The occurrence of defensive alkaloids in non-integumentary tissues of the Brazilian red-belly toad Melanophryniscus simplex (Bufonidae)

Impact of commercial formulations of herbicides alone and in mixtures on the antioxidant system and body condition parameters in tadpoles of Rhinella icterica (Spix 1824)

Pesticide usage has increased over the last decades, leading to concerns regarding its effects on non-target organisms, especially amphibians. Tadpoles of Rhinella icterica were collected in a pesticide-free place, acclimated in the laboratory (21 days) and exposed (7 days) to three herbicides (20µg/L atrazine (A); 250µg/L glyphosate (G); 20µg/L quinclorac (Q)) and their mixtures. Only 2% mortality was observed over the 28 days of the study. Despite this, significant variations were observed for markers of oxidative balance and body condition when comparing all experimental groups. K and Kn factor showed the lowest values in the group A+G+Q, as well as the activity levels of GST and SOD. In contrast to this, the CAT activity was higher in the same group (A+G+Q). The mixture of the three herbicides proved to be more harmful, which points to the need for more restrictive laws for the use of mixed herbicides.

Maternal Provisioning of Alkaloid Defenses are Present in Obligate but not Facultative Egg Feeding Dendrobatids

Poison frogs sequester alkaloid defenses from a diet of largely mites and ants. As a result, frogs are defended against certain predators and microbial infections. Frogs in the genus Oophaga exhibit complex maternal care, wherein mothers transport recently hatched tadpoles to nursery pools and return regularly to supply developing tadpoles with unfertilized (nutritive) eggs. Developing tadpoles are obligate egg feeders. Further, female O. pumilio and O. sylvatica maternally provision their nutritive eggs with alkaloid defenses, providing protection to their developing tadpoles at a vulnerable life-stage. In another genus of poison frog, Ranitomeya, tadpoles only receive and consume eggs facultatively, and it is currently unknown if mothers also provision these eggs (and thus their tadpoles) with alkaloid defenses. Here, we provide evidence that mother frogs of another species in the genus Oophaga (Oophaga granulifera) also provision alkaloid defenses to their tadpoles. We also provide evidence that Ranitomeya imitator and R. variabilis eggs and tadpoles do not contain alkaloids, suggesting that mother frogs in this genus do not provision alkaloid defenses to their offspring. Our findings suggest that among dendrobatid poison frogs, maternal provisioning of alkaloids may be restricted to the obligate egg-feeding members of Oophaga.

Egg toxic compounds in the animal kingdom. A comprehensive review

Covering: 1951 to 2022Packed with nutrients and unable to escape, eggs are the most vulnerable stage of an animal's life cycle. Consequently, many species have evolved chemical defenses and teamed up their eggs with a vast array of toxic molecules for defense against predators, parasites, or pathogens. However, studies on egg toxins are rather scarce and the available information is scattered. The aim of this review is to provide an overview of animal egg toxins and to analyze the trends and patterns with respect to the chemistry and biosynthesis of these toxins. We analyzed their ecology, distribution, sources, occurrence, structure, function, relative toxicity, and mechanistic aspects and include a brief section on the aposematic coloration of toxic eggs. We propose criteria for a multiparametric classification that accounts for the complexity of analyzing the full set of toxins of animal eggs. Around 100 properly identified egg toxins are found in 188 species, distributed in 5 phyla: cnidarians (2) platyhelminths (2), mollusks (9), arthropods (125), and chordates (50). Their scattered pattern among animals suggests that species have evolved this strategy independently on numerous occasions. Alkaloids are the most abundant and widespread, among the 13 types of egg toxins recognized. Egg toxins are derived directly from the environment or are endogenously synthesized, and most of them are transferred by females inside the eggs. Their toxicity ranges from ρmol kg^-1 to mmol kg^-1, and for some species, experiments support their role in predation deterrence. There is still a huge gap in information to complete the whole picture of this field and the number of toxic eggs seems largely underestimated.

