Tortoise: Habits, Diet and Other Facts
The Testudines, chelonians or tortoises are an order of reptiles belonging to the Testudinata clade.
There are 14 families that add up to around 356 species, occurring in tropical and temperate regions of the globe, some of which are threatened with extinction. The popular terms tortoise (freshwater), tortoise (land) and turtles (marine) do not reflect taxonomic classification. The order of Testudines is phylogenetically divided into two major groups: Cryptodira (anteroposterior neck retraction) and Pleurodira (lateral neck retraction). The phylogenetic position among the Amniota remains an enigma, being considered by some authors to be close to lepidosaurs (evidenced by morphological data) or archosaurs (evidenced by molecular data).
Anatomia e fisiologia
Skull and neck retraction
Lateral view of the skull and jaw of Chelonia mydas. Source: Hofling et al. (2019)
Turtle skulls do not have temporal fenestrae, that is, they have an anapsid condition. Goodrich (1930), Boulenger (1918) and Broom (1924) emphasized that the arrangement of bones in the temporal region of turtles does not resemble that of true Anapsida. Furthermore, studies of the ontogeny of the adductor musculature of the mandible reveal similarities between turtles and Diapsid. Thus, today it is known that Testudines are Diapsida with secondary loss of temporal fenestrae.
Throughout evolution, there has been a tendency towards the loss of some dermal bones in the skull of turtles: lacrimal, epipterygoid and ectopterygoid are lost in many groups; the nasal occurs only in the family Chelidae. In the circum-orbital series, one element disappears: according to some authors, the post-frontal, according to others, the post-orbital.
All current Testudines have an expanded optical capsule that reduces the space within the adductor chamber, decreasing the volume of the adductor musculature of the mandible. This, added to the anapsid condition, would generate a problem of low bite force. Thus, during evolution there was an independent origin of different solutions to increase adduction force: in Pleurodira, the adductor muscle of the mandible passes over a pulley-shaped projection of the pterygoid (pterygoid trochlear process), while in Cryptodira the trochlear process is formed by the optical capsule itself.
Carapace (left) and plastron (right) of Testudines, above schematic with keratinized epidermal plates and below schematic with dermal bone plates. Source: Pough (2008)
Today's tortoises have a horny, toothless beak. The fossil record reveals that rmphotheca arose early in the evolution of turtles, although some basal groups still had teeth (such as Odontochelys semitestacea). The secondary palate is present (consisting of horizontal plates on the medial part of bones such as the premaxillary, maxillary, palatine and pterygoid), but poorly developed. Unlike the secondary palate of Crocodylia, in which the choanae are located at the height of the pterygoid bones, in Testudines the choanae are located rostrally at the level of the vomer.
Neck retraction occurs laterally in Pleurodira and anteroposteriorly in Cryptodira. In some species of Chelidae (Pleurodira), the neck may be longer than the hull. In some species of Trionychia and Kinosternidea (Cryptodira), there is the ability to stretch the head over the carapace for defense or hunting. In the case of current sea turtles, there was a loss of the ability to retract the neck.
The shell of turtles is formed by the carapace (dorsal) and plastron (ventral). Both have keratinized epidermal plates above dermal bone plates (no overlap in position and number). The carapace is composed of neural plates fused to the vertebrae, costal plates fused to flattened ribs, and peripheral plates. The plastron is composed of pairs of epidermal plates (such as gular, humeral, pectoral, abdominal, femoral, and anal); located underlyingly there may be the following dermal bone plates: paired epiplastrons derived from clavicles; endoplastron derived from the interclavicle; pairs of hyoplastron, hypoplastron and xifiplastron. The girdles are internal to the rib cage.
Flexible areas (hinges) may be present on the hooves of many Testudines, such as Kinosternon. Leatherback turtles (Dermochelys coriacea) have a carapace formed by cartilage with thousands of small polygonal bones.
If turtles evolved from a common marine ancestor, the presence of heavy abdominal ribs (gastralia) may have contributed to buoyancy control, resulting in the emergence of the plastron before the carapace (evidenced by the Odontochelys fossil). The carapace may have evolved as a defensive structure, but there are also recent hypotheses that turtle ancestors were fossorial, in which shell ossification may have been a way of dealing with soil pressure when digging. However, it should be remembered that other fossorial vertebrates show a reduction in ossification, forming a relatively soft body, which does not support the hypothesis.
Chelonians have a toothless keratinized horny beak. Salivary glands are well-developed lobuloalveolar structures. In the case of fruit-eating turtles, there is an increase in amylase-secreting salivary glands. In the stomach, the presence of food stimulates acid secretion (but the visual and chemical stimulus itself can induce the secretion of gastric juice). In warmer months, proteolytic activity is faster. In Galápagos tortoises (Geochelone nigra), 1 cm diameter gravels are found in the feces, but the exact role of these gastroliths in mechanical digestion is still uncertain. The intestinal tract of herbivorous turtles is larger than that of carnivores to expand the surface area of mutualism with bacteria.
The turtle pancreas also has exocrine and endocrine function, but, as in other reptiles, the exocrine pancreas consists of more branched tubules than the acini and lobules of mammals. The liver, on the other hand, is less organized in lobules than in mammals.