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Picture an animal with tusks.
You’re probably thinking of elephants, but maybe walruses, boars…
Well they all have one thing in common: they’re all mammals.
To better understand how tusks evolved, researchers publishing this week
in the Proceedings of the Royal Society B studied teeth in an ancient relative of mammals.
And in the process, they offered up a more definitive answer
to what the heck tusks even are.
To understand how mammals got their tusks, the researchers looked back in time.
Back to approximately 270 million years ago,
when ancestors of mammals known as dicynodonts roamed the planet.
Dicynodonts came in all sizes.
They had stocky bodies, short necks, big heads, turtle-like beaks,
and prominent front teeth.
In fact, the name “dicynodont” actually means “two canine teeth.”
But one day out in the field, the researchers behind this new study
were having lunch surrounded by dicynodont teeth and wondered:
why do we call these “tusks” when they’re more like teeth?
They seemed to have features that tusks shouldn’t technically have.
The definition of a tusk is surprisingly wibbly
but it’s not just any old ginormous tooth that sticks out of an animal’s mouth.
So the researchers proposed a clearer definition.
To start with, it does need to be a protrusion from the animal’s mouth cavity.
It also needs to grow throughout the animal’s life.
A big old pointy tooth that just grows in and then stops doesn’t count.
Because tusks are generally used for something, like fighting or digging,
so the animal benefits from growing them back if they break.
So the researchers defined certain features associated with continuous growth.
A tusk, they said, has to be connected to the jaw by ligaments.
And it has to be made of a material called dentin,
instead of coated in a protective layer of much harder enamel.
So rodents, for example, would not qualify,
because their protruding teeth have some enamel.
With all this in mind, the researchers took a closer look
at the earliest “tusks” we know of: dicynodont teeth.
They rounded up fossils from 10 different species from museums around the world.
They took very thin slices of their tusks and then imaged them in high resolution.
And they found that not all of the teeth were tusks after all.
Some of them had a thin cap of enamel.
Others were fused to the jaw rather than attached with ligaments,
meaning that there was no way the tooth could be continuously growing.
And prominent teeth that were not tusks
were more common in earlier species of dicynodonts.
But surprisingly, there was not a smooth progression from teeth to tusks overall.
The researchers say this suggests that tusks evolved repeatedly
in different species of dicynodonts.
We know that’s the case in mammals too.
When it comes to why tusks evolved in this way,
it might be that there’s a trade-off between dentin,
which can grow continuously, and enamel, which is one-and-done.
If you’re using tusks as a tool, it’s helpful if they can keep growing back
even when the rest of your teeth can’t.
Rodents hit a halfway spot with a partial enamel covering that can grow back.
But the researchers found that enamel-less tusks tend to evolve eventually.
While animals like sharks are infamous for always having a new set of teeth
in their back pocket, many mammals only get one or two sets in their lifetime.
And it seems like once dicynodonts evolved fewer sets of teeth,
the stage was set for them to acquire continuously growing tusks as well.
While this doesn’t tell us for sure that mammals acquired their tusks this way,
the researchers think it’s possible that similar evolutionary pressures
led to mammalian tusks.
So the more we can learn about these very early tusks, the more we might
eventually be able to learn about the ones mammals have today.
And the more we say the word “tusks,” the less it starts sounding like a word.
Tusks…
Tusk- it’s gone.
While we’re on the topic of mammalian traits and where they came from,
let’s talk about something that most of us consider pretty uniquely human: music.
Plenty of studies have shown that there are elements to music
that are universal across human cultures.
Now, in a study published this week in the journal Current Biology,
researchers are showing that an endangered species of singing lemurs
has one of these elements: rhythm.
The lemurs in question are called indris.
They live in small family groups in the rainforests of Madagascar,
where they often sing coordinated duets and choruses of eerie wails.
Over the course of twelve years, the researchers involved in this study recorded
samples of songs from 39 indris in their natural habitat.
Then they did some statistical analysis of the lemurs’ haunting songs.
There are lots of ways to measure time when it comes to music,
like rhythm, which refers to the pattern of sounds,
or tempo, which describes the speed at which they happen.
In this case, the researchers were looking for a feature known as categorical rhythm.
One common category of rhythm humans use is a 1:1 ratio,
where each note is as long as the next, like singing along with a metronome,
or the “lub-dub” of a heartbeat.
Humans also often use a 1:2 ratio,
where some notes are twice as long as the next notes.
The 1:1 ratio is the only ratio we’ve seen definitively in non-human species.
Songbirds use it for example, though they can sometimes approach 1:2.
But when the researchers analyzed the lemur songs,
they observed clear groupings of rhythms.
And when they looked closer, they found both 1:1 ratios and 1:2 ratios.
Basically, their rhythms were more human-like.
But not perfectly.
In humans, we tend to think of rhythm and length of notes
in relation to a measurement of time we call “the beat.”
The researchers point out, however, that we can’t necessarily assume
that the lemurs have a sense of the beat.
But this provides more evidence for the idea
that there really are universals in music.
Even though theyare fellow primates, indris are such distant relations of ours
that the researchers think we probably each evolved our singing skills separately.
Nonetheless, they still think the lemurs could help us understand
how musicality evolved in humans.
So while the indris might not be quite ready for karaoke,
they can teach us a lot about the way we make music.
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