Week 12: Communication Learning; CH 9 pg 287 – 294 & 299 – 326, CH3 pg 97 -104
Pg 287 – 294
The Origin and Modification of a Signal
Spotted hyenas and their “pseudo penis” – looking at 4 different species of hyenas and their
behaviour – engage in chemical communication (use of anal glands to mark habitats) & they also inspect
the anogenital regions of their companions. L.Harrison Matthews proposed that the pseudopenis was a
result of high levels of male sex hormones in female hyenas. Gould introduced this hypothesis by stating
that the penis and clitoris are both made from the same embryonic tissue and become what they are
based on the exposure to certain hormones (testosterone and androgen = penis, absence of androgens
= clitoris). However, when a female embryo comes into contact with androgens, experimentally or
naturally, she develops an elongated clitoris that resembles a penis. According to a paper written by
Racey & Skinner, female spotted hyenas had circulating levels of testosterone equal to that of males.
Some other experiments were done testing the levels of testosterone in females and males and
found that females had less testosterone then males although it did become about the same when the
female was pregnant. Since the mothers placenta converts androstenedione to testosterone it could be
exposing the female embryo to male hormones, however there are other hormones that target to
inactivate these male hormones when the embryo is female. Another experiment done was to inject a
pregnant female with anti-androgenic chemicals to prevent the androgens from creating a pseudo penis
is females, however even with them, the females still had pseudo penis albeit a bit smaller.
The only other explanation is to suggest that mutation was the governing result of the pseudo
penis in female hyenas and that the mutation served to either lower male-like androgen concentrations
present in ancestral females or modified their effects, thereby reducing the damaging developmental
by-products of exposure to androgens for females.
The Adaptive Value of a Signal
Why are male-like characteristics in females (pseudopenis) preferred over those females that do
not, especially when females can become sterile when exposed to male hormones, when 10% of them
die after their first child birth, and about 60% of all hyena cubs die as well.
Proximate cause - a # of by-product hypotheses for the trait. One being that increased
androgen levels in females spread since it helped make adult females larger and more aggressive, not
because of acquiring a pseudopenis. More androgen exposure does lead to a more aggressive female in
comparison to males. Females also keep a hierarchical system for their own sex, which gain them
priority to the meat of the kill, and also must keep males in their place. Top females are able to hunt
within the clan, which lead them to be more successful, reproductively speaking, while subordinate
females must hunt outside the clan which can be risky. Top females also produce more androgens
during the second half of pregnancy, allowing these benefits to be passed on to their cubs. The first hypothesis proposed (social-bonding) for the adaptive trait of the pseudopenis was that
an erect pseudopenis aids in reducing tension among the highly aggressive female members of a clan,
promoting social bonds that kept the group together to work for the mutual benefit of all.
A second hypothesis proposed (sexual mimicry) was that the pseudopenis allows for some
mimicry of males by females in order to divert aggression away from them, since females are tricked
into treating an individual with a pseudopenis as if she were male. Problems with this hypothesis
include the assumption that hyenas identify males and females by visual cue when really they rely on
olfactory and auditory cues.
An Adaptationist Hypothesis
Historically a male’s erect penis would signal to the female that he is non-threatening and would
encourage the female to accept him as a mate rather than reject. Females would be able to use their
pseudopenis in the same regards, as to signal subordination to a mate while also establishing a strict
dominance hierarchy among females. Also, the pseudopenis poses and advantage to low ranking
females who can signal their subordination to other females to stay in the clan rather than being casted
out. Social harmony is displayed in this theory from the subordinate’s acceptance of its status relative
to a more dominant individual. Social mimicry is displayed in this theory to the extent that females
behave like males to demonstrate their rank relative to other females and not to deceive their
companions about their actual sex.
p299 - 326
Sensory Exploitation of Signal Receivers
Male whistling moths use ultrasound to communicate since an ancestor of theirs had bat
predators which made ultrasounds more reproductively advantageous. Relating this to the pseudopenis
of female hyenas, it is easier to understand the function of something new when it has played a role
before in the past, like the whistling moths. In one group of bowerbirds, males use a harsh and raspy
“skraa” call when courting females, which was almost certainly used for male-male aggressive signals.
This co-opt of signals has made it advantageous for females to make and adaptive choice about mates.
Sensory Exploitation – using a novel signal to tap into pre-existing perceptual mechanism in a
signal receiver. Ex. Courtship of male water mites, which “exploit” the predatory behaviour of females
waiting to ambush their prey (copepods). When the predatory female is in her attack position (called
“net stance”) the male will vibrate a foreleg in front of her, then she grabs him, let’s go (not harming the
male) and the male places spermatophores near the female. If she is receptive, she will pick them up in
her genital opening.
It is not a perfect theory since one suggestion is that more hungry females may be more willing
to attack and mate just to gain the nutrient-rich spermatophore.
