|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.
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.
* This does not include seaturtles.
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
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 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.|