As a predator approaches, prey make
escape decisions based on trade-offs between fitness consequences of fleeing
and not fleeing (Cooper & Wilson, 2007). Sometimes, escaping is a high cost
when there is an opportunity to stay and increasing their fitness instead. This
is especially evident in some males, which are less wary of approaching
predators when fleeing means that they are abandoning courtship or mate guarding
(Cooper & Wilson, 2007).
For example, male striped plateau
lizards (Sceloporus virgatus) permit
closer approach before fleeing when given the opportunity to court or perform
aggressive behaviour, whereas females flee much sooner as they cannot justify
taking the risk of staying (Cooper & Wilson, 2007).
Female
Sceloporus virgatus
Author
unknown
The primary measure of weariness is
the flight initiation distance (FID - the distance between predator and prey
when escape begins) and the escape theory predicts that FID increases with
degree of predation risk of a predator and decreases with cost of escaping (Cooper
& Wilson, 2007). This demonstrates a clear trade-off between staying to
increase fitness/exposing itself to more danger and fleeing to survive/decreasing
the opportunity to increase its fitness.
Another option, which is rather
strange… is to essentially commit suicide.
This is an act of altruism which is
commonly found in eusocial groups of insect species, especially ants, and it
obvisouly decreases the actor’s fitness whilst benefiting the rest of the
colony (Shorter & Rueppell, 2012). Whilst other insects display defences
such as coloration, toxins, stinging, biting, spraying venom, wing shimmering, and
structure building, the Carpenter Ant (Camponotus saundersi) displays an
instantaneous defence known as autothysis, which is basically self-explosion
which results in the release of a caustic secretion; killing the attacker (Shorter
& Rueppell, 2012). The ant has an
enlarged mandibular gland which contains poisonous chemicals and a glue-like substance,
and when threatened they contract their abdominal muscles which such strength that
these glands explode and the ant dies.
Camponotus saundersi
committing suicide for the good of the colony
Author: Mark Moffett
Workers use this type of event either
in one-on-one confrontations of territorial defence far away from the nest (Shorter
& Rueppell, 2012). In turn, the rest of the colony is protected and
reproduce, creating more generations of Kamikaze Ants!
That’s a pretty extreme cost, in my opinion…
References:
Cooper Jr, W.E. & Wilson, D.S. 2007, "Sex and social costs of
escaping in the striped plateau lizard Sceloporus virgatus",Behavioral Ecology,vol.
18, no. 4, pp. 764-768.
Shorter, J.R. & Rueppell, O.
2012, "A review on self-destructive defense behaviors in social
insects",Insectes Sociaux,vol.
59, no. 1, pp. 1-10.
I assume that most of you are looking at this, asking yourselves; “But...WHY???”
I know I was when I first saw it anyway.
This rather interesting looking animal is called a
Saiga (Saiga t. tatarica) and it has evolved one of the most
extraordinary noses (or proboscis) among the mammalian kingdom. (Frey, et al. 2007)The Saiga is a relatively young
species that appear to have originated in central Asia in the late Tertiary (Pliocene)
or early Quaternary (Pleistocene). (Frey, et al. 2007)Recent individuals are quite
morphologically similar to fossils from Pleistocene Saigas, and so
reconstruction of the evolution of this nose is difficult. (Frey, et al. 2007)
In fact, the biological role of this nose has not even been fully explained. The
most accepted hypothesis is that this contraption initially evolved to filter
out dust particles from inhaled air when large herds of these species migrate
and kick you copious amounts of dust. (Frey, et al. 2007)The video below shows just how
much dust is kicked up during their cross-country migrations.