Dose-dependent alkaloid sequestration and N-methylation of decahydroquinoline in poison frogs

Sequestration of chemical defenses from dietary sources is dependent on the availability of compounds in the environment and the mechanism of sequestration. Previous experiments have shown that sequestration efficiency varies among alkaloids in poison frogs, but little is known about the underlying mechanism. The aim of this study was to quantify the extent to which alkaloid sequestration and modification are dependent on alkaloid availability and/or sequestration mechanism. To do this, we administered different doses of histrionicotoxin (HTX) 235A and decahydroquinoline (DHQ) to captive-bred Adelphobates galactonotus and measured alkaloid quantity in muscle, kidney, liver, and feces. HTX 235A and DHQ were detected in all organs, whereas only DHQ was present in trace amounts in feces. For both liver and skin, the quantity of alkaloid accumulated increased at higher doses for both alkaloids. Accumulation efficiency in the skin increased at higher doses for HTX 235A but remained constant for DHQ. In contrast, the efficiency of HTX 235A accumulation in the liver was inversely related to dose and a similar, albeit statistically nonsignificant, pattern was observed for DHQ. We identified and quantified the N-methylation of DHQ in A. galactonotus, which represents a previously unknown example of alkaloid modification in poison frogs. Our study suggests that variation in alkaloid composition among individuals and species can result from differences in sequestration efficiency related to the type and amount of alkaloids available in the environment.

Melanophryniscus admirabilis tadpoles' responses to sulfentrazone and glyphosate-based herbicides: an approach on metabolism and antioxidant defenses

Melanophryniscus admirabilis is a frog endemic to the southern Atlantic Forest (Brazil), with restricted distribution and considered as critically endangered. The aim of this study was to evaluate possible alterations in biomarkers of metabolism (glycogen, proteins, and uric acid) and oxidative balance (superoxide dismutase, catalase, glutathione S-transferase, and lipoperoxidation) of tadpoles of Melanophryniscus admirabilis exposed to commercial herbicide formulations containing sulfentrazone (Boral® 500 SC: 130 and 980 μg a.i./L) and glyphosate (Roundup® Original: 234 and 2340 μg a.i./L). Mortality was not observed in any of the groups studied. Our results show that a 96-h exposure to the herbicides decreased glycogen levels, indicating increased energy demand for xenobiotic metabolism. Protein levels increased in the Boral group but decreased in the higher concentration of Roundup, and uric acid levels did not change significantly between the experimental groups. Lipoperoxidation decreased in the Boral group and in the higher concentration of Roundup. Decreased levels of superoxide dismutase in both treatments and of catalase in the lowest concentration of the herbicides were observed. Glutathione S-transferase activity increased in the Roundup group; this enzyme seems to be crucial in the metabolization of the herbicides and in the survival of the tadpoles. Our results suggest that M. admirabilis has a high antioxidant capacity, which guaranteed the survival of tadpoles. Nevertheless, exposure to pesticides could impose a serious risk to this species, especially considering its restricted distribution, habitat specificity, and high physiological demand to metabolize xenobiotics.

Pyridoxal-5'-phosphate-dependent bifunctional enzyme catalyzed biosynthesis of indolizidine alkaloids in fungi

Indolizidine alkaloids such as anticancer drugs vinblastine and vincristine are exceptionally attractive due to their widespread occurrence, prominent bioactivity, complex structure, and sophisticated involvement in the chemical defense for the producing organisms. However, the versatility of the indolizidine alkaloid biosynthesis remains incompletely addressed since the knowledge about such biosynthetic machineries is only limited to several representatives. Herein, we describe the biosynthetic gene cluster (BGC) for the biosynthesis of curvulamine, a skeletally unprecedented antibacterial indolizidine alkaloid from Curvularia sp. IFB-Z10. The molecular architecture of curvulamine results from the functional collaboration of a highly reducing polyketide synthase (CuaA), a pyridoxal-5'-phosphate (PLP)-dependent aminotransferase (CuaB), an NADPH-dependent dehydrogenase (CuaC), and a FAD-dependent monooxygenase (CuaD), with its transportation and abundance regulated by a major facilitator superfamily permease (CuaE) and a Zn(II)Cys₆ transcription factor (CuaF), respectively. In contrast to expectations, CuaB is bifunctional and capable of catalyzing the Claisen condensation to form a new C-C bond and the α-hydroxylation of the alanine moiety in exposure to dioxygen. Inspired and guided by the distinct function of CuaB, our genome mining effort discovers bipolamines A-I (bipolamine G is more antibacterial than curvulamine), which represent a collection of previously undescribed polyketide alkaloids from a silent BGC in Bipolaris maydis ATCC48331. The work provides insight into nature's arsenal for the indolizidine-coined skeletal formation and adds evidence in support of the functional versatility of PLP-dependent enzymes in fungi.

