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Evolution Basics: Coevolution and Predator / Prey “Arms Races”

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August 16, 2013 Tags: Genetics
Evolution Basics: Coevolution and Predator / Prey “Arms Races”

Today's entry was written by Dennis Venema. You can read more about what we believe here.

Note: This series of posts is intended as a basic introduction to the science of evolution for non-specialists. You can see the introduction to this series here. In this post we examine how a predator / prey relationship shapes the evolution of both species.

In the last post in this series, we discussed the convergent evolution of echolocation in certain bats and toothed whales. For bats, echolocation allows them access to a rich food source (insects) at a time of day when there are few competing predators, and ample prey (since many insects are more active at night for the same reason – there are fewer nocturnal predators). As bats slowly gained the ability to echolocate and hunt at night, this innovation would naturally affect their prey species. Any variation within prey populations that was advantageous for avoiding predation would be selected for, and increase in frequency over time – and contribute to a change in average characteristics of the prey species. These shifts in prey characteristics would then select for variation within bats, and so on – effectively joining predator and prey into a relationship where each acts as the major selective force on the other. Such a relationship is an example of coevolution – a close relationship between two species where each shapes the evolution of the other. For predators and prey, one can imagine their coevolution as an “arms race,” with improving weapons for predation matched by improving defenses.

(As an aside, it’s important to note that in many cases, a stable, long-term coevolutionary, “arms race”-type relationship may not form between predators and prey. In this case, extinction of one of the species can result – and, judging from the fossil record, extinction is a very common feature of biodiversity over long timescales. Both predators and prey species can go extinct, releasing their “partners” from their coevolutionary influence. The ongoing coevolutionary relationships we do observe, then, are the ones that are stable enough to have persisted for some time.)

Bat versus moth

Since echolocation is such an effective tool for preying on nocturnal insects, it’s not surprising that there are a number of adaptations in various insect species that improve their odds of avoiding being eaten by bats. The most basic of such adaptations is a means by which insects can hear the ultrasonic frequencies that bats use for echolocation – in other words, some sort of auditory sensory organ. These sensory organs, known as tympanal organs, employ a membrane that vibrates in response to sound waves, and associated nervous system cells that convert the vibrations into changes in electrical potential that are perceived by the insect’s brain as sound. Since even a rudimentary ability to detect ultrasonic sound would be a distinct advantage when faced with an echolocating predator, it’s not overly surprising that tympanal organs that can detect ultrasonic frequencies have arisen independently numerous times in distinct insect lineages (providing yet another widespread example of convergent evolution). Tympanal organs can be quite simple – a small membrane and a few associated nervous system cells – lending credence to their being evolved over and over again.

Beyond merely hearing a bat’s approach, insects have a range of adaptations that are coupled to detecting ultrasonic frequencies. In many cases, erratic flight patterns (such as dives, loops, and even complete freefalling) are triggered by detecting echolocating bats in close proximity. Detecting the weaker signals of more distant bats often produces a more basic survival mechanism – simply flying away from the source of the sound. In both cases, natural selection acting on variation within insect populations is the likely source of these adaptions – as bats began to use echolocation for hunting insects, insects that possessed variation that reduced the chances of successful predation would reproduce at a greater rate, honing these responses over time to the form we observe in the present day.

Predator and prey: tiger moths of the species Bertholdia trigonia fight bat echolocation with ultrasonic noise of their own. (Image source: bat / moth)

Moth versus bat

One innovative mechanism that some moth species have developed as a defense against echolocating bats is to use their tympanal organs to produce ultrasonic sound in response to detecting the ultrasonic cries of echolocating bats. In some moth species, this acoustic response warns bats that the moths are toxic and unfit for consumption. Since bats can learn to avoid such moths, it makes sense for toxic moth species to advertise their presence so they are not mistaken for palatable species. Since vivid colors and other visible displays (such as are often found on other toxic insects) are not an option for alerting would-be predators at night, characteristic sounds are employed instead. To achieve this, the tympanal membrane is actively vibrated by the moth, producing ultrasonic waves that the bat will hear (since bats are listening to these very frequencies for the purposes of echolocation). Bats come to associate the resulting acoustic signature of the moth with their toxic, unpalatable taste, and in the future avoid other moths making the same sound. On average, then, moths making this sound are more likely to avoid predation by bats (though some members of the moth species will have to pay the ultimate price for educating naïve bats).