This type of locomotion (called the ‘amble’) is most energetically advantageous
across an open flat semi-arid steppe environment, and it involves the neck being
kept horizontal, meaning that the head is almost permanently immersed in a dust
cloud. (Frey, et al. 2007) It
has been suggested that a selection pressure for the evolution of a dust filter
has led to the extraordinary size and shape of these noses in both sexes. (Frey, et al. 2007)
Diagram showing the anatomical
structure of the Saiga proboscis. This includes the lateral vestibular recess,
alarobasal fold, nasal septum and the vocal tract. The nasal passage is
indicated in red and the oral passage is indicated in blue. (Frey, et al. 2007)
There is also a sexual dimorphism evident in nose sizes, as males have a
longer proboscis, especially when performing a rut. (Frey, et al. 2007) This display occurs from December
to January, where male establish territories and gather (sometimes up to 50)
females, defending them in severe fights against rival males. (Frey, et al. 2007)
Diagram showing the nose extension in an adult male saiga
during the rut. The image on the top is the resting position and on the
bottom is the calling posture. (Frey, et al. 2007)
Females also call, but
their noses are often more relaxed during this process, and it is used for a
different reason, most likely to call to their young. (Frey, et al. 2007)Another suggested reason for the
evolution of this nose is heat loss. The environment these animals live in can
be as hot as 50oC, which is enough to cause brain damage in most
animals, however the passage of air through the proboscis cools down the
inhaled air and in turn cools down the body of this amazing animal. (Frey, et al. 2007)
The following is a hypothetical step-by-step model for the evolution of
the Saiga nose, taking into consideration is specialization for vocalizing and
the ecological constraints found in dry-steppe environment. (Frey, et al. 2007)
Increasing use of the amble as a
mode of fast locomotion by a smaller-sized bovid promoted the initial evolution
of the unique nose, with slit-like nostrils
A low head position is advantageous
for ambling (better extension of forelimbs through greater freedom of shoulder musculature).
However, due to the large dust clouds produced by this mode of transport, the
slit-like nostrils have to be opened more widely. This mean means that small
nasal vestibulum is more susceptible to getting clogged with dust, causing a reduced
efficiency of breathing (tradeoff!)
This is followed by the evolution
of slightly larger nasal vestibula, at the expense of the nasal cavity proper in
both sexes. These changes lead to a large difference in the dimensions of the
nasal cavities compared to a typical bovid nose.
These transformations may have led
to a reduced efficiency of counter-current exchange of heat and water vapour
effected by the respiratory region, and so the advantages of the derived nose structure
must overcompensated those disadvantages (another trade-off!)
The increasing demand for
getting rid of mucus and dust from the nasal vestibulum caused regular forced
expirations through the nose, which lead to a preferential calling through the
nose rather than through the mouth in both sexes.
Increasing use of the nasal
airway by adult males during rutting calls instigated the evolution of sexual
dimorphism by mechanisms of sexual selection (a mechanism discussed in my
previous blogs)
Nasal, instead of oral, roaring
was favoured by selection. Roaring loudly through the nose signals an exaggerated
body size to conspecifics when compared to roaring through the mouth (a
constraint to the further reduction of the nasal cavity proper and enlargement
of the nasal vestibulum means the acoustic signal is an honest indicator of
body size which provides reliable information for rivals as well as females).
I realise this post is a bit off topic, considering my blog is about
trade-offs and constraints. However, I mentioned in my first post how intrigued
I am by the mechanisms that cause species to develop weird and wonderful body
parts and behaviours that just don’t seem to make sense at first glance. The
Saiga is a perfect example of a creature that, when first seen, evokes feelings
of curiosity and wonderment as to how and why something has evolved like this. Upon
further investigation, as we can see, there is a very legitimate reason for
having such a ridiculously large nose!
This amazing creature is unfortunately
critically endangered due to high levels of poaching, and their numbers have
declined by 95% in just the past 15 years. (The Saiga Conservation Alliance, 2014) The Saiga Conservation
Alliance aims to restore populations and save them from imminent extinction.
They welcome support from the public. Their link is: http://www.saiga-conservation.com/
if you are interested in further information.
References:
Frey, R., Volodin, I.
& Volodina, E. 2007, "A nose that roars: anatomical specializations
and behavioural features of rutting male saiga",Journal of
Anatomy,vol. 211, no. 6, pp. 717-736.
Last week I explored the concept of animal
communication through honest signalling, in which a signaller sends a signal of
quality to a receiver. In order for an individual to be able to create and
carry a costly trait, they must have a high fitness. Predators learn that they
should try and go for a weaker individual instead, intraspecific competitors
learn it would be futile to approach them for fights, and members of the
opposite sex are honestly signalled that they would be a mate with a higher
fitness.
