As far as we know, humans and marine mammals are the only mammals that modify what they say in response to
what they hear. The vocal repertoire of most mammals is essentially inherited, and learning plays little, if any, part
in the sounds they produce--hybrid monkeys make hybrid calls and even deaf kittens make normal meows.
remains unknown, however, about the role of learning in the development of the whistles that wild dolphins use to
communicate with one another.
From captive animals, researchers discovered decades ago that a dolphin can whistle on the day it is born. These
early whistles are quavery and variable. Before it is a year old, however, each dolphin develops its own distinctive
Dolphin whistles rise and fall in frequency like a tune, but they last only a second or so, and the frequencies are
more than ten times higher than a dial tone. Humans must slow down recordings of dolphin whistles to hear them
Studying whistle development in the wild has proved challenging, for marine biologists. Dolphins can whistle
underwater without exhaling bubbles. This trick comes in handy for them allowing them to recycle air between
breaths but frustrates researchers trying to tell who's whistling.
In the coastal waters off Sarasota, the Dolphins home ranges are small; researchers can set out each day to look
for particular animals and be reasonably sure of finding them. Still, determining which is the whistling dolphin in a
free-swimming group is seldom possible, so marine biologists temporarily restrain the animal in a net corral, attach
underwater microphones, or hydrophones, to their heads with suction cups, and record them.
Over the years, recordings have confirmed that, like their captive cousins, wild dolphins develop signature
whistles, and that the signatures stay the same for more than a decade. Mother and calf Dolphins usually start
exchanging whistles as soon as they are out of each other's sight.
Male Dolphins shape their whistles to become like those of their mother; females modify theirs to become distinct
from hers. These gender differences may reflect differences in the lives male and female calves will lead. For most
of their lives, female Dolphins associate primarily with members of their own matrilineal group.
If several females within a group had similar whistles, a young calf might have trouble maintaining contact with its
mother. Males, in contrast, eventually leave their natal group, so there may be little advantage in developing
whistles different from a mother's; they may even benefit from sounding like her.
Recognition of mother-son relationships could limit inbreeding. Also, adult males form coalitions that compete with
other males for access to females; having similar whistles could help brothers recognize one another during
interactions within and between coalitions.
The duration and loudness of signature whistles may vary when a dolphin is alarmed, but the whistles still retain
their individual distinctiveness. Dolphins usually produce their own signature whistles, but' they occasionally make
different whistles. Some of these appear to be one-shot bursts of sound, but others are precise imitations of the
signature whistles of companions.
On land as in the sea, mammals are capable of recognizing one another, and very often the recognition is based, at
least in part, on voice. Slight variations in the vocal tracts of terrestrial animals lead to predictable differences in the
voices of individuals.
In water, however, and especially for diving animals, these involuntary characteristics of voice are not as reliable.
Vocal tracts are gas-filled cavities, and as an animal dives, these gases halve in volume for every doubling of
Since some parts of the vocal tract are more elastic than others, changes in volume will lead to changes in shape
that, in turn, may alter the sound. If dolphins and other diving animals do indeed rely upon individually distinctive
calls, and use their vocal tracts to produce them, then they may need to create the calls by learning to modify
acoustic features under voluntary control such as the frequency modulation of whistles.
Marine Biologists Study Mutualism- A relationship between two different species in
which they both derive survival benefits
Anemones Help Hermit Crabs Find Homes
Hawaiian fishermen had set traps for deepwater prawns and inadvertently captured hundreds of the anemone-
toting crabs. The anemone were attached to the thin golden shells of deep-sea crabs. They are among the most
extraordinary examples of co-evolution.
Several dozen species of hermit crabs carry sea anemones on their shells, and marine biologists had assumed that
the crabs use the anemones for protection. Several scientists wanted to test this hypothesis. They placed an
octopus in a tank with shallow-water Mediterranean hermit crabs lacking anemones. The octopus easily pulled the
crabs from their snail shells, devouring all six within a week. When they introduced an octopus to six crabs with
anemones, however, none were harmed. In a typical encounter, the octopus reached out toward a crab with one
arm, then extended another, then a third, but upon contact, the octopus abruptly withdrew its arms and tucked the
other five under its body as it scuttled away in pain.
The anemone atop the crab had stung the octopus so badly that it would probably never touch another hermit crab.
Like their cousins the jellyfish, sea anemones have tentacles that are studded with thousands of microscopic
stinging capsules, or nematocysts, which fire hollow, harpoon-like threads that may inject toxin into an attacker's
Hermit crabs carry anemones only when they are needed. Kept in predator-free tanks for months, crabs lightened
their load by abandoning their anemones. If a researcher introduces an octopus into the tank, the hermit crabs
without anemones will attempt to steal them from crabs that had them, and crabs with only a few anemones on
their backs would try to acquire more.
