[Paleopsych] TLS: Gaden S. Robinson: Ant matters
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Gaden S. Robinson: Ant matters
The Times Literary Supplement, 4.7.30
Size, possessiveness and other essentials for an exuberant
PHEIDOLE IN THE NEW WORLD. A dominant, hyperdiverse ant genus. By
Edward O. Wilson. 794pp + CD. Harvard University Press. £85.95 (US
$125). - 0 674 00293 8
FOR LOVE OF INSECTS. By Thomas Eisner. 448pp. Harvard University
Press. Pounds 19.95 (US $29.95). - 0 674 01181 3
Man has three close relatives - the chimpanzee, gorilla and
orang-utan. Our ethnocentricity as systematists and our unease at
contradicting the concept of man as a special creation are just two of
the reasons behind our considering ourselves not just a separate
species or even genus, but a family separate from our cousins. Had the
classification of the living world been written by an ant, we would,
all four of us, be lumped together in a single genus. The mammals and
the birds each contain about as many species as a typical insect
family - around five thousand. Most bugs have a host of close
biological relatives; we have decided that we don't. And we find it
for the most part very hard to visualize and explain the entirely
different scale of species diversity that is encountered in the
invertebrate world. Faced with an illustration and explanation such as
Edward O. Wilson's Pheidole in the New World, we can only be stunned.
"Pheidole is among the largest of all genera of plants and animals",
says Ed Wilson, and he is right. There are very few genera with more
than 750 species and even fewer with upwards of 1,000. Wilson
introduces the term "hyperdiverse" in this monograph to describe such
groups. These small ants are just a few millimetres long; they are
predominantly soil-and litter-dwelling, but a significant number of
species are arboreal. Most are scavengers and predators, but some are
seed-collectors, and a few species have adopted highly specialized
biologies, such as Pheidole titanis, a specialized predator of
termites. The genus ranges from New England southwards into temperate
Argentina, and from Southern Europe south to the Cape, east to the
Pacific and through Australia and Melanesia as far as Samoa.
Including those in the present contribution, 900 different species
have been recognized, and Wilson suggests that a worldwide total of
1,500 is not an unreasonable prediction, making Pheidole the planet's
most diverse ant genus (a distinction currently held by Camponotus
with about 930 species). Among the 624 species that are described in
Pheidole in the New World, more than half, 337, are new to science.
This is one-fifth of the ant species of the Americas, more than 6 per
cent of the world's ant fauna. In the rainforest of Central America, a
few square kilometres can be home to more than fifty species. Pheidole
is not just diverse - it is numerically abundant in a wide range of
habitats across the world. The sweet smell of biological success must
be included somewhere among Pheidole's pheromones.
Some biologists are chary of hyperdiverse genera. To monograph such
groups is an enormous challenge. Many see them as unwieldy, and some
insist that they should be divided into groups of a "convenient" size.
The history of the "splitting" of hyperdiverse genera to make their
components more "manageable" has been a history of disasters resulting
in taxonomic mayhem. In the early 1970s, attempts were made to divide
up the moth genus Coleophora (with about 1,200 species) and some
eighty new genera were proposed. It took ten years to restore sanity
and the status quo, and to repack the species of Coleophora back into
their monogeneric box.
Wilson, rightly, will have none of this. We should not expect the man
who invented the word "biodiversity" to be awed by large genera. His
arrangement of Pheidole, divided into informal species groups, is a
pragmatic solution that will be applauded by both fellow taxonomists
and the fieldworkers who will use his monograph. The species of
Pheidole are too "tightly packed", morphologically, with too many
sequential evolutionary radiations, and with too much "convergence in
character traits between species groups" to be carved up. "Pheidole
today is like a large diamond set on the bench mount of a gem cutter.
To cleave it before seeing into its interior could be disastrous."
Pheidole is the product of "an exuberance of evolution". Wilson's
monograph is the product of a master craftsman. It reeks of authority.
Opening sections explain anatomy, terminology and abbreviations. There
are 100 pages of keys. Each one page species treatment includes line
drawings of the major and minor workers in lateral view, frontal views
of the heads, and details of the thorax and petiole; the location of
the type-specimens; the derivation of the name; diagnosis,
measurements, colour, geographical range and biology. Here are 624
treatments - a gigantic undertaking. And there is more. The CD is a
searchable database that can be used as an identification tool
supplementary to the keys. Possible inputs are measurements, colour
and country of origin. Or the user can scroll between closely related
species and compare high-resolution colour images of the lateral views
of major and minor workers and frontal views of heads.