This concept has been widely researched and
discussed in the last few decades, and the accepted idea has gone from organisms displaying implicit
honesty, to no honesty and no long-term
communication at all, and finally to cost-reinforced honesty (which is the
conventional explanation of today) (Rowell et
al. 2006).
You see, as always,
there are going to be some which manage to break the rules and sneak through
the system. If they can do this without creating too much fuss whilst in the
meantime increasing their fitness, then why not?
These cheeky individuals can
cheat by giving a dishonest signal. An example where deception in intraspecific
fights is important is when stomatopods of the species Gonodactylusbredini fight over ownership of a cavity (Adams & Caldwell, 1990). This species exhibits a display called the
meral spread, which involves leaning out (usually keeping their body in the
burrow) to reveal the meral spot, a small, colourful depression on the
underside of the appendage, and to make the individual appear large and
threatening (Adams & Caldwell,
1990).
Normally the size of this display provides accurate
information about the aggressiveness of the signaller. This is used
when defending homes, and also when females defend their eggs (Adams & Caldwell, 1990).
Peacock mantis shrimp (different species of mantis shrimp) engaging in the meral display Photo by: Linda Cline
However,
research has shown that newly moulted individuals sometimes also exhibit this display to bluff to
their components (Adams & Caldwell,
1990). These weakened
individuals are left with a very vulnerable soft exoskeleton that cannot
withstand blows, and so as they lack the ability to actually defend themselves
in a fight, the display in this case can be interpreted as bluffs (Adams & Caldwell, 1990). The body is not seen and so competitors do
not see that the individual is weakened.
Bluffing has also
been seen in a species of Green Tree Frogs, Rana clamitans, wheresome small
males exaggerate their quality by lowering their acoustic pitch to resemble
that of larger males (Bee et al. 2000).
Dishonest
advertisement is also seen in species of Fiddler crabs such asUca lactea mjoebergi,
which have been shown to lie about their fighting ability. Males
have an enlarged major claw (which looks very impractical in my opinion) which
is used both to attract females and fight rivals (Backwell et al. 2000, Lailvaux et al. 2009).
Most often, those possessing larger claws win the fights, and so we expect
larger claws to be an honest signal of increased fitness and fighting ability (Backwell
et al. 2000, Lailvaux et al. 2009).
Male
fiddler crab, Uca mjoebergi Photographer: Tanya Detto
When a male crab loses a claw,
it occasionally re-generates a new one. This new claw (termed a leptochelous) is about the same size as the original, however
much weaker (Backwell et al. 2000,
Lailvaux et al. 2009). However, what’s
interesting is that rival crabs are unable to distinguish between the original
and weaker claw and are still intimidated by its size and deterred from combat (Backwell
et al. 2000, Lailvaux et al. 2009). As the crab with the
regenerated claw would be unable to back up his ‘claims’ that he is a strong
male if he ever actually got into a fight, this is seen also a prime example of
a bluff.
Watch the video below to see
two male Fiddler crabs battling it off:
So
if escaping battles and predators and getting the ladies with minimal energy
and cost is this easy, why doesn’t everyone do it?
Imagine a group of
foraging birds, which signal a warning call to each other whenever predators
are near. The receiver (predator) receives an honest signal that its prey are
aware that it is there, and so gets the message that he/she should move on and
try and find a group of unsuspecting birds instead. If cheats gave random false
alarms, just in case a predator is nearby, then individuals would soon learn
that there is no point in fleeing as it is probably a false alarm and it’s
likely that there isn’t anything preying on them, just like the other 43 false
alarms of that morning.
Likewise, in the case
of the mantis shrimp mentioned above, if mantis shrimp continuously bluffed
opponents successfully every time, then all mantis shrimp would evolve to bluff
by using the technique of meral spread display, and it would eventually no
longer be effective and fall into disuse.
Over time every dishonest signal
would weaken the integrity of the whole system(Adams
& Mesterton-Gibbons, 1995; Johnstone & Grafen, 1993; Rowell et
al. 2006). For this reason,
in order to establish and maintain what is called an Evolutionary Stable
strategy (ESS), the proportion of dishonest signals must be low. That is, bluffing
with the display must only work for certain individuals (Adams
& Mesterton-Gibbons, 1995; Johnstone & Grafen, 1993; Rowell et
al. 2006).