Transcriptomic Signatures of Experimental Alkaloid Consumption in a Poison Frog

In the anuran family Dendrobatidae, aposematic species obtain their toxic or unpalatable alkaloids from dietary sources, a process known as sequestering. To understand how toxicity evolved in this family, it is paramount to elucidate the pathways of alkaloid processing (absorption, metabolism, and sequestering). Here, we used an exploratory skin gene expression experiment in which captive-bred dendrobatids were fed alkaloids. Most of these experiments were performed with Dendrobates tinctorius, but some trials were performed with D. auratus, D. leucomelas and Allobates femoralis to explore whether other dendrobatids would show similar patterns of gene expression. We found a consistent pattern of up-regulation of genes related to muscle and mitochondrial processes, probably due to the lack of mutations related to alkaloid resistance in these species. Considering conserved pathways of drug metabolism in vertebrates, we hypothesize alkaloid degradation is a physiological mechanism of resistance, which was evidenced by a strong upregulation of the immune system in D. tinctorius, and of complement C2 across the four species sampled. Probably related to this strong immune response, we found several skin keratins downregulated, which might be linked to a reduction of the cornified layer of the epidermis. Although not conclusive, our results offer candidate genes and testable hypotheses to elucidate alkaloid processing in poison frogs.

Induction of phytoalexins and proteins related to pathogenesis in plants treated with extracts of cutaneous secretions of southern Amazonian Bufonidae amphibians

Cutaneous secretions produced by amphibians of the family Bufonidae are rich sources of bioactive compounds that can be useful as new chemical templates for agrochemicals. In crop protection, the use of elicitors to induce responses offers the prospect of durable, broad-spectrum disease control using the plant's own resistance. Therefore, we evaluated the potential of methanolic extracts of cutaneous secretions of two species of amphibians of the family Bufonidae found in the Amazon biome-Rhaebo guttatus (species 1) and Rhinella marina (species 2)-in the synthesis of phytoalexins in soybean cotyledons, bean hypocotyls, and sorghum mesocotyls. Additionally, changes in the enzyme activity of β-1,3-glucanase, peroxidase (POX), and polyphenol oxidase (PPO) and in the total protein content of soybean cotyledons were determined. In the soybean cultivar 'TMG 132 RR', our results indicated that the methanolic extract of R. guttatus cutaneous secretions suppressed glyceollin synthesis and β-1,3-glucanase activity and increased POX and PPO activities at higher concentrations and total protein content at a concentration of 0.2 mg/mL. On the other hand, the methanolic extract of R. marina cutaneous secretions induced glyceollin synthesis in the soybean cultivars 'TMG 132 RR' and 'Monsoy 8372 IPRO' at 0.1-0.2 mg/mL and 0.2 mg/mL, respectively. The methanolic extract of R. marina cutaneous secretions also increased the specific activity of POX and PPO in 'Monsoy 8372 IPRO' and 'TMG 132 RR', respectively, and decreased the activity of β-1,3-glucanases in 'Monsoy 8372 IPRO'. At 0.3 mg/mL, it stimulated phaseolin synthesis. The extracts did not express bioactivity in the synthesis of deoxyanthocyanidins in sorghum mesocotyls. The study in soybean suggests that the bioactivity in defense responses is influenced by cultivar genotypes. Therefore, these results provide evidence that extracts of cutaneous secretions of these amphibians species may contribute to the bioactivity of defense metabolites in plants.

