“Creatures who are entitled to regard the brontosaur and mastodon as brief zoological fads”
“Snapping turtles, embodiment of turtles who shared the earth with the dinosaurs for a time and are now obliged to share it with the human species, might well report that the former companions were far less stressful.”
Scientific Name: Chelydra serpentina serpentina Family: Chelydridae
Common Names: Common Snapping Turtle, Snapper, Mud Turtle
Snapping turtles, as we know them today evolved about 40 million years ago, and they are the ancestors of about 80% of all the turtles today. Today’s snapping turtles have hardly changed from 215 million years ago when Proganochelys, the most primitive turtle known, lived. Proganochelys had most features of today’s turtles, although it was unable to pull its head and legs into its shell. In comparison the age of the dinosaurs was approximately 150 million years ago, 100 million years more recent than the first turtle. Turtles were one of the few reptile groups, which survived the impact of a six mile wide meteorite on earth and the following nuclear winter about 65 million years ago, which is known as the K-T boundary.
Description of Species:
Snapping turtles are ectotherms like all reptiles. Ectotherms do not produce their own body heat, but draw warmth from the environment. However, they still regulate their body temperature, especially by sunning themselves in the summer. Snapping turtles prefer a body temperature somewhat lower than ours, of 82.5 F (28.1°C).
Like most reptiles, snapping turtles lay eggs. The incubation temperature depends on their gender. Hatchling snapping turtles are about the size of a quarter. An adult female snapping turtle's carapace usually measures from 9 to 14 inches. An adult male's carapace is usually from 10 to15 inches in lengh. And 10-year-old individual measures about 5.5 inches long. Growth rates in northern populations are slower and body sizes larger than in southern populations. The largest snapping turtle ever recorded was 18.5 in (47 cm) in carapace length. Weights of 35 to 45 pounds are reached. The heaviest snapping turtle ever caught in the wild weighed 68 lbs. The older snapping turtles get, the slower they grow, so the biggest individuals could be up to 100 years old! It is possible to determine how old a snapping turtle is from the rings on the shell. The oldest observed age for snapping turtles so far is 84 years, while the oldest age based on ring counts was 88. Since growth continues throughout life, very old individuals can conceivably grow very large.
There are two parts to a turtle shell, the carapace (on top) and the plastron (underneath). The carapace of snapping turtles is fairly flat, wider towards the back than in the front, smooth on the anterior edge, and serrated on the posterior edge. It is usually overgrown by algae. In juveniles the carapace is highly ornamented, in adults mostly smooth. The carapace of snapping turtles has 22 marginal scutes along the edge, a very wide nuchal scute right behind the head, 2 supracaudal scutes (above the tail), 5 vertebral scutes (down the enter line) and 8 costal scutes (4 on each side between the marginals and vertebrals). The plastron of a snapping turtle is very much reduced and is connected over two long and narrow bridges to the carapace.
Snapping turtles can therefore not hide as well in their shells as other turtles can, but have much better mobility. When they walk on land they can raise their body up from the ground, and only their tail is dragging. Both carapace and plastron are extensions of the vertebrae and therefore living bone. They are covered with keratinous material. As the carapace grows each of the scutes grows from the focus outward, creating annual growth lines in the keratin.
The head of snapping turtles is approximately triangular, and the mouth is large, with sharp jaw surfaces adapted mostly for cutting. Turtles have no teeth. Several barbles are located on the chin. Those probably serve as an increased surface for oxygen uptake. The neck is very long, and so is the tail which has three rows of spines (tubercules) on the upper (dorsal) side. The four legs are very short and massive with large curved scales on the front edges. As a turtle moves forward they lie flat against the skin, but when it pushes backwards with its legs in a swimming motion they stand up and provide an increased surface to push against. The feet are webbed and very large. They have 5 claws on each of the two front feet and 4 on each of the hind feet. The coloration of snapping turtles ranges from black to yellowish. Sometimes they can even be orange and black! And the jaws usully have black stripes.
Snapping turtles have an extremely good eyesight both above and under water. They can even see straight above their heads because of the position of their eyes. They also hear very well. Their ears are located in the normal location on the skull, but lack external structures and are therefore not readily visible. Anecdotal accounts also testify to the fact that snapping turtles probably have an extremely good sense of smell or taste.
Snapping turtles are a bottom walking species, which means that instead of swimming they crawl or bounce along over the bottom in shallow water. They can only float as long as they hold a significant amount of air in their lungs.
Identifying Males and Females:
Males and females look the same in general, but there are ways to tell them apart. Here’s an easy way to tell them apart. A male turtle's plastron is concave, and a female turtles plastron is flat and smooth. Simple as that.
