The Evolutionary Biology of Fat

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The Evolutionary Biology of Fat We all know fat
is bad, right? So, whats going on? Why does
natural selection let this happen? Why do we get
fat? Basic misconception Fat is not bad it is,
or at least was, good!
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Why is fat good? It is a mechanism for storing
surplus. Twice as many calories per unit
weight. Would that be useful to foragers? To
animals generally? Think about it no grocery
stores, no mac machines, no refrigerators or
freezers. The only resources that were yours
were the ones stored on your body.
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Note that you like fat it tastes
good. Presumably our gustatory (taste)
preferences (both positive and negative) have
been shaped by selection. Individuals with the
right kinds of cravings and revulsions would have
been healthier and left more offspring.
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Note also that the onset of sexual maturity in
females is linked to stored body fat. 17 for
menarche (first menstrual period). 22 for stable
ovulatory cycles. Why? Pregnancy and lactation
are costly. Pregnancy costs 50,000 kcals above
her baseline. Approximately 1000 kcals/day for
lactation. In hunter-gatherer populations,
lactation suspends ovulatory cycles. For Western
women, loss of body fat has the same effect.
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Why is sexual maturation not tied to body fat in
men? So, men dont need fat? They do, but not as
much as women. Boys and girls differ in body fat
composition at birth (!) and continue to diverge
up until puberty and beyond
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BMI             Weight in Kilograms             
(Height in Meters) x (Height in Meters)
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Two long chain polyunsaturated fatty acids
(LCPUFAS), arachidonic acid and docosahexaenoic
acid may be limiting resources in neural
development. These two LCPUFAS each constitute
10 of the dry weight of the human brain.
LCFUFAS are preferentially stored in the hips
and thighs.
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Three directions to go from here 1. Whats going
on with female fat storage? 2. What implications
does this have for sexual selection in
humans? 3. Are we supposed to be as fat as we are?
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• 1. What implications does this have for sexual
  selection in humans?
• Sexual selection is selection for traits that
  increase mating success.
• Getting more mates.
• Getting better mates.
• Fat signals fertility in women. Men should
  evolve to use fat as a criterion of mate choice.
• Females should evolve to unambiguously advertise
  their fat stores.

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Recap Why are men choosy, compared to males of
other primate species? Because the contribute
parental investment. If they just inseminated and
deserted they would mate with any female (perhaps
even female chimps!) Because parental investment
restricts mating opportunities, there will be an
advantage to getting better mates.
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So women evolve to advertise and men to value
body fat. Women have more fat than non-human
primate females Fertile women have a unique
distribution of body fat. different from men,
and different prom pre-pubertal and
post-menopausal females The waist/hip ratio
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2. Are we supposed to be as fat as we are? No!
So how did we get this way? Selection favors
traits that address the challenges of the
prevailing environment. What environments have
shaped our adaptations?
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Humans were hunter-gatherers (food harvesters,
not food producers) for 6 million years. We
have been farmers and herders for a few thousand
years. The December 31, 900 AM analogy EEA
Environment of Evolutionary Adaptedness.
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How much should a hominid eat? How much fat
should a hominid eat? As much as they could get
their grubby hands on! Hominids did not die from
the effects of obesity. Analogy with zoo
animals.
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Fat content of wild vs domesticated
animals. Both artificial selection and livestock
feeding practices elevate the fat content of
domesticated animals 25-30 fat by weight is
typical. In contrast a sample of 15 different
species of wild East African herbivores had a fat
content of 3.9 Concentrated sources of fat and
calories.
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Fat content of wild vs domesticated
animals. Both artificial selection and livestock
feeding practices elevate the fat content of
domesticated animals 25-30 fat by weight is
typical. In contrast a sample of 15 different
species of wild East African herbivores had a fat
content of 3.9 Add to that dairy practices
(triple-crème brie at 60) and other methods of
extracting concentrated fats.
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Plus concentrated sources of other
calories, notably refined sugar And the fact
that our food can be procured a little caloric
expense All this excess gets converted to FAT.
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So, we are well adapted, but not to the
present. We are well adapted to the conditions
that prevailed over most of human
evolution. This is called the mismatch-hypothesis
Obesity is only one of its many manifestations.
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Final exam