Unraveling fine-scale habitat use for secretive species: When and where toads are found when not breeding

A good understanding of species-habitat associations, or habitat use, is required to establish conservation strategies for any species. Many amphibian species are elusive and most information concerning amphibian habitat use comes from breeding sites where they are comparatively easy to find and study. Knowledge about retreat sites is extremely limited for most species and for the greater part of the year. For such species, it is especially important to factor in detection probability in habitat analyses, because otherwise distorted views about habitat preferences may result, e.g., when a species is more visible in habitat type B than in A, even though A may be preferred. The South American red-belly toad, Melanophryniscus pachyrhynus, is a range-restricted species from Southern Brazil and Uruguay that inhabits open areas with rocky outcrops and is usually seen only during explosive breeding events. Here we studied the fine-scale habitat use of the red-belly toad outside of the breeding season to identify retreat sites and test for the importance of accounting for species imperfect detection, using Bayesian occupancy models. We identified shrub density and the number of loose rocks as important predictors of occupancy, while detection probability was highest at intermediate temperatures. Considering the harsh (dry and hot) conditions of rocky outcrops, shrubs and loose rocks may both work as important refuges, besides providing food resources and protecting against predation. Rocky outcrops have been suffering changes in habitat configuration and we identify nonbreeding habitat preferences at a fine scale, which may help to promote population persistence, and highlight the importance of accounting for imperfect detection when studying secretive species.

Loss of skin alkaloids in poison toads, Melanophryniscus klappenbachi (Anura: Bufonidae) when fed alkaloid-free diet

Toads of the genus Melanophryniscus contain toxic alkaloids sequestered from a diet of mainly ants and mites. Wild-caught specimens of M. klappenbachi were fed an alkaloid-free diet and their alkaloid profile was analyzed during 38 months in captivity. Individual alkaloid patterns varied considerably. Over time, the concentration of two alkaloids, pumiliotoxin 251D and 3,5-disubstituted indolizidine 195B, significantly declined, suggesting that the toads depend on continuous access to alkaloid-containing prey to maintain natural levels of their chemical defense.

Diversity within diversity: Parasite species richness in poison frogs assessed by transcriptomics

Symbionts (e.g., endoparasites and commensals) play an integral role in their host's ecology, yet in many cases their diversity is likely underestimated. Although endoparasites are traditionally characterized using morphology, sequences of conserved genes, and shotgun metagenomics, host transcriptomes constitute an underused resource to identify these organisms' diversity. By isolating non-host transcripts from host transcriptomes, individual host tissues can now simultaneously reveal their endoparasite species richness (i.e., number of different taxa) and provide insights into parasite gene expression. These approaches can be used in host taxa whose endoparasites are mostly unknown, such as those of tropical amphibians. Here, we focus on the poison frogs (Dendrobatidae) as hosts, which are a Neotropical clade known for their bright coloration and defensive alkaloids. These toxins are an effective protection against vertebrate predators (e.g., snakes and birds), bacteria, and skin-biting ectoparasites (e.g., mosquitoes); however, little is known about their deterrence against eukaryotic endoparasites. With de novo transcriptomes of dendrobatids, we developed a bioinformatics pipeline for endoparasite identification that uses host annotated RNA-seq data and set of a priori parasite taxonomic terms, which are used to mine for specific endoparasites. We found a large community of helminths and protozoans that were mostly restricted to the digestive tract and a few systemic parasites (e.g., Trypanosoma). Contrary to our expectations, all dendrobatid frogs regardless of the presence of alkaloid defenses have endoparasites, with their highest species richness located in the frog digestive tract. Some of these organisms (e.g., roundworms) might prove to be generalists, as they were not found to be co-diversifying with their frog hosts. We propose that endoparasites may escape poison frogs' chemical defenses by colonizing tissues with fewer alkaloids than the frog's skin, where most toxins are stored.

