NOT ALL FAMILIES ARE THE SAME

by Whit Gibbons


November 28, 2004


The season 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.

First, the 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.

One fascinating 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 crowded conditions.

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?

The salamander 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."

An explanation 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.

Bill Amos 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.

Pilot whales 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.

Among pilot 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.

Ironically, 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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