A Trip Which Sparked Curiosity (Part 2)


The Infamous Horned Bill – Combining Physics And Biology In One Beak

Continuing on my travelimages around the Natural History Museum, I came across an odd shaped beak, cut in half. The main beak was structured how one would expect it to be structured – a curved shape with a spiderweb of bone fibre strands inside – however, the addition of, what I now know to be called, a casque confused me. This hollow mound atop the beak ignited the curiosity inside me, leading me to write my second post in this series about the gorgeous Rhinoceros Hornbills, and more importantly, the more practical functions of their striking beaks.

The Casque

The normal part of the beak is predominantly used how every other beak is used, nothing much unusual to report here, so rather than take you all through the “boring” part of the beak, I decided to only discuss the more extraordinary casque.

Acoustics

The main theory about the function of this weird structure is that it is used almost like a resonance chamber[1], to amplify the sound of the Rhinoceros Hornbill’s call.

The casque is a perfect shape to allow a certain number of whole sound waves to fit into it. This length is different for each species as their calls are different pitches, and the sound waves produced, therefore, have different wavelengths. When the bird then calls, some of the sound waves produced enter the casque. As more sound waves enter, the crests of these waves line up and produce a resultant wave with the combined amplitude of the initial waves. This has a resonating effect and the sound is therefore amplified.

Reinforcement

The casque is said to have reinforcement properties as well. Not only does it add structural integrity to the beak, but also adds weight to make the beak a more effective hammer[2], which is useful when cracking open the tough exoskeletons of insects, and when the female and offspring break out of the enclosed nest (see ‘brief side note’ below).

Another use for this splendid casque is in aerial jousting [3] – a competition males engage in for territory and mates – which is pretty much exactly what it sounds like. Two males fly at each other at great heights and clash casques, so the reinforcement comes in handy; protecting the birds from major injury.

Sexual Attraction

The casque not only has purely practical purposes. A large element is attracting a mate. It is the casque in Rhinoceros Hornbills which exhibits sexual dimorphism – the size of casque differs between male and female[3]. This differs between species; in some species the males have bigger casques, but in others the roles are reversed, and some species are even monomorphic (only one size of casque is present). However, the interspecies differences are driven by sexual selection pressures, as some species’ casques may have evolved to allow them to excel in other areas.

The casque’s colours are also important in sexual attraction. When the Hornbills are born, their beaks and casques are white, like our fingernails. However, as they grow older, they rub their beaks against an oil gland just underneath their tail feathers, imagesecreting an orange oil onto the beak. The cumulative applications of this oil are what create the tremendous bursts of colour on the beaks. This allows individuals to recognise each other and sometimes is helpful in visual signalling as well [2].

Although not always the case, sexual dichromatism can occur as well. As I discussed in my previous post about the King of Saxony bird of paradise, sexual dichromatism is something which occurs in most birds. Some species of Hornbills exhibit this dichromatism through the colours and patterns created on their casques [2]. These factors all combine to create the perfect display in order to look as attractive to the female as possible.

Brief side note

As I mentioned previously, the females and their offspring have to break out of the enclosed nest which the Hornbill parents create. I thought I ought to discuss this further, not least because I find it a particularly fascinating element of the Hornbill’s behaviour.

Once the female and male have mated and the female is ready to lay her eggs, the pair find a hollow tree cavity, build a nest, and proceed to seal the female inside. They do this by creating a imagepaste made from fruit, faeces, and mud, squashing this through their beaks against the side of the cavity entrance. The pair completely seal the cavity except from a slit which is left for the male to pass regurgitated fruit through to keep the female alive while she sits on her eggs, and the offspring alive once they have hatched until they are ready to leave the nest. The female will also use this slit to expel faeces and uneaten food, in order to keep the nest clean [4].

Three months after the eggs have been laid, the female breaks out of the nest. Working with her life partner, she reseals the cavity so the offspring can be kept safe for another three months; until they are able to break out of the nest by themselves. Both parents take care of the offspring until this point [4].

Conclusion

Many of us may have grown up seeing these beautiful birds in zoos and on the TV, so the Rhinoceros Hornbill’s beak may be something that we are used to seeing, however, I hope that this post has given a bit more of an insight into the adaptations which make this bird’s beak perhaps one of the most interesting. I have been surprised at the level of sophistication that the seemingly useless mound that is the casque has demonstrated as a structure, delving briefly into the world of physics and then returning to the familiar realms of biology to fully discover the functions of the beak’s most peculiar feature.

 

[1]WORLD LAND TRUST. (2016) Rhinoceros Hornbill. [Online] Available from: http://www.worldlandtrust.org/education/species/rhinoceros-hornbill [Accessed: 18th November 2016]

[2]JACKSON, T. (2013) What is the function, if any, of a hornbill’s casque? [Online] Available from: http://africageographic.com/blog/what-is-the-function-if-any-of-a-hornbills-casque/ [Accessed: 19th November 2016]

[3]NAISH, D. (2014) The Splendid and Remarkable Anatomy of Hornbills [Online] Available from: https://blogs.scientificamerican.com/tetrapod-zoology/the-splendid-and-remarkable-anatomy-of-hornbills/ [Accessed: 24th November 2016]

[4]NATIONAL AVIARY (2016) Rhinoceros Hornbill [Online] Available: https://www.aviary.org/animals/rhinoceros-hornbill [Accessed: 27th November 2016]

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