Snapping turtles are highly aquatic and spend most of their lives in the water, except when they crawl out on logs to sun themselves. This behavior (called basking) occurs frequently in northern populations where turtles use it to warm themselves up to increase their metabolic rate and rate of energy assimilation so that they will have enough fat reserves for the winter. They prefer obstructed or covered areas to live in and prefer the bottom of rivers and lakes with soft mud (very important), organic debris, dense vegetation, and water lilies. However, they can readily adapt to a wide variety of habitats and are found practically in any permanent or semi-permanent non-moving or slow-moving body of water (ponds, lakes, marshes, swamps, ditches, puddles, salt marshes), as long as the water depth is great enough to allow them to hibernate below the ice, and cover or camouflage is available. Snapping turtles live a sedentary life style. They prefer to be able to reach the surface with their head while sitting on the bottom, and are therefore usually found only in water up to 3 feet (1m) deep, with depth of only 20 inches (50 cm) preferred in many cases, and a maximum possible water depth of 8 feet (2.5 m) (they only swim across deep areas but do not live in them). Hatchlings and juveniles live small streams with very shallow water and densely vegetated areas where they can reach the surface while standing on the bottom. Both hatchlings and juveniles are poor swimmers, and all snapping turtles can drown if the can no longer reach the surface. As the turtles mature they migrate to ponds, rivers, marshes, and the shallow areas of large lakes to establish their adult territory. Adults generally avoid the shallow juvenile and hatchling habitats, a behavior called habitat partitioning. Snapping turtles can go for up to 2 weeks without water, which enables them to make long overland migrations, or to swim in the ocean. They can even live in salt water or brackish water but have to periodically return to fresh water to rehydrate.
Snapping turtles are one of the few species, which can also live in significantly polluted habitats, including sewer systems.
Snapping turtles can live in any still or slow moving body of water from southern Canada (Nova Scotia, New Brunswick, southern Quebec, southern Alberta) to the Gulf of Mexico (Florida and the Texas coast), and from the Atlantic Ocean to the Rocky Mountains. Maine is very close to the northern edge of their range.
Population densities are scientifically given in individuals per hectare (10000 m2). To estimate densities in the American system all those numbers need to be divided by 2.4711 since one hectare equals that amount of acres.
Densities of snapping turtles range from 1 to 75 adults per hectare Density is negative correlated with latitude (it decreases as you move further north). This is probably due to two factors: lower reproductive success (and other life history constraints discussed later) further north, and less primary productivity (plant and subsequent animal growth) in the water. Reproductive success is determined by predation pressure on nests and hatchlings, climatic influences on hatching success, and nest-site availability. Density has been found to be generally higher in high productivity marshes and ponds (especially eutrophic systems) than in low productivity lakes and bogs (mesotrophic to oligotrophic systems). The highest observed densities for eutrophic ponds were 66 individuals per hectare. Densities appear to be higher in smaller bodies of water than in larger ones. Densities have been measured for many different populations. While 62.5 individuals per hectare were found in a West Virginia Pond and 59/ha in a Tennessee pond, only 16/ha were found in the Hudson River tidemarshes, 5/ha in Michigan, 1-2/ha in South Dakota and 1.5 to 2.4/ha in Algonqin Park, Ontario, their northern fringe. But also in Ontario densities of 60-70/ha were found in a eutrophic lake. Overall densities for northern populations are generally 1.5 individuals per hectare, which is far below habitat carrying capacity.
Large male snapping turtles have fixed home ranges, which are avoided by smaller individuals. They show high fidelity to those and stay often in exactly the same spot for many years. Over 10 years have been observed. The largest males are the most sedentary ones. They will even return to exactly the same location over several miles if relocated. Long distance movements for males would be disadvantageous since less females might be present in a different area, so they do not disperse. Home range boundaries are re-determined each spring by size-based dominance and aggressive interactions. Spacing is probably also determined by aggressive interactions. Large males dominate smaller males within their territory to insure their own mating success. The most desirable locations are those through which females migrate on their way to the nesting sites. The territoriality of large males increases the dispersal of immature individuals and regulates densities. Smaller males sometimes live in the same lake year after year, but do not maintain the same home range. Often sub-adult males and small adult males tend to disperse to different areas, especially when they approach maturity and get ready to found their own home territory. Home ranges for both sexes can overlap extensively. Territoriality has not been observed between individuals sharing areas. Home ranges range from about 3.2 m2 to 8.9 ha in size. Large males generally hold larger home ranges than smaller males. Home range size appears to be similar for males and females. In northern populations home ranges are relatively large. Home ranges in small lakes are generally smaller than in large lakes. In a large lake home ranges are arranged along the shores with an occasional crossing to the other side.