One species of moth, the tiger moth Bertholdia trigonia, takes this approach to a whole new level. Not only does it emit ultrasonic noise in response to an approaching bat, it does so in a way that directly interferes with the bat’s ability to echolocate. Moths that are merely using a warning sound to advertise their toxicity emit signals in a pattern that does not produce interference with bat echolocation, but Bertholdia emits ultrasonic bursts well suited for a “jamming” purpose. Recent work on these moths and their bat predators has teased apart a possible “warning” effect and a “jamming” effect by using bats familiar with the moths and actively pursuing them as prey. Bats fully intent on capturing the moths were hampered in their ability to do so when the moths were capable of ultrasonic bursts. Often, the bats would lose the moth in the final stages of its attack, and revert back to “search mode” instead of successfully capturing the moth. Moths unable to employ ultrasonic countermeasures (through surgical removal of their tympanal organs) were easy prey by comparison. This example of active interference with an echolocating predator is thus far the only example known in nature, and represents a striking example of a trait shaped through coevolution. A current hypothesis is that this “jamming” signal arose from a prior “warning” signal, since warning and/or jamming are not mutually exclusive effects for the acoustic signature of an ancestral moth species, especially if that species was not toxic to all potential predators.

Faced with this innovation, the ball is now in the bat’s court, so to speak – and bats that possess variation that allows them to capture these moths more readily may leave more progeny than variants that do not.

In the next post in this series, we’ll explore an even more intimate type of relationship between species: those between parasites and their hosts.

For further reading:

Corcoran, A.J. et al., (2009). Tiger moth jams bat sonar. Science 325: 325 – 327. http://www.sciencemag.org/cgi/content/full/325/5938/325

Yager, D.D. (1999). Structure, development and evolution of insect auditory systems. Microscopy Research and Technique 47: 380 – 400. http://www.ncbi.nlm.nih.gov/pubmed/10607379


Dennis Venema is professor of biology at Trinity Western University in Langley, British Columbia. He holds a B.Sc. (with Honors) from the University of British Columbia (1996), and received his Ph.D. from the University of British Columbia in 2003. His research is focused on the genetics of pattern formation and signaling using the common fruit fly Drosophila melanogaster as a model organism. Dennis is a gifted thinker and writer on matters of science and faith, but also an award-winning biology teacher—he won the 2008 College Biology Teaching Award from the National Association of Biology Teachers. He and his family enjoy numerous outdoor activities that the Canadian Pacific coast region has to offer. Dennis writes regularly for the BioLogos Forum about the biological evidence for evolution.

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Roger A. Sawtelle - #82472

August 16th 2013


Thank you for pointing out how advantageous the nocturnal sky niche was for the bat. 

One would think that birds would be in the best position to do this, and owls of course fo, but it seems that birds were not able to develop echolocation which made it possible to catch insects like moths.  When bats became a part of the moths’ environment, then moths were pushed to adapt to them. Useful adaptation to the environment is the key. 

Now what does this have to do with Malthus and the “struggle for survival?”  It certainly has much to do with the ability of life forms, animal and vegetable, to adapt to new ecological niches.  It is a shame that Darwinian theory has fallen so far behind evolutionary fact and reality.

While the evolution of the bat and the coevolution of the moth is certainly remarkable, there are many examples of different species evolving together through cooperation or symbiosis.  We have sharks and the pilot fish, lichens which were responsible for the oxygenation of our atmosphere, bats and bees that that pollenate, the billions of life forms that live in our bodies. 

I question as to whether predation has been a major cause of extinction (except where humans were involved.)  What we see today is that habitat is the main influence.  When the habitat is poor, both prey and preditor suffer.  When the habitat flourishes both flourish.  When the habitat disappears, both vanish. 

Natural Selection is non random because it is based on the environment and ecology which are not random.  Convergence is not random because it is also based on ecology.  Indeed evolution itself is based mainly on the ecology and variation.                  

Lothar Lorraine - #82487

August 18th 2013

Hello Dennis,


the greatest problem is God’s creation of pain and suffering for innocent animals. 


Only their resurrection could compensate for the evil they underwent.



Lovely greetings from Germany

Liebe Grüße aus Deutschland


Lothars Sohn - Lothar’s son

Roger A. Sawtelle - #82513

August 24th 2013


In case you missed it please see my post #82512 under the previous blog.

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