Positive Relationship between Abdominal Coloration and Dermal Melanin Density in Phrynosomatid Lizards

Phrynosomatid lizards show considerable variation among species in the occurrence of a secondary sexual trait, blue abdominal coloration. The production of blue skin may be controlled by at least two cellular components, melanin in melanophores, and guanine in iridophores. To examine the hypothesis that a mechanism producing variation in abdominal coloration is variation in the presence of melanin in the melanophores within the dermal layer of skin, we used light microscopy to compare melanin density of five species of phrynosomatid lizards with ancestral and derived abdominal coloration. Our results show that the skin of adults with blue abdominal coloration has more dermal melanin than white skin regardless of species or sex. We experimentally tested this relationship by examining the dermal melanin in skin from female Sceloporus undulatus consobrinus with exogenously elevated levels of testosterone or 5α-dihydrotestosterone. These females displayed malelike abdominal coloration and malelike melanin density. Our results suggest that melanin density plays a role in the presence of blue abdominal coloration in these phrynosomatid lizards.

Fig. 1.

A melanophore from a female Sceloporus jarrovii jarrovii. (A) In this orientation, the classic stellate nature of the cell is readily apparent. (B) Processes of the melanophore can be seen extending up from the more basal dermal layer into more superficial layers

Fig. 2.

Comparison 1. Are there sex differences in melanin density in sexually dimorphic species? Males with blue abdominal coloration have a higher density of melanin than female lizards with white abdominal coloration

Fig. 3.

Comparisons 3 and 4. Are there within species differences in melanin density? No difference in melanin density was detected when males and females were compared in the monomorphic-blue species (Sceloporus jarrovii jarrovii) or the monomorphic–white species (Sceloporus virgatus or Uta stansburiana)

Fig. 4.

Comparison 5. Do mascinulized females have the same melanin density as males? Female Sceloporus undulatus consobrinus implanted with either testosterone or DHT have similar levels of melanin compared to nonmanipulated male S. u. consobrinus and differ from female S. u. consobrinus implanted with a blank

Fig. 5.

Comparison 5. Do masculinized females have the same melanin density as males? Images were photographed at 40×. The arrow is pointing to the melanin layer in male Sceloporus undulatus consobrinus (A) and female S. u. consobrinus implanted with testosterone (B) or 5α-dihydrotestosterone (C), and to the lack of melanin in female S. u. consobrinus (D). The scale bar is 50 μm

Contributor Notes

Department of Life Science, Indiana State University, Terre Haute, Indiana 47809. Present address: (VSQ) United States Forest Service, 5985 Highway K, Rhinelander, Wisconsin 54501. (VSQ)vquinn@fs.fed.us Send reprint requests to VSQ.

Section editor: C. Guyer.

Accepted: 04 Jun 2003