Females are more mobile than males. In many cases they occupy the same home range year after year, in others they shift home ranges after nesting, or have very large home ranges and occupy different parts of them in different years. Some females are believed to be transient and without fixed home ranges. Those individuals only nest once in one location and then move on. Adult females are believed to be the dispersal stage for snapping turtles. They are generally more mobile than males. Since they can retain sperm in their body for many years, they do not need to find mates every year to reproduce, and dispersal is not disadvantageous for them.
Home range preferences also depend on size and age. Juveniles for example prefer weedy shallows. When water levels are unstable home ranges also shift constantly as turtles move out of areas which are drying up. When whole ponds dry up its inhabitants will move over land to the next water. Those migrations are usually very quick, and individuals move up to 620 yards (567 m) in 2.5 hours. Roadways are often utilized to facilitate migrations. In tide marshes snapping turtles move between a summer habitat and a winter habitat. Home ranges which have become unsuitable are also left. For example the surviving turtles emigrated from a lake after extensive otter predation had killed part of the population off. Home range fidelity appears to increase with age. Both immature turtles and small adults seem to tend to disperse, since they are often seen in semi-permanent bodies of water or on land far from any permanent habitat.
Snapping turtles do require permanent bodies of water to survive, but can survive without water for at least 2 weeks. This allows them to make extensive terrestrial migrations from one body of water to the other and to also migrate through salt water to coastal islands and into estuaries. Still the rate of immigration appears to be extremely low in snapping turtle populations. (1 to 2 % annually) Since hatchlings and juveniles live in tributary streams a significant amount of this immigration rate will be young individuals from upstream. Hatchlings and juveniles are too small to move against a current and probably slowly make their way down into the lakes as they mature. Larger turtles often move into an area from downstream.
Food and Food Acquisition:
Snapping turtles are generally omnivorous. The main and most important part of their diet is aquatic vegetation (65%), so they can be considered primarily herbivorous. The second most important part of their diet is animal material including fish. Snapping turtles are only capable of taking slow non-game fish (small trash fish) and are absolutely no hazard to game and sport fish. They also eat carrion, frogs and other amphibians, invertebrates (mollusks and crustaceans), occasionally water fowl, and very rarely other birds and small mammals. However, they are not a threat to waterfowl populations. Snapping turtles can be considered ecologically important scavengers. Live prey is mostly eaten in the spring when vegetation is still sparse, and by juveniles. Once aquatic vegetation is available in the summer, it forms the main part of the adult’s diet. Feeding starts once the water temperature rises above 16°C. Snapping turtle eat only once their body weight in food per year.
The hunting technique of snapping turtles involves very little active movement. Snapping turtles usually forage on the bottom or lie still in the mud, waiting for something to swim by close. Even when they are lying still, snappers can still see perfectly what is above their heads. When they stalk prey they move extremely slowly, with the skin on their legs and between their toes folding to the body during the forward motion to reduce drag.
Once they strike the hooks on their jaw hold the prey firmly.
Addressing concerns of sportsmen I have to say that there is a strong indication that snapping turtles might have a positive impact on game fish populations by reducing the number and therefore disturbance by and competition from slow moving, bottom feeding fish (trash fish).
Interactions with Waterfowl:
It is frequently believed that snapping turtles have a significant impact on waterfowl populations. In fact research indicates that turtles present no hazard to waterfowl populations. Birds are only an incidental food and rarely taken, and even then only in localized areas. The frequency of birds in the diet varies widely depending on the habitat. The only instances where snapping turtles do impact waterfowl is in localized spots where turtle densities are very high and access to young waterfowl is easy, such as shallow streams with high bird densities. Even in those areas only a small percentage of the duckling population is killed. Even if turtle densities are reduced to below 0.1 turtles per acre (0.24/ha), duck brood survival increases only minimally. Turtle densities of more than 0.5 turtles per acre (1.3/ha) used to be considered detrimental for waterfowl populations, but an extensive study failed to find any differences in brood size and survival of birds between areas where snapping turtle densities had been reduced to 0 and areas where they had been left at natural levels. Only if snapping turtle populations are artificially raised to more than two times their natural density do they begin to have a small impact, which might then be due to exceeded carrying capacities. It turns out that external forces such as weather have a much higher influence on waterfowl brood survival than turtle predation. In many areas waterfowl populations also peak before turtles even begin to feed and are therefore definitely not impacted. Studies performed on turtle - waterfowl interactions have found that control of turtles would be undesirable and impractical. While turtles have practically no impact on waterfowl populations, turtle control has a hugely negative impact on the turtles since mostly nesting females are eradicated. Annual mortality rises suddenly sharply from the age of maturity on from about 1% before the onset of nesting to 47% for a mature female. In comparison the mortality rates for males stay at a normal 4%. This skews the sex ratio dramatically towards males. If in isolated cases an agency wants to control turtle populations it should be considered that large males, not females are the most important predators on birds. They could easily be hand-captured by divers and moved to locations where lower waterfowl populations exist.
