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