Editing Turtle (section)

== Anatomy and physiology==
<!--"Form and function" arguably more comprehensive-->

===Size===

The largest [[Extant taxon|living]] species of turtle (and fourth-largest [[reptile]]) is the [[leatherback turtle]], which can reach over {{convert|2.7|m|abbr=on}} in length and weigh over {{convert|500|kg|abbr=on}}.<ref>{{cite journal |last1=Chen |first1=Irene H. |last2=Yang |first2=Wen |last3=Meyers |first3=Marc A. |year=2015 |title=Leatherback Sea Turtle Shell: a Tough and Flexible Biological Design |journal=Acta Biomaterialia |volume=28 |pages=2–12 |doi=10.1016/j.actbio.2015.09.023 |pmid=26391496|doi-access=free  |issn = 1742-7061 }}</ref> The largest known turtle was ''[[Archelon|Archelon ischyros]]'', a [[Cretaceous|Late Cretaceous]] sea turtle up to {{convert|4.5|m|ft|0|abbr=on}} long, {{convert|5.25|m|ft|0|abbr=on}} wide between the tips of the front flippers, and estimated to have weighed over {{cvt|2200|kg}}.<ref>{{cite web |url=http://www.bhigr.com/pages/info/info_arch.htm |title=The ''Archelon'' |publisher=Black Hills Institute of Geological Research |access-date=23 December 2018 |archive-date=March 12, 2016 |archive-url=https://web.archive.org/web/20160312042116/http://www.bhigr.com/pages/info/info_arch.htm|url-status=live}}</ref> The smallest living turtle is ''[[Chersobius signatus]]'' of South Africa, measuring no more than {{convert|10|cm|in|abbr=on}} in length<ref>{{cite book |last1=Bonin |first1=Franck |last2=Devaux |first2=Bernard |last3=Dupré |first3=Alain|year=2006 |title=Turtles of the World |publisher=Johns Hopkins University Press |page=230 |isbn=978-0-8018-8496-2}}</ref> and weighing {{convert|172|g|abbr=on}}.<ref>{{cite book |last=Frazier |first=Jack |year=2020 |contribution=Galapagos tortoises: Protagonists in the spectacle of life on Earth |page=26 |title=Galapagos Giant Tortoises |editor-last=Gibbs |editor-first=James |editor-last2=Cayot |editor-first2=Linda |editor-last3=Aguilera |editor-first3=Washington Tapia |publisher=Academic Press |isbn=978-0-12-817554-5 }}</ref>

===Shell===
{{main article|Turtle shell}}
[[File:Turtle skeleton cross-section, labelled as infographic.svg|thumb|left|upright=1.3|[[Sagittal section]] of a tortoise skeleton|alt=Photograph of one half of a tortoise skeleton, cut in half vertically showing the vertebrae following curving along the carapace]]

The shell of a turtle is unique among [[vertebrate]]s and serves to protect the animal and provide shelter from the elements.<ref name=Firefly>{{cite book |last1=Iverson|first1=John |last2=Moll|first2=Edward O|year=2002 |contribution=Turtles and tortoises |title=The Firefly Encyclopedia of Reptiles and Amphibians |editor=Halliday, Tim |editor2=Adler, Kraig |publisher=Firefly Books |pages=118–129 |isbn=978-1-55297-613-5 }}</ref>{{sfn|Orenstein|2012|p=22}}<ref>{{Cite web |last=Hutchinson |first=J. Howard |year=1996 |title=Introduction to Testudines: The Turtles |url=http://www.ucmp.berkeley.edu/anapsids/testudines/testudines.html |publisher=University of California Museum of Paleontology |access-date=June 4, 2003 |archive-date=June 29, 2016 |archive-url=https://web.archive.org/web/20160629134148/http://www.ucmp.berkeley.edu/anapsids/testudines/testudines.html |url-status=live }}</ref> It is primarily made of 50–60 bones and consists of two parts: the domed, dorsal (back) [[Turtle shell#Carapace|carapace]] and the flatter, ventral (belly) [[Turtle shell#Plastron|plastron]]. They are connected by lateral (side) extensions of the plastron.<ref name=Firefly/>{{sfn|Orenstein|2012|p=16}}

