Herein we briefly provide a background context for and summary overview of the articles comprising this special issue of the Southeastern Naturalist on Macrochelys spp. (alligator snapping turtles). We also make a case for standardization in collecting and reporting morphometric data on these species, clarify the current taxonomic status within the genus, provide updated information on the distribution of Macrochelys in the US, and include a current bibliography and categorical analysis over time of the rapidly growing collection of literature focused on these iconic turtles.

Historically believed to harbor unrecognized diversity, the taxonomy of the declining genus Macrochelys (alligator snapping turtles) is debated. The original species, M. temminckii, was recently split into M. temminckii, M. apalachicolae, and M. suwanniensis. However, the status of M. apalachicolae is contested. In this study, we generated thousands of genome-wide loci to quantify population structure and genetic differentiation across the range of Macrochelys spp. Our data indicate that M. apalachicolae is genetically distinct, with little gene flow between M. apalachicolae and other species, thus adding evidence that M. apalachicolae may be a distinct species. We also find genetic variation partitioned among river drainages, with very high intra- and interspecific genetic divergence among river drainages. We suggest that translocations and re-introductions only move turtles in this genus within their natal river drainages to preserve existing patterns of genetic diversity.

With the conservation status of Macrochelys (alligator snapping turtles) being examined at the national level, our objective was to compile categorical data on threats from anthropogenic interactions. We included information from (1) author-collected anecdotes on human–turtle interactions and (2) radiographs to assess the prevalence of ingested fishing hooks. We placed 173 interactions involving 192 incidents into 9 IUCN threat categories and found bycatch involving fish hooks to be 4 times more numerous than the second-most numerous threat, turtle persecution. Fishing bycatch resulted in a high proportion of turtle mortalities (39%), and bycatch incidents in several cases preceded the highest-mortality threat (53%), persecution of individuals involving shooting or blunt trauma. We recommend fishing bycatch-mitigation measures and educational efforts to help conserve Macrochelys.

Head-start programs have been established to aid in the recovery of Macrochelys temmincki (Alligator Snapping Turtle) throughout the Mississippi River drainage. As the success of the breeding programs increase, additional release sites are needed where suitable habitat persists for reintroduction of Alligator Snapping Turtles, but methods to evaluate habitat suitability remain loosely defined. We developed a standardized field survey and habitat suitability model that can be used to assess and prioritize potential reintroduction sites. We measured important habitat features in the field and indexed them using a simple model that translated continuous data to an ordinal scoring system, resulting in an overall site-suitability score. We developed the model using 2 potential measurements of submerged deadwood density and determined habitat suitability among 17 sites in Kansas, Oklahoma, and Mississippi where Alligator Snapping Turtles were absent, present, and reintroduced. The model using side-scan sonar correctly predicted the presence of good habitat at sites where wild and reintroduced populations of Alligator Snapping Turtles were present, and scores were significantly related to Alligator Snapping Turtle abundance.

Macrochelys suwanniensis (Suwannee Alligator Snapping Turtle) is restricted to the Suwannee River drainage in Florida and Georgia. Over the last century, there have been scattered reports in the published and gray literature of M. suwanniensis sightings from the Okefenokee Swamp. This large wetland straddles the Georgia–Florida border and serves as the headwaters of the Suwannee River. Here, we discuss historic records of M. suwanniensis within the Okefenokee Swamp, provide details of recent sightings (2000–2021), including an extension of the known distribution to the northern portion of the swamp, and discuss the conservation and management implications of these observations.

We trapped 4 large alluvial rivers (Ochlockonee, Apalachicola, Choctawhatchee, and Escambia), 10 of their tributaries, and 22 other streams during 2014–2018 to determine the distribution of Macrochelys (alligator snapping turtles) in the Florida panhandle. We trapped three 5-km sections of the 4 large rivers and 130–1347-m sections of 32 smaller streams. We captured 179 turtles in 855 trap nights (TN) for an overall catch per unit effort (CPUE) of 0.21, recording the first observations from Cypress, Bear, Dry, and Wetappo creeks. We failed to capture Macrochelys in 13 streams but suspect populations occur in most streams of all types between the Sopchoppy and Perdido rivers with adequate water depths and cover in the form of deep holes, undercut banks, or fallen trees. We had low trapping success (CPUE = 0.012) in 243 TN in the Choctawhatchee drainage despite apparently suitable habitat. The male:female ratio was 1.44:1, which was significantly different from 1:1. The 4 large rivers had proportionally larger females than smaller streams.