In closing I would also like to address Washington County’s favorite snapping turtle rumor. Despite of what people say, there has never been any record of a snapping turtle eating a loon, and extraordinarily extensive studies were undertaken right here in Maine examining this question. We should always keep in mind that snapping turtles and waterfowl have coexisted for in some cases millions of years before the advent of man, and that there is no reason to expect that snappers will suddenly exterminate them. In fact they may play an important role in the prevention of the spread of diseases from sick ducklings.
In the winter snapping turtles hibernate in shallow water, buried in the mud in places which do not freeze to the bottom, usually in groups. During that time their body temperature is reduced to about 34°F (1°C), just above freezing. If the temperature drops further the turtles can freeze to death or be killed by the forming ice. During this time they hardly move, although some have been observed moving under the ice. Hibernating snapping turtles do not breathe for, in the northern part of their range, more than six month since ice covers their hibernating site. While hibernating snapping turtles can get oxygen by pushing their head out of the mud and allowing gas exchange to take place through the membranes of their mouth and throat. This is known as extrapulmonary respiration. If they cannot get enough oxygen through this method they start to utilize anaerobic pathways, burning sugars and fats without the use of oxygen. The metabolic by-products from this process are acidic and create very undesirable side effects by spring, which are known as oxygen debt. Hibernating snapping turtles are also very vulnerable to predators since they cannot move enough to defend themselves. At the site of the long-term study of snapping turtles in Algonquin Park, Ontario, the whole population of snapping turtles was effectively exterminated during a single winter, when otters ate the viscera out of the hibernating and defenseless turtles.
Snapping turtles in our latitude (Maine) begin to wake up in early May, when the water is about 41 to 50 F (5-10°C). Snapping turtles are diurnal, meaning active at daytime. The turtles begin to move around and bask, and in the females, eggs develop inside the body. When the water reaches about 60 F (16°C), by mid May, the turtles begin to eat. Male snapping turtles establish their home ranges, and some fighting can occur when dominance is reestablished for the year. Large males usually hold the same home territory year after year. The ideal territories are those containing several hibernacula (hibernation sites). As soon as the males wake up in the spring they expel the other males and mate with the females. Other males with less ideal sites move around looking for females elsewhere. Mating in snapping turtles is very aggressive, with the male chasing the female, and the female trying to escape and hide. The male grasps the posterior end of the female’s carapace, then mounts her, holds on to the edges of her shell with all four legs, often biting her head and neck, and inseminates her.
Around mid June nesting will start. The females undertake extensive true nesting migrations in search for a preferably sandy, sunny nest site. The males also start to migrate and aggregate in areas close to major nesting sites and on migration bottlenecks (narrow passages in migration routes) where females are abundant in order to intercept them and mate. They will move up to 2.6 mi. (4.2 km) away from their home range often to sites up river but return home right away once nesting is over. While males move for example up into the creeks, which the females use to migrate, they never leave their home body of water. Some females however will migrate 2.5 to 8.7 miles (4 to 14 km) one way through lakes and rivers to ideal spots. Migrating individuals often move relatively fast, and migration speeds from 2.1 mi. (3.4 km) to 8.6 mi. (13.8 km) in 10 days have been measured for movements to and from the nesting site. The mean speed of travel can be up to 1 mile (1.7 km) per day. Small streams are often used as trails to nesting sites and so are connections between different lakes. They are often rocky and shallow, and females only use them for their migrations. Many females will generally move upstream to nest, but others seem to prefer going downstream. Overland migrations also occur in some locations, and there the turtles always use the same fixed routes year after year. Those migration paths are significantly too long (547 to 1777 yards or 500 to 1625 m) to see a destination (up to 1 mile / 1.6 km away from the water), so snapping turtles must have homing abilities although we do not know how they work. They certainly are somehow capable of complex navigation such as overland travel through dense forest and over uneven terrain. Some overland crossings are extremely hard, such as a once observed trek of two females for 547 yards (500m) through forest with dense undergrowth. Nests have been found even 800m from the closest water. Some snapping turtles even use roads to migrate to nesting sites. The maximum round trip distance measure for a nesting migration was 10 mi. (16 km).
