Why are blue-footed boobies blue? 

Or more accurately: Why do blue-footed boobies have blue feet?

The Galápagos Islands are full of the weird and the wonderful. One of their most iconic species is the blue-footed booby (Sula nebouxii), a marine bird that is found all along the Pacific coast. It is best known for its blue feet (which it was very creatively named after), making it really cool to look at but, why are they blue? To answer this question, let’s break it down into the how and the why.

The how: What are the mechanisms that cause blue-footed boobies’ blue feet?

Our first answer is potentially a killjoy one; they’re not actually blue, they just appear that way! 

Colours most often come from pigments, which absorb specific wavelengths of light, but the blue in blue-footed boobies doesn’t seem to come from a blue pigment. Blue pigments are actually very rare in nature – the funky blue mandarin fish is one of the few examples of an animal that makes any.

Instead, the blue in blue-footed booby feet is likely a structural colour which comes from light being reflected. The way different structures on animal surfaces are organised can affect the way that wavelengths of light reflect off their surface – like in the diagram:

The blue-footed booby has a layer of collagen under its foot skin, and the structure of this collagen likely makes them appear blue by reflecting only blue light wavelengths. This is the case for the majority of blue animals.

Pigments might still play a role though because blue-footed booby feet sometimes appear slightly greener which might be due to yellow pigments (blue + yellow = green!). These pigments are carotenoids which animals can’t actually make themselves. Instead, they get them from their diets and the food they eat can influence what colour they are.

For example, flamingos aren’t born pink – they’re born grey but become pink because of the carotenoids in their food. Similarly, if you eat too many carrots you could turn orange (but please don’t, that would be bad for you)

.In blue-footed boobies, these carotenoids from their diet can affect how bright their feet are. Biologists tested this by changing how much food blue-footed boobies were given – when they didn’t get food, their feet became duller but when they were fed again with fresh fish, their feet became brighter.. This is super cool and also super important to keep in mind for our next question…

The Why: Why have blue-footed boobies evolved blue feet?

The reason that anything in nature looks or works the way it does is because of evolution. Darwin’s theory of natural selection says that traits that increase survival will be passed on so the fact that blue-footed boobies have evolved blue feet suggests they might be helpful in some way. But what advantage do blue feet give them? To answer this we need to understand sexual selection.

What is sexual selection?

Great question! It can be thought of as a special type of natural selection where traits that increase reproduction (instead of survival) will be passed on. This can include animals choosing a mate based on preferences for certain traits, which increases the chances of animals with those traits reproducing and so, the trait is passed on. For example, peahens prefer peacocks that have larger, more colourful tails which means large colourful tails get passed on over time!

As it turns out, female blue-footed boobies prefer brighter feet! 

We know this because biologists carried out some fun experiments – they used make-up to make male feets look duller (who says biology isn’t a very serious science?). When males had duller feet, the females were less likely to mate with them. Brutal!

Biologists also did this to males that had already mated with females (because blue-footed boobies don’t lay all their eggs at once). When the feet of these males were made duller, females actually made their second eggs smaller so that they’d hatch smaller chicks. Even more brutal!

This suggests to us that females are deciding who to mate with based on foot colour and if the blue isn’t as bright as they like they want to reproduce with those males less. So, foot colour might be a sexually selected trait because it increases the chances of reproduction for the males!

Why do female blue-footed boobies like blue feet?

As lovely as the blue is, it’s not just that they really like the colour. The stakes are quite high for them; they are choosing males to father their offspring and want to make a good decision. So, if they are basing this off foot colour, foot colour likely contains information that is quite valuable. It is likely a signal.

What are signals?
Signals are behaviours or structures that have specifically evolved to change the behaviour or state of others by conveying information. The response of the receiver must also have evolved due to signals – it is important to understand what receivers have to gain from responding to signals.

As we already know, carotenoids from their diet can influence blue-footed booby foot colour. What’s even more interesting is that these carotenoid changes can influence the immune response and foot colour also correlates to immune response.

Females might also be interested in how good of a parent males might be and foot colour might also signal this. When biologists swapped baby birds between nests, they found that the foot colour of the foster father was a pretty good indicator of condition (even though they weren’t genetically related).

So, to summarise: blue feet have potentially evolved because male foot colour might signal their condition, females want to reproduce with good condition males so they choose males based on foot colour.

What about female blue-footed boobies’ foot colour?

Male blue-footed boobies also seem to prefer brighter feet on females but the story is slightly less straightforward.

When female feet were made duller, the effects are mostly after they’ve already formed a pair, Blue-footed boobies form pairs then lay eggs but there is a courtship period before they lay an egg. In this period, females with duller feet received less courtship nest presentation (when birds add materials to nests) both from males they were paired with, and other males. 

In the period after egg-laying, making the female foot colour duller also had impacts on how much males incubated eggs. However, this was also affected by egg colour and size, which can indicate offspring quality. When females had duller feet, males incubated more in nests with a large egg but when they had colourful feet, males incubated both small and large eggs. Males also spent less time incubating small-dull eggs than small-colourful and large eggs.

So it seems that female foot colour is signalling something about their condition (and as an extension, the offspring’s) and the way males respond depends on the phase of reproduction they’re in. However, it also seems that foot colour isn’t the only useful indicator; egg size also is. 

One explanation might be that females have to decide between investing in their offspring (egg size) and signalling (foot colour) so foot colour might not give the whole picture. The mechanisms and reasons for this aren’t completely understood yet which just means there’s more left to learn about blue-footed boobies – exciting!

It’s not just blue-footed boobies that use signals

Nature is full of quirks! Beyond blue-footed boobies, evolution has brought about an array of interesting signals throughout nature. Animal signals are incredibly cool and incredibly diverse; they come in all shapes and sizes (literally) and some of them are also less honest than others. For example, some mantis shrimp display a claw that looks threatening to scare off intruders even when they are actually weak.

The signals that animals display will evolve to reflect the context in which animals live, which might be the environment or how they interact with species.

For example, yellow warblers (a species of small bird) produce alarm calls to scare off parasites. However, the yellow warblers that live alongside blue-footed boobies on the Galapagos don’t produce these calls. Why? They don’t need to!

They have been geographically isolated from their parasites for thousands of years, while the warblers on the mainland have not. These alarm calls either evolved before the Galapagos warblers became isolated, and they then lost the behaviour, or the calls evolved only in the mainland warblers afterwards. Either way, they reflect the lack of parasites on the islands!

Understanding why animals look or behave the way they do could be very valuable to us. For example, in the blue-footed boobies, their feet could show when their condition is declining. Since we know feet colour has likely evolved to signal their condition, if the foot colour changes, this could be because their food quality and availability has changed. This could signal to us that they’re in trouble!

So, as well as being incredibly interesting, learning more about nature and how species have evolved could also be important to protecting them.