Macrochelys temminckii (Alligator Snapping Turtle) is classified as a species of moderate conservation concern by the State of Alabama and is proposed for listing as threatened under the Endangered Species Act by the US Fish and Wildlife Service. Using the most up-to-date and best available distribution and biological data is paramount in the decision-making process for any petitioned or proposed-listed species. We provide an additional 21 Alligator Snapping Turtle records from Alabama collected as bycatch during fishery surveys. We incidentally captured 19 Alligator Snapping Turtles from the Alabama (n = 12), Tombigbee (n = 6), and Conecuh (n = 1) rivers and observed 2 additional specimens from the Tallapoosa River drainage. For both the Alabama and Tombigbee rivers combined, 89% of our Alligator Snapping Turtle captures were in backwater areas (i.e., off channel), which are new location records in these river drainages. Researchers and managers should consider contacting and collaborating with colleagues across scientific disciplines to compile and provide the best available data for current species assessments.

Macrochelys temminckii (Alligator Snapping Turtle) has experienced historical harvesting pressures that impact a number of current populations. Filling distribution gaps and obtaining demographic data can better inform conservation and management. We summarized multi-year samples targeting the Alligator Snapping Turtle in large river systems and major tributaries of Alabama. Although catch per unit effort was lower than observed in other states, we filled distribution gaps in several river systems. Though studies from other states documented effects of historical harvesting pressure on Alligator Snapping Turtle populations, mean adult mass of both sexes, mean carapace length, and capture ratio of adults to immatures suggested reduced pressures in Alabama and possible population stability. Nevertheless, smaller tributaries and creeks need to be surveyed for previously undetected populations with mark–recapture and movement studies.

Macrochelys (alligator snapping turtles), the largest freshwater turtles in North America, were recently proposed for threatened status under the US Endangered Species Act. Many previous surveys have focused on catching these large turtles in large river systems, but few surveys have focused on targeting hatchlings and juveniles, particularly in smaller rivers and creeks. I trapped extensively within the Pascagoula River drainage using small, baited crayfish traps, and a considerable focus of the study was in small rivers and streams. Juvenile Macrochelys temminckii (Alligator Snapping Turtle; 42.2–192 mm) were detected in small streams and large rivers (30.4 km²–22,000 km²), and the small traps were effective at capturing young alligator snapping turtles. Smaller streams are logistically harder to trap from a boat, but small streams should not be overlooked when sampling for this species, as these streams may have served as refugia during commercial harvest over the past century.

Macrochelys temminckii (Alligator Snapping Turtle) was recently proposed as a threatened species under the US Endangered Species Act due to current and projected future declining populations range wide. Despite a presumed statewide distribution in Mississippi, the Alligator Snapping Turtle had been documented in only 32 of 82 counties between 1857 and 2012, thus leaving a substantial gap in our knowledge of this species' distribution. Therefore, we collected credible records from museums, literature, and the general public, as well as directly assessed the current distribution and relative abundance of the Alligator Snapping Turtle in all major river systems (i.e., Pascagoula, Pearl, Tombigbee, and Mississippi river drainages) in Mississippi. From 2017 to 2021, we systematically trapped 77 sites across the state totaling 839 captures of 787 individual Alligator Snapping Turtles over the course of 4750 trap nights. State-wide catch per unit effort (CPUE) averaged 0.177, with the Big Black River (0.348) and Tombigbee River (0.028) drainages having the highest and lowest average CPUE, respectively. In Mississippi, CPUE was positively correlated with greater surrounding wetland area and river size, and negatively correlated with terrain ruggedness. This survey, along with the collaboration of other biologists and the public, verified an additional 189 locality records in 56 counties, with Alligator Snapping Turtle presence reconfirmed at 29 of 30 historical localities. Currently, it appears that there have been no local extirpations of the species in Mississippi, and state-wide CPUE was higher than those of the surrounding states.

