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John M. Hranitz et al. 
mean size of the largest size class on the island decreased from 
1988 to 1989, and the smaller of the two size classes observed in 
1988 was not observed in 1989. Three size classes were observed 
on the mainland in 1988 (38-42, 45-48, and 52-57 mm) and in 
1989 (39-50, 54-62, and 66-76 mm) (Fig. 1). 
There were no juveniles on the island in 1988 or on the 
mainland in either year. Of the 111 juveniles captured on Assateague 
Island in 1989, 40 were from coniferous forests, and 71 were from 
meadows. Forest juveniles (25 8 ± 0.5 mm SVL) were significantly 
(P <> 0.05) larger than meadow juveniles (23.5 ± 0.4 mm SVL). 
Genetic diversity was low at each location, and there were no 
noteworthy differences in allele frequencies between the island and 
mainland toad populations at the seven loci studied. Six of seven 
enzyme loci ( Ap , Ldh- 4, Ldh- 5, Mdh- 2, Pgm- 1, and Pgm-2) were 
monomorphic in both populations. Mdh-1 had two alleles, MDH-1 100 
and MDH-1 105 , in each population. The frequency of MDH-1 100 was 
0.96 on the island and 0.12 on the mainland, and the frequency of 
MDH-1 105 was 0.04 on the island and 0.88 on the mainland. Poly- 
morphism (P) for toads' at both locations was 0.142. Heterozygosity 
(H) for MDH-1 was 0.08 on the island and 0.12 on the mainland. 
Mean heterozygosity (H) was 0.01 for island toads and 0.03 for 
mainland toads. Deviations from Hardy-Weinberg expectations at the 
Mdh-1 locus were not significant in the island (x 2 = 0.0434, P < 
0.01, 1 df) or mainland (x 2 = 1.469, P < 0.01, 1 df) samples. 
DISCUSSION 
Differences between the size structure of island and mainland 
populations of animals (Edgren 1961, Carlquist 1974, Gibbons et al. 
1979, Scott 1986) and between adjacent mainland toad populations 
(Oldham 1985) are not unusual. Our results show differences in 
body size between toads of Assateague Island and the adjacent main- 
land, providing an example of an insular population of amphibians 
with smaller body sizes than mainland conspecifics. However, the 
differences between the island and mainland populations of toads 
occurred within a narrow window of time in which mainland samples 
of toads were small; thus, further comparative study is required. 
Reasons for smaller body sizes of island animals are reviewed 
by Carlquist (1974). Small immigrants might be more successful at 
colonizing an island, smaller body size might be in response to 
selective predator pressures, or small body size might be an adapta- 
tion to smaller food objects or food supplies. Although the eastern 
