Tongue Morphology in Horned Lizards (Phrynosomatidae: Phrynosoma) and its Relationship to Specialized Feeding and Diet

In lizards, the tongue is joined to the mandible by the median genioglossus medialis muscle and the larger, paired genioglossus lateralis muscles. These muscles run through a frenulum and along the sides of the tongue, forming its walls. In horned lizards, however, the genioglossus lateralis muscles fail to join the tongue for most of its length, forming separate ridges evident in the floor of the mouth lateral to the body of the tongue. This unique tongue morphology co-occurs with horned lizards’ ability to consume large numbers of potentially lethal harvester ants, a diet enabled by a feeding mechanism in which ants are rapidly immobilized with strings of mucus before immediate swallowing. Circumstantial evidence implicates the unusual morphology of the genioglossus lateralis muscles in the mucus-binding system.

Phylogenomic Assessment of Biodiversity Using a Reference-Based Taxonomy: An Example With Horned Lizards (Phrynosoma)

Phylogenomic investigations of biodiversity facilitate the detection of fine-scale population genetic structure and the demographic histories of species and populations. However, determining whether or not the genetic divergence measured among populations reflects species-level differentiation remains a central challenge in species delimitation. One potential solution is to compare genetic divergence between putative new species with other closely related species, sometimes referred to as a reference-based taxonomy. To be described as a new species, a population should be at least as divergent as other species. Here, we develop a reference-based taxonomy for Horned Lizards (Phrynosoma; 17 species) using phylogenomic data (ddRADseq data) to provide a framework for delimiting species in the Greater Short-horned Lizard species complex (P. hernandesi). Previous species delimitation studies of this species complex have produced conflicting results, with morphological data suggesting that P. hernandesi consists of five species, whereas mitochondrial DNA support anywhere from 1 to 10 + species. To help address this conflict, we first estimated a time-calibrated species tree for P. hernandesi and close relatives using SNP data. These results support the paraphyly of P. hernandesi; we recommend the recognition of two species to promote a taxonomy that is consistent with species monophyly. There is strong evidence for three populations within P. hernandesi, and demographic modeling and admixture analyses suggest that these populations are not reproductively isolated, which is consistent with previous morphological analyses that suggest hybridization could be common. Finally, we characterize the population-species boundary by quantifying levels of genetic divergence for all 18 Phrynosoma species. Genetic divergence measures for western and southern populations of P. hernandesi failed to exceed those of other Phrynosoma species, but the relatively small population size estimated for the northern population causes it to appear as a relatively divergent species. These comparisons underscore the difficulties associated with putting a reference-based approach to species delimitation into practice. Nevertheless, the reference-based approach offers a promising framework for the consistent assessment of biodiversity within clades of organisms with similar life histories and ecological traits.

Taxonomy of Horned Lizards, Genus Phrynosoma (Squamata, Phrynosomatidae)

In this article, I revise the taxonomy of the species and subspecies of the genus Phrynosoma through phylogenetic and species delimitation approaches based on four mtDNA markers (ND1, ND2, ND4, and 12S). The resulting taxonomy recognizes 12 species (P. asio, P. bracconieri, P. cornutum, P. coronatum, P. douglasii, P. hernandesi, P. mcallii, P. modestum, P. orbiculare, P. platyrhinos, P. solare, and P. taurus). Several of these species are divided into subspecies as follows: P. coronatum (P. c. coronatum, P. c. blainvillii, P. c. cerroense, and P. c. frontale), P. cornutum (P. c. cornutum and P. c. bufonium), P. hernandesi (P. h. hernandesi, P. h. ditmarsi, and P. h. ornatum), P. orbiculare (P. o. orbiculare, P. o. bradti, P. o. boucardii, P. o. cortezii, P. o. dugesii, and P. o. durangoensis), P. platyrhinos (P. p. platyrhinos and P. p. goodei), P. taurus (P. t. taurus and P. t. sherbrookei). In this coherent and objective approach, those taxa treated here as subspecies have diverged to a much lesser degree than those that are herein recognized as separate species. Typically, those taxa recognized as subspecies are one another’s closest relatives (i.e., they together form a monophyletic group that represents the species) and are distributed allopatrically. In this approach, all separate evolutionarily significant units are recognized as named taxa—either species or subspecies—thereby reflecting the importance of identifying and naming such units for conservation. I provide a checklist of the recognized species and subspecies of Phrynosoma along with synonymies and distribution maps.

Avoiding being stung or bitten – prey capture behaviors of the ant-eating Texas horned lizard (Phrynosoma cornutum)

ABSTRACT Horned lizards (Phrynosoma) are specialized predators, including many species that primarily feed on seed harvester ants (Pogonomyrmex). Harvester ants have strong mandibles to husk seeds or defensively bite, and a venomous sting. Texas horned lizards possess a blood plasma factor that neutralizes harvester ant venom and produce copious mucus in the pharynx and esophagus, thus embedding and incapacitating swallowed ants. We used high-speed video recordings to investigate complexities of their lingual prey capture and handling behavior. Lizards primarily strike ants at their mesosoma (thorax plus propodeum of abdomen). They avoid the head and gaster, even if closer to the lizard, and if prey directional movement is reversed. Orientation of captured ants during retraction is with head first (rostral), thus providing initial mucus coating of the mandibles. Prey capture accuracy and precise handling illustrates the specificity of adaptations of horned lizards in avoiding harm, and the challenges lizards face when feeding on dangerous prey.

Soft Release Translocation of Texas Horned Lizards (Phrynosoma cornutum) on an Urban Military Installation in Oklahoma, United States

Wildlife translocation is an often-used technique to augment populations or remove animals from harm’s way. Unfortunately, many translocation efforts fail to meet their goals for myriad reasons, particularly because translocated animals make large, erratic movements after release, which can result in high mortality rates. Soft release, holding animals in acclimation pens for some period of time at the recipient site before release, has been proposed as a technique to reduce these large movements and increase the survival of translocated animals. Here, we compared the survival and movement patterns of soft-released Texas horned lizards (Phrynosoma cornutum) with resident lizards, as well as hard-released lizards from a prior study. Juvenile lizards that were soft-released had high survival rates similar to resident lizards, despite still moving more frequently and occupying larger home ranges than residents. Conversely, soft-released adult lizards had survival rates similar to those that were hard-released, and much lower rates than resident adults. Curiously, soft-released adults did not have significantly higher movement rates or home range sizes than residents. Our results suggest that caution should be used before adult Texas horned lizards are translocated. However, juveniles responded well to soft release, and future research should explore whether they are more resilient to translocation in general, or if soft release provided a specific survival advantage. Contrary to our predictions, the survival of translocated animals was not related to their post-release movement patterns, and the mechanism underlying the observed survival patterns is unclear.