72 Science & technology The EconomistNovember 23rd 2019
2 is doing also help. “Room scale” vrsystems
let people move around in the real world
while perceiving similar movement in the
virtual one. Following a user’s movement
can be done in one of two ways. Outside-in
tracking relies on external cameras observ-
ing beacons of various sorts scattered
around a user’s body. Inside-out tracking is
the opposite: the beacons are scattered
around the room and detectors on a user’s
body employ them as reference points.
On top of all this, there is the design of
the lenses that sit inside a headset in front
of a user’s eyes to adjust optically for the
fact that what is actually a nearby image is
supposed to be some distance away. Since
the shape of these lenses is fixed and the
amount of adjustment required varies with
what is being looked at, distortion is inev-
itable. But distortions are particularly no-
ticeable when users move their eyes, says
Paul MacNeilage of the University of Neva-
da, Reno. Some headsets therefore now
track a user’s gaze and move the lenses
within the headset in response.
Make the input too credible, though,
and you run into a different problem—the
contrast between what a user’s eyes are see-
ing and what the motion-sensors in his in-
ner ear are detecting. To deal with that,
some designers program in a “virtual
nose”, just visible to the user, to serve as a
point of reference.
These tactics help. But they do not get
rid of cyber-sickness entirely. That is
where the second hypothesis, unstable
posture, comes in. And it is one that has the
virtue of offering an explanation of a mys-
tery about the condition—why women are
more likely to be affected than men.
Thomas Stoffregen of the University of
Minnesota, who has studied the matter
and found women four times as suscepti-
ble as men, cites the example of driving a
car to explain the unstable-posture hy-
pothesis. When turning the steering
wheel, he observes, drivers need to keep
their heads oriented to the road. They need
to stabilise their bodies, particularly when
the car is changing direction and pushing
the body in different ways. “When you
spend a lot of time in cars, you get used to
doing that,” he says. “It’s a skill.” But in vir-
tual environments, where there are no
forces to act as signals, people have not
learned to adjust their bodies properly.
They lean when the virtual car turns, but in
fact they are leaning away from stability.
He finds this particularly affects women,
who have lower centres of gravity than
men. That may cause them to sway more.
And increased swaying, he has found, cor-
relates with higher rates of cyber-sickness.
It is a neat idea. But Bas Rokers of the
University of Wisconsin-Madison believes
there is a simpler explanation for women’s
experience of cyber-sickness, which is that
headsets are not designed for them. For vrtoworkproperly,setsneedtobeadjusted
tothedistance betweenthepupilsof a
user’seyes.Inonepopularbrand,however,
DrRokersfoundthat90%ofwomenhave
aninterpupillarydistancelessthanthede-
faultheadsetsetting,and27%ofwomen’s
eyesdonotfittheheadsetatall.
IfDrRokersiscorrect,a bigpartofthe
problemofcyber-sicknessmightbedealt
withbya smallchangetohelmetdesign.If
women’sratesofthecomplaintcouldbe
reducedtothelevelexperiencedbymen,
thena lotmorepeoplecouldenjoyvrrath-
erthanenduringit.Andthen,perhaps,it
reallymightachieveitspotential. 7C
rowd animalstogether and one likely
outcome is parasitic infestation. This
has been a problem since the beginning of
animal husbandry. Many scholars, for in-
stance, suspect that the origins of religious
dietary laws forbidding the consumption
of pig meat lie in pigs’ susceptibility to
worms that are harmful to human beings.
But they are also harmful to animals. They
are, for example, the largest cause of natu-
ral death among the world’s sheep.
Nowadays, at least where farmers have
access to modern veterinary facilities, the
usual approach to infestation is to admin-
ister vermicidal drugs (see picture). These
often work. But, as with any such approach
to parasites and pathogens, extensive use
has encouraged the evolution of resis-
tance. Many worms have become immuneto at least one of the commonly used ver-
micides. Some are immune to all three.
What has not been tried until recently is
to apply the principles of selective breed-
ing that are employed to improve meat,
milk, wool and hide production to the
question of parasite control. That has
changed with work done in Britain by Han-
nah Vineer at the University of Bristol and
Eric Morgan at Queen’s University Belfast.
As they report in the International Journal
for Parasitology, selective breeding of sheep
for resistance to nematode-worm infesta-
tion works. And, crucially, it works without
detriment to the desirable characteristics
of lamb weight, ewe weight and milk yield.
That worms like nematodes are, to hu-
man sensibilities, revolting creatures with
revolting lives is surely the result of mil-
lions of years of co-evolution that has fa-
voured avoiding any contact with them. In-
festation starts when a host accidentally
consumes a nematode’s eggs. The parasites
then hatch, grow and mature in the host’s
stomach or intestines, where they con-
sume nutrients which that host would oth-
erwise absorb. Once mature, they release
eggs that are broadcast into the world in the
host’s faeces, and the cycle starts again.
Dr Vineer and Dr Morgan knew from
earlier work that the number of eggs so
broadcast varies a lot from animal to ani-
mal. This led them to wonder if selectively
mating together individuals that passed
few eggs in their faeces, and so seemed re-
sistant to infestation, might result in
strains that were parasite-free.
To find out, they teamed up with two
farmers in south-west England who had al-
ready been experimenting informally with
such breeding programmes. Following up
on a decade of this informal work, farmers
and academics spent a further four years
systematising and recording in detail what
was happening. They discovered that the
approach worked. On one farm the faecal
nematode-egg count per animal dropped
by a quarter. On the other it fell by a third.
In neither case were desirable characteris-
tics of body weight or milk yield harmed.
Falls in infestation of a quarter to a third
are not as dramatic as those caused by ver-
micides. However, the hostile physiologi-
cal environment that has brought about
such falls is likely to have many dimen-
sions, making it harder to evolve around
than the toxic effect of a single drug mole-
cule—or even three of them. So, though
this is a small pilot study, it certainly looks
worth following up. If larger investigations
confirm Dr Vineer’s and Dr Morgan’s find-
ings, then explicitly breeding sheep, and
possibly other livestock, for parasite resis-
tance would seem a good idea. Animals
would start out healthier and would need
less worming-by-drug. And that would
make resistance to those drugs less likely
to evolve in the first place. 7Nematode parasites kill a lot of sheep.
Breeding better sheep might stop thisAnimal husbandryTurning the worm
Take that, nematodes