Digit ratio in the common toad Bufo bufo: the effects of reduced fingers and of age dependency

Abstract

Introduction: Despite the growing number of studies describing digit ratio patterns in tetrapods, knowledge concerning certain basic issues is still scarce. In lower vertebrates such as tailless amphibians (Anura), the numbering of individual fingers on the forelimbs and their homology with the fingers of other vertebrates pose an unsolved problem. Based on reviewed data on anuran limb development, we argue that the correct finger numbering scheme should be based on the assumption that the first finger, not the fifth finger, was reduced on the forelimbs. We analyzed the digit ratio in the common toad (Bufo bufo, Bufonidae), a species characterized by well-developed sexual dimorphism whereby females are larger than males, using both numbering schemes present in the literature.

Results: We found that the digit ratio on hindlimbs differed significantly between the sexes only in the cases of left 2D:3D, with lower digit ratios in females, and of left 3D:4D, with lower digit ratios in males. We found that sex was the only significant variable for forelimbs, differentiating 2D:3D on the left forelimb, with lower digit ratios in females; 2D:4D on the right forelimb, with lower digit ratios in males; and 3D:4D on both forelimbs, with lower digit ratios in males. These results relate to variant II reflecting the hypothesis that the first digit was reduced during phylogenesis. There was no relationship between the body size (SVL) of individuals and any digit ratio, excluding 2D:4D on the right forelimbs in models with age variables. Additionally, for a subset of data where individual age was known, the models indicated that age was linked to significant differences in 2D:4D and 3D:4D on the left hindlimbs, while age, SVL, and sex influenced 2D:4D on the right forelimbs.

Conclusion: We emphasize the importance of the problem of the correct numbering of forelimb digits in Anura and, under the assumption that it was the fifth digit that was reduced, argue that earlier results on digit ratio in this group should be interpreted with caution. The detected relationship between digit ratio and age in amphibians expands our knowledge, indicating that the age of individuals should be included in future digit ratio studies. This relationship may also apply to studies using digit ratio as a noninvasive indicator of endocrine disruption in amphibians.

Keywords: 2D:3D; 2D:4D; 3D:4D; Anura; Digit identity; Morphology; Sexual dimorphism.

Fig. 1

Two alternative finger numbering protocols for anuran forelimbs, reflecting the hypotheses that the fifth (variant I) or the first (variant II) digit was reduced during phylogenesis. The upper and lower parts of the figure show which fingers are used to measure digit ratios in each of the adopted protocols, with the results presented in the form of box plots. It should be noted that 2D:3D in variant I corresponds to 3D:4D in variant II; similarly, 2D:4D corresponds to 3D:5D, and 3D:4D corresponds to 4D:5D. Although variant I is predominantly used in research, it is variant II that probably reflects the actual finger numbering in anurans (see in text: Limb and finger development in anurans). Asterisks indicate the statistical significance level of the difference in digit ratios between sexes (gray box plots - male; white box plots – female; levels of significance: ***p < 0.001, **p < 0.01). The illustration of an anuran forelimb was adapted from [27]

Fig. 2

Panel a: relationship between size (SVL, snout-vent length) of the common toad Bufo bufo and right forelimb 2D:4D (variant II) for each sex (for each individual, SVL was centered around the mean size for its sex); Panel b: relationship between age (determined using skeletochronology) and right forelimb 2D:4D (variant II) for each sex; Panel c: relationship between age and left hindlimb 2D:4D for each sex; Panel d: relationship between age and left hindlimb 3D:4D for each sex. Age results were obtained from the second dataset containing only individuals from site 2 (N = 99; individuals after 3, 4, 5, 6, 7, or 8 hibernations. For more details, see Fig. S1)

Fig. 3

Phenotypic integration: relationship between the sexual dimorphism index (SDI) of body size (SDI_SVL, snout-vent length) and digit ratio (SDI_{digit ratio}) between three study sites marked as squares (panels: a, b) and four age cohorts marked as dots (panels: c, d). The digit ratio used was as follows: Panels a, c – right forelimb 2D:4D (variant II); Panels b, d – left hindlimb 3D:4D. Age results were obtained from the second dataset containing only individuals from site 2