We began with “Is honesty the best policy? Testing signal
reliability in fiddler crabs when receiver-dependent costs are high” (Bywater,
C., et al, 2012). The authors of this
paper took notice that signal reliability has only been manipulated in the
lab. While it has been proven costly to
the unreliable signaler, the writers pointed out that no studies have been done
in the natural habitat. This paper was one
of the first to examine signaling and receiver-dependent costs in the wild, and
their findings advanced the field of Animal Behavior. They showed a correlation between signal
reliability and population density. A
signal was more likely to be unreliable when the signaler was in a low-density
population. I began by asking why this
is. Through their study Bywater, et al
found that in low population density, individuals were less likely to come into
contact with a competitor. Because of
this, they allocated less energy to developing muscle, yet retained large claw
size. This leads to an unreliable
signal. Crabs first present their claw
to any competitor or potential mate. A
large claw size is more likely to deter a competitor and attract a female. However, if a competitor wished to engage in
physical contact, they might find that claw size is not representative of claw
strength. This is an unreliable signal.
It was
often mistaken during the discussion that claw strength is dependent on claw
size. This is not what Bywater, et al
found. In actuality, it was the shape of
the claw that determined strength. We
then postulated why a large, shapely claw might not be representative of
strength. It was brought up that
resource availability might be an effect on claw strength, a factor Bywater, et
al admittedly did not take into account.
A rocky terrain vs. a smooth terrain would definitely affect population
density and potential competitor interaction.
We further discussed the ability of the crab to better the
representation of its claw strength after molting. It is possible that the claw could get bigger
after a molting season, and using information from a previous season, it could
allocate more muscle to the claw. In
this way, the crab could become a more reliable individual. We also agreed during discussion that
Bywater, et al missed out on some great data by leaving out crabs that lost a
claw and then replaced it. They assumed
that because they are generally weaker, they are automatically unreliable
signals. But if they had included these
crabs as isolated events, their findings might have been surprising and the
discussion could have delved deeper in what it means to develop a stronger
claw.
The
discussion on the second paper “A mechanism of extreme growth and reliable
signaling in sexually selected ornaments and weapons” (Elmen, D., et al) was
admittedly better. This paper examined
horn growth in rhinoceros beetles. They
examined its dependence on the insulin/IFG pathway in comparison to genital and
wing size. When insulin receptor gene
was knocked out, they found that genital size was not affected significantly;
wing size was affected, but not as drastically as horn size. This is thought to be controlled by the
intracellular signaling as a product of blocking insulin receptors. From this we can define two terms: handicaps
vs. indices. Handicaps are those signals
that are unfaithful and incur a high cost on the signaler. Indices are those that are impossible to
replicate. The insulin/IFG pathway is an
example of indices. This supports
Condition Dependence. This is a process
defined by Animal Behaviorists as one that keeps a signal honest through many
pathways. This paper supports that horn
size in rhinoceros beetles is a reflection of cellular processes (insulin/IFG
pathway).
Now
that we know the trait is dependent on insulin, we can look at the proximate
question (ultimate was assumed) on the trajectory of evolutionary change. If there is an increase in sensitivity to
insulin, then selection pressure could select for increased insulin
sensitivity. This brings up the
question, what came first, the chicken or the egg? In other words, did selection lead to sensitivity
or did sensitivity lead to selection?
Either answer could be correct, but could be determined by looking at
phylogenetic similarities to insulin sensitivity. If it arose early in rhinoceros beetle
evolution, the former can be assumed. If
sensitivity arose late in their evolutionary lineage, then the latter can be
assumed. Since we did not have a
rhinoceros beetle phylogeny, we moved onto another question. One that was even more complicated, but
proved fun to speculate on.
Someone brought up the question,
what is quality? Elmen, et al repeatedly
defined beetles as high-quality individuals and low-quality individuals, but
failed to define quality. It is assumed
that quality is a mark of some previous defined standard, but that remains
unclear throughout the paper. We assume
that quality is reflective of horn size, but what does horn size say about the
individual. Are they more likely to
reproduce? Are they more
competitive? Do they gain more
resources? Is there a top barrier to
horn size, and how does that affect the individual? Ultimately, this part of the discussion led
to more questions than answers. It felt
like we were going in a circle when it was brought to our attention that,
indeed, we were. Someone in the group
mentioned a certain trait controlling quality.
From this I drew a little diagram that encapsulated our discussion of
the term quality. Trait→ Reproductive
Success→ Quality→ Trait. This was just
too mind-boggling to handle so I diverted our attention back to the paper.
Figure 2 D, E, and F
plotted body size (g) against Aedeagus (genital) length (mm), wing length (mm),
and horn length (mm) respectively. These can be seen above, courtesy of Elmen et al. These graphs show a simple linear regression
for control animals vs. knockout individuals (those that received dsln-R
injections to knockout insulin receptors).
These graphs were simple, yet showed something very interesting and
significant. The slope of the line did
not change for each graph while the y-intercept did. I brought this to the attention of the class
and asked why this was important and what it says. Put simply, y=mx+b. Body size remains proportional to the three
measurements taken (slope). This shows
that insulin affects growth proportionally.
The only thing that changed was the base line (y-intercept) at which the
control and knockout individuals began to develop the three measurements
taken. Because there was only one
variable, a goal of every experiment, Elmen et al were able to measure the
magnitude of difference between the control and knockout individuals. This is where their significant data came
from and how they were effectively able to support their hypothesis that sexual
selected ornaments and weapons are highly controlled. When looking at these graphs, we see that
genital length differed between the two groups by 0%. They were able to conclude that genital
formation is independent of insulin/IFG.
Wing size differed slightly (2%) showing a slight dependence on
insulin/IFG pathway. They were probably
very excited when they analyzed horn length.
The knockouts showed a 16% change in length when compared to those of the
control group. This shows an astounding
dependence on insulin/IFG. It remains to
be seen as to why such an important ornament is dependent on insulin, but data
shows time and time again that we have a pretty clear idea of how.
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