Why is this book so heavy? Why are there so many species of Pheidole?
Wilson devotes an introductory chapter to discussing the origins of
hyperdiversity. It is an important essay. If the numbers of species
within, say, the genera of a family are rank-ordered as a histogram,
its shape will be a hollow curve and this signature is a
characteristic of the living world as a whole. A large number of
genera contain a single species while only very few contain large
numbers of species.
Hyperdiversity is a rare extreme of one end of the hollow curve. A
succession of scientists (for more than eighty years) have tried to
pin down both the how -the precise mathematical form of the curve, and
the why - the underlying model or evolutionary mechanism that
generates this elegant mathematical generality among groups of
organisms. The hollow curve approximates to a logarithmic series and
belongs among the family of curves that derive from the negative
binomial; but the model that generates this curve is obscure. However,
empirical models that assume sequential origin of groups through time
and a cyclical peaking and subsidence of diversity can, says Wilson,
approximate the observed distribution.
Factors that have been demonstrated to contribute to high species
diversity are "small size, right demographic factors, preemption
during colonization and subsequent incumbency, and a suite of key
adaptations potent in opening new niches or excluding competitors". So
nothing new there then. Small organisms are suggested to be more
adaptable to change and the pressure of environmental stress, while
large organisms do better in stable environments. Oscillation between
environmental stability and instability will cull the size extremes.
The diversity of organisms in the small-to-medium-size classes is
boosted by higher speciation rates and a better fit to the fine
fractal topography of environments. The most diverse groups have other
common features too - short generation times and a brief average
lifespan, superior dispersal ability and high resource availability.
Sewall Wright's 1931 theory still holds, that species that are divided
into demes (demonstrably discrete populations) speciate more rapidly
than species with amorphous populations. But speciation may be offset
by extinction. And the Pheidole story is complicated by these being
atypical organisms - they are social.
Two competing hypotheses may apply here: one says that social
organization is primitive but successful and drives diversity, the
other that sociality preadapts organisms to coping with new
environmental challenges, aka speciation opportunities. Speculation is
rife and there is little here to hang one's hat upon. Wilson returns
to his original thesis of pre-emption and incumbency. The former
involves possibly two key factors - dispersal ability and
breakthrough; the latter is the ability to create and use new niches
or to elbow one's way into and take over existing niches.
Incumbency is simply holding the fort - possession is nine-tenths of
the evolutionary game. Wilson likens pre-emption and incumbency to
industrial development. And in terms of growth, success in both
industry and the natural world breeds success. Whether the analogy
extends to long-term sustainable diversity is another matter. In the
end, Wilson admits that the "hypotheses, correlative analyses and
speculations . . . have been very broad, even abstract . . .". He
focuses on the big question "What are the adaptations that promote
ecological dominance within a clade (the complete group of taxa
derived from a single common ancestor)?". It is a deceptively simple
question but one that is fiendishly difficult to answer, or even to
plan how to answer. And answering that cannot of itself take us to
conquer the Santa Rosalia question of why there are so many species of
Pheidole. What is so special about Pheidole?
The hypothesis that Wilson eventually tenders is that the breakthrough
novelty in Pheidole is the existence of the large-headed major-worker
caste with reduced sting and increased abilities in defensive
secretions produced by exocrine glands.
As a group, Pheidole places extreme reliance on these majors for
colony defence and they form "a highly mobile strike force" while the
minor workers are the drudges. The minors "have the appearance of a
'throwaway' caste, that is, small, light, cheaply manufactured and
short-lived". Pheidole is "an unusually resilient superorganism able
to expend and replace minor workers readily while utilizing the major
subcaste both for defense and as an emergency labor force in the event
of severe depletion of the minors". We are left admiring the elegance
of his summation but still very conscious that the precise "why" of
hyperdiversity has not been satisfactorily answered.
