The Tiny Turtles of North America

Clemmys guttata (Spotted Turtle), a native of the Eastern United States and among the smallest turtles. Taken from my Flickr.

Thanks to Turtles of the United States and Canada, I wondered if North America has an unusually high diversity of small turtles. While Spotted Turtles have a maximum strait carapace length of only 14.25 cm (5.6"), there are 8 smaller species on the continent and several of similar size. But, what is "small" for a turtle? To find out - and possibly demonstrate an ascension from geek to nerd - I used the Turtles of the World website to obtain maximum size data for 305 species* and placed them in size categories at 5 cm intervals (e.g. 10-15 cm, 15-20 cm), rounding up. The number crunching resulted in this:

* I upgraded Trachemys scripta subspecies to full species and used data from TotUSaC where available.

Minimum = 9.6 cm (Homopus signatus)
Maximum = 291 cm (Dermochelys coriacea)

The mode size category is 20-25 cm* (~19% of the total), although the average is 38.39 cm with a standard deviation of 32.44 cm. Since no turtles are less than a standard deviation from the average (thanks to seaturtles and island-dwelling tortoises) I'll consider the bottom 10% "small", which is just under 15.5 cm and roughly includes individuals in the 5-10 and 10-15 cm categories. As for how size varies within major clades:

* I apologize to any of my countrymen reading this, providing conversions from metric makes this even more unreadable. 

Kinosternidae range 12-37.9 cm. Mode 15-20 cm (52%). Average 17.01 cm (stdev = 5.5 cm).

Geoemydidae range 13-80 cm. Mode 20-25 cm (35%). Average 27.02 cm (stdev = 14.96 cm).

Emydidae range 11.5-60 cm. Mode 25-30 cm (25%). Average 28.47 cm (stdev = 9.38 cm).

Pleurodira range 12-89 cm. Mode 30-35 cm (22%). Average 29.99 cm (stdev = 11.29).

Testudinidae range 9.6-130 cm. Mode 20-25 cm (12.5%). Average 50.3 cm (stdev = 36.96 cm).

Trionychidae range 25-200 cm. Mode 60-65, 65-70% (25% total). Average 77.06 cm (stdev = 43.87 cm)

Including only these clades, one standard deviation less than the average is now about 18 cm, or 15 cm sans the freakishly big softshells. The remaining "family"-level clades have few species and extremely large body sizes (Cheloniida, Dermochelyidae, Chelydridae, Carettochelyidae, Dermatemydidae), with the notable exception of Platysternidae. So, let's consider turtles below 15 cm (6") small; I'll get back to this in a bit, but first, how do North America's turtles stack up against other continents*?

* This does not include seaturtles. 

Eurasia range 13-200 cm. Mode 20-25 cm (23%). Average 38.6 cm, stdev = 34.09 cm

North America range 11.6-80 cm. Mode 15-20, 20-25 cm (41% total). Average 26.84 cm, stdev = 12.66 cm

Central/South America range 12-89 cm. Mode 20-25 25-30 cm (33% total). Average 32.52 cm, stdev = 15.63 cm

Africa range 9.6-101.5 cm. Mode 20-25 cm (20%). Average 30.53 cm, stdev = 20.69 cm

It certainly appears that North America's turtles are smaller, and consistently so. It's worth noting that Turtles of the United States and Canada often had considerably larger figures than Turtles of the World, so it is possible that the maximum sizes for the other continents are understated. Now, to put that jumble of numbers into a more comprehensible form:

Note that these totals are percentages. Eurasia has 78 species, North America 68, Central/South America 55, and Africa 39. The patterns still occur with actual numbers, but are less apparent.

And now for that ever-trendy third dimension.

North America certainly does appear to have a trend towards smallness, although admittedly it isn't a strong one. Or is it. In accordance with action film tropes, there's a major aspect of this discussion I've suspiciously failed to mention - some turtles display the most extreme sexual size dimorphism of any tetrapod*. Barbour's Map Turtle (Graptemys barbouri) females are over twice the size of males (max strait carapace length of 33 cm vs. 13.5 cm) and Spiny Softshell (Apalone ferox) females display an even more pronounced dimorphism (54 cm vs. 21.6 cm) (Ernst and Lovich 2009); such linear differences imply females weigh at least 15 times as much as males. According to Turtles of the World, females from the geoemydid species Hardella thurjii reach an SCL of 61 cm while males only reach 17.5 cm - with females 3.5 times the linear dimensions of males, this implies a difference of at least 40 times in mass! For the most part it appears that North American emydids (i.e. most of them) have particularly pronounced sexual dimorphism, especially the numerous map turtles (Graptemys) (Gibbons and Lovich 1990). Unfortunately, digging up data for the remaining 200+ species of turtle without readily available data on sexual size dimorphism is far too much for this already ludicrous blog post, so I'm going to assume that there aren't dozens of hyper-dimorphic geoemydids out there.

* The record is typically given to the Green Anaconda (Eunectes murinus), although a survey of 177 males and 48 females found that in regards to means, females were 1.41 times longer and 4.68 times as massive; in regards to largest sizes, the female was 1.55 times longer and weighed 5.77 times as much. The study was found on Jesus Rivas's website, although I am not sure where it was published.

North American turtle sexual size dimorphism. Not every case involves females larger than males.

Counting males and females as separate "forms", suddenly a quarter of United States and Canada* turtles are in the 10-15 cm category, which seems unlikely for any other region to exceed. As to what conditions in North America would be ideal for small turtles - I have no idea. It seems unlikely to be predation (most of the species overlap in range with alligators, and all overlap with Chelydra) or climate... maybe it could just be a phylogenetic fluke?

* The data was not available for most Mexican turtles, hence I couldn't say "North American"

Glyptemys muhlenbergii, taken from Sophro's Flickr stream.

Stay tuned to follow-ups regarding the world's smallest turtle(s) and turtle sexual size dimorphism!

References:

Ernst, C. H., and Lovich, J. E. (2009). Turtles of the United States and Canada. John Hopkins University Press: Baltimore, Maryland. ISBN 13:978-0-8018-9121-2

Gibbons, J. W., and Lovich, J. E. (1990). Sexual Size Dimorphism in Turtles with Emphasis on the Slider Turtle (Trachemys scripta). Herpetological Monographs 4, 1-29. Available.