The [[carapace]] is fused with the vertebrae and ribs while the plastron is formed from bones of the [[shoulder girdle]], [[sternum]], and [[gastralia]] (abdominal ribs).<ref name=Firefly/> During development, the ribs grow sideways into a carapacial ridge, unique to turtles, entering the [[dermis]] (inner skin) of the back to support the carapace. The development is signaled locally by proteins known as [[fibroblast growth factor]]s that include [[FGF10]].<ref name="Cebra-Thomas Tan 2005">{{cite journal |last1=Cebra-Thomas |first1=Judith |last2=Tan |first2=Fraser |last3=Sistla |first3=Seeta |last4=Estes |first4=Eileen |last5=Bender |first5=Gunes |last6=Kim |first6=Christine |last7=Riccio |first7=Paul |last8=Gilbert |first8=Scott F. |title=How the Turtle Forms its Shell: a Paracrine Hypothesis of Carapace Formation |journal=Journal of Experimental Zoology Part B: Molecular and Developmental Evolution |volume=304B |issue=6 |year=2005 |pages=558–569 |issn=1552-5007 |doi=10.1002/jez.b.21059 |pmid=15968684 |bibcode=2005JEZB..304..558C |s2cid=2484583 |url=http://doc.rero.ch/record/15491/files/PAL_E2884.pdf }}</ref> The shoulder girdle in turtles is made up of two bones, the scapula and the [[coracoid]].<ref name=gaffney>{{cite journal |last=Gaffney |first=Eugene S. |year=1990 |title=The Comparative Osteology of the Triassic Turtle ''Proganochelys'' |journal=Bulletin of the American Museum of Natural History |issue=194 |pages=1–263 |hdl=2246/884 |url=http://digitallibrary.amnh.org/handle/2246/884 |access-date=May 16, 2021 |archive-date=May 16, 2021 |archive-url=https://web.archive.org/web/20210516221736/http://digitallibrary.amnh.org/handle/2246/884 |url-status=live |oclc=263164288 }}</ref> Both the shoulder and pelvic girdles of turtles are located within the shell and hence are effectively within the rib cage. The trunk ribs grow over the shoulder girdle during development.<ref name="Schoch Sues 2019"/>
[[File:How the Turtle Gets its Shell.svg|thumb|upright=1.6|right|[[Embryogenesis|Development]] of the shell. The ribs are growing sideways into the carapacial ridge, seen here as a bud, to support the carapace.<ref name="Cebra-Thomas Tan 2005"/>|alt=Drawing of a section through a turtle embryo showing formation of the shell, with the ribs growing sideways]]
The shell is covered in [[epidermal]] (outer skin) scales known as [[scute]]s that are made of [[keratin]], the same substance that makes up hair and fingernails. Typically, a turtle has 38 scutes on the carapace and 16 on the plastron, giving them 54 in total. Carapace scutes are divided into "marginals" around the margin and "vertebrals" over the vertebral column, though the scute that overlays the neck is called the "cervical". "Pleurals" are present between the marginals and vertebrals.{{sfn|Orenstein|2012|pp=16–17}} Plastron scutes include gulars (throat), humerals, pectorals, abdominals, and anals. [[Pleurodira|Side-necked turtles]] additionally have "intergular" scutes between the gulars.{{sfn|Orenstein|2012|p=16}}<ref>{{cite book |last=Pritchard |first=Peter C. H. |chapter=Evolution and Structure of the Turtle Shell |editor-last=Wyneken |editor-first=Jeanette |editor-last2=Bels |editor-first2=V. L. |editor-last3=Godfrey |editor-first3=Matthew H. |title=Biology of Turtles |year=2008 |publisher=CRC Press |isbn=978-0-8493-3339-2 |oclc=144570900 |page=56}}</ref> Turtle scutes are usually structured like [[mosaic]] tiles, but some species, like the [[hawksbill sea turtle]], have overlapping scutes on the carapace.{{sfn|Orenstein|2012|p=16}}

The shapes of turtle shells vary with the adaptations of the individual species, and [[sexual dimorphism|sometimes with sex]]. Land-dwelling turtles are more dome-shaped, which appears to make them more resistant to being crushed by large animals. Aquatic turtles have flatter, smoother shells that allow them to cut through the water. Sea turtles in particular have streamlined shells that reduce [[drag (physics)|drag]] and increase stability in the open ocean. Some turtle species have pointy or spiked shells that provide extra [[Anti-predator adaptation|protection from predators]] and [[camouflage]] against the leafy ground. The lumps of a tortoise shell can tilt its body when it gets flipped over, allowing it to flip back. In male tortoises, the tip of the plastron is thickened and used for butting and ramming during combat.{{sfn|Orenstein|2012|pp=22–26}}

Shells vary in flexibility. Some species, such as [[box turtle]]s, lack the lateral extensions and instead have the carapace bones fully fused or [[ankylosis|ankylosed]] together. Several species have hinges on their shells, usually on the plastron, which allow them to expand and contract. [[Softshell turtle]]s have rubbery edges, due to the loss of bones. The leatherback turtle has hardly any bones in its shell, but has thick [[connective tissue]] and an outer layer of leathery skin.{{sfn|Orenstein|2012|pp=22–23, 26–27}}

