Tropical Natural History 20(2): 134–143, August 2020 2020 by Chulalongkorn University Origins of the Extinct, Subfossil Galápagos Giant Tortoises (Chelonoidis niger) of the Post Office Lava Tube (Inferior) of Floreana Island: Voluntary or Accidental Occupancy? DON MOLL1*, LAUREN E. BROWN2 AND ALAN RESETAR3 1Department of Biology, Missouri State University, Springfield, Missouri 65987, USA 2School of Biological Sciences, Illinois State University, Campus Box 4120, Normal, Illinois 61790, USA 3Amphibians and Reptiles, Field Museum of Natural History, 1400 South Lake Shore Drive, Chicago, Illinois 60605, USA * Corresponding author. Don Moll ([email protected]) Received: 17 January 2020; Accepted: 5 April 2020 ABSTRACT.– The origins of the remains of giant tortoises that have accumulated in vast quantities on the floors of caves in some locations where giant tortoises once lived have received only scant attention to date. The subfossil Galápagos Giant Tortoises (Chelonoidis niger) of Floreana Island’s Post Office lava tube (inferior) are the main focus of this paper with supporting data obtained from other tortoise populations and locations. These remains have historically been explained, if at all, as the result of accidental falls (e.g., via pitfall traps). With the accumulation of greater knowledge of tortoise ecology and paleoecology the likelihood of voluntary cave entry and exit, sometimes by large numbers of giant tortoises (e.g., the Aldabra Atoll Giant Tortoises), also seems plausible. If followed by occasional blockages of exits by geological phenomena such as roof collapse the tortoises could be trapped within as well. An inquiry into the available evidence for the occurrence of both phenomena is the subject of this paper. KEY WORDS: giant tortoises, lava tubes, subfossils, Floreana tube of Floreana Island (= Charles or Santa INTRODUCTION Maria) in the Galápagos Archipelago. Hundreds of their remains are scattered on The Giant Galápagos Tortoises (Chelo- the floor of this cave-like lava tube. noidis niger species complex) of Ecuador’s Townsend (1928) and Steadman (1986) Galápagos Islands are among the largest of suggested the tortoises fell through the extant land tortoises. They are also long ceiling “pitfalls” and became trapped, famous for their role as ships’ fare in the age resulting in death from injuries sustained in of sail due to their ability to survive without their fall or eventual starvation. We herein sustenance for months in the ships’ holds. A challenge their explanation of the tortoises’ more detailed knowledge of their biology means of entry, and suggest an alternative has gradually accumulated in more recent explanation for some of the tortoises present times (e.g., Blake et al., 2012; Blake et al., in the lava tube. 2013; Blake et al., 2015; Caccone et al., 2002; Caccone et al., 1999; Pritchard, 1996, 2007; Rick and Bowman, 1961). However, MATERIALS AND METHODS relatively little has been published concerning the extinct subfossil forms Literature Search including the specimens of Chelonoidis The methodology of Brown et al. (2008) niger obtained from the Post Office lava was employed to obtain a maximum number MOLL ET AL. — ORIGINS OF THE EXTINCT, SUBFOSSIL GALÁPAGOS GIANT TORTOISES 135 of relevant references concerning the subject 1980). They are formed by flowing lava and matter of this paper. Searches employing may be subterranean or formed by crusting- numerous combinations of search words and over of surface lava channels. They may the search engines Google, Google Scholar have complex branching and can have a (Advanced Scholar Search), and JSTOR more or less horizontal or downslope surface Advanced Search were used along with opening (i.e., window) through which the traditional library-based searches of paper liquid lava within may escape, leaving a sources at Illinois State University’s Milner hollow tube. It has also been observed that if Library, and in LEB’s and DM’s personal a surface window does form, the still- herpetological and paleontological libraries. flowing lava may rebuild it and not flow Scientific and common names follow out. At other times, overflow may occur those designated by the Turtle Taxonomy draining the tube (Greeley, 1971a; Jagger, Working Group (2017). 1921). Roof collapse may eventually block Collecting History and Location of the tube when it is void of lava. When this Specimens occurs near the original surface opening, a In 1928, Townsend (Director of the New secondary opening may split off creating a York Aquarium) visited Floreana, one of the steep, vertical, well-like hole. southern Galápagos Islands (Latitude: -1° Post Office lava tube was described in 17' 30.60" S; Longitude: -90° 26' 1.79" W), detail by Townsend (1928) and Steadman where he observed hundreds of subfossil (1986). There are two elongate branches fragments of tortoises in a lava tube cave (“inferior” and “superior”) that are now (Post Office lava tube [inferior]) on the separated from one another by a massive northwest side of the island, and