Fig. 1.

Endocrine basis of species differences in larval period and plasticity. a Spadefoot toads have evolved the largest differences in development rate among anurans. Old World spadefoot toads (Pelobates spp.) tend to breed in long lasting ponds and have a relatively longer larval period. The closely related New World species have evolved faster developmental rates, and S. couchii in particular shows the fastest developmental rate known in anurans. Phylogenetic relationships among spadefoot genera with corresponding larval period durations and a typical pond for each genus are shown. b Simulated pond drying initiated at Gosner stage 35 (early prometamorphosis) and maintained until measurement at Gosner stage 42 (metamorphic climax) induced marked developmental acceleration in P. cultripes (in yellow), an intermediate response in S. multiplicata (in blue), and no significant response in S. couchii (in red). Boxes in the boxplots indicate the median and the upper and lower quartiles, whereas the whiskers indicate the minimum and maximum values excluding outliers. Sample size was 10 for all species and treatments. c, d Developmental acceleration in P. cultripes in response to simulated pond drying is regulated by c increased thyroid hormone (TH; p = 0.00019) and d increased corticosterone (CORT; p = 0.0027). We observed a similar degree of CORT increase regulating adaptive plastic responses in S. multiplicata (d; p = 0.00083). However, S. couchii exhibited a higher and invariant tissue content of both hormones c, d underlying its minimal adaptive developmental plasticity. Simulated pond drying was initiated at Gosner stage 35 (early prometamorphosis), and hormone tissue contents were assessed using tadpoles at Gosner stage 38 (late prometamorphosis). Sample sizes were 7 and 9 for TH in P. cultripes in high and low water levels, and 10 for S. couchii for both treatments. Sample sizes for CORT data were 9 and 8 for high and low water levels in P. cultripes and 10 for the other species and treatment combinations. Asterisks indicate statistically significant differences between tadpoles within a species raised in high vs. low water. Photo credits: I. Gomez-Mestre