===Head and neck===
[[File:Mocsári teknős Kiserdő 2016.jpg|thumb|Head and neck of a [[European pond turtle]]|alt=Closeup of the head and neck of turtle|left]]
The turtle's skull is unique among living [[amniote]]s (which includes reptiles, birds and mammals); it is solid and rigid with no openings for muscle attachment ([[temporal fenestrae]]).{{sfn|Franklin|2011|p=18}}{{sfn|Orenstein|2012|p=33}} Muscles instead attach to recesses in the back of the skull. Turtle skulls vary in shape, from the long and narrow skulls of softshells to the broad and flattened skull of the [[mata mata]].{{sfn|Orenstein|2012|p=33}} Some turtle species have developed large and thick heads<!-- for their size-->, allowing for greater muscle mass and stronger bites.{{sfn|Franklin|2011|p=28}}

Turtles that are carnivorous or [[durophagy|durophagous]] (eating hard-shelled animals) have the most powerful bites. For example, the durophagous ''[[Mesoclemmys nasuta]]'' has a bite force of  {{convert|432|lbf|N|abbr=on}}. Species that are [[insectivorous]], [[piscivorous]] (fish-eating), or [[omnivorous]] have lower bite forces.<ref>{{cite journal |title=Evolution of Bite Performance in Turtles |last1=Herrel |first1=Anthony |last2=O'Reilly |first2=James C. |last3=Richmond |first3=Alan M. |s2cid=54067445 |journal=Journal of Evolutionary Biology |volume=15 |issue=6 |pages=1083–1094 |year=2002 |doi=10.1046/j.1420-9101.2002.00459.x|citeseerx=10.1.1.484.5540 }}</ref> Living turtles lack teeth but have beaks made of [[rhamphotheca|keratin sheaths]] along the edges of the jaws.{{sfn|Orenstein|2012|pp=33–34}}<ref name=Firefly/> These sheaths may have sharp edges for cutting meat, serrations for clipping plants, or broad plates for breaking [[mollusks]].{{sfn|Orenstein|2012|p=34}} Sea turtles, and several extinct forms, have evolved a bony [[secondary palate]] which completely separates the oral and nasal cavities.<ref>{{cite journal|last1=Abramyan|first1=J|last2=Richman|first2=J. M.|year=2015|title=Recent insights into the morphological diversity in the amniote primary and secondary palates|journal=Developmental Dynamics|volume=244|issue=12|pages=1457–1468|doi=10.1002/dvdy.24338|pmid=26293818|pmc=4715671}}</ref>

The necks of turtles are highly flexible, possibly to compensate for their rigid shells. Some species, like sea turtles, have short necks while others, such as [[snake-necked turtle]]s, have long ones. Despite this, all turtle species have eight [[cervical vertebrae|neck vertebrae]], a consistency not found in other reptiles but similar to mammals.{{sfn|Orenstein|2012|p=36}} Some snake-necked turtles have both long necks and large heads, limiting their ability to lift them when not in water.{{sfn|Franklin|2011|p=28}} Some turtles have folded structures in the [[larynx]] or [[glottis]] that vibrate to produce sound. Other species have [[elastin]]-rich [[vocal cord]]s.<ref>{{cite journal|last1=P. Russell|first1=Anthony|last2=M. Bauer|first2=Aaron|year=2020|title=Vocalization by extant nonavian reptiles: A synthetic overview of phonation and the vocal apparatus|journal=The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology|volume=304|issue=7|pages=1478–1528|doi=10.1002/ar.24553|pmid=33099849|s2cid=225069598|doi-access=free}}</ref><ref>{{cite book|author1=Capshaw, Grace|author2=Willis, Katie L.|author3=Han, Dawei|author4=Bierman, Hilary S.|year=2020|title=Neuroendocrine Regulation of Animal Vocalization|contribution=Reptile sound production and perception|pages=101–118|editor1=Rosenfeld, Cheryl S.|editor2=Hoffmann, Frauke|publisher=Academic Press|isbn=978-0-12-815160-0}}</ref>

