Red Baboon Bottoms As Sexual Traffic Lights

Vicki - Seasonal or sexual swellings are the big red bottoms that you see when you look at certain baboons and macaques, and are invariably the things that visitors love to point at. These big bottoms are a signal that the females are ready for breeding. Different individuals will have different sizes of sexual swelling, different colours and turgidity. What we're particularly interested in is that in different zoos it would seem that these characteristics clump together. For example, in zoo A, you might get particularly large swellings, whereas in zoo B, they might be a lot smaller. We're interested in finding out what factors contribute to these differences between institutions. It could be genetic or it could be nutrition. If it's the latter, we need to moderate our husbandry to ensure that the swellings remain pretty consistent between institutions.

Chris - Because there are obvious implications for when these animals go back into the wild.

Vicki - Yes, there are implications for breeding in zoos and also for conservation. In terms of reintroducing animals back into the wild, if these swellings have been sustained by very very high nutrition, then in the wild, these swellings might not be sustainable because the animals can't get enough food. Equally, these bottoms can become so large that the animals may have trouble escaping form predators or locomoting. It sounds very amusing, but it could be quite an important thing that we need to look at.

Chris - So are you thinking that breeding these animals in zoos, you're slowly selecting for a bigger and bigger bottom genetically. You might be breeding a population of super bottoms.

Vicki - The bigger my bottom, the more fertile I am, therefore I'll have more offspring. This will mean I'll have babies that will go on to have bigger bottoms. At some point there has to be an environmental factor that stops these bottoms getting bigger than they can be sustained. Not only can this tell us what is the function of these bottoms as signals, we might also be able to tell which of our females is the most fertile. It might be by size, or it might be the colour. The reddest bottom may be the most attractive to a male. These are very pure reasons for us to be interested in this. On the applied side, it's very important for us as a zoo to understand the implications if we are breeding animals that don't have this environmental cut off. There bottoms shouldn't be allowed to just keep getting bigger.

Chris - Now to get to the bottom of the problem, if you'll pardon the pun, why do their bums actually go red? What's going on?

Vicki - It's something that when the males goes past he cannot miss that this female is ready for breeding and for certain primates, the colours change as the female gets closer to ovulation. If you're a male in charge of lots of different females, then they may be coming into season at different times and you want to make sure that you don't leave that female over the couple of days of ovulation. Sometimes the bottoms can get reddest when that female is ovulating. The make wants to stay net to her so no other males can mate with her first.

Chris - So it's a bit like a sexual traffic light isn't it?

Vicki - It most definitely is, but red doesn't mean stop in this scenario.