In fact, at the ESS the very strongest and
the very weakest members of the population threaten while animals of
intermediate strength do not(Rowell et al. 2006). Think about the mantis
shrimp again... When strong mantis
shrimp display, they may deter the conspecific, however even if they don’t they
are probably able to win the battle as their display is in fact honest of their
superior fighting ability. When a newly molted mantis shrimp bluffs, it takes
the risk that the conspecific might attack, and just hopes that it doesn’t
actually have to get in a fight. This is disastrous if the bluff doesn’t work.
For this reason, for individuals of mediocre strength the risk isn’t worth it
and they are better off not using the technique to ward off competitors and
investing their energy in something else (Adams & Mesterton-Gibbons,
1995; Johnstone & Grafen, 1993;
Rowell et al. 2006). Hence we see a system where only the strongest and the weakest mantis shrimp benefit from bluffing and
the bluffing technique will continue to ward off opposing conspecifics as long
as this low frequency of individuals bluffing is maintained (Holden, 1995).
References:
Adams E.S &
Mesterton-Gibbons M 1995, "The cost of threat displays and the stability
of deceptive communication", Journal of Theoretical Biology, vol.
175, no. 4, pp. 405-405.
Adams, E.S. & Caldwell,
R.L. 1990, "Deceptive communication in asymmetric fights of the stomatopod
crustacean Gonodactylus bredini", Animal Behaviour, vol. 39, no. 4,
pp. 706-716
Backwell, P.R., Christy,
J.H., Telford, S.R., Jennions, M.D. & Passmore, N.I. 2000, "Dishonest
signalling in a fiddler crab", Proceedings. Biological sciences / The
Royal Society, vol. 267, no. 1444, pp. 719-724.
Bee, M.A.,
Perrill, S.A. & Owen, P.C. 2000, "Male green frogs lower the pitch of
acoustic signals in defense of territories: A possible dishonest signal of
size?",Behavioral
Ecology,vol. 11, no. 2, pp.
169-177.
Johnstone, R.A. &
Grafen, A. 1993, "Dishonesty and the handicap principle", Animal
Behaviour, vol. 46, no. 4, pp. 759-764.
Lailvaux, S.P., Reaney,
L.T. & Backwell, P.R.Y. 2009, "Dishonest Signalling of Fighting
Ability and Multiple Performance Traits in the Fiddler Crab Uca
mjoebergi", Functional Ecology, vol. 23, no. 2, pp. 359-366.
Rowell, J.T., Associate
Editor: Peter D. Taylor, Editor: Michael C. Whitlock, Ellner, S.P. & Reeve,
H.K. 2006, "Why Animals Lie: How Dishonesty and Belief Can Coexist in a
Signaling System", The American Naturalist, vol. 168, no. 6, pp.
E180-E204.
Sunday, 27 April 2014
Liar, liar, pants on fire…
… or telling the truth is probably wiser!
Before you read my post, watch this video of Springbok leaping
high into the air when escaping the predation of the fast cheetah. I promise you, you will be entertained.
Now, some
of you might be thinking the same thing as I did when I first saw the video.
Why on earth would these animals not expend their energy on running away as fast as possible instead
of wasting their time and energy on ballerina displays, as impressive as they
might be…
Well,
funnily enough, impression is exactly what these Springbok’s are trying to do.
Through this behaviour (called ‘stotting’ displays), they convey a message to
their predator that there is no point in chasing them because they are far too
fit and strong. The process of signalling quality to a receiver (in this case a
predator) is a concept known as the honest signalling theory, or, the handicap
principle (Nur & Hasson 1984).
Honest Signalling Theory/Handicap Principle
Whether
you are exploring the reef, hiking through valleys or strolling through a
forest, you are constantly being bombarded with signals, most of them intended
for non-human receivers. These could be in the form of calls, colours, smells,
just to name a few. Several of these signals are costly to produce and send,
and I have discussed some of these in my past blogs.
Check out the other examples
below:
Baby Northern Cardinals begging loudly for food whilst
being fed by their parents:
Performing
these loud cries might be useful in signalling to their parents that they are
hungry, however it would also undoubtedly attract predators to the nest.
Big Bull Elk rubbing the velvet off their huge antlers
These
horns are impressive, attractive to females and useful in battle, but
incredibly energetically expensive to create and carry (some of them weigh up
to 18kg!).
