ALL FAMILIES ARE THE SAME
by Whit Gibbons
November 28, 2004
between Thanksgiving and Christmas brings to the forefront the importance
of families and the interactions among family members. In ecology, many
studies address situations in which animals modify their behavior when
they belong to a family unit. In fact, the nature of the family is so
vital that a common theme can be found even between cannibalism in salamanders
and the social structure of whales.
salamanders. Tiger salamanders are native to more than thirty states,
ranging from Delaware to Florida and from Washington State to New Mexico.
Having a typical amphibian life cycle, the adults live on land and migrate
to wetlands to breed. The young, called larvae, have gills and live in
the water, eating small aquatic animals. The juveniles eventually metamorphose
and leave the water to take up permanent residence on land.
aspect of ecology is the variability that exists within species. In some
populations of tiger salamanders, the larvae become cannibalistic, feeding
on other tiger salamanders. The cannibal salamanders are larger than noncannibalistic
ones and have specialized structures in the mouth that aid in eating other
salamanders. Cannibalism occurs most frequently when larvae develop under
As if such
a lifestyle were not intriguing enough, studies by David W. Pfennig of
Cornell and James P. Collins of Arizona State University have unveiled
another facet of tiger salamander biology. Salamanders reared in genetically
unrelated groups are more likely to develop into cannibals than are groups
of siblings. That is, tiger salamanders that are brothers and sisters
are less likely to eat each other. How do they know who their siblings
are, and what does this have to do with whales?
experiments involved placing larvae in various groups. Some groups were
all siblings and some were unrelated. All larvae, whether related or not,
were of the same size, so the variation in body size could not be used
as a cue to whether larvae had developed from the same set of eggs. Instead,
the investigators hypothesize that larval salamanders release chemical
cues that can be used to distinguish close kin from others. Presumably,
genetically similar salamanders have a similar "smell."
of the ability of larval salamanders to detect--and not eat--their siblings
resides in evolutionary theory. Briefly, this predicts that organisms
can increase their own genetic success by helping their relatives. Obviously,
eating your brother, who shares many of the same genes, would not help
achieve this goal. The whale phenomenon, although of a different nature
from salamander cannibalism, is also explained by evolutionary theory.
of Cambridge University and colleagues Christian Schlotterer and Diethard
Tautz of the University of Munchen in Germany study the biology of long-finned
pilot whales in the Faeroe Islands, located between Iceland and the British
Isles. Pilot whales don't cannibalize each other, but some males exhibit
a behavior that is different from what is considered normal.
form large social groups called pods. The investigators used molecular
techniques to establish that pod members were closely related, forming
an extended family. A pod normally has more adult females than males.
In most mammal species in which females live in groups, males disperse
from their homesite before they become breeding adults, avoiding genetic
inbreeding. The departure of young males also reduces the competition
that they would face against larger and more experienced older males.
whales, however, many of the males do not leave their family pod, which
may number more than a hundred. Instead they remain with it. But genetic
studies revealed that males in a pod rarely or never breed with the females
of the same pod, who might be their mothers or sisters. Mating is presumably
carried out when different pods encounter each other in the ocean. No
one knows exactly how the nonbreeding males "help" their relatives
in a pod, but defense from marine predators or assistance in a communal
feeding effort have been suggested.
from a conservation perspective the cohesive family structure of long-finned
pilot whales makes them prey to the most dangerous predator of all, humans.
Hundreds of these whales are killed each year because boats can be used
to herd pods into coastal areas, which means the whole family can be eliminated.
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