Thursday, August 2, 2018

Sexual dimorphism in toothed whales: which are the differences between male and female cetaceans?

The odontoceti is a suborder that includes all the cetaceans (whales and dolphins) with teeth. In general, this group looks to follow the mammalian pattern of increasing sexual dimorphism with the increasing of body size. For example, the sperm whale (Physeter macrocephalus) is the largest toothed whale and the cetacean with a bigger sexual dimorphism (different characteristics in males and females of the same specie). 

Toothed whales present very few secondary sexual characteristics and are not very evident  - males are bigger than females and in some families there are differences in the number of teeth and size of the dorsal fin. Although in most of the cases is almost impossible to distinguish a male from a female dolphin in wild life, unless there is a clear view of the genital area.  The bottlenose dolphins, the common dolphins, Atlantic spotted dolphins and Risso’s dolphin are some of the examples. Males have two slits, the long one is the genital slit and the smaller one is the houses the anus. In females there is a continuous slit that are both the genital and anus slit and two mammary slits.

Some examples:

Orca (Orcinus orca) – dorsal fins of males can reach up to 2 m and in females only 1 m. In females the fin is slightly curved and in males completely vertical.

Pilot whale (Globiceplala spp.)– males can have a more swollen head with better defined melon. The dorsal fin in males is strongly hooked and very long-based.

Beaked whales (Ziphiidae family)– It is the second biggest group in the cetacean world and is the more mysterious one. In this group there is 21 species and all of them are different but in general only in males the tusks come out from the gums and females and juveniles are toothless. Only in a few species we can see functional teeth in both sexes.
Sometimes we can also distinguished a male from a female because of their behaviour. Just like us, cetaceans present a really strong bond between mother and calf, so when we see a baby is always associated with a female (no necessary the mother).  In sperm whales, for example, females live in groups with juveniles (males and females) and calves, adult males live solitary lives. So, in our tours when we see a baby we know immediately that the individual associated is a female. 

SHIRIHAI, Hadoram. Whales, Dolphins and Seals: A Field Guide to the Marine Mammals of the world: Bloomsbury, 2006.

Written by Inês Coelho

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