The Alligator Snapping Turtle (Macrochelys temminckii) is a species of conservation concern across its range due to past commercial harvest for consumption. Northern Louisiana was originally surveyed in the 1980s, but the survey data were not compiled and published. After almost 40 years, we resurveyed the region to validate the historic data as well as provide current data on the distribution of Alligator Snapping Turtles. From the 1980s, we provide demographic data for 300 Alligator Snapping Turtles collected from 17 Louisiana parishes. From 2018–2020, we captured and measured 62 Alligator Snapping Turtles from 19 trapping sites. We recommend continued surveying and monitoring efforts in Louisiana to track populations during the post-commercial–harvest period.

Long-term studies on long-lived species are rare, as is the case for the Alligator Snapping Turtle (Macrochelys temminckii). Here, we developed a database of historical accounts drawing spatial and temporal data from local ecological knowledge (LEK) and non-LEK sources. Across all sources, we compiled 215 spatial and 263 temporal records. To test applicability of LEK, we conducted field surveys at sites selected based on LEK accounts, non-LEK accounts, and habitat. Our surveys yielded occupancy rates of 66.7%, 68.8%, and 40%, respectively, while detectability models estimated proportion of occupied sites to be 58–77%. Catch per unit effort did not vary when compared to previous assessments in Texas. Turtles were most active from February to September. Incorporation of LEK when developing study designs will ultimately enhance communication between researchers and local experts.

Texas contains the southwestern range edge of Macrochelys temminckii (Alligator Snapping Turtle), but there is relatively little published information on this species within the state. To document its range and assess temporal changes in its distribution and demography, we sampled 23 sites from 1999 to 2001. We then resurveyed 22 of these sites and sampled 29 additional sites in 2020–2021. Detection outcomes were consistent between 18 of the 22 resurveyed sites. Sex ratios and body-size distributions were similar across surveys. Catch per unit effort (CPUE) was lower in areas with trotlines, corroborating known interactions between turtles and fishing gear. Patterns in CPUE indicate Gulf of Mexico-draining watersheds are important systems for the species, while CPUE was lower in Mississippi-draining watersheds.

Understanding population demographics of Macrochelys temminckii (Alligator Snapping Turtle) requires long-term studies. Most previous studies do not provide demographic population data. We conducted a mark–recapture study of M. temminckii inhabiting Buffalo Bayou in Harris County, TX, from October 2016 through August 2021. We captured 110 unique individuals during 465 trap nights and recorded 45 recaptures. We generated 8 biological metrics. Capture per unit effort varied from 0.267 to 0.547 turtles per trap-night, the population estimate was 173 turtles (95% CI:138–250), the sex ratio was 1.00 M:1.28 F, and density and biomass estimates were 4.43 turtles/river km and 89.9 kg/km, respectively. Given that anthropogenic risks to this urban population may increase, continued monitoring and research is warranted.

Macrochelys temminckii (Alligator Snapping Turtle) were commercially harvested for consumption, and Louisiana was the last state to ban this commerce. There are no demographic studies on healthy populations within the state. We conducted trapping surveys at Black Bayou Lake National Wildlife Refuge near Monroe, LA, from 2004 to 2020. We captured 90 individual Alligator Snapping Turtles over 17 years. We compared survey data between the 2004–2005 survey period and the 2019–2020 survey period to estimate changes in demography. We found the population to be stable, but the low number of juveniles captured raises concerns for the future of the population. Continued monitoring is recommended to evaluate the stability of the population.

We studied the determination of sex for intermediate size classes and adult sexual dimorphism in a Georgia population of Macrochelys temminckii (Alligator Snapping Turtle). We sexed turtles when possible based on size and morphometrics. We took blood samples from captured individuals and measured plasma testosterone using enzyme-linked immunoassay. We examined females and individuals for which sex was unknown using ultrasound to determine sex and reproductive status. We used testosterone concentration to confirm individual sex assignments. For all biometric measurements, adult males were larger than females. Testosterone concentrations were higher in adult and subadult males than in females. Head width did not differ between the sexes when adjusted for body size. Plastron-to-vent length (cm) and testosterone concentrations were significantly greater in adult and subadult male turtles compared to females and were valuable in determining sex of smaller adults >25 cm straight-midline carapace length.

