals specialize at 5:1 ratio

In more or less monogamous human populations, there is evidence that

women tend to marry up the socioeconomic scale. As a consequence,

some women on the upper end of the scale would be left w/o men to marry

while the same would be true for men at the opposite end of the scale

Since there is a strong tendency for the socioeconomic status of the parents

to determine the socioeconomic status of the children....

... women at the upper end should produce sons while women at the lower

end should produce daughters

Male Female

Elephant seal 100 8

Red deer 24 14

Man 888 69

In animals....

• Population sex ratio must be 1:1 so that specialization in sons is cancelled
• by reciprocal specialization in daughters
• (2) One sex exhibits higher variance in Rep. Success than the other

(3) Parents can assess – consciously or not (“inherited”) – whether their offspring

are likely to fall at the upper or lower bound of Rep. Success distribution

e.g., in the above example, if you know your offspring are likely to succeed as

competitive elephant seals, you will have more grandchildren if you have sons

sons

daughters

daughters have higher success

sons have higher success

Sex ratio of offspring is

biased toward males when

females are high ranking

Sex ratio of offspring is

biased toward females when

females are low ranking

In other species, especially primates, males

disperse, whereas females remain at their natal

sites and inherit their mother’s status....

How do you expect them to differ from red deer?

High ranking females produce

more daughters than sons

• Trivers-Willard Effect doesn’t predict an unambiguous

directional change with regard to female status

An experimental test:

Virginia Opossum

Experimental mothers were

fed sardines during during the

breeding season and gestation

no food supplementation

and male biased

sex-ratio

In Humans:

1,014 random males of the US èlite born between 1860-1939

- 1,180 sons and 1064 daughters

- ratio 1.11:1 (differs from US mean of 1.06)

1,757 male of the German èlite

- 1,473 sons and 1,294 daughters

- 1.138:1 (differs from German mean of 1.05)

1,179 males of the British èlite

- 1,789 sons to 1,522 daughters

- 1.1754 (versus 1.06 mean)

Ulrich Mueller (1993)

100

80

Percent of

older children

breast fed

60

40

1550

< 10K

> 60K

M

1450

Interval (days)

before younger

child

1350

1250

F

1150

< 10K

> 60K

Gaulin and Robbins (1991) Trivers-Willard Effect in contemporary NA Society

Data derived from a

National questionnaire, N=906

M

F

Duration of

breast-feeding (mo.)

Percent of

older children

breast fed

Interval (days) before

younger child

10

2400

100

F

8

2000

80

F

M

6

1600

M

60

M

4

1200

40

F

Absent

Present

Absent

Present

Absent

Present

Male

How do they do it- Part II

As Trivers has noted,the sex ratio at birth is only one indication of how parents

distribute their investment (or alter their sex ratio) among male and female offspring

Dickemann (1979) has argued that historical preferential female infanticide among

high-status families of Europe and Asia reflects T-W investment bias

Similarly, Voland (1988) has shown among 17-19th century German households a

male based child mortality among all classes except the land-holding class

Boone (1988): 15-16th century Portuguese nobility – highest classes invested more

into sons (via estates), whereas the lower classes into daughters (via dowry)

And on and on and on....

See Pp. 281-282 of your text for infanticide in humans, with supporting data from Canada

Parent-offspring conflict

Classically viewed from perspective of parents

Parents allocate investment to their young so as to maximize the number of surviving young,

(or max LRS) and one imagines the offspring as passive vessels

Once we see the offspring as an acting participant wanting to maximize its RS we realize that

the offspring will presumably want more investment from the parents than the parents are

selected to give

Herein lies the conflict....

offspring

survival

rate

parent

parental investment

Fundamental Tradeoff between present and future reproductive effort, such that too much

effort to the present compromises a parent’s survivorship leading to early death....

What’s true about future RS if you die early?

If the mother has fewer future offspring, is there a fitness cost to the present offspring?

Tradeoffs among the offspring and among the parents...

Parents – if I give too much PI now I will have fewer young in the future

Offspring – if I demand too much PI now I will have fewer siblings in the future

The key to seeing the conflict between parents and offspring is

The offspring is related to itself by r = 1.0 and to its future siblings by

r=0.50 (full sibling) or r = 0.25 (half sibling)

• Offspring should be selected to demand PI until the cost to the mother is more

than twice the benefit to itself (or 4 times if half-sibs) because the mother’s

future offspring are devalued by the current offspring

What’s even COOLER

Hymenoptera colonies – Queen’s perspective sons and daughters have r=0.5

- Worker sister perspective, brother’s r=0.25, sister’s r =0.75

Queen favors a sex ratio (ratio of PI) of 1:1 in the reproductives, whereas sisters favor 1:3 (M:F)

How does POC manifest itself?

