The Terrific Skink bite force suggests insularity as a likely driver to exceptional resource use

Abstract

Natural history museum collections hold extremely rare, extinct species often described from a single known specimen. On occasions, rediscoveries open new opportunities to understand selective forces acting on phenotypic traits. Recent rediscovery of few individuals of Bocourt´s Terrific Skink Phoboscincus bocourti, from a small and remote islet in New Caledonia allowed to genetically identify a species of land crab in its diet. To explore this further, we CT- and MRI-scanned the head of the holotype, the only preserved specimen dated to about 1870, segmented the adductor muscles of the jaw and bones, and estimated bite force through biomechanical models. These data were compared with those gathered for 332 specimens belonging to 44 other skink species. Thereafter we recorded the maximum force needed to generate mechanical failure of the exoskeleton of a crab specimen. The bite force is greater than the prey hardness, suggesting that predation on hard-shelled crabs may be an important driver of performance. The high bite force seems crucial to overcome low or seasonal variations in resource availability in these extreme insular environments. Phoboscincus bocourti appears to be an apex predator in a remote and harsh environment and the only skink known to predate on hard-shelled land crabs.

Figure 1

(a) Map of New Caledonia, Isle of Pines and satellite islands including the islet where Phoboscincus bocourti is found. The map was generated using QGIS v3.10 software (https://www.qgis.org/en/site/), with the Cross-blended hypsometric tint Earth layer downloaded from Natural Earth (https://www.naturalearthdata.com/). The New Caledonia and Isle of Pine layers were generated using QGIS built in features. (b) Picture of P. bocourti, and crab remains found in its feces and genetic confirmation of crab identification as Geograpsus grayi. Picture credits; (P. bocourti—Ivan Ineich, and G. grayi—Martin Höhle).

Figure 2

Figure showing different top mandible (in green) and bottom mandible (in white) for Phoboscincus bocourti, P. garnieri and Amphiglossus astrolabi (Beamline ID19).

Figure 3

Reconstruction of the three major groups of jaw adductors from CT and MRI scanning and segmentation of the holotype of Phoboscincus bocourti. (a,b) Lateral and dorsal views of the most internal sets of jaw adductors: pseudotemporalis complex composed by the muscles pseudotemporalis profundus in red, pseudotemporalis superficialis in orange. The m. pterygoideus is colored in salmon. The medial portion of the mAME medialis (mAME-3) is shown in blue. The mAME profundus (mAME-4) complex is also composed by two bundles respectively colored in aquamarine green (mAME-4a) and fuchsia (mAME-4b). (c,d) lateral and dorsal views of the mAME superficialis (mAME-1) and the radial portion of the mAME medialis (mAME-2) complexes. The mAME-1 is composed by the anterior bundle (dark blue) and the posterior bundle (dark green). The mAME-2 is instead shown colored in violet. In the lateral views the transparency of the skull has been increased to enhance the visibility of the position of the muscles.

Figure 4

Graph showing head width versus bite force to compare the bite force data estimated for Phoboscincus bocourti and 332 specimens across 44 skink species at the tip of the jaw and at the gape angle of 30°. The skinks depicted on the graph are: P. bocourti on top (photo credit Ivan Ineich), Tiliqua scincoides second right, T. rugosa third bottom, Corucia zebrata fourth bottom (photo credit David Utrera), T. multifasciata fifth bottom (Photo credit for Tiliqua spp. Anders Zimny).