Dramatic dietary shift maintains sequestered toxins in chemically defended snakes.

Unlike other snakes, most species of Rhabdophis possess glands in their dorsal skin, sometimes limited to the neck, known as nucho-dorsal and nuchal glands, respectively. Those glands contain powerful cardiotonic steroids known as bufadienolides, which can be deployed as a defense against predators. Bufadienolides otherwise occur only in toads (Bufonidae) and some fireflies (Lampyrinae), which are known or believed to synthesize the toxins. The ancestral diet of Rhabdophis consists of anuran amphibians, and we have shown previously that the bufadienolide toxins of frog-eating species are sequestered from toads consumed as prey. However, one derived clade, the Rhabdophis nuchalis Group, has shifted its primary diet from frogs to earthworms. Here we confirm that the worm-eating snakes possess bufadienolides in their nucho-dorsal glands, although the worms themselves lack such toxins. In addition, we show that the bufadienolides of R. nuchalis Group species are obtained primarily from fireflies. Although few snakes feed on insects, we document through feeding experiments, chemosensory preference tests, and gut contents that lampyrine firefly larvae are regularly consumed by these snakes. Furthermore, members of the R. nuchalis Group contain compounds that resemble the distinctive bufadienolides of fireflies, but not those of toads, in stereochemistry, glycosylation, acetylation, and molecular weight. Thus, the evolutionary shift in primary prey among members of the R. nuchalis Group has been accompanied by a dramatic shift in the source of the species' sequestered defensive toxins.

New species of Blaesodactylus (Squamata: Gekkonidae) from Tsingy karstic outcrops in Ankarana National Park, northern Madagascar.

We describe a new gecko of the genus Blaesodactylus from a karstic outcrop in deciduous dry forest of Ankarana National Park, northern Madagascar. Blaesodactylus microtuberculatus sp. nov., the fifth recognized species of Blaesodactylus, is distinguished from all other congeners, B. ambonihazo, B. antongilensis, B. boivini and B. sakalava by a combination of small, homogeneous gular granules, unspotted venter and lack of tubercles on distal part of original tail. Mitochondrial (ND2 and ND4) and nuclear (RAG-1) DNA identify a consistent divergence between B. microtuberculatus and its allotopic sister species B. boivini. We highlight habitat partitioning in these allotopic congeners where Blaesodactylus microtuberculatus inhabits karstic outcrops in Tsingy massif, and B. boivini dwells on tree trunks in deciduous dry forest.

Novel cooperative antipredator tactics of an ant specialized against a snake.

Eusocial insects can express surprisingly complex cooperative defence of the colony. Brood and reproductive castes typically remain in the nest and are protected by workers' various antipredator tactics against intruders. In Madagascar, a myrmicine ant, Aphaenogaster swammerdami, occurs sympatrically with a large blindsnake, Madatyphlops decorsei. As blindsnakes generally specialize on feeding on termites and ants brood by intruding into the nest, these snakes are presumably a serious predator on the ant. Conversely, a lamprophiid snake, Madagascarophis colubrinus, is considered to occur often in active A. swammerdami nests without being attacked. By presenting M. colubrinus, M. decorsei and a control snake, Thamnosophis lateralis, at the entrance of the nest, we observed two highly specialized interactions between ants and snakes: the acceptance of M. colubrinus into the nest and the cooperative evacuation of the brood from the nest for protection against the ant-eating M. decorsei. Given that M. colubrinus is one of the few known predators of blindsnakes in this area, A. swammerdami may protect their colonies against this blindsnake by two antipredator tactics, symbiosis with M. colubrinus and evacuation in response to intrusion by blindsnakes. These findings demonstrate that specialized predators can drive evolution of complex cooperative defence in eusocial species.

Evolution of nuchal glands, unusual defensive organs of Asian natricine snakes (Serpentes: Colubridae), inferred from a molecular phylogeny.

A large body of evidence indicates that evolutionary innovations of novel organs have facilitated the subsequent diversification of species. Investigation of the evolutionary history of such organs should provide important clues for understanding the basis for species diversification. An Asian natricine snake, Rhabdophis tigrinus, possesses a series of unusual organs, called nuchal glands, which contain cardiotonic steroid toxins known as bufadienolides. Rhabdophis tigrinus sequesters bufadienolides from its toad prey and stores them in the nuchal glands as a defensive mechanism. Among more than 3,500 species of snakes, only 17 Asian natricine species are known to possess nuchal glands or their homologues. These 17 species belong to three nominal genera, Balanophis, Macropisthodon, and Rhabdophis. In Macropisthodon and Rhabdophis, however, species without nuchal glands also exist. To infer the evolutionary history of the nuchal glands, we investigated the molecular phylogenetic relationships among Asian natricine species with and without nuchal glands, based on variations in partial sequences of Mt-CYB, Cmos, and RAG1 (total 2,767 bp). Results show that all species with nuchal glands belong to a single clade (NGC). Therefore, we infer that the common ancestor of this clade possessed nuchal glands with no independent origins of the glands within the members. Our results also imply that some species have secondarily lost the glands. Given the estimated divergence time of related species, the ancestor of the nuchal gland clade emerged 19.18 mya. Our study shows that nuchal glands are fruitful subjects for exploring the evolution of novel organs. In addition, our analysis indicates that reevaluation of the taxonomic status of the genera Balanophis and Macropisthodon is required. We propose to assign all species belonging to the NGC to the genus Rhabdophis, pending further study.

Correction: The Acoustic Properties of Low Intensity Vocalizations Match Hearing Sensitivity in the Webbed-Toed Gecko, Gekko subpalmatus.

[This corrects the article DOI: 10.1371/journal.pone.0146677.].

The Acoustic Properties of Low Intensity Vocalizations Match Hearing Sensitivity in the Webbed-Toed Gecko, Gekko subpalmatus.

The design of acoustic signals and hearing sensitivity in socially communicating species would normally be expected to closely match in order to minimize signal degradation and attenuation during signal propagation. Nevertheless, other factors such as sensory biases as well as morphological and physiological constraints may affect strict correspondence between signal features and hearing sensitivity. Thus study of the relationships between sender and receiver characteristics in species utilizing acoustic communication can provide information about how acoustic communication systems evolve. The genus Gekko includes species emitting high-amplitude vocalizations for long-range communication (loud callers) as well as species producing only low-amplitude vocalizations when in close contact with conspecifics (quiet callers) which have rarely been investigated. In order to investigate relationships between auditory physiology and the frequency characteristics of acoustic signals in a quiet caller, Gekko subpalmatus we measured the subjects' vocal signal characteristics as well as auditory brainstem responses (ABRs) to assess auditory sensitivity. The results show that G. subpalmatus males emit low amplitude calls when encountering females, ranging in dominant frequency from 2.47 to 4.17 kHz with an average at 3.35 kHz. The auditory range with highest sensitivity closely matches the dominant frequency of the vocalizations. This correspondence is consistent with the notion that quiet and loud calling species are under similar selection pressures for matching auditory sensitivity with spectral characteristics of vocalizations.