Sequestered Alkaloid Defenses in the Dendrobatid Poison Frog Oophaga pumilio Provide Variable Protection from Microbial Pathogens

Most amphibians produce their own defensive chemicals; however, poison frogs sequester their alkaloid-based defenses from dietary arthropods. Alkaloids function as a defense against predators, and certain types appear to inhibit microbial growth. Alkaloid defenses vary considerably among populations of poison frogs, reflecting geographic differences in availability of dietary arthropods. Consequently, environmentally driven differences in frog defenses may have significant implications regarding their protection against pathogens. While natural alkaloid mixtures in dendrobatid poison frogs have recently been shown to inhibit growth of non-pathogenic microbes, no studies have examined the effectiveness of alkaloids against microbes that infect these frogs. Herein, we examined how alkaloid defenses in the dendrobatid poison frog, Oophaga pumilio, affect growth of the known anuran pathogens Aeromonas hydrophila and Klebsiella pneumoniae. Frogs were collected from five locations throughout Costa Rica that are known to vary in their alkaloid profiles. Alkaloids were isolated from individual skins, and extracts were assayed against both pathogens. Microbe subcultures were inoculated with extracted alkaloids to create dose-response curves. Subsequent spectrophotometry and cell counting assays were used to assess growth inhibition. GC-MS was used to characterize and quantify alkaloids in frog extracts, and our results suggest that variation in alkaloid defenses lead to differences in inhibition of these pathogens. The present study provides the first evidence that alkaloid variation in a dendrobatid poison frog is associated with differences in inhibition of anuran pathogens, and offers further support that alkaloid defenses in poison frogs confer protection against both pathogens and predators.

Toxins and pharmacologically active compounds from species of the family Bufonidae (Amphibia, Anura)

ETHNOPHARMACOLOGICAL RELEVANCE

Among amphibians, 15 of the 47 species reported to be used in traditional medicines belong to the family Bufonidae, which demonstrates their potential in pharmacological and natural products research. For example, Asian and American tribes use the skin and the parotoid gland secretions of some common toads in the treatment of hemorrhages, bites and stings from venomous animals, skin and stomach disorders, as well as several types of cancers.

OVERARCHING OBJECTIVE

In addition to reviewing the occurrence of chemical constituents present in the family Bufonidae, the cytotoxic and biomedical potential of the active compounds produced by different taxa are presented.

METHODOLOGY

Available information on bioactive compounds isolated from species of the family Bufonidae was obtained from ACS Publications, Google, Google Scholar, Pubmed, Sciendirect and Springer. Papers written in Chinese, English, German and Spanish were considered.

RESULTS

Recent reports show more than 30% of amphibians are in decline and some of bufonid species are considered to be extinct. For centuries, bufonids have been used as traditional folk remedies to treat allergies, inflammation, cancer, infections and other ailments, highlighting their importance as a prolific source for novel drugs and therapies. Toxins and bioactive chemical constituents from skin and parotid gland secretions of bufonid species can be grouped in five families, the guanidine alkaloids isolated and characterized from Atelopus, the lipophilic alkaloids isolated from Melanophryniscus, the indole alkaloids and bufadienolides known to be synthesized by species of bufonids, and peptides and proteins isolated from the skin and gastrointestinal extracts of some common toads. Overall, the bioactive secretions of this family of anurans may have antimicrobial, protease inhibitor and anticancer properties, as well as being active at the neuromuscular level.

CONCLUSION

In this article, the traditional uses, toxicity and pharmacological potential of chemical compounds from bufonids have been summarized. In spite of being reported to be used to treat several diseases, neither extracts nor metabolites from bufonids have been tested in such illness like acne, osteoporosis, arthritis and other illnesses. However, the cytotoxicity of these metabolites needs to be evaluated on adequate animal models due to the limited conditions of in vitro assays. Novel qualitative and quantitative tools based on MS spectrometry and Nuclear Magnetic Resonance spectroscopy is now available to study the complex secretions of bufonids.

