Transcript Slide 1

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
Males produce lots of sperm and a few males monopolize mating
Male Female
50
Elephant seal
Red deer
Man
Percent of
copulations
100
24
888
8
14
69
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?
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 malebiased
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
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
In animals....
(1) 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
Male Female
Elephant seal
Red deer
Man
100
24
888
8
14
69
(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
An experimental test:
Virginia Opossum
(Austad and Sunquist 1986)
Experimental mothers were
fed sardines during during the
breeding season and gestation
period, while controls received
no food supplementation
supplemented
young had greater mass
and male biased
sex-ratio
Possible mechanisms leading to biased sex ratios
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)
Gaulin and Robbins (1991) Trivers-Willard Effect in contemporary NA Society
Data derived from a
National questionnaire, N=906
100
M
Percent of
older children
breast fed
80
F
60
40
< 10K
> 60K
1550
Interval (days)
before younger
child
M
1450
1350
1250
F
1150
< 10K
> 60K
Duration of
breast-feeding (mo.)
Percent of
older children
breast fed
10
100
Interval (days) before
younger child
2400
F
80
8
F
M
60
2000
M
6
1600
M
4
40
Absent
Present
Male
1200
Absent
Present
F
Absent
Present
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....
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....
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....
offspring
survival
rate
parent
parental investment
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
twice the benefit to itself (or 4 times if half-sibs) because the mother’s
future offspring are devalued by the current offspring
(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.
Time course of Parent-Offspring Conflict
2
B/C
of
PI
No conflict here,
adaptive to provide PI
from either’s perspective
No conflict here,
adaptive to cease PI
from either’s perspective
1
time
conflict here; parents selected to stop
offspring selected to be demanding
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.
(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:
(1) Increased fetal trophoblast “invasion” with the uterine lining
(2) Counter 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 nonreceptive regions of the uterus results
in enhanced trophoblast invasion, which suggests this process is usually suppressed in normal
pregnancy
(3) Fetal tissue (in humans) and estrogens (horses) function to increase
utero-placental blood flow (via dilation of vessels), thereby supplying the fetus with more resources.
Suppression of these estrogens in horses leads to smaller foals.
Pre-eclampsia and gestational diabetes