Graptemys nigrinoda Cagle, 1954

Common Name & Carapace Shape

Graptemys nigrinoda, commonly called the black-knobbed map turtle, has a slightly domed carapace.

Carapace Vertebral Knobs

The first four vertebral sections of this carapace bear black, backward-projecting knob-like processes, with the second and third processes larger than the first and fourth. In older females, these knobs shrink down to small swellings.

Carapace Base Color

The overall color of the carapace is dark olive-brown.

Pleural Plate Markings

Each pleural plate of the shell contains yellow-green circular rings outlined in black.

Hatchling Carapace Traits

Hatchlings share the same base color pattern as adults, but their colors are more vibrant and contrasting, and their knob-like processes are compressed laterally.

Head Markings

This species has a small dark brown head marked with yellow stripes, including yellow crescents that point toward the posterior end of the turtle behind each eye.

Limb Coloration

These yellow stripes continue onto the legs, and the underside of each leg is lighter than its dorsal surface.

Sexual Dimorphism Traits

Clear sexual dimorphism exists in Graptemys nigrinoda: adult females are approximately twice the size of males, have higher carapaces than males, while males have longer tails than females.

Carapace Length Range

Recorded carapace length ranges from 7.6 to 10.2 cm (3.0 to 4.0 in) for males, and 10.2 to 19.1 cm (4.0 to 7.5 in) for females.

Endemic Range

The black-knobbed map turtle is endemic to the southeastern United States.

Specific Distribution Areas

It occurs in the Mobile Bay drainage in Alabama, and in the Tombigbee River system in Mississippi, as well as in the Black Warrior River north to Jefferson County, Alabama.

Habitat Restriction

It can only survive in fresh water, so it is restricted to freshwater river systems.

Annual Activity Period

Black-knobbed map turtles are active seasonally from April to late November.

Basking Behavior

They routinely bask in early morning and early afternoon each day. Basking is thought to serve the purposes of thermoregulation, plus removing parasites and algal growth.

Anti-Predator Response

When approached, these turtles jump into nearby water, then seek shelter between the branches of fallen trees on the river bottom.

Adult River Habitat Traits

Most inhabited riverbeds have sand and clay bottoms and moderate currents.

Hatchling Habitat Preference

Hatchlings prefer slower, more sluggish waters off the main river channel.

Foraging Behavior Knowledge

Little is known about this species' overall foraging behavior.

Observed Prey Items

It has been observed eating beetles and dragonflies that have fallen into the river.

Sex-Based Diet Differences

A study of stomach contents by Lahanas found a difference in the percentage of food types between sexes: males had approximately 58% animal matter and 40% plant matter, while females had 70% animal matter and 29% plant matter.

Diet Composition Details

The top three sources of animal matter for this species are freshwater sponges, bryozoans, and molluscs, and the only plant matter found was a type of freshwater alga. This species feeds primarily on insects.

Sexual Maturity Age

Males reach sexual maturity at 3 to 4 years old, while females reach sexual maturity at 7 to 8 years old.

Clutch Production

Females produce clutches of roughly five eggs, and can lay three or four clutches in a single year.

Nesting Period

Nesting takes place from May to August, occurs at night on sandbanks.