An orca's dorsal fin is held erect by collagen, which normally hardens in late adolescence.

Most captive orcas have a dorsal fin that is partially or completely collapsed to one side. Possible explanations for this phenomenon include:

• alterations in water balance caused by the stresses of captivity dietary changes;

• lowered blood pressure due to reduced activity patterns;

• overheating of the collagen brought on by greater exposure of the fin to the ambient air.

Collapsed or collapsing dorsal fins are rare in most wild populations and usually result from a serious injury to the fin, such as from being shot or colliding with a vessel.

Captive orcas - From Wikipedia, the free encyclopedia