Why did male and female huia have different beaks?

Why did male and female huia have different beaks?

The beaks of male and female huia are so different that the two sexes were originally described as two separate species. But what is the reason behind that difference? Natural History Curator Rodrigo Salvador takes us through recent research.

Huia, or Heteralocha acutirostris, were distributed through most (if not all) of the North Island of Aotearoa New Zealand. Their numbers declined fast after Polynesian settlement, to the point that by the time Europeans arrived the species was already restricted to the southern end of the North Island.

The last known huia specimen dates from around 1907 and is now kept here at Te Papa, one of our saddest and most meaningful taongataonga treasures Māori | Noun | listen (treasure).

Huia had the most extreme male/female difference in beak shape known in all birds. Accordingly, many naturalists at the time tried to figure out why.

The longer sickle-shaped beak of the female huia, Heteralocha acutirostris, collected North Island, New Zealand. Purchased 1949. CC BY 4.0. Te Papa (OR.005516)
Male huia, Heteralocha acutirostris, collected 1899, Makara, Wellington, New Zealand. Gift of Mr L.J. Vangioni, 1942. CC BY 4.0. Te Papa (OR.011063)

Most believed that a huia pair would feed cooperatively, with each sex complementing the diet of its mate with the different types of prey their beaks allowed them to catch. That idea was – and still is – widespread in popular accounts of the huia, but it is a misinterpretation of 19th-century reports.

Others held that the beak shape was a secondary sexual character used in courtship, like a peacock’s tail. But female huia had the extreme beak shape, not the male. So, that doesn’t quite fit with what is known about sexual selection in birds.

A third idea was that the beak shapes allowed huia to feed on different prey to avoid competition for food within the pair. That is the most likely idea and the one we wanted to test.

But given that the huia is extinct and all, it is very difficult to get an actual answer. So, we turned to chemistry.

The “magic” of isotopes

Chemical elements typically come in more than one form. Broadly speaking, most atoms of any given chemical element will have the same weight – but a few atoms can be heavier.

For instance, carbon has three naturally-occurring forms, with molecular weights of 12, 13 and 14, and each of these forms is called an isotope. The carbon isotope of weight 12 (or 12C) is by far the most common, while 13C is quite uncommon and 14C is very rare (not to mention radioactive).

Everything is made up of chemical elements, so we can measure the proportions of isotopes of a given element in all sorts of material. We can, for instance, measure the isotopes in an animal’s body. But why would we want to do that?

The isotopes contained within our bodies reflect what we have eaten and also the environment where we live and where our food was grown. We are what we eat, so to speak.

As such, we can use the information of isotopes to extract ecological data. This is a very useful tool for gaining knowledge on species that are difficult to observe in the wild. Or impossible to, like, say, an extinct New Zealand bird.

Museum feathers

So, we measured the isotopes of carbon and nitrogen in huia specimens. Carbon and nitrogen are good sources of information about what the animals were eating. So, we could use that as a way to tell if male and female huia were feeding on different things.

We used bits of feathers from taxidermied museum specimens for measuring the isotope content.

Our specimens here at Te Papa weren’t near enough, unfortunately. So, we obtained additional huia feathers from museums all around the world: Cardiff, Christchurch, Copenhagen, Helsinki, Invercargill, Paris, Pittsburgh, Stockholm, and Vienna.

Isotope data

Our data showed that the isotope signatures of male and female huia were not as distinct as we would have expected if they were feeding on completely different things. The isotope signatures were different, but they had a large overlap.

An x-y graph with circles and squares in different colours plotted on it
Graph showing the isotopes of nitrogen (δ15N) vs carbon (δ13C), with circles representing females and squares representing males. The ellipses delineate the “average” isotopic niche of each sex, showing that they are mostly distinct, but overlapping a bit. The coloured points indicate pairs of huia, that is, a male and a female that were collected together in the same place and at the same time, possibly representing a breeding pair of birds.

We also measured isotopes from specific amino acids that make up the proteins in the huia’s feathers. That extra data allowed us to calculate the trophic position of male and female huia. You can think of the trophic position as the level an animal occupies in the food chain.

Even so, male and female huia occupy identical trophic positions. Females, however, showed less variability in their trophic position.

An x-y graph with four diagonal dotted lines and silhouettes of beetles and a branch on it
Graph showing the nitrogen isotope values from specific amino acids, glutamic acid (Glx, which comes from glutamate) and phenylalanine (Phe). Circles represent females and squares represent males. The diagonal lines indicate the trophic levels (TP), from plants to herbivores and carnivores: TP = 1, fruits, berries, seeds, nectar; TP = 2, aphid, adult and larval lepidopteran; TP = 3, coccinellid, mantid, ant. This shows that the huia are feeding mostly on plant-eating insects and plants.

So, are male and female huia different or not?

We interpreted our results as a partial dietary segregation between males and females.

While both males and females fed on herbivorous insects and on plant material (berries and seeds), the females would have a more specialised diet, feeding on a smaller variety of food items when compared to the males.

The unique features of the huia make them a good model to tackle questions of sexual segregation in diet. It was, after all, the avian example chosen by Darwin to exemplify sex-related differences driven by natural selection rather than by sexual selection.

Two taxidermied birds sitting on a branch prop with a grey background. The birds have orange wattles and white tips on their tails.
A taxidermized pair of huia. Huia, Heteralocha acutirostris, collected no data, New Zealand. Gift of the Napier Museum, 1949. CC BY 4.0. Te Papa (OR.005533)

Another important aspect of our research is to show the value of museum specimens: it’s possible to study aspects of behaviour and ecology even in extinct and poorly known species through those specimens. This was only possible thanks to the abundance of curated bird specimens being kept and cared for in museums collections worldwide.

Research

This research was published in the scientific journal Oecologia: Extreme bill dimorphism leads to different but overlapping isotopic niches and similar trophic positions in sexes of the charismatic extinct huia. It was led by former Te Papa researcher Barbara Tomotani (now at NIOO-KNAW), in collaboration with Te Papa curators Rodrigo Salvador and Colin Miskelly, former Te Papa head of science Susan Waugh, and colleagues from NIWA, Muséum national d’Histoire naturelle, and La Rochelle Université.

The research was funded by the Dutch Research Council (NWO); part of the isotope analysis was funded by Birds New Zealand.

1 Comment

  1. Perhaps the male and female huia had different beaks, because they evolved different beaks, and this had different functional consequences. If one tries to ‘explain’ the origin of structure in terms of functions that cannot exist without the structure in the first place, one ends up with teleological non-science. That is the danger here.

Leave a comment