Caterpillars of tiger moths eat poisonous plants. As adults, they make sounds that inform bats they are poisonous and not to eat them.
Tiger moths are in the Arctiid branch of the noctuid moth family., In these moths, the caterpillars are often adapted to feed on plants containing cardiac glycosides that are poisonous to other animals. Like monarch butterflies whose caterpillars also consume cardiac glycosides, the adult tiger moths retain these plant toxins and are themselves distasteful and poisonous to many of their predators.
These moths often have distinct yellow or cream stripes on a dark background (thus the term “tiger”), or have brightly colored hind wings that they keep hidden until disturbed or while flying.
The bright colors of these moths advertise their toxicity and provides them protection from many predators. Some larvae of these moths are the well-known woolly bear caterpillars (that are not really able to predict how severe the next winter will be), others are tussock moths with toxic spines.
Bats Avoid Tiger Moths
When attacked by bats, tiger moths do not drop to the ground like other noctuids. Instead, they produce a series of ultrasonic clicks which appear to advertise the moth is distasteful.
The moth’s rapid clicks warn the bat that the moth is distasteful and a bat that has previously eaten these moths will abort its attack.
How the Moth Detects the Bat
Like other noctuids, tiger moths are able to hear bats because they are equipped with a pair of ears on the thorax behind their wings. Other noctuid moths have two types of neurons that respond to bat clicks.
Type I neurons respond to soft, widely spread, low pitched clicks and cause the moth to turn parallel to the bat’s flight. Type II neurons respond to the high intensity, rapidly delivered, high pitched buzz the bat makes just before capture. When Type II neurons fire, most noctuids stop beating their wings in unison and spiral or drop directly to the ground.
In the tiger moths, Type I neuron response does not change the moth’s behavior noticeably. But when the bat is closing in and its Type II neurons fire, a tiger moth produces its ultrasonic clicks.
How the Moth Makes its Clicks
A tiger moth’s thorax has a pair of raised hardened semicircles of cuticle called “tymbals” (see the second picture).
When the moth hears an attacking bat closing in for the capture, the tymbal muscles cause the tymbals to pop in and out, producing up to 450 ultrasonic clicks in a tenth of a second.
When the bat hears these clicks, its sonar is jammed, it becomes disoriented and misses the moth.
Bats Do not Have to Learn to Avoid Poisonous Tiger Moths
Like most predators, a young bat is naive about distasteful prey. When learning to forage for itself, it samples all prey that it detects and can capture. If it approaches a tiger moth and hears a response from the insect, the bat is not deterred. It captures the moth, bites into it, and tastes the bitter toxins the moth has stored.
Although dead, the moth is dropped and the bat resumes its search for food. Most young bats do not make the same mistake twice. The production of sound by the moth is an unusual event that is easily remembered.
Thus, any prey individual that produces sound in the future is protected from being eaten because a bat that has previously bitten into a nasty tasting tiger moth remembers the unpleasant experience and avoids capturing the moth.
Clicks are Both Necessary and Sufficient to Avoid Capture
In the event a tiger moth does or can not click or does so too late for the bat to respond, the bat captures the moth normally. If upon biting into the moth, the bat has captured a poisonous moth, it drops the moth, and resumes its search for food. Otherwise, the moth is eaten. Thus, the click is necessary for the bat to turn away from the poisonous insects.
Some tiger moths do not feed on poisonous plants. Even though they are palatable, they take advantage of the behavior of their poisonous relatives and deceptively produce clicks that reduce being eaten by the bats. This appears to be case of auditory mimicry and the clicking alone is sufficient for the bat to reject a palatable moth, but may be a shift of function from warning to jamming the bat’s radar.
Toxins Even Protect the Eggs
Tiger moths can transfer toxins into or place them on the surface of eggs as they are being laid. When egg eating predators taste these toxins, they leave the eggs alone. There are several mechanisms for transfer, some of them quite unusual.