Ideally nests will be located close to small streams, where hatchlings will then spend their first couple of years, before they move into larger bodies of water. Many of the nesting areas are man-made such as dams, roads, or gravel pits. Before the advent of man snapping turtles nested more or less solitary on muskrat houses and abandoned beaver lodges. This change in nesting pattern might dramatically increase the amount of nest predation, since communal nesting sites are much easier to locate for predators. Preferred nesting sites where many turtles aggregate do exist. Dams for example are favorite nesting sites for resident and non-resident females alike, and several hundred individuals may nest on a single dam. It is believed that younger females probably migrate around until they find an ideal nesting site early in their reproductive life. This searching behavior might be especially important in northern regions where good locations might be scarce and limiting. A strong nesting site fidelity exists, so once a female has found a good nesting site, she often remains faithful to it throughout her life. Not all females nest every year. Since about 50 - 75% of the population returns to a given nesting site in any given year, most females probably do return to their nesting site whenever they are ready to lay eggs, which in some areas may only be every other year. Some females might also have several favorite sites between which they alternate. Generally nesting site fidelity ranges from 75 to 92 %. It has also been suggested that hatchling snappers may imprint on their site of birth and return to it once they are ready to lay eggs themselves, but nobody really knows.
Females will usually move very quickly to their nesting site and then remain in the water just offshore until they are ready to lay their eggs. Sometimes females may wander around for days, trying to find an ideal spot. The embryos in the eggs stop growing soon after fertilization until the eggs are laid, giving the female time to find the ideal spot. The female will dig a nest chamber first with her front legs, then with her hind legs, holding on to a ridge with her front legs. After about 1.5 hours the nest chamber will be completed, and the female will lay from 22 to 62 one inch (2.54 cm) eggs. After laying the eggs the female will fill the nest hole again with sand, press it down, camouflage the nest somewhat, and leave. Up to 90% of the nests will be destroyed by predators such as raccoons, skunks, foxes, and mink, often already during the first night. The female will move on right after nesting. Sometimes she will move to a different location to live for the year, but may return to her old nesting place the next summer. Other females return right after nesting to their old home ranges. The return trip home generally takes longer than the trip out (1 to 9 days). Some individuals remain at the nesting site for several weeks and really take their time returning home, only arriving in their original body of water at some point later in the summer.
By late August - beginning of September snapping turtles already begin to slow down again. This means that their maximum active period in Maine is only 5.5 month, compared to the 9 to 11 month active period further south. The period of greatest activity is even shorter, only from mid May to mid August. Even the spermatogenic cycle of the males (the period during which they can mate) is extremely short in our latitudes because of the short period of optimum temperatures. Testicular regression begins already by mid September. The eggs will hatch after 90 to 120 days, which is usually in September in this area. The hatchlings will dig out of the nest and will somehow head straight for the nearest water, even if they cannot see it. Cold temperatures in our area constrain and slow embryonic development. If the weather is too cold after hatching, the hatchlings may sometimes try to overwinter in the nest, which works well further south, where the ground does not freeze. In our latitude those turtles die before the spring. This is probably the main factor preventing the species from expanding much further north. Hatchlings are in and out of the nest heavily hunted by the same predators which eat the eggs.
While some snapping turtles are still moving around a little at the beginning of October, they will have all have moved to their hibernating areas by mid-October. Some individuals have hibernacula within their home range, but about 50% have hibernacula outside of their home range to which they travel up to 2.4 mi. (3.9 km). They then return to their home territory in the spring. Hibernating sites have to offer some access to either dissolved or atmospheric oxygen to prevent a serious oxygen debt by spring. In areas where ice forms hibernating areas need to be deep enough so they don’t freeze to the bottom. About 50% of the turtles choose to hibernate again at sites where they have previously overwintered, although not necessarily the previous year. Turtles often switch between different types of hibernacula from year to year, but have a favorite site of each type to which the return again and again. Even if individuals change their summer home range, they still return to their hibernacula. There are only a few favorite sites in each body of water, so many snapping turtles aggregate and hibernate literally in a pile together, staying less than 2 m away from the other turtles, and often stacked directly on top of each other. While the exact reasons for the group hibernating behavior are unclear, it unfortunately exposes them to predation by otters. Males hibernate on top of females, so it is possible that they serve to protect the females from both predation and the environment (such as thickening ice). In some areas where ponds are temporary snapping turtles might just bury in the mud during the summer. As water levels rise in fall and temperatures drop they simply continue to hibernate until spring.
Life History Parameters:
Snapping turtles have a problematic life history. Once snapping turtles reach about 3 inches carapace length, they have no more natural predators. Because the survival of eggs and juveniles is so low, that only a few in a thousand make it, adults have to lay many eggs over their lifetime to successfully reproduce.
