Do Allegheny Mountain Dusky Salamanders (Desmognathus ochrophaeus) Avoid ‘Stressed’ Conspecifics?
The production of glucocorticoids by the hypothalamic-pituitary-interrenal axis of amphibians and reptiles allows individuals to respond to acute stressors, but chronic elevation can have negative fitness consequences. Amphibians offer a unique opportunity to study the potential effects of elevated glucocorticoids, such as corticosterone (CORT), as lipid-soluble glucocorticoids readily pass through the skin and into the bloodstream. Interestingly, glucocorticoids are also secreted and excreted by amphibians to such an extent that relative rates of CORT release have been used to assess the relative ‘health’ of populations. Given that CORT has the potential to be both absorbed and secreted through the skin of amphibians and may have a negative impact on fitness, I attempted to assess the degree to which salamanders could identify ‘stressed’ conspecifics whose CORT levels were experimentally elevated through the use of a dermal patch. Plethodontid salamanders provide a convenient opportunity to evaluate the discrimination of ‘stress’ levels in amphibians because they utilize well-developed vomeronasal organs to facilitate a range of behavior, and their plasma CORT levels are readily manipulated via transdermal delivery. I tested the ability of Desmognathus ochrophaeus to detect and avoid substrates sullied by conspecifics with elevated CORT. The test was conducted via a simple choice experiment in which male and female test subjects were exposed to substrates sullied by an individual with baseline levels of CORT or the same individual with experimentally elevated CORT. Overall, I found that females did not differentially respond to substrates sullied by ‘high-stress’ females or males. Males did not avoid substrates sullied by high-stress males but did spend less time than expected on substrates from high-stress females. The differential response by males toward stimuli from high-stress females was replicated in a second study conducted approximately seven months after the first. These findings suggest that male D. ochrophaeus are capable of discerning among females, but not males, in ‘high-stress’ and ‘low-stress’ states. Female salamanders did not respond differently to substrates based on the stress level of stimulus donors.
Diagrammatic representation of the collection of ‘low-stress’ and ‘high-stress’ substrates (1–3) and choice tests using sullied substrates (4). To collect low-stress stimuli, 1) each salamander was transferred from its home dish to a stimulus-collection dish and provided with a dermal patch with 1.5 μL of a blank (sesame oil) for 60 min. The patch was removed and each individual was returned to its home dish. To experimentally elevate ‘stress,’ 2) each salamander received a dermal patch saturated with 1.5 μL of CORT solution for 30 min. This application occurred while each salamander was retained in its home dish after the collection of the low-stress stimulus. To collect the high-stress stimulus, 3) each ‘stressed’ salamander was transferred to a new clean stimulus-collection dish using a cotton swab. Each salamander was free to move throughout the stimulus-collection dish for 60 min. To conduct choice tests, 4) two filter paper semicircles were placed on opposite sides of 15 cm test dishes while maintaining an approximately 3 mm gap between each. The two filter papers used as substrates for each test came from the same stimulus donor; one semicircle was provided from the low-stress substrate collection (LO), the other from the high-stress collection (HI).
Boxplots of time, in seconds, that (A) female and (B) male Allegheny Mountain Dusky Salamanders spent on ‘high-stress’ substrates sullied by female and male conspecifics. The horizontal line indicates the median, box edges the interquartile range (IQR), and whiskers 1.5 × IQR. Points represent individual measurements. An * indicates that the time salamanders spent on high-stress substrates was significantly less than the expected value of 600 sec if choices of substrate were random using one-sample Wilcoxon signed rank tests with continuity correction.
Boxplot of time, in seconds, that male Allegheny Mountain Dusky Salamanders spent on ‘high-stress’ substrates sullied by female conspecifics. The horizontal line indicates the median, box edges the interquartile range (IQR), and whiskers 1.5 × IQR. Points represent individual measurements. An * indicates that the time salamanders spent on high-stress substrates was significantly less than the expected value of 600 sec if choice of substrate was random using one-sample Wilcoxon signed rank tests with continuity correction.
Contributor Notes
Associate Editor: J. Lamb.