===Limbs and locomotion===

Due to their heavy shells, turtles are slow-moving on land. A [[desert tortoise]] moves at only {{cvt|0.22|–|0.48|km/h|abbr=on}}. By contrast, sea turtles can swim at {{cvt|30|km/h|abbr=on}}.<ref name=Firefly/> The limbs of turtles are adapted for various means of locomotion and habits and most have five toes. Tortoises are specialized for terrestrial environments and have column-like legs with elephant-like feet and short toes. The [[gopher tortoise]] has flattened front limbs for digging in the substrate. Freshwater turtles have more flexible legs and longer toes with [[Webbed foot|webbing]], giving them thrust in the water. Some of these species, such as [[snapping turtles]] and [[mud turtle]]s, mainly walk along the water bottom, as they would on land. Others, such as terrapins, swim by paddling with all four limbs, switching between the opposing front and hind limbs, which keeps their direction stable.<ref name=Firefly/>{{sfn|Orenstein|2012|p=38}}

[[File:Green Sea Turtle swimming.jpg|thumb|[[Sea turtle]]s have [[streamlined]] shells and limbs adapted for fast and efficient swimming.<ref name="Davenport 1984"/>|alt=Marine turtle swimming]]

Sea turtles and the [[pig-nosed turtle]] are the most specialized for swimming. Their front limbs have evolved into flippers while the shorter hind limbs are shaped more like rudders. The front limbs provide most of the thrust for swimming, while the hind limbs serve as stabilizers.<ref name=Firefly/>{{sfn|Orenstein|2012|pp=38–40}} Sea turtles such as the [[green sea turtle]] rotate the front limb flippers like a bird's wings to generate a propulsive force on both the upstroke and on the downstroke. This is in contrast to similar-sized freshwater turtles (measurements having been made on young animals in each case) such as the [[Caspian turtle]], which uses the front limbs like the oars of a rowing boat, creating substantial negative thrust on the recovery stroke in each cycle. In addition, the streamlining of the marine turtles reduces drag. As a result, marine turtles produce a propulsive force twice as large, and swim six times as fast, as freshwater turtles. The swimming efficiency of young marine turtles is similar to that of fast-swimming fish of open water, like [[mackerel]].<ref name="Davenport 1984">{{cite journal |last1=Davenport |first1=John |last2=Munks |first2=Sarah A. |last3=Oxford |first3=P. J. |title=A Comparison of the Swimming of Marine and Freshwater Turtles |journal=Proceedings of the Royal Society of London. Series B. Biological Sciences |volume=220 |issue=1221 |date=February 22, 1984 |issn=0080-4649 |doi=10.1098/rspb.1984.0013 |jstor=35758 |pages=447–475 |bibcode=1984RSPSB.220..447D |s2cid=84615412 }}</ref>

Compared to other reptiles, turtles tend to have reduced tails, but these vary in both length and thickness among species and between sexes. [[Snapping turtles]] and the [[big-headed turtle]] have longer tails; the latter uses it for balance while climbing. The [[cloaca]] is found underneath and at the base, and the tail itself houses the reproductive organs. Hence, males have longer tails to contain the penis. In sea turtles, the tail is longer and more [[prehensile]] in males, who use it to grasp mates. Several turtle species have spines on their tails.{{sfn|Orenstein|2012|p=40}}{{sfn|Franklin|2011|p=18}}

=== Senses ===
[[File:Red Eared Slider's Eye.jpg|thumb|The [[red-eared slider]] has an exceptional seven types of color-detecting cells in its eyes.<ref name="Fritsches et al 2013"/>|alt=head of a red-eared slider turtle|left]]

Turtles make use of [[Visual perception|vision]] to find food and mates, avoid predators, and orient themselves. The [[retina]]'s light-sensitive cells include both [[Rod cell|rods]] for vision in low light, and [[Cone cell|cones]] with three different [[photopigment]]s for bright light, where they have full-color vision. There is possibly a fourth type of cone that detects [[ultraviolet]], as hatchling sea turtles respond experimentally to ultraviolet light, but it is unknown if they can distinguish this from longer wavelengths. A freshwater turtle, the [[red-eared slider]], has an exceptional seven types of cone cell.<ref name="Fritsches et al 2013"/><ref name="Granda Dvorak 1977 pp. 451–495">{{cite book |last1=Granda |first1=Alan M. |last2=Dvorak |first2=Charles A. |title=The Visual System in Vertebrates |chapter=Vision in Turtles |series=Handbook of Sensory Physiology |publisher=Springer Berlin Heidelberg |publication-place=Berlin, Heidelberg |year=1977 |volume=7 / 5 |issn=0072-9906 |doi=10.1007/978-3-642-66468-7_8 |pages=451–495|isbn=978-3-642-66470-0 }}</ref><ref name="Jacobs 1981">{{cite book |last=Jacobs |first=Gerald |chapter=Reptiles: The Turtle |title=Comparative Color Vision |publisher=Academic Press |publication-place=New York |year=1981 |isbn=978-0-12-378520-6 |pages=102–105}}</ref>