There are moments in history when one wishes one could have been a fly
on the wall or, in this case, a fly on the windshield. In the summer
of 1952, Wilson was a graduate student at Harvard and joined forces
with another student, Tom Eisner, to "undertake a major exploratory
venture". With a $200 grant and an old Chevrolet, they travelled
12,000 miles through forty-eight states to "get acquainted with the
American landscape and its insects". It was both an entomological and
an intellectual odyssey, and the two became and remain great friends
as well as two of the most influential and powerful forces in
entomology. Wilson's generous foreword to Thomas Eisner's For Love of
Insects sets the stage for a remarkable memoir by a remarkable man.
Opening with Martin Rees's opinion that "what makes things baffling is
their degree of complexity, not their sheer size . . . . a star is
simpler than an insect", Eisner's raison d'etre for his science is
deceptively simple. "Insects survive because they have special
strategies for doing so . . . . How it is that one goes about
deciphering these strategies, and how serendipity, group effort and
sheer good luck combine to provide momentum in such research, are what
I have attempted to convey . . .". Eisner is a chemical ecologist, and
his career has been spent unravelling some of the most bizarre stories
of how insects use a combination of chemicals, sometimes simple and
sometimes complex, and sophisticated morphological structures to
defend themselves, their colonies, their mates and even their
offspring. And it is not just the use of chemicals in defence -
insects communicate using scent or pheromones, sound, and visual
signals, or a combination of these.
When defensive chemistry is involved also in reproduction and the
protection of offspring the story can become quite extraordinary, and
the investigation of the extraordinary calls for extraordinary
methods. Eisner's book compels and fascinates at a variety of levels.
It probes the ways in which insects use chemicals, and documents the
ways in which an investigator poses the questions and teases out the
answers. Eisner has covered an immense territory in his career - he
has been an unashamed opportunist, observing a phenomenon that others
may have taken for granted, asking why and how and being richly
rewarded for his curiosity. He tells his stories in the most
accessible way. His experiments are simple - he approaches problems
using a logical technique he calls "biorationality" - the formulation
of a sequence of linked questions which, if answered, explain the
function and adaptive significance of a structure or behaviour,
compare parallels or homologies in other organisms, and explain the
origin or evolution of the feature. The sheer elegance of his approach
is spellbinding. And the photographs that document his explorations
are remarkable - every experimental tale here is beautifully
illustrated. A single example from the many will give a little of the
flavour of his stories.
Utetheisa moths have a wingspan of a little more than an inch, and are
speckled black, white and bright rose; the English vernacular name of
the African species that is an occasional migrant to British shores is
the Crimson Speckled. They are diurnal, and that behaviour, combined
with their bright coloration, makes them prime suspects to be
distasteful to predators. In 1966, Eisner observed a Utetheisa
ornatrix, the Florida species, fly into a spider's web. Instead of
struggling, the moth froze and waited. The spider inspected it and
then cut it free. "Being witness to that act of rejection . . . was a
compelling experience", says Eisner.
Spiders consistently rejected Utetheisa. Eisner noticed that when
disturbed, the moths emitted a foam from the back of the neck. The
foam proved to be haemolymph, insect "blood", mixed with air. But
spiders rejected moths whether or not they foamed. Tests with
separated wings and bodies showed that all parts of Utetheisa were
rejected by spiders. The foam seemed to be an added deterrent,
suggesting Utetheisa was thoroughly nasty on the inside as well as the
Utetheisa ornatrix caterpillars feed on Crotalaria, the rattlepod.
This wild bean is toxic and has an unpleasant reputation as a cattle
poison, for its seeds and leaves contain pyrrolizidine alkaloids
Other species of Utetheisa feed on plants of the borage family which
are known also to contain PAs. Could adult Utetheisa carry poisons
sequestered during larval feeding, as does the Monarch butterfly?
Eisner's lifelong collaborator, Jerry Meinwald, and his research group
took up the challenge and showed that the moths indeed harboured a
high concentration of PAs. But did PAs confer protection from
predators? Eisner's group were able to rear Utetheisa on an artificial
diet made either from pinto beans (PA-free) or Crotalaria beans
(PA-rich). Adults reared on pinto beans did not contain PAs and were
eaten by spiders. Adults reared on the Crotalaria diet were armed with
PAs and rejected by spiders. The final test was to taint mealworms
(usually eaten with gusto by spiders) with PAs and offer them to
spiders; they were rejected. Female Utetheisa passed PAs into their
eggs and these protected the eggs from predation by Leptothorax ants.
