How Do Energetic Costs of Signaling Mediate Growth and Energy Allocation in Nestling Birds?

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How Do Energetic Costs of Signaling Mediate
Growth and Energy Allocation in Nestling Birds?

• Eli Awad, Ian Santino, Elise Lauterbur

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Evolutionary Signaling Theory

• Signal A behavior or trait, fashioned and
  maintained through natural selection because it
  conveys information to other organisms.
• Why signal? or the No organism is an island
  hypothesis.
• Who drives the system? or Manipulators vs. Mind
  Readers.
• F-I-T-N-E-S-S and Trade-offs.
• Why study it?

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Begging Behavior and the Problem of Signaling

• Systems have evolved so that nestlings transmit
  information about their condition to their
  parents via begging signals vocal, physical,
  etc.
• These signals affect parental behavior, mainly
  through food provisioning. Many studies have
  shown that parents feed begging babies, and
  babies rely on their parents for food.
• So no problem, right?

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Lies and the Lying Liars That Tell Them
According to the basics of signaling theory

• What if a nestling begged for food, even when it
  was full?
• What if a nestling begged constantly?
• Why dont we see this happening in nature?

• It would get fed even if it didnt need it.
• It would get fed constantly.
• Glad you asked

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Consequences of Deceit

• Short Term
• Baby gets fed more than it needs, which is a
  good thing from its little point of view
• Parents expend more energy to feed the baby and
  increase their risk of death from exhaustion and
  predation. Also may increase babys exposure to
  parasites

• Long Term
• Parents may attenuate to the over-expressed
  signal.
• Parents that can detect deceit, and only feed
  their babies what they need will survive longer
  and have more offspring, spreading their mind
  reading genes through the population.

SO?
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Signals must have some cost that renders the
signaling system reliable

• Cost
• Direct Overworked parents are less efficient
  providers, increased feeding trips also mean
  increased risk of exposure to parasites.
• Indirect A nestling faces a trade-off between
  increasing its own fitness at the expense of the
  fitness of its parents and siblings. In the long
  term, the indirect fitness costs may outweigh the
  immediate energetic benefits.

• Reliable
• In other words The information in the begging
  signal corresponds to the actual condition of the
  nestling.
• We expect any signaling system to contain at
  least a kernel of reliability. Otherwise, what
  possible evolutionary significance could it have?

So we asked If a signal is costly, then how can
we model the effects of the costs on a nestlings
growth and size at fledging (read fitness)?
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Trial and Error

• Gradient Model?
• Michaelis-Menton?
• Lotka-Volterra?

What about a Lotka-Volterra Hybrid with a twist?
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Scaffold

• We began with a Lotka-Volterra scaffold
• Logic A parent with high energy will put lots of
  energy into feeding its baby. The baby will in
  turn use this energy to grow and signal more,
  leading to a decline in the parents energy,
  followed by the babys energy decline. And on and
  on.

BUT
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Growth, Metabolism, and Signaling

• Though signaling may show Lotka-Volterra behavior
  as a function of parent and baby energy, growth
  and metabolism, in nature, do not.
• So separate flows out of a stock of Baby
  Energy for growth, metabolism, and signaling,
  with a separate stock for Signaling Energy that
  functions in the Lotka-Volterra part of the
  model.
• Have growth flow as a function of baby energy.
• Have metabolism flow as a function of baby
  size.

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Parent
Feed Me!
No Feed ME!
Parent Foraging
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Baby
A function of Latent Energy x Nestling size
Determines fraction of total energy put towards
signaling
Determines Signal Threshold function of Nestling
Size / Latent Energy
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Two babies, or not two babies

• Made another baby sector. Same as the first one,
  but we can vary when it is born, as well as its
  Deceit Coefficient.
• Can be turned on or off.
• This had the potential to seriously affect the
  models performance.

