24 THE SCIENTIST | the-scientist.com
TOP: JILLIAN ASWIFT; BOTTOM: PATRICK VKIRCHNOTEBOOKcan gain insight into the state of the whole
ecosystem during the rodents’ lifetimes.
To find bones that could retell the tale
of settlement in Oceania, Swift recently
collected specimens on those quiet
islands surrounding Mangareva. She
also borrowed other researchers’ speci-
mens from those same islands and from
a site on one of the Marquesas Islands
and pored over samples from sites on the
island of Tikopia from the Bernice Pau-ahi Bishop Museum in Hawaii. The rat
species she focused on, Rattus exulans,
is commensal with humans, and would
have hitched rides in the canoes of set-
tlers from other Polynesian islands. Using
specimens dating back more than 1,000
years, Swift and her collaborators ana-
lyzed the isotopic composition of nitro-
gen and carbon in the bones’ collagen to
look for changes over time in the land-
scapes’ nutrient flows.The analyses revealed a consistent
pattern following human colonization
of each island, even though these ini-
tial settlements were spread over hun-
dreds of years. “I was doing this island
to island, site to site... and then put-
ting it all together I realized there was
this nearly universal decline in [the ratio
of nitrogen-15 to nitrogen-14] through
time across all of these islands,” she says.
“These nutrient flows are changing so
substantially, and it seems to be occur-
ring everywhere and really tied into these... massive landscape changes that occur
as soon as people enter into new regions.”
(PNAS, 115:6392–97, 2018). The carbon
composition changed, too, though not
in such a consistent w ay. On Agakauitai,
near Mangareva, for example, the ratio of
carbon-13 to carbon-12 isotopes increased
after colonization, but on Tikopia and one
of the Marquesas islands the ratio fell
after settlement.
The results don’t reveal how, exactly,
humans changed the isotopic composi-
tion of the islands as they settled in. Peo-
ple can alter that composition through
many different activities, such as graz-
ing animals or fertilizing crops, and “the
really tricky part is teasing those different
influences apart,” says archaeologist Paul
Szpak of Trent University in Peterbor-
ough, Ontario, who was not involved in
the study. Szpak and colleagues recently
linked increases in relative nitrogen-15
composition to intensifying agricultural
practices in Ireland during the Bronze
Age (Sci Adv, 4:eaas9383, 2018).
Still, Swift has some ideas about how
humans might have wrought the changes.
Other studies on the Polynesian islands
have found charcoal generated when
people cleared forests by burning them,
she notes; that shift in island flora would
have affected the ratios of nitrogen and
ISLAND DIGS: The remains of rats on Polynesian
islands such as Agakauitai, French Polynesia
(left, top panel), provide zooarchaeologists with
a record of environmental changes, and by proxy,
human activity, in the region. One recent study
excavated the animals’ bones (left, lower panel) to
track those changes over more than 1,000 years.