He’s a dark horse, the dark-spotted giraffe

New research finds that coat colour in male giraffes may function as a signal of social status, not age as previously thought. 

The study, led by Madelaine Castles of University of Queensland, Australia is the first to suggest giraffe colour is a secondary sexual trait.

Their patches can range from pale brown to completely black, with a tendency for males to get darker as they age.

“We now know that – rather than simply indicating age – colour may display males’ physical condition and be used as a way to signal competitive ability to others,” Castles says.

“Male giraffes’ colour may function in a similar way to the lion’s mane, as lions with dark manes are usually dominant and are preferred by females.”

The use of colour as a signal is known as a secondary sexual ornament, and in a dominance-based, polygynous mating system such as used by giraffes (Giraffa camelopardis) and lions (Panthera leo), these signals allow males to demonstrate their worth, whilst reducing aggressive encounters.

In lions, it is the darkness of the mane, whereas in mandrills, the colour intensity of their skin patches signals the males’ dominance and suitability as a mating partner.

The use of colour, especially dark colours as a signal, is “uncommon in mammals and has rarely been examined”, the researchers write. 

The prediction that colour is associated with behaviour in male giraffes required the team to rule out whether dark coat colour was associated with age, and secondly to assess their sociability or behaviour. 

To test this, they collected photographs, demographic and grouping data on giraffes living in Etosha National Park, Namibia, over a 12-year period.

From these data, Castles analysed the coat colour and sociability of 66 males. This involved assessing patch darkness of 1793 photographs of male giraffes. Calculations of gregariousness and sociability were also measured by time the giraffes spent alone or within a group.

The findings, published in the journal Animal Behaviour, show that, in general, coats darkened with age and the oldest males were the darkest.

But many males remained light coloured, and nine individuals actually decreased in colour during the study period – therefore age was clearly not the determinant, but rather something more complex.

It appeared that the giraffes’ behaviour varied according to the colour of their coats.

“We think that darker, more dominant male giraffes use an often-successful but risky mating tactic, roaming between groups of giraffes looking for sexually receptive females”, Castles explains.

“In contrast, the lighter, less dominant males may be making the best of a bad situation so to speak, by remaining with females in the hope of getting lucky when a dominant male is not around.”

In lions, mane colour and length are associated with a complex range of factors such as past injury, testosterone and nutrition levels, and even ambient temperature of the habitat.

The next step towards understanding colour in giraffes, therefore, says co-author Anne Goldizen is to “find out how colour could be a signal of a male’s condition”.

“Colour could be linked to testosterone, to heat stress, diet, genetics or a combination of multiple factors.

“And giraffes have recently been moved from ‘of least concern’ to ‘vulnerable’ on the IUCN Red List, so further research on these gentle giants is critical,” she adds.