What are secondary sexual traits?
The horns of wildebeest are an example of a secondary sexual trait, which are characteristics that are not directly involved in the reproductive process but can have an influence on it.
Traits such as the tail of a male peacock, or the inflatable membrane of a hooded seal, can demonstrate the health or virility of an individual to potential mates and competitors.
Where these signals are directly linked to the health of an individual, or take a lot of effort to produce, they are considered 'honest signals' because they are difficult or unrewarding for weaker animals to fake them.
This means these characteristics show distinctive growth patterns that should be visible in both living and extinct organisms.
'Features such as horns which are used for display exhibit a characteristic known as positive allometry, where larger animals have proportionally bigger traits,' Andy explains. 'Male bovids all have these traits, but so do around half of females.'
'These traits are under strong selection in males to help them compete, so in females we would expect there to be a substantial difference in this pressure.'
The researchers investigated whether this pattern could be observed using 75 blue wildebeest skulls, with the vast majority coming from the Museum. Each skull was digitised, with its dimensions being used to calculate its precise 3D shape.
When shape alone is considered, there is a clear distinction between the horn shape of males and females, with both sexes demonstrating a shared allometry. However, these differences disappear when corrected for size.
'When you look at the distribution of the data, a small male looks to all intents and purposes like a female,' Andy says. 'All the animals pass through the same size range, but females generally don't grow to the largest end of the spectrum.'
It suggests that males only have distinctive horns because they tend to grow larger than females, and that the horns of both sexes experience similar levels of selection.
It has been suggested that selection for female horns could relate to predator defence, fighting other females, or even male mimicry, but the evidence for these theories is currently limited.