Encyclopedia Britannica, Inc.
1989 Britannica Book of the Year

Life Sciences

Detailed observations and experimental studies of animals during 1988 revealed unusual and unanticipated phenomena, serving as a continuing reminder of the complexity of natural systems. In addition, fossil finds and interpretations advanced scientists' understanding of evolutionary possibilities and phylogenetic relationships.

A study of marbled salamanders (Ambystoma opacum) by David E. Scott of the University of Georgia's Savannah River Ecology Laboratory uncovered important connections between the conditions a larval salamander experiences in the aquatic habitat and the acquisition of adult traits related to reproductive output. Manipulation of larval densities in large field enclosures at several breeding sites over a three-year period demonstrated that larvae reared at low density under natural conditions grow faster than larvae reared at high density. They also attain larger body size, have a shorter larval period and higher probability of survival, and emerge from the pond with more body fat. The effects were due, in part, to differences experienced by the two groups in competing for limited food resources. In laboratory studies salamanders from low-density enclosures survived longer than those from high-density enclosures. In field populations adults from low-density larval environments returned to breeding sites at a larger size and earlier age, two traits associated with increased reproductive output. Variation in the number of days a breeding site holds water also exerts effects on larval traits that carry over to adults. The effects are most pronounced in drought years, when body size at metamorphosis is small and survivorship is low.

-- by J. Whitfield Gibbons