So why
don’t these animals send a more subtle message to the receiver instead of
partaking in these elaborate displays? These signals obviously work well in
convincing the receiver as they have evolved through natural selection, but
how?
The
Handicap principle proposed by Amotz Zahavi in the 1970’s suggests thatif an organism can signal
something costly, it means that it has a higher fitness than other individuals
as it can afford the signal in the first place. That is, it is strong and
healthy enough to deal with the cost and pass the test of survival, when
compared to another individual who has an inferior fitness and therefore cannot
afford the costly traits. Through this honest signalling organisms are
effectively conveying their ‘quality’ to other individuals (Zahavi & Zahavi
1977).
This can
be to attract females, such as with the peacock or large rack of antlers.
Peacock’s with dull feathers that drag on the ground and collect mites signal
to females the lower quality and fitness of that male, and she will find
someone else with beautiful feathers (making the grooming worth it in the
end!). Likewise, small antlers signify to females that the male cannot “afford”
to grow bigger ones and carry them and is therefore of lesser quality than the
spunky male with the oversized bone/skin/horns projecting from his head (Nur
& Hasson 1984, Grafen 1990, Johnstone 1995)
Which one would you choose?
It can
also be to deter predators, in the case of Gazelles and Springbok. Jumping high
signals to the predator that they are fit and strong, and that they should
probably focus more on chasing the one who keeps tangling his hooves and who
can’t even jump 2 feet of the ground (how shameful!). The hard to catch prey
benefit by distinguishing themselves from less healthy individuals and escape
predation, and the predators benefit by understanding this signal and not
wasting their time on prey who are more likely to escape. Other prey do not benefit
y being distinguished, however the cost of the leaping signal is so high that
they cannot pretend to be a harder-to-catch individual, meaning that is an
honest signal! (Nur & Hasson 1984, Yachi 1995)
In the
case of baby birds, they are informing their parents about how hungry they are.
These birds might all pretend that they are hungry and all make noise. However,
this takes energy and could also possibly attract predators to the nest. This means
nestlings end up honestly conveying their hunger levels as the ones who are truly
hungry continue making noise (need for food outweighs the risk of predation),
whereas the satisfied ones will stay silent due to the cost of predation (Rodríguez-Gironés
et al 1996).
Signals
have to be honest, or the receiver would eventually evolve to ignore them,
leading to the signallers evolving not to send them in the first place as they
would be completely useless. And in order for the signals to be honest, they
must also be costly. Each signaller makes a choice of the size of their
handicap they want to produce, taking into consideration both the cost and the
benefit (this is often not a conscious decision, but one that is encoded in the
genes and acted upon through natural selection). Stronger individuals can
assume greater handicaps at lower costs and hence will grow ‘bigger and better’
ornaments, whereas the same handicap would hinder a weak individual. Consequentially,
handicap size is a reliable signal of strength to the receiver.
References:
Grafen, A. 1990, "Biological signals as
handicaps", Journal of theoretical biology, vol. 144, no. 4, pp.
517-546.
JOHNSTONE, R.A. 1995, "SEXUAL SELECTION,
HONEST ADVERTISEMENT AND THE HANDICAP PRINCIPLE: REVIEWING THE EVIDENCE", Biological
Reviews, vol. 70, no. 1, pp. 1-65.
Nur, N. & Hasson, O. 1984,
"Phenotypic plasticity and the handicap principle", Journal of
Theoretical Biology, vol. 110, no. 2, pp. 275-297.
Rodríguez-Gironés, M.A., Cotton, P.A. &
Kacelnik, A. 1996, "The Evolution of Begging: Signaling and Sibling
Competition", Proceedings of the National Academy of Sciences of the
United States of America, vol. 93, no. 25, pp. 14637-14641.
Shigeo Yachi 1995, "How can Honest
Signalling Evolve? The Role of Handicap Principle", Proceedings of the Royal
Society of London. Series B: Biological Sciences, vol. 262, no. 1365, pp.
283-288.
Zahavi,
A. & Zahavi, A. 1977, "The cost of honesty. Further Remarks on the Handicap
Principle", Journal of Theoretical Biology, vol. 67, no. 3, pp.
603-605.
