We all have bacteria that live in our bodies, helping us by breaking down food, flushing out toxins, and regulating
our weight. But for hyenas
and other animals with specialized scent glands, bacteria may have an
additional, important role: They could be allowing hyenas to communicate with each other.
https://gizmodo.com/could-the-microbes-in-your-gut-be-making-you-gain-weigh-1255281287
Top image via Chris Eason/Flickr.
The Birth of the Fermentation
Hypothesis
Back in the mid-1970s, two groups of researchers
were analyzing the anal
scent glands of different animals, including small Asian mongooses, red
foxes and lions, when they stumbled upon something interesting. They found that
a prominent
component of the animals’ communicative scents were short-chain fatty acids,
a type of molecule produced by fermenting bacteria.
This discovery birthed the “fermentation hypothesis,”
said Kevin Theis, an ecologist at Michigan State University. The hypothesis
proposes that the scented components in many mammalian chemical signals are the
byproducts of the fermentation by symbiotic bacteria. It also goes on to
suggest that the variation in chemical signals among mammals with specialized
scent glands is due to the underlying diversity of bacterial
communities inside the glands.
https://gizmodo.com/the-truth-about-why-microbes-make-you-sick-471481771
“Scent glands are always warm, moist and nutrient
rich,” Theis told io9. “They’re good places for bacteria to
grow.”
On the whole, the hypothesis makes sense, but there has long
been a lack of evidence to back it up. Reason being: The techniques necessary
to really analyze the bacterial communities in scent glands weren’t available
yet. But then new DNA sequencing technology came on to the scene.
Last year, Theis and his colleagues used next-gen sequencing
to survey
the bacteria in the scent gland secretions of female spotted hyenas (Crocuta crocuta). “We found a
greater diversity of bacteria than the 15 previous surveys of scent glands
combined,” Theis said. “It really speaks to the power of the
technology.”
The research also revealed that most of the microbes were
members of bacterial groups that ferment and produce odors. What’s more, the
bacterial communities in the secretions differed between social groups,
suggesting that this variation is behind the group-specific odors that spotted
hyenas are known to produce. Though the study provided some support for the
fermentation hypothesis, it didn’t try to tie the bacteria to the odor profiles
of the scent gland secretions. So Theis conducted another study.
Pasting in Wild
Hyenas
For the new research, just published in the journal PNAS, Theis and his colleagues focused
on wild spotted hyenas and striped hyenas (Hyaena
hyaena). These two species of hyenas live very different lifestyles.
Spotted hyenas live in very large hierarchical groups, or
clans, which contain 40 to 80 individuals. Adult males and females work
together to maintain and defend their territory against other clans. Striped
hyenas, by contrast, live in very small groups, consisting of just a few sexually
mature females and males. However, the animals don’t usually have much
interaction with their group members because they prefer to rest, travel and
forage alone.
Despite their differences, both species engage in a
conspicuous chemical signaling behavior called “pasting,” which
involves depositing an odorous secretion — “paste” — from
anal scent glands on to grass stalks.
A hyena sniffs the paste left by another hyena. Courtesy of Kay E. Holekamp.
“In general, we think pasting has a
territorial, communicative function,” Theis said. Females may use their
scents to facilitate social cohesion, whereas males may use pasting as a kind
of dominance display. Even cubs rub their sacs on grass stalks, even though
they don’t produce paste of their own — this behavior may be a way for
them to acquire their symbiotic bacteria from other hyenas’ paste (if the
bacteria are, in fact, behind the paste’s odors).
For spotted hyenas, the paste
odors provide information about the identity, age, sex, and reproductive state of the animal. And research has shown that the major smelly constituents of the paste include volatile
fatty acids (VFAs), esters, hydrocarbons, alcohols and aldehydes.
Communicating With
Bacteria
Theis and his team analyzed the bacterial communities and
VFAs from the paste of male spotted and striped hyenas, as well as females that were pregnant, lactating or neither. They found that both spotted and striped
hyenas had paste full of fermentative bacteria, though the two species had
their own distinct genera of bacteria, whose metabolisms are known to yield
varying concentrations of different short-chain fatty acids, including acetic, propionic,
and butyric acid.
Importantly, the composition of the bacterial communities
varied between species along with the profiles of the odorous VFAs in the secretions. And
within a clan of spotted hyenas, the bacterial communities and VFAs in the
pastes both differed between the sexes and female reproductive states.
The results add support for the fermentative hypothesis, by suggesting that symbiotic bacteria are behind the
species-specific odors of striped and spotted hyenas and further underlie the
paste odors that are specific to sex and reproductive states in spotted hyenas. But this is not to say that bacteria are the sole producers of the hyenas’
paste odors. “I would expect that there are some synergistic effects
between the hyenas and the bacteria,” Theis said. “Even if the
animals are not directly contributing to the scent, they could still be
modulating their bacterial communities,” which has an overall affect on
the paste scent.
The researchers are now interested in showing that the
bacteria in the paste are really the source of the secretions’ VFAs, either by culturing the bacteria and measuring the VFAs they produce, or by digging into
the bacteria’s genome. If this holds true, the team plans to create synthetic
mixtures of the VFAs — if the hyenas can discriminate between the VFAs, it
would suggest that the bacteria really do play a part in the hyenas’ chemical
communication.
It’s likely the case that bacteria are involved in the chemical communication of other mammals, too, Theis said. Or the fermentation hypothesis may be even more
broadly applicable than that. “It may not even be just a mammalian
thing,” he said. “It could extend to animals in general.”
Check out study over
in the journal PNAS.
