(including horses and ground sloths) became extinct in
what is now Costa Rica. These included an elephant- like
group called the gomphotheres. Gomphotheres were in
all probability essential dispersers of seeds from large-
fruited plants. As a result of gomphothere extinction,
plants such as Guanacaste (Enterolobium cyclocarpum)
were left without any agent of dispersal. Their fruits,
once well adapted, are no longer as well adapted.Adaptations in Environmental
ContextAn adaptation must be viewed in the context of the
environment. Consider the Brown- throated Three-
toed Sloth (Bradypus variegatus; plate 8- 15), a common
species in much of the Neotropics. Charles Waterton,
an eccentric British explorer who traversed the Amazon
during the early 19th century, noted that the three- toed
sloth appears poorly suited to survive when it is seen
struggling over the ground. Tree sloths, however, do not
normally perambulate on the ground, at least not today
(large ground sloths once did, but they were well adapted
by size and strength to function terrestrially). Today’s
tree sloths are arboreal, skillfully and slowly moving
upside down from branch to branch. In his well- known
account Wanderings in South America (1825), Waterton
wrote of the sloth, “This singular animal is destined by
nature to be produced, to live and to die in the trees;
and to do justice to him, naturalists must examine him
in this his upper element.” Waterton then went on to
describe in detail how well adapted the sloth is for its
arboreal life. He put adaptation in context. For more on
the wonderful world of sloths, see chapter 16.Species
The species is the basic unit of biology. When we survey
an ecosystem, we typically measure the species richness
(number of species) of trees, bromeliads, birds, moths,
ants, tree frogs, or whatever groups we find to be of
most interest. Thus it is important for ecologists to
understand what, exactly, a species happens to be. So,
what is a species? The question is complex, because
it is often difficult to know whether organisms are
members of the same species or are of different species.
For sexually reproducing species, which is pretty much
most of what you will see on a visit to the Neotropics,the question is answered by observing who breeds with
whom. Male howler monkeys do not attempt to mate
with female woolly monkeys. But they know another
howler when it is time to form a pair bond and breed.
The same is true of the colorful heliconius butterflies
(chapter 11). A Heliconius erato will not mate with a
Heliconius melpomene, even though the two species
look remarkably similar. Organisms, including plants,
are genetically adapted to accurately recognize others
within their species.
This profound reality forms the basis of the most
widely used definition of a species: populations of
actually or potentially interbreeding organisms. In
this definition, called the Biological Species Concept
(BSC), species designation is based on the notion that
the organisms themselves will reveal to which species
they belong through their reproductive habits. Under
the BSC, the species category becomes a natural unit
because the organisms to which it pertains actually
recognize it themselves, at least in a manner of speaking.
It is doubtful that a howler monkey knows it is a
monkey, compared with, say, a Kinkajou or opossum,
or if it appreciates that it is a mammal and not a bird.
But its genetics make it competent to recognize and
interact meaningfully with another howler monkey.
The BSC is a theoretically strong definition,
heuristically satisfying, but often difficult to apply inPlate 8- 15. The Brown- throated Three- toed Sloth is always
slow moving but also always highly adept at navigating the
tree canopy. It looks awkward when on the ground, but it is
not adapted to be on the ground. Photo by Gina Nichol.chapter 8 evolutionary cornucopia 119