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Single Nestling
Maximum Size
Honest Signaling 35 g
Deceit Over-Represents Need 24 g
Deceit Under-Represents Need 63 g
Over-representing need harms the nestling in the
long run, but under- representing need can be
beneficial.
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Pair of Nestlings
Maximum Size
Pair 1 Both honest Baby 1 24 g
Pair 1 Both honest Baby 2 24 g
Pair 2 Baby 1 Over-Represents Baby 1 16 g
Pair 2 Baby 1 Over-Represents Baby 2 21 g
Pair 3 Baby 1 Under-Represents Baby 1 23 g
Pair 3 Baby 1 Under-Represents Baby 2 32 g
Pair 4 Both Over-Represent Baby 1 12 g
Pair 4 Both Over-Represent Baby 2 12 g
Pair 5 Both Under-Represent Baby 1 44 g
Pair 5 Both Under-Represent Baby 2 44 g
Died at 2322 time units
Died at 2292, due to parent mortality
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The Interesting (Expected!) Minorities
Pair of Nestlings, Nestling 1 Deceit Set at 0.3,
Nestling 2 is Honest
Energy Kcal Size g Signal Kcal
Max. size 23 g
Baby 1
Baby number 1 tries to save energy by
signaling less, but all the extra food is
given to his honest brother
Energy Kcal Size g Signal Kcal
Baby 2
Max. size 32 g
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Baby 1 is honest and baby 2 is just a little
dishonest (deceit set at 1.1)
Max. size 23 g
Energy Kcal Size g Signal Kcal
Baby 1
At this slight level of deceit, it actually does
benefit the liar to lie
Max. size 25 g
Energy Kcal Size g Signal Kcal
Baby 2
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Discussion
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Discussion

• Energy is used for signaling instead of for
  metabolism and growth.
• A nestlings size at fledging (the end of the
  simulation) is assumed to be proportional to its
  lifetime fitness.
• It is never beneficial to a single nestling to
  over-represent need. For a pair of nestlings, the
  over-representer will usually die, but there are
  instances where over-representing does benefit
  that individual.
• It benefits the babies to signal less than is
  necessary, because more energy is used for
  growth, which increases overall fitness.
• But nestlings cant under represent their need
  too much, or else they wont get fed and will
  die.
• If the nestling(s) greatly over-represent, mom
  has to work extra hard, and can die from
  exhaustion of her energy reserves.

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Implications

• Our model supports current theory that energetic
  signaling costs can maintain signal reliability
  over evolutionary time.
• The under-representation paradox brought up by
  our model is an unexpected one. This is partially
  a result of signal intensity being proportional
  to nestling size.
• The presence of a competing nestling can alter
  its siblings optimal signaling strategy, which
  we also see in nature.

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Shortcomings and Potential Futures

• Unrealistic values for energy levels, in addition
  to massive energetic fluctuation.
• Signaling costs have been shown to exist in
  nature, but nowhere near the level we have
  modeled them at.
• Entirely theoretical, no empirical values used
  for coefficients.
• Future research could focus on examining the
  benefits of under-representation, as well as
  attempting to formulate a working model with
  empirical data.
• Additional layers of complexity, such as the
  parents fitness, as well as seeing how a babys
  rearing effects it later in life when it becomes
  a parent.

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The End!
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References

• Kilner, R.M., D.G. Noble, and N.B. Davies. 1998.
  Signals of need In parent-offspring
  communication and their exploitation by the
  common cuckoo. Nature 397 (6721) 667-672.
• Kilner R., and R.A. Johnstone. 1997. Begging the
  question Are offspring solicitation behaviours
  signals of need?. Trends in Ecology and
  Evolution 12 (1)11-15.
• McCarty, J.P. 1996. The energetic cost of begging
  in nestling passerines. Auk 113 (1) 178-188.
• Ottosson, U., J. Backman, and H.G. Smith. 1997.
  Begging affects parental effort in the pied
  flycatcher, Ficedula hypoleuca. Behavioral
  Ecology and Sociobiology 41 (6) 381-384.
• Searcy, W. A. and S. Nowicki. 2005. The Evolution
  of Animal Communication Reliability and
  Deception in Signaling Systems. Princeton
  University Press.
• Special thanks to Keith Tarvin for his advice!