Because adults have no natural enemies, they usually live a long life and die of old age during the winter. Like most animals, they will live longer in captivity than in the wild. In captivity, they can live to be 80 years and up. In contrast, they live an avarage of about 60 years in the wild. Here's two reasons why they live longer in captivity.
1) If they are old, they cant keep their body temperature as balenced as normal. Therefore, they wont freeze if they are inside.
2) When a snapper gets old, it can't hunt as good, and they cant swim fast enough to catch their prey. If it is kept in captivity, you can kill the fish, so it will float and it will be able to catch their food.
-Reproductive age and output:
A positive correlation exists between female body size and the number of eggs which can be laid in any given year. To maximize survival and therefore lifetime reproductive output, females have a constantly low reproductive effort with each clutch being only about 7% of their body mass. Depending on size, this translates into 11 to 87 eggs, with a mean clutch size of 28-49. Mean clutch size for northern population is 34. Clutch size relative to body size stays constant year after year. This enables the turtles to retain enough energy to survive through the winter. Snapping turtles also conserve energy by not reproducing ever year (reproductive frequency 0.85), and only about 72% of the females lay a clutch every year. It is unclear if and how females choose the years when they will reproduce if the do not do so annually, and climate might possibly play a role. To maximize the number of eggs that can be laid during a lifetime, a large body size at maturity becomes important, because once a female starts laying eggs her growth slows down due to her energy investment into the eggs. So it is selectively advantageous to start laying more eggs later than to start sooner with fewer eggs. A larger and older female will be able to lay large clutches for the remainder of its life, which in an undisturbed system would be very long. Snapping turtles delay their maturity until their initial clutch size will be about 22 eggs. Because females are selected for an ideal body size at maturity, male body size will follow since males have to be capable of mating with females and since it would be a waste of energy for them to be ready to mate while the female of the right size was still immature. Males fight for dominance and forcibly inseminate females, so a large size at maturity is also necessary for them. Consequently snapping turtles reach maturity only at the relatively large size of 20-25 cm. This size may be the minimum length at which enough energy is present to support both reproduction and survival, especially in a northern population with an extended wintering period. In northern areas this takes approximately 12-19 years for males and approximately 11 to 19 years for females, with age at first nesting being generally around 19 years. Generally faster growth rates in males may result in a slightly earlier maturity than in females. The average age of reproducing females is even higher, 34 - 40 years in northern populations. In comparison, by the time a snapping turtle nests for the first time, more than 10 generations will have passed for a deer. Further south growth is faster and age at maturity already occurs at age 4 to 12. Size at maturity however stays the same. There the age of nesting females might range from 8 to 24. Clutch sizes also increase with latitude, indicating that in northern populations a large body size at maturity with a resulting larger clutch size and a longer reproductive lifespan were favored over earlier maturity as it occurs further south.
Reproductive success is highly variable due to the unpredictable environment. Especially the weather during the incubation period plays an important role since embryos develop only at temperatures above 68°F (20°C). Because snapping turtles have TDS, temperature dependent sex determination, an incubation temperature of exactly 82°F (28°C) is necessary to maintain a 50:50 sex ratio. In northern populations short cool summers with high amounts of precipitation cause frequent years with complete reproductive failures. However, because survival from year to year is naturally so high for adults, reproductive failures in one year have normally little impact on lifetime reproductive success and population stability (this of course holds no longer true if adult mortality increases). Nests will also fail to hatch if they are located in a shaded area. Predation on nests is also extremely high. An average of 65% of nests are destroyed by mammalian predators (skunks, raccoons, mink, red foxes), but yearly variation is high. Especially favored nesting areas where many turtles nest together in a small area are vulnerable to predation, while isolated nests often stay undisturbed. Only about 14% of all clutches emerge annually. This low reproductive success is probably the factor which keeps densities in northern populations so low. However, the lucky undisturbed nests in good years can produce up to 50 hatchlings. Still only about 15 hatchlings will leave a successful nest (emergence success is only about 20 to 45%). If the air and surface temperature is too low hatchlings attempt to overwinter in the nest, a strategy which is successful in the south, where even the unhatched eggs can overwinter. In the north this strategy is fatal, and the hatchlings freeze to death. The northern extent of the species is therefore limited by summers which are long enough for hatching or soil which stays above freezing to allow eggs and hatchlings to overwinter. All those factors together cause huge fluctuations in reproductive success from year to year. It is possible that only one year of ideal climatic conditions for nesting and hatching out of 5 to 10 may be enough to maintain or increase the population, but only if nest predation is also low in that year. Predation on hatchlings and juveniles is still heavy especially during the first year, and only slightly lower during the 2nd and 3rd year. They get eaten by raccoons, mink, weasel, skunks, herons, and large fish while they are still under three inches (7.6 cm) in length. The probability of survival from egg to adulthood is 1 in 1445 individuals, the probability of survival from hatching to adulthood 1 in 133. This results for female snappers in a probability of death between hatching and breeding age of 99.17%. Annual recruitment into the breeding population (the number of juveniles reaching maturity in any given year) is only 1 to 1.8%.
