The numbers
Researchers sampled the middle Xingu River in Brazil from 2012 to 2014 and sexed 154 redtails: 66 males and 88 females.
| Males | Females | |
|---|---|---|
| Average length | 63.5 cm | 68.0 cm |
| Longest | 119.0 cm | 125.4 cm |
| Average weight | 6.7 kg | 8.7 kg |
| Heaviest | 26.6 kg | 40.0 kg |
The weight difference was statistically significant; the length difference was not. Males also grow differently: they get longer faster than they get heavy, while females fill out evenly as they grow. The authors suggest that faster growth in length lets males mature sooner, and that females carry more weight because of the energy that goes into eggs.
Telling them apart by eye
In adults, look at the head:
- Female A bigger, flatter, wider head. In side view the profile runs almost straight from the dorsal fin to the tip of the snout. Grows faster and ends bigger.
- Male A narrower head with a rounded profile from dorsal to snout. Grows slower and ends smaller.
It is a guide, not a test, and easiest with two adults side by side. If your biggest fish is your flattest-headed one, it is very likely a female.