[music]
Miles O’Brien: At the Pfennig lab at UNC
Chapel Hill, it’s time to feed the spadefoots.
[music]
Miles O’Brien: Adult toads have
a distinct taste for crickets.
While brine shrimp are on the menu for the tadpoles.
[nat - croak!]
Miles O’Brien: They’re called “spadefoots”
because of this little shovel- like structure on their feet.
They’re sand burrowers, as you can see here.
We sped this video up, but not by much.
Karin Pfennig: This research is trying to get
at that fundamental aspect of the living world,
which is why the world is as diverse as it is.
Miles O’Brien: So, what could this humble
little toad tells us about that? Turns out - a lot.
Spadefoots are master “shape-shifters,”
able to make drastic changes to their forms
and their behaviors in response to the environment.
It’s a concept called “plasticity.”
With support from the National Science Foundation,
evolutionary biologists David and Karin Pfennig
and their teams study spadefoots to better understand
the role plasticity plays in evolution.
David Pfennig: A lot of times in science,
if you want to understand something,
you go find a really extreme example
of the phenomena that you’re interested in.
Miles O’Brien: The Pfennigs go every year
to Arizona to study the toads in the field,
and work with them year-round in the lab.
David and his group focus on plasticity in tadpoles.
Certain kinds of spadefoots come in two very different types,
or morphs - omnivores and carnivores.
David Pfennig: They’re so different that
for many, many years, for decades,
biologists thought that they were a different species.
Miles O’Brien: Omnivores are the smaller ones.
They can eat pretty much anything.
But, if the tadpole happens to hatch in a pond rich in shrimp,
some, but not all, will beef-up bigtime.
And, it’s not just shrimp.
Carnivores will eat other tadpoles.
But, remember, they’re the same species.
Once they grow to adulthood, they look the same again.
David Pfennig: There are different strategies
for achieving the same end,
which is basically propagating one’s distinctive
characteristics in the next generation.
Miles O’Brien: Back in the lab,
they’re examining which genes are involved in this plasticity.
David Pfennig: We want to know what genes
are being turned on
and which are being turned off in each of the two morphs.
We want to get insights into what the genes
are that might be involved and ultimately producing,
determining or influencing
whether you become one morph or the other.
Miles O’Brien: Karin and her team look
at a different type of plasticity - behavioral -
how adult females choose their mates.
They’ve set-up a simulated pond in the lab.
The female sits on the platform in the middle of this tank,
listening to mating calls from different species of spadefoots.
Karin Pfennig: What females will actually do
is evaluate the environment
and decide whether to mate with males of their own species
or change their decision, which is a form of plasticity,
and mate with males of the other species.
[nat - croak!]
Miles O’Brien: She sees this behavior
in the field.
This is a so-called “Plains” Spadefoot.
This guy is a “Mexican” Spadefoot.
A female “Plains” can tell them apart by their calls.
[nat - croak!]
Miles O’Brien: All things being equal,
she’ll mate within her own species,
unless her pond is drying up. Then, she might “hybridize”
and mate with the Mexican Spadefoot
that’s evolved to survive in a drier environment.
Karin Pfennig: If the pond is shallow
and it’s going to dry up really fast
so that if they mated with their own species
those tadpoles would die because they wouldn’t get out,
then they “hybridize,” and they can produce fast
developing tadpoles that will get out.
Miles O’Brien: It comes with a cost.
If a Plains spadefoot interbreeds,
all the males hatched from that clutch of eggs
will grow up to be sterile.
Karin Pfennig: From an evolutionary
perspective,
it’s better for a female to have some offspring that,
you know, half of them might be sterile,
but the females are going to be able to breed,
and that’s better than having no offspring at all.
Miles O’Brien: The Pfennigs say understanding
plasticity is important
because variation between individuals
is what spurs the evolutionary process.
David Pfennig: You know, traditionally,
we just assume that the reason that individuals vary
is because they have different genes,
and what we want to know is
“well, is there more to variation than just genes?”
Miles O’Brien: Turning to the little
spadefoot toad for insights into the interplay
between genetics, behavior and the environment.
For Science Nation, I’m Miles O’Brien.
Miles O’Brien, reporting.