Eggs laid by PA-free females were not protected. Ants actually
developed a long-term aversion to PA-tainted eggs, and were able to
remember the experience and avoid Utetheisa eggs for at least a month
after being exposed. Larvae of lacewings were also able to
discriminate between eggs with and without PAs - they sampled eggs by
piercing them and sucking out the contents with their hollow jaws.
PA-laden batches, each of ten eggs, were avoided after sampling of
just two or three eggs.
Caterpillars deficient in protective PAs were clearly "aware" of this
deficiency and would attack and eat eggs and larvae of their own
species that contained PAs. Clearly PAs played an enormous part in the
life of Utetheisa.
Eisner's team began to tease apart the secrets of courtship, mating
and reproduction. They found that, unsurprisingly, Utetheisa females
"called" males using an airborne pheromone, as do most Lepidoptera
species. But what was remarkably different was that the pheromone was
emitted in precisely timed pulses and they were able to show how the
glands operated to achieve this. The reason for pulsing is still
unclear, but it seems to save on chemical resources as well as provide
a way of refining the male's search strategy at close range.
At this point the lazy researcher might have just assumed "female
attracts male and Bob's your uncle". Eisner instead became interested
"in the behavior that comes into play once the male encounters the
female . . . . Utetheisa, before mating, engages in 'pillow talk'. We
decided that it might be worth listening in .
. .". Videotape of mating encounters showed how the male flew upwind
to the female, hovering beside her and making contact with antennae
and legs; the male then abruptly flexed his abdomen, thrusting the tip
towards the female; she raised her wings, exposing her abdomen, and
the male landed beside her, made genital contact and copulated. The
abdominal action lasted barely a third of a second.
More refined high-speed photography and some careful dissection showed
that two balloon-like structures set with specialized scales that
formed brushes in the male abdominal tip were extruded and presented
to the female in that one-third of a second. These brushes had been
shown to be pheromone-carriers in other moth species and Jerry
Meinwald's team quickly showed that the coremata (the correct term for
the brushes) were loaded with hydroxydanaidal (HD), a derivative of
Moths that were PA-free did not carry HD. Male moths with no HD, or
which had had their coremata surgically removed, were frequently
rejected by females.
What was the message carried to the female in the HD? Eisner's team
showed that the HD concentration in the coremata of the adult male was
proportional to the quantity of PAs it had ingested and stored as a
larva. And then it became clear.
They found that eggs laid by a PA-free female could contain PAs and be
protected against predation; all the female had to do was to mate with
a PA-loaded male (PA-free males and females produced PA-free eggs).
Males transmit protection to their offspring by giving the female a
gift of PA during copulation (as a component within the liquid of the
spermatophore, the sperm "packet"); about one-third of the egg's PA is
derived from the father. The male's wafting of his HD-loaded scent
brushes in front of the female proclaims his PA loading quantitatively
- he offers a nuptial gift with, as it were, the value marked on the
box. His gift, added to her PA loading (for in nature all females have
at least a trace of PA), protects their eggs and also protects her
while she lays them - she mobilizes his PA gift immediately; a PA-free
female becomes distasteful to spiders within five minutes of
Mating provides female Utetheisa not just with PAs but also with
nutrients, notably proteins, and minerals in the spermatophore.
Females mate more than once, and one final twist in the extraordinary
story of this moth is that the female can choose which spermatophore
to use to fertilize her eggs; she chooses the largest.
And spermatophore size is correlated positively with the physical size
of the male and his level of PA loading. Eisner's team were able to
demonstrate that body mass is a heritable feature, and thus close the
circle. The female selects a mate that will give her larger sons with
potentially increased fitness and larger daughters with greater
fecundity. And she makes that choice using the male's pheromone his HD
level - as a chemical yardstick. She does not or cannot discriminate
between males of different sizes or with different PA concentrations.
She bases her genetic future on a one-third-of-a-second flash of the
The dustjacket portrait of Thomas Eisner sums up this book. He is
riding a bicycle backwards, no hands. As the author, so the subjects:
who dares, wins.
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