Ex. Male and female guppies – male guppies are especially attractive to females when they have
bright orange spots on their skin which they can only express when eating a certain plant that is rich in carotenoids. A hypothesis for why females find this so attractive is based on a by-product of a sensory
preference that had evolved in another context. Female guppies would occasionally feed on orange
fruits that would fall in the stream where they lived, so females could have gained a sensory bias to the
colour orange because of a foraging benefit associated with the ability (orange skin). Females do tend
to approach the orange disc more often and actually try to bite them. Experiments were done across
many species that altered their natural attributes to those that could not be produced naturally
(extremely artificial). In all cases females preferred the artificially enhanced male rather than the
natural one. Another explanation for this bias is that an ancestor in the past may have used these
attributes to assess a mate, and the species now still have that existing bias even though mates cannot
grow such attributes. Ex. Lizard Sceloporus virgatus, which lack the large blue abdominal patches of
many other members of its genus. The blue patches were used in male threat posture, althought the
display is still not lost. When painted blue patches on the male, and they did their display, other males
would back off more readily whereas with no paint this was not the case.
Another hypothesis – are there signal receivers that have a preference for signals that neither
they nor their ancestors ever possessed or responded to? One way to test is to attribute a species with
a trait that would have not been in their ancestral past. Burley and Symanski outfitted zebra finches and
long-tailed finches with ridiculous white feathers glued to their head. Even though this trait has never
been seen now and in the past, females associated more with the male finch with the novel trait as
opposed to the other males.
Some arguments against the sensory bias hypothesis for female preference in swordtails
(covered in lecture) have to do with a detailed phylogeny of the xiphophorus that swords have evolved
and been lost several times in this genus of fish. Sexual display traits are likely to change rapidly over
evolutionary time, which could make it difficult to reconstruct evolutionary history via the standard
comparative approach. Also some researches also express that an arbitrary preference such as an
elongated tail could actually just be an adaptive preference for a more general trait, such as a large
However at the ultimate level of things females can benefit for going after these males that can
exploit their pre-existing sensory bias since they can gain sons that can inherit the trait or a male with
exaggerated courtship signals were physiologically capable of helping to rear a choosy female’s
offspring. If evolution favoured females who developed a resistance to such exploitation displays that
we would see a change towards females who do not select for such traits. However it can be costly to
females to do so under some circumstances. Ex. Is in the female garter snakes. They open their cloaca
in order to deter predators away using the odorous chemicals and feces released from an open cloaca.
However males copulate with females by inserting their intromittent organ into the cloaca. Even though
females subject themselves to males when opening their cloaca they also deter predators which if they
didn’t they would probably die. Adaptionist Questions about Communication
Darwinian puzzles in communication appear when observers see signal gives or receivers
apparently disadvantaged by their actions. If giving or receiving a signal lowers fitness, then the
behaviour for it should disappear over time. Heinrich started studying ravens when he heard one raven
call out to others while feasting on the carcass of a moose. The puzzle here was, why attract
competition to your food source instead of reaping all of the benefits (the food) for yourself. One
theory was that the raven was calling other ravens within its family (relatives) but according to DNA
fingerprinting they were all unrelated. Another hypothesis was that the call would attract a type of
carnivore (wolf or bear for example) to open up the carcass so the ravens could get to the inside easier
and feast. Exp. Wave a goat carcass by day to attract a raven and observe the behaviour. Results:
ravens did not call out/yell when carcass was not open, but they did when it was open, which proves the
hypothesis wrong. His other hypothesis was that the raven was attracting other ravens in order to
decrease its chance of being attacked by a hiding predator (coyote or fox). However, even after
acquiring a group of other ravens for security, the raven still was calling out and yelling, which also fails
as a hypothesis. While still carrying out the goat experiment, he then observed that the calling only
happened when 3 or more ravens had arrived at the carcass location (did not happen with 2 or less).
Since ravens tend to form pairs on certain territories, the pair would often challenge the intruder for the
carcass and try to drive them off their territory. Because of this he hypothesized that the intruder raven
would call out in order to gain protection from the resident pair, in order for the intruder to feast
instead of being kicked out. The “gang up on the territorial residents” hypothesis leads to a number of
predictions: 1) resident territory owners should never yell, 2) non-resident ravens should yell, 3) yelling
should facilitate a mass assault on a carcass by non-resident ravens, 4) resident pairs should be unable
to repel a communal assault on their resources, and 5) a food bonanza (resource) should be eaten by
either a pair alone or by a mob of ravens. He found that all these predictions were true and conclude
that yelling, although brings a cost of sharing the food, it also brings the benefit of feasting and also not
getting attacked by resident ravens.
Why Do Baby Birds Beg So Loudly?
Comparing the begging approach of some baby birds to others, the ones that beg the loudest
attracted the most amount of predators and were eaten/or eggs were ta