Sea turtles orient themselves on land by night, using visual features detected in dim light. They can use their eyes in clear surface water, muddy coasts, the darkness of the deep ocean, and also above water. Unlike in terrestrial turtles, the [[cornea]] (the curved surface that lets light into the eye) does not help to focus light on the retina, so focusing underwater is handled entirely by the lens, behind the cornea. The cone cells contain oil droplets placed to shift perception toward the red part of the spectrum, improving color discrimination. Visual acuity, studied in hatchlings, is highest in a horizontal band with retinal cells packed about twice as densely as elsewhere. This gives the best vision along the visual horizon. Sea turtles do not appear to use [[polarized light]] for orientation as many other animals do. The deep-diving leatherback turtle lacks specific adaptations to low light, such as large eyes, large lenses, or a reflective [[Tapetum lucidum|tapetum]]. It may rely on seeing the [[bioluminescence]] of prey when hunting in deep water.<ref name="Fritsches et al 2013">{{cite book |last1=Fritsches |first1=Kerstin A. |last2=Warrant |first2=Eric J. |chapter=Vision |editor-last=Wyneken |editor-first=Jeanette |title=The Biology of Sea Turtles |publisher=CRC Press |year=2013 |isbn=978-1-4398-7308-3 |oclc=828509848 |pages=31–58}}</ref>

Turtles have no ear openings; the [[eardrum]] is covered with scales and encircled by a bony [[otic capsule]], which is absent in other reptiles.{{sfn|Orenstein|2012|p=36}} Their hearing thresholds are high in comparison to other reptiles, reaching up to 500 [[Hz]] in air, but underwater they are more attuned to lower frequencies.<ref>{{cite book|last=Willis|first=Katie L. |title=The Effects of Noise on Aquatic Life II |chapter=Underwater Hearing in Turtles |series=Advances in Experimental Medicine and Biology |year=2016 |volume=875 |pages=1229–1235 |doi=10.1007/978-1-4939-2981-8_154 |pmid=26611091|isbn=978-1-4939-2980-1 }}</ref> The [[loggerhead sea turtle]] has been shown experimentally to respond to low sounds, with maximal sensitivity between 100 and 400&nbsp;Hz.<ref name="MartinAlessi2012">{{cite journal |last1=Martin |first1=Kelly J. |last2=Alessi |first2=Sarah C. |last3=Gaspard |first3=Joseph C. |last4=Tucker |first4=Anton D. |last5=Bauer |first5=Gordon B. |last6=Mann |first6=David A. |title=Underwater Hearing in the Loggerhead Turtle (''Caretta caretta''): a Comparison of Behavioral and Auditory Evoked Potential Audiograms |journal=Journal of Experimental Biology |volume=215 |issue=17 |year=2012 |pages=3001–3009 |issn=1477-9145 |doi=10.1242/jeb.066324 |pmid=22875768 |s2cid=459652 |url=https://scholarcommons.usf.edu/etd/3233 |doi-access=free |url-access=subscription }}</ref>

Turtles have [[Olfactory system|olfactory]] (smell) and [[vomeronasal organ|vomeronasal]] receptors along the nasal cavity, the latter of which are used to detect chemical signals.{{sfn|Orenstein|2012|pp=35–36}} Experiments on green sea turtles showed they could learn to respond to a selection of different odorant chemicals such as [[triethylamine]] and [[cinnamaldehyde]], which were detected by olfaction in the nose. Such signals could be used in navigation.<ref name="MantonKarr1972">{{cite journal |last1=Manton |first1=Marion |last2=Karr |first2=Andrew |last3=Ehrenfeld |first3=David W. |title=Chemoreception in the Migratory Sea Turtle, ''Chelonia mydas'' |journal=The Biological Bulletin |volume=143 |issue=1 |year=1972 |pages=184–195 |issn=0006-3185 |doi=10.2307/1540338 |jstor=1540338 |url=https://www.biodiversitylibrary.org/part/24653 |access-date=June 6, 2021 |archive-date=June 6, 2021 |archive-url=https://web.archive.org/web/20210606225146/https://www.biodiversitylibrary.org/part/24653 |url-status=live }}</ref>