Geological processes influence water chemistry and biological productivity along the Suwannee River and divide the mainstem of the river into 6 distinct ecological reaches (ERs) in Florida. Because of these distinctions, we hypothesized that the Macrochelys suwanniensis (Suwannee Alligator Snapping Turtle) population varies among ERs. During 2011–2013, we sampled two 5-km sites in each ER to investigate spatial variation in relative abundance, sex ratio, size-class distribution, and body size of M. suwanniensis. Capture rates and male body sizes were greater in the middle ERs than in the upstream and downstream ERs. Sex ratios were male-skewed only in the middle ERs. Additional research is needed to understand the ecological drivers of M. suwanniensis population dynamics in this river.

We investigated nesting behavior and terrestrial activity in a captive population of Macrochelys temminckii (Alligator Snapping Turtle) housed outdoors in semi-natural environmental conditions in Oklahoma. The nesting season lasted from 12 May to 15 June 2012, and turtles were most active between the hours 2300–0300. Nesting duration averaged 182.5 min. Over 50% of nest construction time was spent excavating the cavity, whereas covering the eggs accounted for ∼28% of the total duration. Individual females averaged 25 non-nesting emergences before successfully depositing eggs. Terrestrial activity was positively correlated with increasing average nighttime temperature. We found thermal profiles generated by temperature data loggers affixed to females useful for studying terrestrial activity during nesting.

We report nest success for 16 Alligator Snapping Turtle nests oviposited at Black Bayou Lake National Wildlife Refuge in 2008 and 2009. We split each clutch into 2 portions for field and laboratory incubation. The field portion was protected from mammalian depredation. For each nest, we report both pipping and hatching success. Our total observed levels of pipping and hatching success were 55.9% and 47.7%, respectively. We obtained a hatching success rate of 85.8% in the lab. Reasons for the difference in hatching success between the field and lab included both abiotic and biotic factors in the nest environment, such as flooding, temperature, and invertebrate predators.

We studied nesting ecology and sex determination in a population of Macrochelys temminckii (Alligator Snapping Turtle) inhabiting the lower Apalachicola River in western Florida. During 1990–1991, we located 105 nests (63 intact, 42 depredated); 83% occurred on prominent dredged spoil substrates. Coupled with data from additional years, mean clutch size of 130 nests was 35.1 (min–max = 17–52). Most nesting occurred from 20 April to 18 May, with about a week annual variation. Hatching extended throughout August. In nests that we protected from predators, hatching success averaged 78% (24 nests) in 1990 and 66% (26 nests) in 1991. Overall sex ratios were roughly 1 male: 2 females, although individual nests varied from all males to all females in conjunction with solar exposure.

We studied the reproductive cycle of wild Macrochelys temminckii (Alligator Snapping Turtle) in southwest Georgia from March 2008 to September 2010. We measured plasma testosterone (T), estradiol (E₂), and corticosterone (C). Seasonal changes in T were observed in males, with maximum concentrations occurring in October. Evidence of mating and male combat were seen in April. Females displayed seasonal changes in T and E₂ concentrations, with peaks occurring in March and April, respectively, (in association with mating and ovulation) and again in September and October, respectively, (in association with follicular development and vitellogenesis). We observed oviductal eggs (mean clutch size = 28.5) in April. Concentrations of C were not significantly different between months. Reproductive patterns coincided with seasonal variation in temperature and water level.

Macrochelys suwanniensis (Suwannee Alligator Snapping Turtle) coexists with 2 other native large omnivorous turtle species (Chelydra serpentina [Snapping Turtle] and Trachemys scripta scripta [Yellow-bellied Slider]) in a 9-km section of the Santa Fe River in northern Florida. A major shift in dominant submersed aquatic vegetation prompted us to quantify trophic position and niche overlap among these 3 species. Here, we examine carbon and nitrogen isotopic values of these turtles and their potential food resources within the changing riverine system. We provide evidence of low isotopic niche overlap between M. suwanniensis and C. serpentina, whereas T. s. scripta occupied a discrete niche having lower carbon and nitrogen values.

Alligator snapping turtles (genus Macrochelys) are the largest freshwater turtles in North America. Despite strong interest in their conservation, we still lack a comprehensive understanding of their natural history, including behaviors associated with terrestrial activity. We reviewed the literature and solicited unpublished observations of terrestrial movements of these highly aquatic turtles. We compiled 206 observations of terrestrial activity from 9 states. The majority of observations were associated with nesting behavior. Observations expanded our knowledge regarding the spatial extent and timing of terrestrial movements and the nesting season. In addition, we compiled observations of terrestrially foraging animals, as well as males and juveniles traveling overland and crossing roads. Collectively, this information clarifies the extent of terrestrial movement by Macrochelys, behaviors that should be considered in conservation plans.

