Age Estimates for a Population of American Toads, 
Bufo americanus (Salientia: Bufonidae), 
in Northern Virginia 
Heather J. Kalb 1 and George R. Zug 
Department of Vertebrate Zoology 
National Museum of Natural History 
Smithsonian Institution 
Washington, D.C. 20560 
ABSTRACT . — Age estimation by skeletochronology was made on 
adult female and male Bufo americanus from a small population in 
northern Virginia. This breeding population consisted predominantly 
of males 3-4 years old and females 4-5 years old. Size and age were not 
closely correlated, i.e. the larger toads were not necessarily older. 
Large males did not appear to have an advantage over the small in 
mating; average SVL of amplectic males (64.5 mm) was similar to that 
of calling males (64.8 mm). First breeding occurred in late March or 
early Arpil and appeared dependent upon water temperatures greater 
than 1 1°C in breeding ponds. 
An old technique for the estimation of age in amphibians and 
reptiles, skeletochronology, has recently gained renewed interest. The 
use of bone layers to estimate the ages of frogs or snakes has occurred 
irregularly since the early 1900s; however, the technique received 
considerable criticism because of the researchers’ inability to demonstrate 
that each bone layer represents the same time period. This criticism has 
been addressed by several European herpetologists (e.g. Castanet 1985), 
who have used known-aged or captive-raised animals to test the annual 
deposition of a single bone layer in elements of the appendicular 
skeleton. 
Hemelaar and van Gelder (1980) studied two populations of the 
European toad, Bufo bufo (L.) and demonstrated that a single periosteal 
layer was deposited each year. Their experimental demonstration was 
simple, but elegant. They marked toads so that each could be recognized 
individually and then removed bone samples in successive years. This 
procedure showed that an extra layer was present in the second year. In 
an attempt to repeat their experiment, Zug initiated a mark and 
recapture study of an American toad ( Bufo americanus Holbrook) 
population in a suburban park in northern Virginia. Owing to the 
transitory nature of the population and an irregular sampling regime, a 
Hemelaar/ van Gelder-type study could not be repeated, although we 
'Present address: Department of Biology, Texas A&M University, College 
Station, TX 77843 
Brimleyana 16:79-86, July 1990 
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