​I finally started working on turtles in 2007 thanks to my former student, Andrew Grosse, and most of what I know and appreciate about turtles, I learned from my former graduate student, Dr. Sean Sterrett. What I write here I attribute to them.

Turtles are evolutionary and ecological phenomenon unparalleled among vertebrates.

​Take a moment to think about where your neck, shoulders, arms, pelvis, and legs sit in relation to your ribs. If you got down on all fours, you realize that these features of your body have a similar arrangement to most other tetrapod vertebrates (amphibians, mammals, reptiles, and birds). the relationships of these features in your body are not much different from that of a lizard, crocodile, or sparrow.  Imagine that your spine, ribs, and sternum (and these chest bones in that you don't have) all expanded and fused to form a bone box. Then imagine that your shoulder girdle (clavicle, scapula), upper arms, pelvic girdle (hip bones, pubis, ischium, sacral vertebrae), and upper legs were all inside your rib cage. That is essentially the difference between you and other tetrapods compared to turtles. I would argue that a turtle is the most dramatic reorganization of the tetrapod body plan of any animal known on earth. And this form has persisted on Earth relatively unchanged for more than 200 million years.

Diamondback terrapin photographed on Jekyll Island, Georgia, J. C. Maerz, 2017

This skeletal rearrangement also represents a massive transformation of the ratio of body tissues among tetrapods. You, and most other tetrapods, are mostly muscle. Your skeleton represents 15% of your wet body mass. If we removed all the water from your body, your skeleton would represent 7% of your dry mass (our bones are 30%-35% water). In contrast, a turtle's skeleton represents 30% of its wet mass and an insane 70%-80% of its dry mass!
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"You are what you need to eat."
The differences in skeletal mass among animals matters physiologically and ecologically. You have heard the saying, "you are what you eat." That isn't really true. In reality, "you are what you need to eat." You and other animals benefit from consuming foods with a ratio more closely matched to your body ratio. When you consume food that has a higher ratio of one element than your body, then you have to excrete the excess element while conserving the more limited element. Imbalance in the ratios of elements such as phosphorus in an animal's diet and limit how fast they can grow.

You are more muscle than bone. Building muscle demands a lot more nitrogen than phosphorus. Humans have a nitrogen to phosphorus ratio of 4:1 - 3:1. Most animals have a nitrogen to phosphorus ratio of 16:1. Building bone requires more calcium and phosphorous. Turtles have a ratio of 1:1. ​There is no other vertebrate known that has a nitrogen to phosphorus ratio like a turtle. Calcium and phosphorus are elements that are naturally rarer in ecosystems and it is far easier to find foods that have a higher nitrogen to phosphorus ratio; so turtles must consume lots of food to support their growth and it takes time to grow bone. Bone is also a naturally slower growing tissue, adding to the time needed to grow for an animal that is mostly bone. Turtles have the slowest growth rates of any known vertebrate, often on the scale of millimeters or less per year. They also have some of the most prolonged ages to maturity, from 6-12 years for many smaller, freshwater species and as long as 30-40 years for some tortoises and marine species.

Green turtle, Great Barrier Reef Marine Park, Queensland, Australia, J. C. Maerz, 2017

So why would an animal evolve such a skeleton if it limits growth so dramatically? Well a shell has lots of uses. Obviously its can serve a good defense, making it difficult for predators to crush or access the softer body parts. Hiding your head in or under the shell makes it even more effective.

Diamondback terrapin, J. C. Maerz, 2009

Saw-shelled turtle, Carnarvon Gorge, Queensland, Australia, J. C. Maerz 2022

Fun fact: most people in North America and Europe are not aware that there are two lineages of turtles that "hide" their necks differently. North Americans are familiar with turtles that retract their necks into the shell (known as Cryptodires). But the rest of the world has a second lineage of turtles known as "side-necked" turtles (Pleurodires). The turtles have necks that do not extend strait but rather curl like a snake and can be folded around and under the shell. Two analogous adaptations for using the shell to protect the neck and head.

The shell also is an arch, making it very strong for attaching muscles. People are often surprised with the power of turtle legs and necks. The shell is also a create big surface for catching the sun during basking, allowing turtles to warm up faster. The skeleton is also a great reservoir for storing rare nutrients like calcium needed to shell eggs. So shells have costs, but they also have many perks. That is the nature of biodiversity. Different organisms exist because of the tradeoffs among different traits.

Female Barbour's map turtle, Baker County, GA, J. C. Maerz 2024

So here is how I think about "what a turtle is" ecologically. Turtles evolved an extreme skeleton that is slow to grow. As a consequence, maturity is substantially delayed, appearing to put turtles at a disadvantage. However, if you seen the turtle skeleton as a long-term investment, then it makes sense. Turtles are among the longest lived vertebrates on Earth, with many living decades and some living well over a century. There is a living tortoise estimated to be over 190 years of age. If you have 20 or 170 years to reproduce, then waiting 10-30 years to mature really isn't a big deal.