In this game of "musical chairs," crabs unlucky enough to end up without at least one sea anemone were eaten,
while the others were not. Researchers found that merely running water from a tank containing an octopus into
one with hermit crabs caused the crabs to begin snatching anemones from one another. The chemical cues that
warn of predators are usually present in the wild, so crabs in nature maintain this protective behavior.
Anemones deter other predators as well. Along the Atlantic and Gulf coasts of the United States, a species of box
crab preys on hermit crabs using a claw that is specially adapted for snipping open snail shells. Predatory crabs kill
about 80 percent of hermit crabs if they lack anemones but less than 10 percent of those bearing anemones.
Anemones Also Benefit From This Relationship
Most sea anemones seek a more or less permanent attachment on rocky areas of the sea floor where currents
bring them the food they need. A hermit crab's shell provides the needed foothold and frees an anemone from its
Mobility, in and of itself, is of no great benefit, but the crab takes its anemone to all the right places; crabs are
excellent scavengers, sensing food at great distances and scurrying to it before other scavengers, such as sea
urchins, isopods, or shrimp, can devour it. The anemone gets a free ride to the food, and as the crab tears away at
its meal, the anemone feasts on scraps that drift about in the water.
In the deep sea, the symbiotic relationship is more than advantageous; it is essential to the survival of both crab and
anemone. By producing shells for the hermit crabs, the anemones can live where the bottom is muddy or sandy,
with no place for firm attachment.
And the deep-sea hermits, which are among the largest of their kind, need not expend energy finding larger homes
as they grow. There are not many large snails in the deep sea, where food is notoriously scarce. Moreover,
calcium carbonate, of which snail shells are made, dissolves easily in the water of the ocean depths, so even if
snails were plentiful, their shells would not last as long as they would in shallow water. In the deep sea, the
anemones' "fake" shells are better than the real thing.
Endangered Sea Life
The endangered Kemp's ridley sea turtle makes its home off Mexico's Gulf Coast, near Rancho Nuevo. Some 500
miles north, at the National Marine Fisheries Service in Galveston, Tex., researchers working on a captive breeding
project hope to repopulate native waters with egg-laying ridley females.
That effort, however, has posed a dilemma for marine biologists because the although a harmless
radioimmunoassay for testosterone can reveal the gender of prepubescent ridley sea turtles with 90 percent
accuracy, the most reliable method of identifying the sex of newly hatched turtles would require killing them.
Waiting the necessary two years for the turtles to come of age for the radioimmunoassay would slow repopulation
efforts; releasing hatchlings of undetermined gender would leave researchers unable to guarantee an ample supply
Fortunately scientists developed an alternative based on genetic fingerprinting techniques. After fragmenting DNA
extracted from a small blood sample scientists apply a genetic probe that selectively binds to gender-specific DNA
fragments. The probe, a DNA sequence first isolated in a highly poisonous Asian snake, can distinguish sexes even
in hatchlings. It also works in the green sea turtle--a ridley relative--and it may assist gender identification in other
reptiles as well.
In a study of 30 ridleys whose sex had been determined by others using a different method, marine biologists
accurately identified the gender of 15 females and 14 males. They also identified the gender of nine of 10 green sea
turtles, the researchers report in a forthcoming.
The DNA probe may also lead to a more precise determination of the relationship between gender and incubation
temperature for ridley eggs. The relatively warm incubation environment produces predominantly female
hatchlings, while temperatures just 5°C to 8°C cooler produce mostly males. The probe should aid researchers in
determining whether turtles raised in hatcheries for release into the wild interfere with the normal ratio of male to
Saving ridleys from extinction may depend on precisely controlled re-population. These creatures face several
dangers in the wild: They often feed near drilling platforms, where underwater shock waves can kill them, and
many drown when entangled in shrimpers' nets.
Marine biologists study marine life to help save endangered species and to learn how mutualism is important when
one of the species that benefit from the relationship becomes scarce. They continue to learn new things about the
species inhabiting the oceans.
Dudzinski, K. M. 2008. Dolphin Mysteries: Unlocking the Secrets of Communication.
Karleskint, G. 2009. Introduction to Marine Biology.
Lasky, K. 2006. Interrupted Journey: Saving Endangered Sea Turtles.
Magby, M. 2012. Sea Anemones.
Phillips, P. 1988. The Great Ridley Rescue.