No Old World monkeys have prehensile tails, and they
seem to do fine. Thus a prehensile tail, while adaptive
to those that have evolved it, is not absolutely required
to survive a life in the treetops.
Testing Adaptations
Because they seem so obvious, adaptations are often
inferred, and such inference may be true, but rigorous
testing is the only way in which adaptation can actually
be demonstrated. For example, orb- weaver spiders
(family Araneidae) throughout the Neotropics and the
temperate zone make large webs in which there are
some areas of obvious, thickened, zigzag strands called
stabilimenta (plate 8- 14). Spiders use considerable
energy to synthesize the silk for the web, especially
the dense stabilimenta. Why should spiders invest in
making stabilimenta? Are these conspicuous zigzag
strands adaptive?
The spiders that make stabilimenta are those whose
webs remain intact throughout the daylight hours
(many spiders make new webs each evening and take
them down at dawn). Stabilimenta make the webs easily
visible to humans, and biologists have hypothesized
that they have the same effect on birds (which are also
visually oriented). A flying bird will thus avoid an orb-
weaver spider’s web, saving the spider from having to
remake a web damaged by a flying bird strike (which
would yield no food for the spider, as birds are too
big to capture and eat). Spiders that invest energy in
making stabilimenta rather than risk having to start
from scratch and make a whole new web would be
less prone to bird accidents and damage from other
animals. Therefore, stabilimenta could represent an
adaptation for energy saving in an environment where
birds pose a risk to the security of the web. As such, the
hypothesis sounds plausible, but, without testing, it is
just a story, an educated guess. How could it be tested?
First, an observer could simply watch spider webs
and note bird behavior. This was done, both in Panama
and in Florida, and birds were observed to take short-
range evasive action when approaching webs with
stabilimenta. Secondly, using webs that do not have
stabilimenta, researchers altered some webs, adding
artificial stabilimenta, and kept other webs without
stabilimenta as controls. The webs without stabilimenta
did not remain intact during the day, while those with
stabilimenta generally did. Further direct observations
implicated birds as the major threat to webs, though
other large animals ranging from butterflies to deer could
also be forewarned by the presence of stabilimenta. This
work demonstrated the adaptiveness of stabilimenta.
Not all traits need be adaptive. Organisms represent
the combined effects of thousands of genes working in
concert. The anatomy, physiology, and behavior of an
organism represent various compromises imposed by
the interactive effects of the genes that formed it. Thus,
when looking at a trait, either anatomical or behavioral,
it is essential to ask how the trait is likely to act to
enhance fitness of the organism. At that point you might
want to perform what Albert Einstein called a “gedanken
experiment,” which means a thought experiment. Ask
how you might test the hypothesis that you are observing
an actual adaptation. Sometimes there is little choice but
to do a thought experiment. Here’s an example.
There are about 30 species of trees and vines in Costa
Rica that produce large and fleshy fruits. These fruits
drop from the tree, and most essentially just rot. That
is puzzling, considering that fruits function to attract
seed dispersers (chapter 10). So why are these fruits not
consumed, their seeds dispersed? Some years ago Daniel
Janzen and Paul Martin hypothesized that the large fruits
represent what they called “ecological anachronisms,”
fruits adapted to be dispersed by animals that are now
extinct. During the Pleistocene, some 10,000 years
ago, about 15 species of large (megafaunal) mammalsPlate 8- 14. The web of this orb- weaver spider from Honduras
displays prominent zigzag structures known as stabilimenta.
Photo by James Adams.118 chapter 8 evolutionary cornucopia