Variation in Testosterone and Corticosterone in Amphibians and Reptiles: Relationships with Latitude, Elevation and Breeding Season Length
Cas Eikenaar, Jerry Husak, Camilo Escallon, Ignacio T. Moore
The results for the amphibians in the study supported the hypotheses that testosterone and baseline corticosterone levels positively correlated with latitude of the study sites (Table 2). However, corticosterone was negatively correlated with elevation and there was no correlation between testosterone and elevation. In the lepidosaurs, there were no significant correlations between testosterone and corticosterone concentrations and elevation and latitude. For both Lepidosaurs and amphibians, both hormone concentrations were negatively correlated with breeding season length. For example, short breeding season amphibians in Tennessee often explosively breed (Scaphiopus, Rana, Bufo) and secondary sex characteristics (Figure 1) and calling behaviors are often mediated by high levels of testosterone (Darcy, 1980). Tropical amphibians with longer breeding seasons have lower testosterone levels primarily because there is less competition and selection pressure to breed at any given time.
Hormones are among the key players in endocrinology, physiology, morphology and behavior across all vertebrate taxa (Bentley, 2001) and this article demonstrates the differences of amphibian and reptile hormones based upon geographic distribution and breeding season lengths. I was surprised to say the very least that researchers would even consider latitude and elevation as factors regulating hormone concentrations in amphibians and reptiles. Hormones such as testosterone are produced in the testes of males but I would have never imagined that breeding season length would have any influence on the concentrations observed. Regardless of my expectations, I found this article to be not only relevant for hormonal observation but the authors did a great job of integrating their observations into several fields of study for herps.
The authors explain the relevance of hormones in life history traits and emphasize the observed differences in testosterone and corticosterone levels when amphibians and reptiles partake in social interactions such as male-male competition, courtship and parental care. The hypotheses tested in the study were that in amphibians and reptiles, testosterone and baseline corticosterone concentrations would be positively correlated with latitude and elevation and negatively correlated with breeding season length. The authors provide good examples for justification including these observations as seen in birds. Breeding seasons in temperate habitats are much shorter and therefore much more energy is invested in a shorter breeding season because of the pressures to find a mate and there is more competition because these breeding seasons are usually synchronized. Observed differences in body size, clutch size and age of reproductive maturity have been documented in amphibians and reptiles at different latitudes and elevations so in my opinion these hypotheses have good justification with a strong literature base.
The authors combined the mean hormone concentrations of amphibians and reptiles from 163 studies worldwide and obtained all topographic information for all the collection sites. I found it interesting that the authors themselves did not mention collecting samples for this publication. Many of the samples were only collected from individuals during one breeding season and were not collected from the same sites over several seasons so perhaps the data used was not entirely representative of all reptile and amphibian hormone concentrations at different geographic distributions and breeding season lengths. The authors also only analyzed Lepidosaurid reptiles and so I think they should specify that their findings only apply to this taxa because the mating systems and geographic distributions of reptiles vary greatly. However, the authors used complex statistical analysis that was easy to interpret if you have any knowledge of p values and correlation coefficients. The samples were consistent as well and only those collected from free-living males that were calling (amphibians) or displaying territory defense (reptiles) were used.
The results for the amphibians in the study supported the hypotheses that testosterone and baseline corticosterone levels positively correlated with latitude of the study sites (Table 2). However, corticosterone was negatively correlated with elevation and there was no correlation between testosterone and elevation. In the lepidosaurs, there were no significant correlations between testosterone and corticosterone concentrations and elevation and latitude. For both Lepidosaurs and amphibians, both hormone concentrations were negatively correlated with breeding season length. For example, short breeding season amphibians in Tennessee often explosively breed (Scaphiopus, Rana, Bufo) and secondary sex characteristics (Figure 1) and calling behaviors are often mediated by high levels of testosterone (Darcy, 1980). Tropical amphibians with longer breeding seasons have lower testosterone levels primarily because there is less competition and selection pressure to breed at any given time.| Figure 1. Secondary sexual characteristic common in male amphibians of temperate regions. Nuptial pads are used to grasp females during amplexus. |
The best part of this article is the discussion. The authors demonstrate the possible causes for the observed differences in hormone concentrations relative to latitude, elevation and breeding season length in amphibians and reptiles. The results from this article can be used as a model for herpotologists from many backgrounds to predict the variations between and within species that have wide geographic distributions. For example, a researcher could potentially predict the differences in secondary sexual characteristics between several species of Bufo by analyzing hormone concentrations. Although after reading the article, I think that there is too much ambiguity to make any solid conclusions about hormone correlations and latitude, elevation and breeding season length for reptiles and more samples from multiple seasons and taxa are needed. The authors do admit that there may have been sampling error so I think this part of the study is to be continued. Despite some minor issues, it is science and this article still holds a good amount of relevant information. This article not only ties together physiology, behavior, endocrinology and ecology & evolution, but it provides a large scale understanding of the hormonal mechanisms underlying life history traits in amphibians and reptiles.
References:
1. Variation in Testosterone and Corticosterone in Amphibians and Reptiles: Relationships with Latitude, Elevation, and Breeding Season Length.
The American Naturalist , Vol. 180, No. 5 (November 2012), pp. 642-654
Article Stable URL: http://www.jstor.org/stable/10.1086/667891
2. Kelley, Darcy B. "Auditory and vocal nuclei in the frog brain concentrate sex hormones." Science (New York, NY) 207.4430 (1980): 553.
3. Bentley, P. J. "Sex Hormones in Vertebrates." eLS (2001).
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