Combat has been described in captive but not in wild Macrochelys temminckii (Alligator Snapping Turtle). Combat may be seldom observed in the wild because of the habitats used by these highly aquatic, cryptic turtles. We report 1 observation of fighting in M. temminckii and 3 observations of fighting in M. suwanniensis (Suwannee Alligator Snapping Turtle) from Florida and Georgia. Combat involved wrestling and biting of heads and front feet, and it was usually accompanied by dramatic bubbling of the water surface from submerged turtles. Observations were made in September, October, and December, outside of the presumed courtship and mating season.

Macrochelys temminckii (Alligator Snapping Turtle) occurs at low densities in the Choctawhatchee River system in Florida. Translocation could help bolster the population, but its effectiveness for this species is not well studied. We released 9 adults into a cutoff of the Choctawhatchee River and radio-tracked them from Sept 2017 to May 2019 to monitor movements and habitat use. Turtles had significantly larger home-range distances than previously reported (12,882.8 ± 8781.5 m for males and 10,621.6 ± 6020.4 m for females), but all established a home range overlapping with 1 or more individuals. On several occasions turtles were found together, suggesting interaction and potential mating opportunities. This study suggests that translocation may be an effective strategy for augmenting populations of this species.

Macrochelys temminckii (Alligator Snapping Turtle) is a shy, well-camouflaged species that often inhabits turbid water. These factors, as well as the success of baited hoop traps in attracting the species at night, have created the perception that the species is predominantly nocturnal and does not bask. However, a growing body of evidence suggests that the species is also diurnally active and does bask. The use of binoculars and high-range cameras combined with extensive time in the field allowed us to document 25 instances of diurnal activity including behaviors such as basking, protrusion of the head above water, swimming, and foraging (i.e., entering baited traps). Our observations contribute to increasing evidence that both adults and juveniles exhibit basking and other diurnal behaviors.

The published literature does not provide a consensus regarding maximum body sizes of North America's largest wild freshwater turtles (genus Macrochelys). The largest known wild M. suwanniensis (Suwannee Alligator Snapping Turtle) currently exists as a curated specimen measuring 713 mm straight-midline carapace length, 801 mm maximum carapace length, 619 mm maximum carapace width, and 236 mm maximum head width. The turtle was never weighed, but we used morphometric data from our studies in the Santa Fe and Suwannee rivers to estimate that it weighed ∼76.4 kg (168 lbs) when captured. Our 95% prediction interval (67.6–86.3 kg [149–190 lbs]) suggests that speculative estimates and anecdotal reports of M. suwanniensis that weigh ≥90 kg (≥200 lbs) are dubious.

The lingual lures of Macrochelys (alligator snapping turtles) are believed to be the only prey-capturing lures within the mouths of modern reptiles. To date, no formal assessment of lure condition in Macrochelys has been published, and few researchers record lure data. Herein, we report damaged or missing lures from 25 Macrochelys temminckii (Alligator Snapping Turtle; 7 adults, 18 juveniles) from a sample of more than 2000 lure assessments in 4 states, indicating this is a rare occurrence. We also describe lingual lure color observed in these assessments and introduce standardized terminology and color categories. We suggest researchers record data on the condition and coloration of the lingual lure to further our understanding of this ecological and evolutionary adaptation.

Understanding feeding performance can inform feeding ecology and niche dynamics. Macrochelys temminckii (Alligator Snapping Turtle) and Chelydra serpentina (Common Snapping Turtle) are closely related, sympatric species with documented interactions. To understand ontogenetic and interspecific differences in bite performance, we measured bite force in an ontogenetic series of 62 Alligator Snapping Turtles and 33 Common Snapping Turtles. Within species, bite force positively correlated with size but scaled differently by species. Alligator Snapping Turtles produced higher maximum bite forces overall throughout ontogeny than Common Snapping Turtles, although Alligator Snapping Turtles reach significantly larger maximum sizes than Common Snapping Turtles, thereby enabling them to produce higher maximum bite forces. Differences in bite force between these species provide biomechanical context for distinctions in the ecologies of both species.