Sexual Dimorphism in Visceral Organ Mass and Hematology in Spotted Salamanders, Ambystoma maculatum, before and after Breeding
Despite having relatively little dimorphism in external morphology, male and female Ambystoma maculatum have substantial differences in both short-term (e.g., resting metabolic rate) and long term (e.g., growth rate) physiological processes. In this study, I investigated differences in the mass of reproductive (gonads and proximal reproductive tract) and somatic (liver and heart) organs, and in hematological parameters related to oxygen carrying capacity (erythrocyte count, hematocrit, and hemoglobin concentration) and osmotic balance (plasma osmolality) between males and females before and after breeding. Pre-breeding females had substantially larger ovaries and oviducts compared to post-breeding females, whereas males had similar reproductive organ masses before and after breeding. Females had smaller livers than males, but similar heart masses. Males had similar erythrocyte counts to females, but higher hematocrits, hemoglobin concentrations, and plasma osmolalities. The internal anatomical and physiological differences between males and females suggest greater investment in gametogenesis in females and higher performance in aquatic environments in males. These findings highlight the importance of physiological dimorphisms in organisms where there is little apparent difference in external morphology between the sexes.
Comparisons of log total length and log dry carcass mass between male and female Spotted Salamanders before and after breeding. Data are presented as mean±SEM. Filled circles represent pre-breeding animals; open circles represent post-breeding animals.
Variation in log-transformed dry somatic organ mass (livers and hearts), dry reproductive organ mass (gonads and reproductive tracts), and carcass water content with log dry carcass mass in male and female Spotted Salamanders before and after breeding. Filled circles = pre-breeding females; open circles = post-breeding females; filled triangles = pre-breeding males; open triangles = post-breeding males. Linear regression models are provided in instances where correlations with log dry carcass mass were significant per unvariate ANCOVA models.
Comparisons of hematology parameters (RBC count, Hct, [Hb], and plasma osmolality) between male and female Spotted Salamanders before and after breeding. Data are presented as least-squares means+SEM. Filled columns = pre-breeding animals; open columns = post-breeding animals. For RBC counts, like letters indicate no significant difference between sex × reproductive condition groupings (P ≤ 0.0125, Bonferroni adjusted t-test). For Hct, [Hb], and plasma osmolality, the results of comparisons of groups within each class variable (sex and reproductive condition) are indicated as follows: “M > F” = the mean value for males is significantly greater than that for females (P ≤ 0.05); Pre > Post = the mean value for pre-reproductive animals is significantly greater than that for post-reproductive animals (P ≤ 0.05); “Pre NSD Post” = there was no significant difference in the mean values for pre- and post-reproductive animals (P > 0.05). See text for F-statistic values.
Contributor Notes
Associate Editor: K. Martin.