Spatial and Temporal Instability of Local Biotic Community Mediate a Form of Aposematic Defense in Newts, Consisting of Carotenoid-Based Coloration and Tetrodotoxin

Abstract

Most animals advertise their unprofitability to potential predators via conspicuous signals. Whether the strength of this aposematic signal indicates the quality and quantity of chemical defenses in animals is controversial. Here, we investigated the relationship between the conspicuousness of an aposematic signal and toxicity, which likely depends, at least in part, on dietary sources, in the newt Cynops pyrrhogaster. Our results indicate that the magnitude of the aposematic signal was not correlated with the amount of tetrodotoxin (TTX) and 6-epi TTX of wild individuals among populations. Using atoxic newts, reared from eggs, we compared the ability to accumulate TTX from diets between mainland and island populations. Newts of a mainland population that exhibited a less conspicuous signal accumulated more TTX than did equivalent newts of an insular population that displayed a more conspicuous signal; this was unrelated to variation in the toxicity of wild individuals of these two populations. We also found toxicity of wild newts changed over approximately one generation (10 years) in both populations. These results indirectly suggest that environmental variance, such as fluctuations in TTX resources in nature, may obscure differences in the ability of wild newts to accumulate TTX, and that this variation may be responsible for a lack of correlation between the strength of a newt's signal and its toxicity in the wild. These results imply that toxicity of wild individuals likely is a phenotypic trait largely dependent on environmental conditions.

The conspicuous postmetamorphic coloration of fire salamanders, but not their toxicity, is affected by larval background albedo

“…Whether the conspicuous coloration of fire salamanders is under natural selection by serving as an aposematic signal remains to be tested, for example, by using clay models in the field, in which not only the proportions of black and yellow but also the color pattern (stripes, spots) should be taken into consideration (e.g., Preißler & Pröhl, 2017). In other amphibians, a correlation between coloration and toxicity has been found at the species (e.g., Summer & Clough, 2001) and population levels (e.g., Maan & Cummings, 2012) but to our knowledge, no evidence of quantitative honesty has been found within a population of amphibians (Mochida et al 2013;Stuckert, Saporito, & Summers, 2018). Although the intra-population variation in alkaloid content found herein was high, we encourage that future studies explore population differences in the relationship between coloration and toxicity of fire salamanders.…”

An Empirical Test Indicates Only Qualitatively Honest Aposematic Signaling Within a Population of Vertebrates

“…Therefore, predators apparently do not use frog brightness as an indicator of toxicity to adjust their attack probability. This is similar to newts (Cynops pyrrhogaster), which do not signal honestly within populations (Mochida et al, 2013). Therefore, although evidence suggests there is general quantitative honesty across vertebrate species (e.g., Summers and Clough, 2001), quantitative honesty likely does not occur within populations, and likely varies extensively across populations (Daly and Myers, 1967;Wang, 2011;Maan and Cummings, 2012).…”

“…Studies investigating the relationship between signal level and toxicity across populations have found mixed results (e.g., Daly and Myers, 1967;Wang, 2011;Maan and Cummings, 2012;Arenas et al, 2015), but there seems to be a more consistent positive relationship between signal and toxicity across species (e.g., Summers and Clough, 2001;Cortesi and Cheney, 2010;Arenas et al, 2015). The only test of quantitative honesty within a vertebrate population found no evidence of quantitative honesty in aposematic newts (Mochida et al, 2013). So the issue of within-population relationships is particularly pertinent, because many insects (e.g., lepidopterans) acquire their toxicity as larvae before metamorphosing into adults (Duffey, 1980), whereas in many vertebrate aposemes, defense is acquired during either development and/or throughout later life (e.g., dendrobatid poison frogs: Daly et al, 1994;other poison frogs: Jeckel et al, 2015;newts: Hanifin and Brodie, 2002;snakes: McCue, 2006;mammals: Newman et al, 2005;Hunter, 2009).…”

Sex differences but no evidence of quantitative honesty in the warning signals of six-spot burnet moths (Zygaena filipendulaeL.)*

“…, ] and the Japanese fire‐bellied newt, Cynops pyrrhogaster [Mochida et al. ]) have previously found either no relationship or negative correlations between aposematic signals and the strength of the defences they are advertising, including both within and between populations of single species of poison frogs (Daly and Myers 1967; Wang ; Crothers et al. ).…”

“…) or sequestration ability (Mochida et al. ). However, more empirical work is needed to truly understand when and why honest signals may or may not be observed in nature.…”