Reptiles
Black-Knobbed Map Turtle
Graptemys nigrinoda · also called black-knobbed map turtles
Basking on a partially submerged log within a central Alabama river, a black-knobbed map turtle displays the distinctive row of raised, dark-tipped knobs running down its shell, a feature that sets it apart even among a genus already known for narrow, river-specific ranges.
Overview
The black-knobbed map turtle, Graptemys nigrinoda, is a river-dwelling turtle native to flowing river habitat within a restricted range in central Alabama. Its distinctive raised shell knobs, naturally narrow single-state range, and dependence on exposed basking and nesting sandbars make it a representative example of the map turtle genus’s characteristic pattern of fine-scale, river-specific endemism.
A shell feature turned up to the maximum
While several map turtle species show a subtle ridge along the shell, the black-knobbed map turtle takes that same feature and makes it dramatically more pronounced, turning a subtle family trait into its own defining characteristic.
Status
Least Concern, present within its naturally restricted Alabama river range, though vulnerable to dam construction and altered water levels affecting basking and nesting habitat.
Black-Knobbed Map Turtle: common questions
Why is it called the black-knobbed map turtle?
This species displays a row of raised, dark-tipped knobs running along the central ridge of its shell, a more pronounced version of the low ridge found in subtler form across several other map turtle species, and this distinctive feature gives the species its descriptive common name.
Why is the black-knobbed map turtle found only in Alabama?
This species is restricted to a limited number of river systems within Alabama, a naturally narrow range typical of many map turtle species in the genus Graptemys, several of which are each confined to just a single river drainage due to how the group evolved in relative isolation along different southeastern river systems.
How much bigger are female black-knobbed map turtles than males?
A female of this species grows considerably larger than a male, a size difference typical of map turtles generally, linked to the greater body size females need to produce and carry eggs, while males can complete their role in reproduction without needing the same larger body size.
Why is the black-knobbed map turtle vulnerable to dam construction?
This species relies on exposed logs, rocks, and sandbars for basking and on undisturbed sandbars for nesting, and dam construction or river management practices that alter natural water levels can destroy or submerge this specific habitat, posing an ongoing risk despite the species not currently being considered broadly threatened.