=== Breathing===
[[File:Indian softshell turtle (Nilssonia gangetica) Babai River nose-breathing at surface.jpg|thumb|A submerged [[Indian softshell turtle]] nose-breathing at river surface|alt=photo of a river turtle with only its nose above water]]
The rigid shell of turtles is not capable of expanding and making room for the lungs, as in other amniotes, so they have had to evolve special adaptations for respiration.<ref name="Cordeiro 2016">{{Cite journal |last1=Cordeiro |first1=Tábata E. F. |last2=Abe |first2=Augusto S. |last3=Klein |first3=Wilfried |date=April 2016 |title=Ventilation and Gas Exchange in Two Turtles: ''Podocnemis unifilis'' and ''Phrynops geoffroanus'' (Testudines: Pleurodira) |journal=Respiratory Physiology & Neurobiology |volume=224 |pages=125–131 |doi=10.1016/j.resp.2014.12.010 |pmid=25534144 |issn=1569-9048 |hdl=11449/158795 |s2cid=37446604 |url=https://repositorio.unesp.br/bitstream/11449/158795/1/WOS000373540800015.pdf |hdl-access=free |access-date=September 25, 2019 |archive-date=July 24, 2021 |archive-url=https://web.archive.org/web/20210724105252/https://repositorio.unesp.br/bitstream/handle/11449/158795/WOS000373540800015.pdf;jsessionid=7500C1BBBBCF7CD5F9A0DE4D94B0AC03?sequence=1 |url-status=live }}</ref><ref name="Lyson 2014">{{Cite journal |last1=Lyson |first1=Tyler R. |last2=Schachner |first2=Emma R. |last3=Botha-Brink |first3=Jennifer |last4=Scheyer |first4=Torsten M. |last5=Lambertz |first5=Markus |last6=Bever |first6=G. S. |last7=Rubidge |first7=Bruce S. |last8=de Queiroz |first8=Kevin |date=7 November 2014 |title=Origin of the Unique Ventilatory Apparatus of Turtles |journal=Nature Communications |volume=5 |page=5211 |doi=10.1038/ncomms6211 |pmid=25376734 |issn=2041-1723 |bibcode=2014NatCo...5.5211L|doi-access=free }}</ref><ref>{{Cite journal |last1=Lee |first1=Stella Y. |last2=Milsom |first2=William K. |year=2016 |title=The Metabolic Cost of Breathing in Red-eared Sliders: An Attempt to Resolve an Old Controversy |journal=Respiratory Physiology & Neurobiology |volume=224 |pages=114–124 |doi=10.1016/j.resp.2015.10.011 |pmid=26524718 |s2cid=5194890 |issn=1569-9048}}</ref> The lungs of turtles are attached directly to the carapace above while below, connective tissue attaches them to the organs.{{sfn|Orenstein|2012|p=41}} They have multiple lateral (side) and medial (middle) chambers (the numbers of which vary between species) and one terminal (end) chamber.<ref>{{Cite journal |last1=Lambertz |first1=Markus |last2=Böhme |first2=Wolfgang |last3=Perry |first3=Steven F. |date=July 2010 |title=The Anatomy of the Respiratory System in ''Platysternon megacephalum'' Gray, 1831 (Testudines: Cryptodira) and Related Species, and its Phylogenetic Implications |journal=Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology |volume=156 |issue=3 |pages=330–336 |doi=10.1016/j.cbpa.2009.12.016 |pmid=20044019 |issn=1095-6433}}</ref>

The lungs are ventilated using specific groups of abdominal muscles attached to the organs that pull and push on them.<ref name="Cordeiro 2016"/> Specifically, it is the turtle's large liver that compresses the lungs. Underneath the lungs, in the [[coelomic cavity]], the liver is connected to the right lung by the [[Root of the lung|root]], and the stomach is directly attached to the left lung, and to the liver by a [[mesentery]]. When the liver is pulled down, inhalation begins.<ref name="Lyson 2014"/> Supporting the lungs is a wall or [[septum]], which is thought to prevent them from collapsing.<ref>{{Cite journal |last1=Klein |first1=Wilfried |last2=Codd |first2=Jonathan R. |year=2010 |title=Breathing and Locomotion: Comparative Anatomy, Morphology and Function |journal=Respiratory Physiology & Neurobiology |volume=173 |pages=S26–S32 |doi=10.1016/j.resp.2010.04.019|pmid=20417316 |s2cid=28044326 |issn=1569-9048}}</ref> During exhalation, the contraction of the [[transversus abdominis muscle]] propels the organs into the lungs and expels air. Conversely, during inhalation, the relaxing and flattening of the [[Abdominal external oblique muscle|oblique abdominis muscle]] pulls the transversus back down, allowing air back into the lungs.<ref name="Lyson 2014"/>