• Disagreement over the sex ratio – Where males require say 2x PI as females, the mother is
• selected to produce a 1:2 (M:F) sex ratio. But males have 2x the RS in this case, so the offspring
• would prefer to be males. This reduces the mother’s RS, but that reduction is devalued from the
• offspring’s perspective leading to conflict. Triver’s determined that:
• Parents “prefer” a 1:2 ratio
• Offspring prefer a 1:1.414 (see Trivers 1974 for calculation)

Who has control?

The Test:

• Trivers look at sex ratios in:
• Prenolepisimparis – 8:1 (M:F) sex ratio among reproductives, but females mass is 25:1.
• Comparing dry weights of the population of each sex produce yielded 1:3 ratio.
• (2) Compared dry weights of the population of fresh formed reproductives of the fire ant
• Solenopsis invicta. Population sex ratio is 1:1, but when adjusted for PI it is 1:3.5

(3) Lastly, sex ratio of ants enslaved by Slave-maker Ants - since enslaved workers have no

self interest in the sex ratio we expect the Queen to wield control.

Leptothoraxcurvispinosa enslaved by L.duloticus produce a 1:1 ratio

L. curvispinosa by themselves = 1:3

(4) Your book adds one more observation: if a new queen takes over the sex ratio shifts

from 1:3 towards 1:1 as workers are no longer producing sisters.

Time course of Parent-Offspring Conflict

No conflict here,

2

B/C

of

PI

No conflict here,

1

time

conflict here; parents selected to stop

offspring selected to be demanding

(2) Weaning and begging

In birds and mammals, young are “weaned” off a diet almost exclusively provided by the parents.

There is a decline in food delivered to chicks as begging and contact-seeking behavior increases

in many birds and mammals. Often, out-right aggression is use as the last act that dissolves the

family.

Offspring as psychological manipulators

Obviously young can not fling their mother to the ground to nurse at will...

Presumably the young have better knowledge of

their condition and need to communicate this to

the parents. Both parties benefit from the

communication, but almost immediately this system

is subject to manipulation.

In brood parasites, the parasite chick begs the loudest – they have nothing to lose in terms

the “mother’s” future reproductive success

(3) In-utero conflict in humans and mammals

“inside the mother the offspring is expected to employ chemical tactics [to compete effectively

with its parent]” (Trivers 1974)

In-utero, there is potential conflict if the amount and duration of PI can be influenced by

genes expressed in the offspring.

In placental mammals, the fusion of fetal and maternal organs or tissue for the basis for

physiological exchange

Crespi and Semeniuk (2004) proposed that constrained antagonistic coevolution between parents

and offspring in traits that influence PI has resulted in extreme diversity of placental mammals...

There is no other mammalian organ whose structure and function are so species diverse as those of the

placenta. This is curious since the ”purpose” of the placenta, presumably, is the same in all species

(Faber et al. 1992)

Uterine Tug-of-war:

• Increased fetal trophoblast “invasion” with the uterine lining

(2) Countered increased invasiveness via at least 3 mechanisms:

- maternal secretions to reduce invasion, including stronger maternal immune response

- evolution of stronger maternal epithelial barriers

- shedding of overly invasive trophoblasts with uterine lining

Interestingly, transplanted trophoblasts (in mice and pigs) to nonrecptive regions of the uterus results

in enhanced trophoblast invasion, which suggests this process is usually suppressed in normal

pregnancy

(3) Fetal tissue (in humans; see your text P. 280) and Oestrogens (horses) function to increase

utero-placental blood flow (via dilation of vessels), thereby supplying the fetus with more resources.

Suppression of these oestrogens in horses leads to smaller foals.

Family groups – Conflicts and Interests among the Family

• Before birth: Optimal sex ratios and adjusting for the
• circumstances
• Conflicts between parents and offspring
• Conflicts among siblings

Parent-offspring Conflict:

Parents – if I give too much PI now I will have fewer young in the future

Offspring – if I demand too much PI now I will have fewer siblings in the future

The key to seeing the conflict between parents and offspring is

The offspring is related to itself by r = 1.0 and to its future siblings by

r=0.50 (full sibling) or r = 0.25 (half sibling)

• Offspring should be selected to demand PI until the cost to the mother is more

than twice the benefit to itself (or 4 times if half-sibs) because the mother’s

future offspring are devalued by the current offspring

Sex Ratios, Weaning and begging, In-utero conflict

Fraser and Thompson (1990)

explored the function of these

teeth as neonatal weaponry thru

removal exps.

200

Mass

Gain

light

100

heavy

Am. Robin chicks

time spent

begging

Control

Chicks

exp con exp con

0-3 days

3-7 days

time begging – food deprived chicks

Sibling Rivalry

Once again, an individual is related to itself by r = 1.0 and to its full siblings by r = 0.50.

Thus each sib values itself more than its siblings and conflicts ensue...

E.g., for resources, i.e., mother’s milk - pigs have slightly sideways facing eye-teeth that they

use to slash each other with milking