As Many Exceptions As Rules: The Bird Jaws of Life

Biology concepts – ecology, radiative speciation, talpid, tomia, temporomandibular joint, cranial kinesis, bilateral symmetry, asymmetry

You’ve heard the phrase, “As scarce as hen’s teeth?” Well, you’ve heard it if you’re as old as I. Hens don’t have teeth, so the phrase means something so rare as to be fictitious. Sort of like Will Ferrell’s chances of winning an Oscar.

Chicks don’t’ peck their way out of their shell. They

use an egg tooth to pip the shell, putting a small hole

in it. This destroys the integrity of the shell, and

they can push their way out.

Are there are any birds with teeth? They would be an exception, and that’s what this blog is all about. Maybe they just don’t have a lot to smile about. Birds are edentulous – without teeth.

Most chicks do have an egg tooth, but it’s not really a tooth. An egg tooth is a small growth on the tip of a bird’s beak just before it hatches. The chick’s talons and beak aren’t sharp enough to break through the egg, so the egg tooth does the job. They pip the outer shell (just break through) and push themselves out.

Of course, that’s how it usually works. There are of course, exceptions. The kiwi bird of New Zealand doesn’t produce an egg tooth. It uses its strong legs – even in the shell they are strong – to kick it’s way out of the shell. A few of the other large birds of Australia do it as well. No teeth here.

There are a few species of birds that look like they have teeth as adults. Anyone who has crossed paths with a Canada Goose with goslings probably knows that they can be toothy trouble makers. Called tomia, geese teeth these look a little like snake teeth; not the fangs, just the teeth. They slope backward to help grasp food.

Tomia aren’t covered with enamel and they don’t have a pulp chamber, so they are definitely not teeth. These serrations (serrated beaks) are made from the same material as the beak itself, and the rubbing of the top and bottom bills keep them sharp. Tomia are good for cutting and grasping, but they are more like fingernails or bone, not teeth.

These are the tomia of a domestic goose. They look

formidable. Tomia are located on the lower and

upper jaws, but also on the roof of the moth and the

tongue. See how they are pointed backward to help

grip things. But they aren’t teeth!

Birds have a common ancestor with dinosaurs, and dinosaurs had teeth. T. Rex himself is a direct ancestor of today’s birds, and boy did he have teeth! The genes to make teeth are still there in birds, they’ve just been turned off.

The common ancestor of all living birds did have teeth, according to new work from UC Riverside. Birds don’t really fossilize well since their bones are so light, so the six genes of teeth production will have to do for study. Did all birds lose teeth in one fell swoop, or did different lines of birds lose teeth independently, at different times?

The researchers found that genes in all birds had the same mutations, so all birds lost their enamel, and therefore their teeth, about 116 million years ago.

Another way we know that the tooth making genes are still there – sometimes they come back! There are chickens that show a certain group of mutations – they grow true teeth. Don’t get too excited, the mutations also kill them before they hatch. The mutation is called talpid, and involves the beta-catenin gene.

A 2006 study showed that teeth of the mutant embryos were very alligator like teeth buds, and turning on beta-catenin in normal embryos brings the same tooth development. Yep, birds used to have teeth and still have them buried deep in their genes.

For some reason, anatomists call the two jaws of man

a mandible (bottom) and a maxilla (top), but the two

jaws of a bird just upper and lower mandibles. See

how much more complex the human jaw joint as

compared to the bird. Also, see how close the human

joint is to your ear, that’s why you hear yourself chew.

The toothiness of ancient birds is just the beginning. Birds have some bizarre mouths as well. In humans, the mandible (bottom jaw) houses the most complex joint in our body. The tempromandibular joint (TMJ, between the temporal bone of the skull and the mandible) can move front and back, hinge up and down, and move down and forward laterally. It’s the only joint in the body that can do all these moves. On the other hand, our maxilla (upper jaw) is completely fixed, it doesn’t move at all.

However, in most birds the upper mandible (our maxilla) can move! The attachment to the facial bones is more complicated in birds than in mammals, the upper mandible attaches to the facial bones via some cartilage, called a syndesmosis joint. There are several types of movement based on just where the upper mandible is attached to the facial bones.

Being able to move the upper jaw is call cranial kinesis (cranial = upper jaw makes up part of the cranium or skull, and kinesis =moving). Snakes are big on cranial kinesis, they need it to eat things bigger than their head. But mammals can’t do it.

Cute yes, but you should pay attention to the top

part of the beak. It moves independently of the lower

mandible. We can’t do that. It’s called cranial kinesis

when the upper jaw can hinge against the facial bones,

and parrots are the birds that do it best.

Mammals and some other animals have developed a secondary palate made of bone. This affixes the maxilla tightly to the skull; there’s no way to get any movement out of it. The secondary palate is a consequence of needing to breathe while eating (necessary because of our high metabolism) and as a result of needing to develop suction for drinking milk as babies.

As far as birds go, many species move their upper jaw a bit, but the parrots move it the most. This helps then to eat the large and oddly shaped nuts that make up their diet. But oddly enough, they don’t move their lower jaw much side to side. Herbivores do, they have flat back teeth for grinding (see picture below).

Since birds don’t have teeth to chew food, they don’t really need to move their lower jaw laterally very much. This is true for most birds, but there is one exceptional genus of birds that can move their jaws laterally quite a bit - and they have to. Their bills grow sideways and cross each other like scissors!

Herbivores use lots of lateral mandiblular movement

to help grind their food. Birds have no teeth, so they

don’t move their jaws lateral much at all.

There are about 5 species of crossbill finches, all of the genus Loxia (Greek for oblique). They are the only birds whose upper and lower bills cross one other. One grows to the right so it can pass the other (which moves to the left). In populations of each species, the dextral individuals (bottom jaw crosses to the right of the upper) are about 1:1 with sinistral individuals (bottom jaw is on the left).