Although many turtles spend large amounts of their lives underwater, all turtles breathe air and must surface at regular intervals to refill their lungs. Depending on the species, immersion periods vary between a minute and an hour.<ref>{{Cite journal |last1=Morera-Brenes |first1=Bernal |last2=Monge-Nájera |first2=Julián |year=2011 |title=Immersion Periods in Four Neotropical Turtles |url=https://revistas.uned.ac.cr/index.php/cuadernos/article/view/212 |journal=UNED Research Journal |volume=3 |issue=1 |pages=97 |doi=10.22458/urj.v3i1.212 |doi-access=free |access-date=June 11, 2020 |archive-date=August 9, 2020 |archive-url=https://web.archive.org/web/20200809164614/https://revistas.uned.ac.cr/index.php/cuadernos/article/view/212 |url-status=live }}</ref> Some species can [[Enteral respiration|respire through the cloaca]], which contains large sacs that are lined with many finger-like projections that take up dissolved [[oxygen]] from the water.<ref>{{Cite journal |last1=Priest |first1=Toni E. |date=December 2002 |last2=Franklin |first2=Craig E. |title=Effect of Water Temperature and Oxygen Levels on the Diving Behavior of Two Freshwater Turtles: ''Rheodytes leukops'' and ''Emydura macquarii'' |journal=Journal of Herpetology |volume=36 |issue=4 |pages=555–561 |jstor=1565924 |issn=0022-1511 |doi=10.1670/0022-1511(2002)036[0555:EOWTAO]2.0.CO;2 |s2cid=85279910 }}</ref>

=== Circulation ===

[[File:Snapping turtle in Mud.JPG|thumb|[[Chelydra serpentina|Snapping turtle]] emerging from period of [[brumation]], in which it buried itself in mud. Turtles have multiple circulatory and physiological adaptations to enable them to go long periods without breathing.<ref name="Wyneken 2008 Circulatory"/>|alt=photo of a turtle climbing out of mud|left]]

Turtles share the linked [[Circulatory system|circulatory]] and pulmonary (lung) systems of vertebrates, where the three-chambered heart pumps deoxygenated blood through the lungs and then pumps the returned [[oxygenated blood]] through the body's tissues. The cardiopulmonary system has both structural and physiological adaptations that distinguish it from other vertebrates. Turtles have a large lung volume and can move blood through non-pulmonary blood vessels, including some within the heart, to avoid the lungs while they are not breathing. They can hold their breath for much longer periods than other reptiles and they can tolerate the resulting low oxygen levels. They can moderate the increase in acidity during [[anaerobic respiration|anaerobic (non-oxygen-based) respiration]] by [[buffer solution|chemical buffering]] and they can lie dormant for months, in [[aestivation]] or [[brumation]].<ref name="Wyneken 2008 Circulatory"/>

The heart has two [[Atrium (heart)|atria]] but only one [[Ventricle (heart)|ventricle]]. The ventricle is subdivided into three chambers. A muscular ridge enables a complex pattern of blood flow so that the blood can be directed either to the lungs via the [[pulmonary artery]], or to the body via the [[aorta]]. The ability to separate the two outflows varies between species. The leatherback has a powerful muscular ridge enabling almost complete separation of the outflows, supporting its actively swimming lifestyle. The ridge is less well developed in freshwater turtles like the sliders (''[[Trachemys]]'').<ref name="Wyneken 2008 Circulatory">{{cite book |last=Wyneken |first=Jeanette |chapter=The Structure of Cardiopulmonary Systems of Turtles: Implications for Behavior and Function |editor-last=Wyneken |editor-first=Jeanette |editor-last2=Bels |editor-first2=V. L. |editor-last3=Godfrey |editor-first3=Matthew H. |title=Biology of Turtles |year=2008 |publisher=CRC Press |isbn=978-0-8493-3339-2|oclc=144570900 |pages=213–224}}</ref>

Turtles are capable of enduring periods of anaerobic respiration longer than many other vertebrates. This process breaks down sugars incompletely to [[lactic acid]], rather than all the way to [[carbon dioxide]] and water as in [[aerobic respiration|aerobic (oxygen-based) respiration]].<ref name="Wyneken 2008 Circulatory"/> They make use of the shell as a source of additional buffering agents for combating increased acidity, and as a sink for lactic acid.<ref>{{Cite journal |year=2002 |last=Jackson |first=Donald C. |title=Hibernating Without Oxygen: Physiological Adaptations of the Painted turtle |journal=Journal of Physiology |volume=543 |issue=Pt 3 |pages=731–737 |doi=10.1113/jphysiol.2002.024729 |pmid=12231634 |pmc=2290531}}</ref>