In the northern parts of their range snapping turtles have a reproductive strategy which scientists call bet-hedging. Its characteristics are high and fluctuating egg mortality, highly variable density independent juvenile mortality, but low and constant adult mortality (long life), delayed maturity, low reproductive effort, annual reproduction, constant reproductive output independent of environmental factors, and smaller than maximum possible clutches. This is generally a response to an unpredictable and highly fluctuating environment, since an individual, which will reproduce with the same amount of effort every year will have a higher probability of actually reproducing during the infrequent favorable years where the whole clutch will survive. While delayed sexual maturity, greater size at first reproduction, and long lifespan give the benefit of an increased number of young per reproductive cycle, a better quality of young (better chance of survival), a decreased reproductive cost, and a consequently decreased mortality as an adult, it also has two distinct costs: the risk of death prior to first reproduction and longer generation times. Adults have to live and reproduce for a long time to keep population levels stable. Enough juveniles have to survive until maturity to eventually replace the adults. The distinct advantage of this strategy is enhanced lifetime reproductive success. In a system where nest predation is so high and unpredictable selection favors females, which divert energy from reproduction into survival and have many small clutches over their lifetime. Size at maturity becomes a much more important factor in lifetime reproductive output than age at first reproduction since in populations with a low rate of increase there is very little effect of small clutches early in life. The strategy is called bet hedging because it literally means saving your bets for later. A species which lives only for a short time and dies soon (such as rabbits) need relatively stable conditions to insure that at least two of their offspring will survive to reproduce again (genes continue on if the mother and the male are replaced by two offspring). Since snapping turtles in the north cannot predict which year will be favorable and allow their clutches to hatch, it doesn’t make sense for them to put their whole lifetime reproductive output into a few years at a small size and die young, since just those young might not make it. But if they instead delay maturity until their body is large enough to support reproduction without reducing the chance of survival, they can possibly lay eggs over maybe 60 to 100 years and be reasonably sure that a few of those will hatch in a summer where the weather is good and nest predation low. But this strategy only works as long as adult mortality is very low. As soon as it increases, adults will die without reproducing, and extinction will occur. It is therefore clear that with this reproductive strategy, adults which die prematurely cannot be replaced. This is especially true since populations with these life history parameters do not have density dependent reproductive responses, which means that a decline in adults will not lead to an increased survival of young, increased recruitment of adults, earlier age at maturity, or greater reproductive output. Most mammalian species can do several of those things to compensate for increased mortality. Therefore this species is basically predisposed to extinction, since it is extremely susceptible to increased adult mortality. The bet-hedging strategy was capable of bringing snapping turtles through several ice ages and climatic shifts since it is simply a constant slow waiting for better conditions, which individuals with such long lifespans can afford. Since they evolved there was only one factor which snapping turtles never had to face: adult death. Due to the increased killings of adults in our time (road mortality as well as harvesting) the whole strategy falls apart, and reproduction does in many cases not occur before the female dies. Protection of adults is therefore the most important conservation issue. Studies have found that snapping turtle populations in the north are apparently non-sustainable and rely on immigration from other areas. Statistical tables of survival data have also shown that northern populations do not reproduce enough to even continue to exist. One possible explanation is that we are already looking at a slow continuous population decline.
Aggression and Danger to humans:
Snapping turtles are not as aggressive as commonly believed. They will defend themselves if cornered and cut off from the water by striking out with their head, which can reach almost all the way back along the shell. In the water snapping turtles will always leave instead of attack. Snappers do not attack people in the water. Over the course of my research I have handled over 50 different wild snapping turtles ranging up to 16 inches in size and have never been bitten. Because of their position in the food chain snapping turtles are not afraid, but they are also not aggressive. An annoyed snapping turtle will back into a corner and lunge at you, stopping about an inch short of you. If you do not move away, it will lunge at you with open jaws but close them a fraction of an inch before it touches you and just bump you. Only if you violate a very tight zone around a snapping turtles head with a small object (something it judges it could get its jaws around) while the turtle is annoyed it will sometimes strike, bite, and sometimes hold on. This can of course also happen if a snapping turtle is injured. If a snapping turtle does not fee threatened it will in most cases tolerate extensive handling including having its mouth and nose touched. Even if a turtle bites it has no intention of hurting you. Snapping turtles often play bite their conspecifics without injuring them. Our delicate skin just gets injured much more quickly. Do not attempt to remove the turtle if for some reason it has bitten and held on. It will let go by itself after a short wile if you leave it in the water. Killing a snapping turtle will cause significantly more injury to you since the jaws will contract after death even more. But again, snapping turtles in the wild do not bite people. Many individuals are very curious and will approach swimmers or boats very closely. They examine things by touching them with their nose, so a very curious snapper might bump a swimmer carefully. Their curiosity for boats often seems to cause injuries by the propellers, since especially turtles in lakes with high boat traffic frequently have scarred backs. There is absolutely no truth to he rumor that snapping turtles will drag swimmers under. They do not even hunt in water deeper than we can stand up in. Most of the snapping turtles I have ever worked with were very curious and friendly individuals. Others appeared to be simply bored with my presence and hurried to move off.