This makes the crossbill bilaterally asymmetric, and the top bill is longer than the bottom, assuring this asymmetry. Why did this crossed bill evolve? It’s based on what they eat. The diet of crossbills consists only of conifer seeds, harvested straight from the maturing cones. Each species dines on the seeds of different conifers, so their bills are slightly different based on what they are digging out.

The top image is the Hispaniola crossbill. This picture

shows the crossing of the upper and lower jaws. This

individual is a dextral crossbill. The lower picture is a

red crossbill feeding. Look carefully at the beak as it is

pushing the scales of the cone apart. The crossbill

moves its lower jaw laterally more than any other bird.

The crossbill turns his head to the side and inserts the crossed bill between two scales on a cone. He moves the lower mandible laterally while turning the head a bit back to vertical. This pries the scales open while it opens the mouth. When the space is wide enough, the tongue shoots out and grabs the exposed seeds. See the video at the end of the post.

Each bird attacks the cone based on which type of crossed bill it has, dextral or sinistral. Therefore, each bird can only access about half the seeds of a cone. This is why populations are 1:1 dextral/sinistral – it allows any population to get at all the seeds. If one morph (dextral or sinistral) predominated, some would starve. Having 1:1 ratio allows both morphs to feed maximally.

Different conifers have differently shaped cones. Over evolutionary time, individual differences were maximized until different species resulted. This allows different populations to live in the same area, because they feed on different trees. Called adaptive radiation, Darwin’s finches did it in the Galapagos Islands because of isolation and different foods. Here the crossbill species do it with in the same area to fill different feeding niches.

Some portion of the instructions for the crossing bill

is genetic, but how much? Look at the chick’s bill. It

isn’t crossed yet. They don’t cross until they are ready

to feed on their own. Is it an acquired characteristic?

Species have different bills, but so do individuals within a population, they are just smaller differences. A study in 2009 showed for the first time that differences in feeding ability of crossbills, based on individual differences (fluctuating asymmetry, will talk more about it in a couple weeks) in bill shape, may be used in mate selection. Those that are able to forage fastest seem to draw the attention of more females, and for longer times.

A second study indicates that differently individuals have different contact calls, and those with the most similar bills would respond best to each other’s calls. This would reinforce mating choice by assortative flocking. Feeding and calling based on bill morphology are two reasons behind ecological speciation in crossbills.

The above evidence of mate selection and radiative adaption suggest that bill shape is genetic, but it isn’t totally genetic. Chicks are born with straight bills, but they bend and cross at some point before the chick is required to search out food for itself. And a 2005 study found no evidence for simple or gender-based inheritance when examining captive bred versus will crossbills. More research is obviously needed.

The wrybill of New Zealand is the only bird in the world

with a beak that bends sideways. It is bilaterally

asymmetric like the crossbill, but in a somewhat weirder

way. It’s supposed to help find food, but other birds

with straight bill find just as much food in the rocks

as they do.

There is one bird that is even more asymmetric than the crossbills. The wrybill (Anarhynchus frontalis; wry is Old English for contorted) lives on the islands of New Zealand. Many birds have bills that bend up or down (or even cross), but this is the only bird whose bill turns to the side – always the right.

A single species of plover, the wrybill (Ngutuapore in native Maori language) is rare; only 5000 live on the North Island and fly to the South Island to breed each year. The turned bill is supposed to be for turning stones over and retrieving crustaceans, worms, and insects from the crevices of shore rocks.

However, the wrybill has been studied very little, and other wading birds do just as well at turning stones over, so the reason for the bill turn isn't understood. We don’t even know just how or if it is an adaptive advantage. Even if we don't know why it exists, it must play some role – the turn is ALWAYS to the right, and even the unhatched chicks have the turned bill. We’ll have to wait for the next turn in their story.

Next week - if we want to continue talking about bilateral asymmetry, we first have to talk about how males and females look different - well some do. But in animals like spotted hyenas, even the experts can tell the guys from the gals.

Meredith, R., Zhang, G., Gilbert, M., Jarvis, E., & Springer, M. (2014). Evidence for a single loss of mineralized teeth in the common avian ancestor Science, 346 (6215), 1254390-1254390 DOI: 10.1126/science.1254390

Smith, J., Sjoberg, S., Mueller, M., & Benkman, C. (2012). Assortative flocking in crossbills and implications for ecological speciation Proceedings of the Royal Society B: Biological Sciences, 279 (1745), 4223-4229 DOI: 10.1098/rspb.2012.1500

Benkman, C., Parchman, T., & Mezquida, E. (2010). Patterns of coevolution in the adaptive radiation of crossbills Annals of the New York Academy of Sciences, 1206 (1), 1-16 DOI: 10.1111/j.1749-6632.2010.05702.x

SNOWBERG, L., & BENKMAN, C. (2009). Mate choice based on a key ecological performance trait Journal of Evolutionary Biology, 22 (4), 762-769 DOI: 10.1111/j.1420-9101.2009.01699.x

For more information or classroom activities, see:

Tomia –

http://www.birdsnways.com/wisdom/ww38eiv.htm

http://dinogoss.blogspot.com/2011/04/youre-doing-it-wrong-birds-with-teeth.html

http://www.birdtricks.com/blog/whats-inside-your-birds-mouth/

http://www.dvrconline.org/raptoradapt.html

Crossbill –

http://www.allaboutbirds.org/guide/Red_Crossbill/id

http://birdweb.org/birdweb/bird/red_crossbill

http://birdnote.org/show/crossbills-beak-does-job

http://www.arkive.org/common-crossbill/loxia-curvirostra/

https://www.youtube.com/watch?v=1NvU8WG9bg0