===Osmoregulation ===
In sea turtles, the bladder is one unit and in most freshwater turtles, it is double-lobed.<ref>{{cite book |last1=Miller |first1=Jeffrey D. |last2=Dinkelacker|first2=Stephen A.|chapter=Reproductive Structures and Strategies of Turtles |editor-last=Wyneken |editor-first=Jeanette |editor-last2=Bels |editor-first2=V. L. |editor-last3=Godfrey |editor-first3=Matthew H. |title=Biology of Turtles |year=2008 |publisher=CRC Press |isbn=978-0-8493-3339-2 |oclc=144570900 |page=234}}</ref> Sea turtle bladders are connected to two small accessory bladders, located at the sides to the neck of the urinary bladder and above the [[Pubis (bone)|pubis]].<ref>{{cite journal |last1=Wyneken |first1=Jeanette |last2=Witherington |first2=Dawn |date=February 2015 |title=Urogenital System |url=http://www.ivis.org/advances/wyneken/16.pdf?LA |journal=Anatomy of Sea Turtles |volume=1 |pages=153–165 |access-date=May 18, 2021 |archive-date=June 8, 2019 |archive-url=https://web.archive.org/web/20190608150657/http://www.ivis.org/advances/wyneken/16.pdf?LA |url-status=live }}</ref> Arid-living tortoises have bladders that serve as reserves of water, storing up to 20% of their body weight in fluids. The fluids are normally low in [[solute]]s, but higher during droughts when the reptile gains [[potassium]] salts from its plant diet. The bladder stores these salts until the tortoise finds fresh drinking water.<ref name="Bentley 2013">{{cite book |last=Bentley |first=Peter J. |url=https://books.google.com/books?id=U0D3BwAAQBAJ&pg=PA143 |title=Endocrines and Osmoregulation: A Comparative Account in Vertebrates |date=2013 |publisher=Springer |page=143 |isbn=978-3-662-05014-9 |access-date=May 18, 2021 |archive-date=May 31, 2021 |archive-url=https://web.archive.org/web/20210531050224/https://books.google.com/books?id=U0D3BwAAQBAJ&pg=PA143 |url-status=live }}</ref> To regulate the amount of salt in their bodies, sea turtles and the [[brackish water|brackish]]-living [[diamondback terrapin]] secrete excess salt in a thick sticky substance from their [[lacrimal gland|tear gland]]s. Because of this, sea turtles may appear to be "crying" when on land.{{sfn|Franklin|2011|p=31}}

=== Thermoregulation ===
[[File:Two basking cooter turtles (5861462496).jpg|thumb|Smaller pond turtles, like these [[northern red-bellied cooter]]s, regulate their temperature by basking in the sun.|alt=cooter turtles basking in sunshine near their pond]]

Turtles, like other reptiles, have a limited ability to [[Homoiotherm|regulate their body temperature]]. This ability varies between species, and with body size. Small pond turtles regulate their temperature by crawling out of the water and basking in the sun, while small terrestrial turtles move between sunny and shady places to adjust their temperature. Large species, both terrestrial and marine, have sufficient mass to give them substantial [[thermal inertia]], meaning that they heat up or cool down over many hours. The [[Aldabra giant tortoise]] weighs up to some {{convert|60|kg|lb}} and is able to allow its temperature to rise to some {{convert|33|C|F}} on a hot day, and to fall naturally to around {{convert|29|C|F}} by night. Some giant tortoises seek out shade to avoid overheating on sunny days. On [[Grand Terre Island]]<!--part of Aldabra-->, food is scarce inland, shade is scarce near the coast, and the <!--Aldabra giant--> tortoises compete for space under the few trees on hot days. Large males may push smaller females out of the shade, and some then overheat and die.<ref name="Pough Janis 2019"/>

Adult sea turtles, too, have large enough bodies that they can to some extent control their temperature. The largest turtle, the leatherback, can swim in the waters off [[Nova Scotia]], which may be as cold as {{convert|8|C|F}}, while their body temperature has been measured at up to {{convert|12|C-change|F-change}} warmer than the surrounding water. To help keep their temperature up, they have a system of [[countercurrent heat exchange]] in the blood vessels between their body core and the skin of their flippers. The vessels supplying the head are insulated by fat around the neck.<ref name="Pough Janis 2019">{{cite book |last1=Pough |first1=F. Harvey |last2=Janis |first2=Christine M. |chapter=16. Turtles |title=Vertebrate Life |publisher=Sinauer Associates |publication-place=New York |year=2019 |edition=10th |isbn=978-1-60535-607-5 |oclc=1022979490 |pages=283–299}}</ref>