collected roof collapse. The inferior branch contains 12 nearly complete shells of extinct Giant most of the tortoise remains. Its opening to Tortoises (Chelonoidis niger). The following the surface was also formed by a roof year, 1929, K. P. Schmidt (Field Museum of collapse. This opening is ca. 12 ft. (3.7 m) in Natural History) and S. N. Shurcliff also diameter and was half-concealed by bushes. explored this same lava tube and found three Ground level beneath the entrance is ca. 20 additional shells in “perfect condition” ft. (6.1 m) deep. One side of the opening has which had apparently been overlooked a steep slope (Townsend, 1928). previously (Shurcliff and Schmidt, 1930). The Field Museum tortoises with nearly RESULTS AND DISCUSSION complete shells include FMNH 13522 (now MCZ Herpetology R-32098) and FMNH The Tortoise Turnstile Scenario 13523 (2 specimens). A number of carapaces Townsend (1928), echoed by Steadman and other subfossil material are held by the (1986), stated that the tortoises found in the American Museum of Natural History and Post Office lava tube (inferior) must have Harvard University’s Museum of Comparative entered by falling through the near-vertical Zoology (D. Dickey, pers. comm.; Poulakakis opening left by the collapsed roof section et al., 2008). described previously. This would presumably Description of Study Area have occurred after the major roof rock fall Lava tubes are cave-like structures blocked the liquid lava exit pathway that associated with effusive volcanic eruptions of low viscosity basalt lavas (Carlquist, 136 TROPICAL NATURAL HISTORY 20(2), AUGUST 2020 previously allowed any tortoises to enter could have been the case for some extinct and leave the lava tube at will. species also. Townsend (1928) suggested that, “With Moll and Brown (2017) reviewed and a steep slope at one side the unluckly [sic] summarized examples of several extant tortoise that tumbled in did not necessarily tortoise species known to utilize different strike bottom with a fatal crash but rather forms of rock shelters, from large caves to rolled down an incline it could not ascend.” crevices. This depended on the size, and As the shells of most Giant Galápagos sometimes age of the species in question. Tortoises are thick, hard, and tough (except Protection from predators, heat and the Pinta Tortoises [Chelonoidis abingdonii]), humidity fluctuation, cold temperatures, and most individuals falling ~6 m into the lava use as hibernacula are common benefits for tube in this manner would be unlikely to be these species. Most interesting in the seriously injured, if at all. Without an context of this discussion was the discovery accessible exit these tortoises would be of two caves on Grande Terre, Aldabra doomed to eventually die of starvation or Atoll by D. Hansen and his research team other stresses in the lava tube. (The collapsed from the University of Zurich that are used roof section in the Post Office lava tube by many Giant Aldabra Tortoises [superior] descended at a shallower angle, (Aldabrachelys gigantea) to escape the resulting in fewer tortoises being trapped potentially lethal afternoon heat on the atoll than in the inferior tube [Steadman, 1986]). (see photos and text in Bittel [2016], Warne When Townsend (1928) and even [2016], and photo by Dr. Dennis Hansen in Steadman (1986) conducted their research Moll and Brown [2017]). Dennis Hansen (in there was very limited ecological or paleo- Bittel, 2016) believed the use of these caves ecological information available concerning by tortoises was quite ancient based upon Giant Galápagos Tortoises, or, indeed, the extent of wear of the rock surfaces in tortoises in general. As more knowledge has and around the caves. The mid-day heat of accumulated it has become apparent that Aldabra Atoll is an existential threat to another explanation for some of the tortoises tortoise survival there and shade is present in the Post Office lava tube is necessary for coping with the hottest parts plausible: the tortoises could have of the day. As Bittel (2016) stated, “For voluntarily entered the lava tube, and were giant tortoises living in the tropics you subsequently trapped within by some either find a way to get out of the sun or you geological phenomenon such as a roof die.” Naturalists have observed small collapse or landslide. juvenile, and larger adult tortoises on Caves provide favorable environments Aldabra Atoll utilizing shade in rock for some species of extant tortoises. The crevices and under clumps of Tussock caves’ other fauna may benefit from their Grass, Sclerodactylon macrostachyum, or presence also. For example, the large other shade-producing plant clumps (Bittel, amount of dung produced by Aldabra 2016; Coe and Swingland, 1984; Pritchard, Tortoises in the caves they visit seems to 2007). The Grande Terre caves, however, feed resident crabs and insects (Baxter, R.; provide considerably more shade space in Blake, S. and Hansen, D. in [Bittel, J., addition to those previously known shaded 2016]).