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Chapter 7

ANT211H5 Chapter Notes - Chapter 7: Fisherian Runaway, Amotz Zahavi, Ronald Fisher

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Sherry Fukuzawa

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Notes From Reading
Truth in Advertising – The Evolution of Body Signals
Signals are essential to animal communication that is, the process by which one animal alters
the probability of another animal behaving in a way that may be adaptive to one or both
Signals of humans and other animals have evolved through natural selection
Among the most familiar to us, animals use auditory signals, such as the territorial songs by
which birds attract mates and announce possession to rivals, or the alarm calls to warn others
Equally familiar to us are behavioural signals: dog lovers know that a dog with its ears, tail, and
hair on the neck raised is aggressive, but a dog with its ears and tail lowered and neck hair flat is
submissive or conciliatory
Olfactory signals are used by many mammals to mark a territory and ants to mark a trail to a food
Females of many primate species advertise their time of ovulation by swollen, brightly coloured
skin on the buttocks or around the vagina
An experiment showed that when a male Long-Tailed Widowbird had a tail that was
experimentally cut down to six inches, attracted few females, while a mail with a tail extended to
twenty-six inches by attaching an extra piece with glue attracts extra mates
Three competing theories attempt to account sexual signals
1. Sir Ronald Fisher  Runaway Selection Model
Females who preferred males with the structure would also gain an advantage:
they would transmit the gene for the structure to their sons, who would in turn be
preferred by other females.
A runaway process of selection would then ensue, favoring those males with
genes for the structure in an exaggerated size and favouring those females with
genes for an exaggerated preference for the structure
From generation to generation the structure would grow in size or
conspicuousness until it lost its original slight beneficial effect on survival
Ex: a slightly longer tail might be useful for flying, but a peacock’s gigantic tail
has no use in flying
The evolutionary runaway process would halt only when further exaggeration of
the trait would become detrimental for survival
2. Amotz Zahavi  Handicap
Many structures functioning as body sexual signals are so big for conspicuous
that they must indeed be detrimental to their owner’s survival
Argues that any male that manages to survive despite such a costly handicap is in
effect advertising to females that he must have terrific genes in other respects
When a female sees a male with that handicap, she is guaranteed that he is not
cheating by carrying the gene for a big tail and being otherwise inferior
3. Astrid Kodric-Brown  Truth in Advertising
Emphasize that costly body structures necessarily represent honest
advertisements of quality, because an inferior animal could not afford the cost
Views them as either favouring survival or being closely linked to traits
favouring survival
The costly structure is thus a doubly honest ad
Human signals include faces, smells, hair colour, men’s beards, and women’s breasts
Like other animal species, we have evolved many body traits that signal age, sex, reproductive
status, and individual quality, as well as programmed responses to those and other traits
Attainment of reproductive maturity is signaled in both human sexes by the growth of pubic and
axillary hair
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