​We see this reflected in turtle life histories. Most turtles have adult female annual survival rates greater than 90%. So once a turtle makes it to adulthood, it has a very high probability of naturally living a long time. Importantly and different from other long-lived animals like humans or blue whales, adult female turtles will continue to lay eggs nearly every year of their adult lives. In fact, unlike humans or whales, turtles do not senesce (senescence is the gradual cessation of cell division that leads to inevitable functional decline). Humans begin senescence in our 20s (that biological clock got louder didn't it!), and whales begin senescence in their 30s and 40s. Turtles never begin senescence. They can get more fecund each year they manage to survive.

Turtles are also different than most long-lived animals such as humans or whales. Humans or whales have few offspring that require intensive prolonged care. In contrast, turtles can lay dozens to hundreds of eggs every year for decades or centuries. You can't really place turtles on the conventional continuum of animal life histories between r-selected (rapid maturity, large numbers of offspring, low adult survival) and k-selected species (delayed maturity, few offspring, high adult survival). Turtles genuinely are an ecological paradox. 

So the essence of turtle success is a strategy that puts a premium on patient investment early in life that pays off in the long-term ("slow and steady"). Critical to this life history is a premium on high survival of adult females, and this is exactly why turtles are struggling critically to persist in a human-dominated world.

Common musk turtle (aka "Stinkpot"), Baker County, GA, J. C. Maerz 2024

There are only about 360 known species of turtle worldwide. More than half are substantially threatened with extinction. Several species are extinct in the wild.

The reality is that too many human activities are incommensurate with turtle ecology. Humans have long harvested adult turtles and turtle eggs for food and they continue to be consumed in parts of the world today. Turtles could be collected relatively easily and stored alive for extended periods without the need for food or water. Unfortunately, for most turtles suitable for human consumption, females are significantly larger than males and therefore preferentially harvested. Turtles remain under heavy over harvesting pressure to sustain food and cultural medicine demands and a growing pet trade. Captive breeding and turtle farming are currently not sustainable and cannot meet global turtle demand. These practice still require the regular take of wild turtles, sustain a demand for turtles, and do not provide sufficient return of turtles to the wild to have a meaningful conservation impact.

F​emale turtles also particularly vulnerable because they must emerge from the water to nest. Increasing shore armoring to deal with sea level rise and increasing storm intensity interfere with female access to nesting areas. Nesting females freshwater turtle and terrapins are often struck by cars as females often attempt to nest in elevated areas alongside roads. Nesting females are vulnerable to harassment and injury by pets and predators subsidized by humans (e.g., raccoons), and predation rates on nests are unsustainably high in many developed areas.

Female diamondback terrapins attempting to nest near a coastal road and female terrapins killed by vehicle strikes while attempting to nest.

Adult female killed by a raccoon while nesting and her recently deposited eggs eaten and scattered. Right photo, Jekyll Island, GA, J.C. Maerz 2009

Turtles also face a range of other threats including habitat loss, impaired water and pollution, bycatch in commercial and recreational fisheries, derelict fishing gear, strikes by vehicles and boats, invasive species, and emerging diseases. Turtles even collide with efforts to develop alternative energy sources such as solar.

​It is hard to know turtle ecology and not see a very bleak prognosis.

But turtles are too unique, too special to write off. When turtle populations are healthy, they have important effects on freshwater ecosystems. Turtles also hold a special place in human culture. Turtles feature in the origin stories of many cultures. Many North American First Nations believe the earth was formed on the back of a giant turtle that carried mud from the bottom of the ocean, leading to the designation of North America as "Turtle Island". The shell's 13 scutes are associated with the 13 moons and 28 smaller markings with the human menstrual cycle. In Hinduism, the turtle, Kurma, is a sacred avatar of Vishnu who supports the world. In some accounts, four elephants stand on the back of a giant turtle to support the earth. In Aboriginal Australian Dreamtime stories, the turtle is depicted as a creature that evolved from a warrior named Wayamba protecting himself with shields. Mayan cultures revered turtles, associating them with water and fertility. The turtle's shell influenced their calendar systems, with temples and carvings dedicated to them. In Chinese mythology, the turtle symbolizes the entire universe, with its dome-shaped back representing the sky and its flat belly representing the earth. In some Polynesian creation myths, humans were born from eggs laid by a great World Turtle. In many cultures, turtles are connected to longevity, fecundity, protection, patience, and stoicism. Turtles routinely poll as one of the most popular animals in contemporary western cultures. The diamondback terrapin is the mascot of my alma mater, the University of Maryland [fear the turtle].

I believe humans have an obligation to conserve turtles. I also believe people have an intrinsic desire to save turtles and are willing to do what it takes to save them from extinction. ​