Northern Red-bellied Cooter Sex Differences

I read today that there is some sexual dimorphism among our Northern Red-bellied Cooters. Though in many of our turtles the females tend to be larger, in Northern Red-bellied Cooters the difference isn't great; a large male and a medium-sized female are probably going to be about the same size, and unless you see turtles pretty close up, judging subtle size differences can be error prone.

But this article suggests that it's the females that have the distinctive reddish-orange plastrons while the plastrons of the males tends to be pinkish. And my field guide says that while the sides of the females look striped, the sides of the males look more mottled. That's why I'm suspicious that the 1st turtle below is a female and that the 2nd turtle picture is of a male (with a friend).
Bright orange plastron + stripey side = Female
March 16, 2023 at the Raritan Water Power Canal
Photo 267710927, (c) jpviolette, some rights reserved (CC BY-NC)

The presumed male Northern Red-bellied Cooter is the 1 on the right; for this discussion you can ignore the Painted Turtle on the left. (Or you can admire it while realizing that it has nothing to do with sex differences in cooters. It's your choice.)

Pinkish plastron + mottled side = Male
March 6, 2023 at the Raritan Water Power Canal
Photo 267703981, (c) jpviolette, some rights reserved (CC BY-NC)

A couple other points:
  • Though I see and recognize the females around here fairly regularly, I don't notice the males as much. This might be because their less distinctive plastrons make me less confident of what species I'm seeing. Perhaps I'm confusing them with Pond Sliders or River Cooters?
  • On the other hand, it's possible that we actually have fewer males. The sex of the turtles is related to the temperature of the eggs, and as the climate warms it seems more likely that more females will be hatching. This is actually 1 reason herpetologists are concerned about the long-term viability of turtles like these.
  • There is also a concern that they could be bred out of existence due to interbreeding with Pond Sliders, a species that's been introduced into our area. (If this can happen, I'm wondering if we've got our taxonomy correct. Pond Sliders are not just a separate species, they're in a different genus. I wouldn't normally expect animals in different genera to be able to hybridize.)

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