The relationship between poison frog chemical defenses and age, body size, and sex

INTRODUCTION

Amphibians secrete a wide diversity of chemicals from skin glands as defense against predators, parasites, and pathogens. Most defensive chemicals are produced endogenously through biosynthesis, but poison frogs sequester lipophilic alkaloids from dietary arthropods. Alkaloid composition varies greatly, even among conspecific individuals collected at the same time and place, with some individuals having only a few micrograms of one or a few alkaloids and others possessing >1 mg of >30 alkaloids. The paucity of alkaloids in juveniles and their abundance in adults suggests that alkaloids accumulate over time; however, alkaloid diversity is highly variable among adult poison frogs and has never been studied in relation to individual age. Using skeletochronology to infer individual ages and gas chromatography-mass spectrometry and vapor phase Fourier-transform infrared spectral analysis to identify the defensive chemicals of 63 individuals, we tested the relationship between defensive chemicals and age, size, and sex in the Brazilian red-belly toad, Melanophryniscus moreirae, a poison frog that possesses both sequestered alkaloids and the biosynthesized indolealkylamine bufotenine.

RESULTS

Adult females were, on average, older and larger than adult males. Juveniles were smaller but not necessarily younger than adults and possessed bufotenine and 18 of the 37 alkaloids found in adults. Alkaloid richness was positively related to age, but not size, whereas the quantities of sequestered alkaloids and bufotenine were positively related to size, but not age. Defensive chemicals were unrelated to sex, independent of size.

CONCLUSIONS

The relationship between alkaloid richness and age appears to result from the gradual accumulation of alkaloids over a frog's lifetime, whereas the relationship between the quantity of defensive chemicals and size appears to be due to the greater storage capacity of larger individuals. The decoupling of age and size effects increases the amount of individual variation that can occur within a population, thereby possibly enhancing anti-predator efficacy. Further, given that both richness and quantity contribute to the overall chemical defense of individual frogs, our results suggest that older, larger individuals are better defended than younger, smaller ones. These considerations underscore the importance of including age in studies of the causes and consequences of variation in poison frog chemical defenses.

Sequestered and Synthesized Chemical Defenses in the Poison Frog Melanophryniscus moreirae

Bufonid poison frogs of the genus Melanophryniscus contain alkaloid-based chemical defenses that are derived from a diet of alkaloid-containing arthropods. In addition to dietary alkaloids, bufadienolide-like compounds and indolealkylamines have been identified in certain species of Melanophryniscus. Our study reports, for the first time, the co-occurrence of large quantities of both alkaloids sequestered from the diet and an endogenously biosynthesized indolalkylamine in skin secretions from individual specimens of Melanophryniscus moreirae from Brazil. GC/MS analysis of 55 individuals of M. moreirae revealed 37 dietary alkaloids and the biosynthesized indolealkylamine bufotenine. On average, pumiliotoxin 267C, bufotenine, and allopumilitoxin 323B collectively represent ca. 90 % of the defensive chemicals present in an individual. The quantity of defensive chemicals differed between sexes, with males possessing significantly less dietary alkaloid and bufotenine than females. Most of the dietary alkaloids have structures with branched-chains, indicating they are likely derived from oribatid mites. The ratio of bufotenine:alkaloid quantity decreased with increasing quantities of dietary alkaloids, suggesting that M. moreirae might regulate bufotenine synthesis in relation to sequestration of dietary alkaloids.

