SHE is big. She is lithe. She dwarfs her mate, but he wants nothing more than
to be with her even if that means an early death. When little men lust after
much taller women, it is for the likes of Freud to decipher. But in the world of
spiders鈥攚ell, this is just the way things are.
Female spiders are generally close to 50 per cent larger than their male
counterparts. And in the case of the golden orb spider Nephila clavipes
the difference is much more marked: females are on average four times as large
as males. Is the male a dwarf or the female a giant? Five years ago, one set of
researchers claimed they had the answer: the males had shrunk. Now, another
research group has shown that answer to be the wrong one. Instead, they say, the
males are normal sized, and it is the females that have blossomed into
giants.
While many other species have big females and small males鈥攅specially
marine creatures like the barnacle鈥攕piders provide the classic examples of
sexual size dimorphism. Evolutionary ecologists Fritz Vollrath of the University
of Aarhus, Denmark, and Geoff Parker at the University of Liverpool decided to
use them to take a crack at working out how the size differences have
evolved.
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Scouring the literature, they found that sexual size dimorphism is least
pronounced in spider species that hunt down their food. Jumping spiders that
pounce on their victims and then tether them with silk are typical of this
category. By contrast, size dimorphism is most obvious in spiders that just sit
in their webs waiting for food to come along. The relationship between size
difference and lifestyle holds good among other animals, too. The hyperparasitic
crab and the angler fish both have sedentary females and very small males.
This might, at first sight, suggest that something about living an easy life
favours diminutive males and buxom females. But when Vollrath tracked N.
clavipes in their natural habitat in the forests of Panama, he realised
nothing could be further from the truth. Among juvenile spiders of both sexes
death rates are moderately high鈥攁bout 25 per cent die every 20 days. But
once the spiders reach sexual maturity at between three and six months, their
death rates diverge markedly. For the plump females, in the secure environment
of their orb-shaped webs, only 7 per cent perish every 20 days. For the males,
adult life is far more hazardous: their death rates approach 90 per cent every
10 days. The males perish in such large numbers because they must leave the web
where they hatched and travel for days to find females, encountering predatory
mud-dauber wasps, dragonflies, hummingbirds and squirrel monkeys on their
way.
Once he reaches a new web and finds a mate, however, life for the male golden
orb spider becomes rosier. Other species of spider can suffer massive mortality
as they fight off rivals. But male N. clavipes tend to engage in far
fewer of these macho battles鈥攁nd this, according to Vollrath and Parker,
is the crux of the matter. Few males end up in the same web, so fights are
relatively rare.
That lack of competition means that male spiders who live high-risk adult
lives relative to the females 鈥渟hould cut their losses and mature earlier鈥�, says
Vollrath. Spiders stop growing when they reach sexual maturity, so that
necessarily means maturing smaller. The small males will be less able to fight
off other males, but there will be fewer males to deal with. By maturing earlier
at least they reduce their death rates as juveniles and so increase their
chances of finding a mate and reproducing. Male Tidarren sisyphoides,
the black and white spiders that hang out in the forests of the southern states
of the US, may be the ultimate product of such selection pressure, says
Vollrath. Not only is the male minuscule compared with the female, it reaches
sexual maturity while still in the egg sac.
N. clavipesmales are not quite so extreme. Nonetheless, so few
survive as adults, conclude Vollrath and Parker, that the evolutionary pressure
towards being big and brawny has taken second place to the pressure to mature
early. The males have become dwarfs.
That conclusion has not gone down well with invertebrate zoologists Jonathan
Coddington at the National Museum of Natural History, and Gustavo Hormiga of
George Washington University, both in Washington DC. Earlier this year, they,
along with Nikolaj Scharff of the University of Copenhagen, published a letter
in Nature (vol 385, p 687), arguing instead that the female golden orb
spiders are giants. The males, they say, are just regular-sized guys.
Vollrath and Parker got it wrong, says Coddington, because they based their
hypothesis on an age-old evolutionary misconception: that males play the dynamic
heroes, while the ladies sit and watch. Most studies of sexual dimorphism commit
the same error, Coddington alleges. It is 鈥渁n egregious case of
misinterpretation鈥�, he says, 鈥渁n entire field running off to prove something
that never happened鈥�.
The Coddington team avoided that trap by using an entirely different
technique known as cladistic analysis. This compares the characteristics of
related species or genera to create an evolutionary map of who is descended from
whom. Coddington has created a database of roughly 49 000 taxonomic observations
on 139 genera of spider from 55 different families, including features such as
the shape of the spiders鈥� body parts, their web-building patterns, fossil
history and the relative size of the males and females.
Biologists use the database to map out sections of the spider family tree by
running a computer analysis on data from the spiders concerned. The program
works out whether each trait of a particular spider species or genus is common
to all its ancestors. If it is, the spider stays in line at the bottom of the
tree. But if enough new traits are identified, the spider is moved to a new
branch鈥攖hat is a new species or a new genus is added to the growing family
tree.
Coddington, Hormiga and Scharff took the size differences between the sexes
of 79 genera of spider from two orb web-weaving families, Tetragnathidae (which
includes Nephila) and Araneidae, and mapped them onto the evolutionary
family tree. They concluded that over the past 100 million years or so, there
have been at least 10 changes in the relative sizes of the male and female
spiders. There was no overall trend: sometimes the difference became bigger,
sometimes smaller; sometimes the females were a different size from their
ancestors, and sometimes it was the males. When Coddington and his colleagues
looked at N. clavipes they discovered that the male had stayed roughly
the same size when it branched from the spider family tree. The female, however,
had got decidedly bigger. It was a giant.
In the face of such evidence, says Coddington, Vollrath and Parker should
give up their claim to have shown that golden orb males are dwarfs. 鈥淵ou can鈥檛
say the Moon is made of green cheese when every time someone looks at it, it鈥檚
made of rock,鈥� says Coddington. He does not say that Vollrath and Parker are
necessarily wrong in their general hypothesis that male spiders leading reckless
lives relative to the females are likely to be dwarfs. But N. clavipes,
with its giant females, is not the species to prove the point.
Vollrath, meanwhile, is fighting back. He points out that uncertainties
surround cladistic analysis of spider species. 鈥淪pider systematics is in a right
awful mess,鈥� he says. 鈥淐ladistic trees are to be taken with a pinch of
蝉补濒迟.鈥�
But Coddington and his colleagues remain confident that their conclusions
will hold, and that the branching of the part of the family tree that includes
orb web spiders is not going to change. And there鈥檚 another point on which they
are even more adamant. From now on, any new theories about sexual dimorphism are
going to have to take into account the possibility that females can change
too.