Energetic Costs of Signaling and Nestling Bird Growth

1. How Do Energetic Costs of Signaling Mediate Growth and Energy Allocation in Nestling Birds? Eli Awad, Ian Santino, Elise Lauterbur

2. 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?

3. 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?

4. What if a nestling begged for food, even when it was full? What if a nestling begged constantly? Why don’t we see this happening in nature? It would get fed even if it didn’t need it. It would get fed constantly. Glad you asked… Lies and the Lying Liars That Tell Them According to the “basics” of signaling theory…

5. 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 baby’s 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. Consequences of Deceit SO?

6. 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? Signals must have some cost that renders the signaling system reliable So we asked: If a signal is costly, then how can we model the effects of the costs on a nestling’s growth and size at fledging (read: fitness)?

7. Trial and Error • Gradient Model? • Michaelis-Menton? • Lotka-Volterra? What about a Lotka-Volterra Hybrid with a twist?

8. 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 parent’s energy, followed by the baby’s energy decline. And on and on. BUT…

9. 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.”

10. Parent Feed Me! Parent Foraging No Feed ME!

11. 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

12. 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 model’s performance.

13. Single Nestling Maximum Size Over-representing need harms the nestling in the long run, but under- representing need can be beneficial.

14. Pair of Nestlings Maximum Size * Died at 2322 time units ** Died at 2292, due to parent mortality

15. 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

16. 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

17. Discussion

18. 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 can’t under represent their need too much, or else they won’t 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.

19. 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 sibling’s optimal signaling strategy, which we also see in nature.

20. 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 parent’s fitness, as well as seeing how a baby’s rearing effects it later in life when it becomes a parent.

21. The End!

22. 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!