Simple Indolizidine and Quinolizidine Alkaloids

This review of simple indolizidine and quinolizidine alkaloids (i.e., those in which the parent bicyclic systems are in general not embedded in polycyclic arrays) is an update of the previous coverage in Volume 55 of this series (2001). The present survey covers the literature from mid-1999 to the end of 2013; and in addition to aspects of the isolation, characterization, and biological activity of the alkaloids, much emphasis is placed on their total synthesis. A brief introduction to the topic is followed by an overview of relevant alkaloids from fungal and microbial sources, among them slaframine, cyclizidine, Steptomyces metabolites, and the pantocins. The important iminosugar alkaloids lentiginosine, steviamine, swainsonine, castanospermine, and related hydroxyindolizidines are dealt with in the subsequent section. The fourth and fifth sections cover metabolites from terrestrial plants. Pertinent plant alkaloids bearing alkyl, functionalized alkyl or alkenyl substituents include dendroprimine, anibamine, simple alkaloids belonging to the genera Prosopis, Elaeocarpus, Lycopodium, and Poranthera, and bicyclic alkaloids of the lupin family. Plant alkaloids bearing aryl or heteroaryl substituents include ipalbidine and analogs, secophenanthroindolizidine and secophenanthroquinolizidine alkaloids (among them septicine, julandine, and analogs), ficuseptine, lasubines, and other simple quinolizidines of the Lythraceae, the simple furyl-substituted Nuphar alkaloids, and a mixed quinolizidine-quinazoline alkaloid. The penultimate section of the review deals with the sizable group of simple indolizidine and quinolizidine alkaloids isolated from, or detected in, ants, mites, and terrestrial amphibians, and includes an overview of the "dietary hypothesis" for the origin of the amphibian metabolites. The final section surveys relevant alkaloids from marine sources, and includes clathryimines and analogs, stellettamides, the clavepictines and pictamine, and bis(quinolizidine) alkaloids.

Contact toxicities of anuran skin alkaloids against the fire ant (Solenopsis invicta)

Nearly 500 alkaloids, representing over 20 structural classes, have been identified from the skin of neotropical poison frogs (Dendrobatidae). These cutaneous compounds, which are derived from arthropod prey of the frogs, generally are believed to deter predators. We tested the red imported fire ant (Solenopsis invicta) for toxicosis following contact with 20 alkaloids (12 structural classes) identified from dendrobatids or other anurans. Individual ants forced to contact the dried residues of 13 compounds exhibited convulsions and/or reduced ambulation. We estimated the cutaneous concentrations of several compounds based on their reported recoveries from skin extracts of free-ranging frogs and our measurements of the skin surface areas of museum specimens. Pumiliotoxin 251D exhibited contact toxicity below its estimated cutaneous concentration in the Ecuadorian frog, Epipedobates anthonyi, an observation consistent with the hypothesized role of this compound in anuran chemical defense. Our results and those of a previous study of mosquitoes indicate that some anuran skin compounds function defensively as contact toxins against arthropods, permeating their exoskeleton.

Sequestered defensive toxins in tetrapod vertebrates: principles, patterns, and prospects for future studies

Chemical defenses are widespread among animals, and the compounds involved may be either synthesized from nontoxic precursors or sequestered from an environmental source. Defensive sequestration has been studied extensively among invertebrates, but relatively few examples have been documented among vertebrates. Nonetheless, the number of described cases of defensive sequestration in tetrapod vertebrates has increased recently and includes diverse lineages of amphibians and reptiles (including birds). The best-known examples involve poison frogs, but other examples include natricine snakes that sequester toxins from amphibians and two genera of insectivorous birds. Commonalities among these diverse taxa include the combination of consuming toxic prey and exhibiting some form of passive defense, such as aposematism, mimicry, or presumptive death-feigning. Some species exhibit passive sequestration, in which dietary toxins simply require an extended period of time to clear from the tissues, whereas other taxa exhibit morphological or physiological specializations that enhance the uptake, storage, and/or delivery of exogenous toxins. It remains uncertain whether any sequestered toxins of tetrapods bioaccumulate across multiple trophic levels, but multitrophic accumulation seems especially likely in cases involving consumption of phytophagous or mycophagous invertebrates and perhaps consumption of poison frogs by snakes. We predict that additional examples of defensive toxin sequestration in amphibians and reptiles will be revealed by collaborations between field biologists and natural product chemists. Candidates for future investigation include specialized predators on mites, social insects, slugs, and toxic amphibians. Comprehensive studies of the ecological, evolutionary, behavioral, and regulatory aspects of sequestration will require teams of ecologists, systematists, ethologists, physiologists, molecular biologists, and chemists. The widespread occurrence of sequestered defenses has important implications for the ecology, evolution, and conservation of amphibians and reptiles.