Shake an Egg
An egg might seem to be its own little world—encapsulated and
cut off from its environment, at the mercy of the forces of nature.
But some eggs are very aware of what's going on around them. Not
only that, certain eggs can also respond to their circumstances and
hatch up to 30 percent prematurely in order to better their chances
Karen M. Warkentin, a biologist at Boston University, and her
colleagues have been studying the red-eyed treefrog, Agalychnis
callidryas, an inhabitant of Central American tropical forests.
The frog lays gelatinous clutches of eggs on leaves overhanging
ponds, so that when the tadpoles hatch, they drop into the water.
However, the eggs' arboreal location makes them vulnerable to
consumption by snakes and wasps.
The usual gestation period for red-eyed treefrog eggs is six to
eight days. Warkentin found that after four days of gestation, if
the clutch is attacked by a predator, the tadpoles will suddenly
start to hatch and fall into the water. A predator can do nothing
but watch as its prey literally slips away while the predator chews
its first mouthful.
But how do the eggs know that they are under attack? It doesn't seem
to be chemical cues or signals from doomed sibling eggs. The eggs,
it turns out, sense vibrations from the attack. As the eggs
are secured in a rather tough gelatinous coating, a predator such as
a snake has to really tear into the clutch in order to pull free a
bite, jarring the entire clutch in the process.
To prove that this was the signal that the eggs cue into, Warkentin
inserted a small accelerometer into clutches of eggs and recorded
the vibrations from snake attacks. She then replayed these
vibrations, using a device called a mini shaker, to other egg
clutches. These tadpoles hatched in response to the vibrations, with
no actual snakes present.
The treefrog eggs are subjected to a number of other, harmless
vibrations, such as those from wind and rain. In the same type of
playback experiments, few eggs would hatch in response to vibrations
from these benign events. Somehow, the eggs can tell the difference
between dangerous and safe vibrations. As Warkentin reported in June
at the Acoustical Society of America conference in Providence, Rhode
Island, the eggs seem to pick up a combination of attributes
of the signal.
Both snake attacks and rain have low-frequency components in their
vibrational signals. However, rain has additional higher-frequency
components that are not found in snake attack signals. In addition,
the duration and spacing of vibrational events are different for the
two signals. For instance, during a rainstorm, longer-duration
vibrations occur when multiple drops fall in rapid succession, but
the interval between drops shortens. A longer snake attack would
likely have increasing intervals between bites.
Warkentin modified the recording of rain and snake attacks so that
each sounded like the other: She clumped the rain vibrations into
short bursts that were widely spaced and evened out the intervals
and durations of snake-bite vibrations. The eggs hatched more to the
"snakeified" rain signal than to actual rain or to the
"rainified" snake signal.
The investigators then created entirely artificial signals from
white noise with various burst and interval lengths. They found the
highest rate of hatching—up to 75 percent—from signals
that had a half-second duration and intervals of 1.5 to 2.5 seconds.
Such stimuli with intervals longer than their durations are
consistent with the patterns of snake attacks, but not with rain.
Either signal on its own was not sufficient to induce hatching.
Neither was continuous vibration. "More vibration is not more
scary to the eggs, it's not that simple," says Warkentin.
It is still unknown how fetal tadpoles sense vibrations, but
Warkentin suspects that they may be using their lateral lines. These
hairlike receptor cells on the skin of tadpoles and fish are
sensitive to water movement.
But why wait until a snake attacks before hatching, instead of
sensing, say, the vibration from a snake approach? "A snake can
sit there looking at the egg mass for an hour and nothing would
happen," says Warkentin. The problem may be that once the
tadpoles hatch, they are not out of the woods. In the water, they
face a host of new predators, such as shrimp and insects. Preemie
tadpoles are not only three-quarters of the size of a
fully-developed tadpole, but have less-developed respiratory,
gastrointestinal and possibly nervous systems, all of which make it
harder for them to escape predators.
The eggs therefore need to be absolutely certain that they are under
attack before they hatch, trading this off with the risk of being
taken in the first mouthful. The eggs begin to hatch on average 16
seconds after the first bite, taking their time to sample the signal
for all the components that confirm that this is a deadly event. A
snake attack is several minutes long, so most tadpoles still have
time to escape.
Warkentin says that more than a dozen amphibian species show induced
hatching, as do some fish, one species of spider and possibly some
crustaceans. "We never used to ask these kinds of questions
about eggs," says Warkentin. "Over the last 10 years, we
have discovered a number of ways that embryos can detect information
about their environment and respond to it, and we still don't know
the full range of the kinds of things eggs can do."