The worst threat to snapping turtles in Maine is vehicle traffic. Each year many females get killed in their search for nesting sites. Often vehicles will not stop or even deliberately hit turtles because snapping turtles are disliked by many people. Nests on road sides and in gravel pits are often destroyed by vehicles and road grading. Hatchlings on their way back to the water are frequently run over.
Snapping turtles are frequently trapped. Because they are seen as competitors for game fish and waterfowl, there are no adequate laws protecting them. Stocks are completely unknown, and trapping virtually unregulated. In Maine for example the number of snapping turtles which can be taken is unlimited. A free commercial trapping permit is only required if the turtles are sold. The only protective law existing for this species is that turtles can no longer be run down with a car. Every year 6000 to 8000 adults are trapped in Minnesota, 5000 to 8000 in southern Ontario. 60,000 snapping turtles from all over the United States are each year killed in a slaughterhouse in Iowa. Demand for snapping turtle meat has increased dramatically since marine turtles and terrapins became protected. According to Developing Management Guidelines for Snapping Turtles, a US Forest Service Report, the “northern population [of snapping turtles] cannot sustain even minimal exploitation by humans”. Other studies have also concluded that northern populations of snapping turtles need to be completely protected to continue to exist. Long lived vertebrates in general have practically no resistance to increased adult or juvenile mortality, especially if they have delayed maturity and an increased reproductive lifespan. Because of the high juvenile mortality the removal of adults produces a decline in numbers, which cannot possibly be replaced by juvenile recruitment. Because of their life history parameters, which rely on adult longevity, snapping turtle populations cannot tolerate increased adult mortality. Harvesting or any form of exploitation could cause quick population crashes and recovery through reproduction would be extremely slow, or probably not possible at all. In fact the protection of adults and juveniles would be much more important than hatchling or nest protection. Even an increase of adult mortality of only 10% annually would halve the number of adults in 15 years. Even much more productive populations further south with a mean clutch size of 45 and an age at first nesting of 10 could only barely sustain an increase in adult mortality of 10%. Historically especially hibernating turtles have been trapped, an especially destructive technique which very quickly destroys a population. Trapping in general always removes the largest and most productive individuals, which cannot possibly be replaced. In areas which are regularly trapped especially females have a lower life expectancy since they are killed on their way to the nesting beaches. Almost all of the larger turtles which remain are males, which further decreases the reproductive output of the population. In fact trappers have already reported drastic declines in northern populations where adults have been trapped. Studies have shown that snapping turtle harvesting activities generally halve the adult biomass within 4 years with recruitment being unable to compensate. Major population declines from harvesting might also be masked for a few years through the growth of the remaining juveniles and their recruitment into the adult class, although no more reproduction is taking place because of the long generation time of this species. If harvest has to take place it should at least be restricted to small and medium-sized males and nesting females should be protected.
Snapping turtles in many cases would also not be suitable for human consumption since they carry extremely high loads of persistent environmental pollutants such as PCBs (up to 10,000 ppm were for example found in their fat, brain tissue, and testes). Those levels would be immediately deadly for a human.
To Help Snapping Turtles:
· Drive slowly during June and be on the lookout for snapping turtles crossing the roads, especially from dawn to noon, and again in the evening.
· If you need to move a snapping turtle quickly simply pick it up by the back edge of its carapace and carry it across the road holding its head away from you.
· Snapping turtles especially during their nesting migrations will frequently snap. However, in the great majority of cases they simply strike in the air or touch you with closed jaws. They do not intend to bite as long as you keep your fingers away from their head.
· Do not hold snapping turtles by the tail because that can cause serious injury to them.
· Do not eat turtle meat.
· Do not collect snapping turtles and sell them to stores. They will only be sold as turtle meat.