|
|
Si el Norte
Fuera el Sur: A Case of Squirrel Monkey Identities1 by |
Scenario
You are a biologist working to protect a recently discovered population of
Central American squirrel monkeys (Saimiri oerstedii) on the Pacific Coast
of Costa Rica. Just a few years ago, this small troop of approximately 30
individuals was found in an isolated patch of secondary forest close to Finca
La Cusinga, on the border of Ballena Marine National Park. The attached map of southwest Costa Rica shows the locations
of Ballena Marine National Park, Corcovado National Park, and Manuel Antonio
National Park, as well as important potential geographic barriers such as
mountains and rivers. Because almost none of the terrestrial ecosystems along
this coast are protected, you have convinced a conservation foundation to
purchase approximately 200 hectares (ha) of forested land in the area inhabited
by the newly found squirrel monkeys. Your objectives are to manage this reserve
for secondary forest, which is S. oerstedii's preferred habitat, and to
establish a viable population of squirrel monkeys.2
The situation of this tiny population is critical for at least two reasons.
First, previous studies of S. oerstedii have shown that groups with less
than 15 members face almost certain extinction, indicating your 30-member troop
is probably at high risk. Second, this group is one of only three remaining
populations of squirrel monkeys in Costa Rica. Historically, S. oerstedii
once occupied a much wider range, inhabiting contiguous tropical wet forest
along the Pacific lowlands of Costa Rica and Panama at altitudes lower than 300
meters (m). A 1983 census put the Costa Rican population at approximately
200,000 individuals, with a relatively continuous distribution. By the end of
the 1990s, only 4,000 S. oerstedii remained. These were confined to two
National Parks, one in Manuel Antonio on the Central Pacific Coast and one in
Corcovado on the South Pacific Coast. Elsewhere, the species was believed to be
extinct.
In designing your recovery plan, you decide your first priority is to
bolster population numbers and increase genetic variability. This will require
introducing animals from another area to mate and reproduce with the La Cusinga
monkeys. After much discussion with officials from the Osa Peninsula and
Central Pacific Conservation Areas3, you decide that the Osa Peninsula is
your best option for a source population. Since the Osa's forests are
relatively less fragmented and therefore support a larger monkey population
compared to Manuel Antonio, you reason that extracting monkeys from the Osa
will have relatively less impact on the overall status of S. oesterdii
in Costa Rica.
After a long and painful permit process, you begin culling young females
from the Osa population and introducing them to the La Cusinga preserve. (You
introduce only young females because in this species it is the females that
disperse.) As a good scientist, you hire skilled undergraduate assistants to
observe the social and reproductive behaviors of this animal in the field to
ensure the success of your program. Within a very short time, you begin to fear
that your plan is failing. Of 30 reproductively mature females introduced to La
Cusinga, not one has reproduced successfully.
Although the animals from the Osa look almost identical to those from La
Cusinga, you suspect they are reproductively isolated, and perhaps even
separate species. Your focus turns to identifying differences between the two
populations, and determining the reasons for reproductive isolation. The data
collected by your field assistants on morphological and behavioral differences
reveals few factors, other than those listed in Table 1,
that distinguish the two populations. (A list of general S. oerstedii
characteristics is given in Table 2.) However, you do
notice that the La Cusinga population more closely resembles Manuel Antonio
monkeys than it does Osa monkeys.
|
Characteristic |
La Cusinga/Manuel
Antonio |
|
|
Body
weight (grams) |
F
600; M 750 |
F
650; M 800 |
|
Head
coloration |
females
- black |
females
- black |
|
Body
coloration |
females
& males orange |
variable,
but generally less brilliant than in Manuel Antonio; more of a copper-yellow |
|
Grooming
of infants by mothers |
common |
never
observed |
|
Spatial
associations between resident males |
remarkably
close |
close,
but not as tight as in Manuel Antonio |
|
Trait |
S. oerstedii |
|
Group
Size |
35-65 |
|
Typical
fruit patch harvested |
Small,
low density |
|
Aggressive
within-group fruit competition |
Extremely
low |
|
Superciliary
patch above eyes |
Forms
a peak, shaped like a Gothic arch |
|
Female-female
social aggression |
Rare |
|
Female-female
affiliative coalitions |
Not
detected |
|
Male-male
social aggression |
Rare |
|
Male-male
affiliative coalitions |
Present |
|
Male-female
social relations |
Egalitarian
and males integrated into troop |
|
Female
troop residence |
Emigrate
before first mating season |
|
Male
troop residence |
Usually
natal |
|
Male
defense of immature troop members from predators |
Vigorous |
|
Interbirth
interval |
1
year |
|
Intratroop
birth synchrony duration |
2
weeks |
|
Infant
age at weaning |
Less
than 6 months |
|
Existence
of a distinct 'travel' call to coordinate troop travel |
Ubiquitous |
|
Dental
anatomy: molar structure |
Narrow
and long |
Additional Background on
the Central American Squirrel Monkey
This case study is based on the biology of the Central American squirrel
monkey, Saimiri oerstedii, whose historical range is found along the
Pacific wet lowlands of Costa Rica and Panama at altitudes lower than 300
meters (m), an area of less than 8,000 square kilometers (km2). According to the Holdridge classification system, the
bulk of this area is characterized by tropical wet forest, with small patches
of tropical dry forest and wet forest transition near the mouth of the Sierpe
and Terraba Rivers.
Molecular, morphological, and behavioral data indicate that S. oerstedii
is distinct from the South American squirrel monkeys. In this case study, a
fictitious scenario is presented in which S. oerstedii is divided into
two reproductively isolated populations--presumably two different species.
Currently, the Central American squirrel monkey is thought to be two
subspecies, S. oerstedii oerstedii found on the Osa Peninsula and S.
oerstedii citrinellus found in Manuel Antonio. The geographic boundary
between the distribution of the two subspecies is the Terraba River. This river
is the largest water drainage in the southern zone of Costa Rica, and together
with the Sierpe River forms the delta of the Sierpe-Terraba Mangrove Forest
Reserve, comprising approximately 227 km2 of protected wetlands.
The major components of the squirrel monkey diet include caterpillars,
grasshoppers, and small berry-like fruits. These undergo large fluctuations in
availability across the annual cycle of the Pacific wet lowlands of Costa Rica.
These items are sparsely distributed, and even during seasons of peak food
abundance a troop's range exceeds 80 hectares (ha), with about 60% of its time
allocated to foraging activities. S. oerstedii are considered unusually
peaceful primates, with very little intra- or inter-group aggression. In
addition, male-male social ties are extremely close in comparison with other
species of monkeys.
The conservation status of S. oerstedii is dismal. A 1983 census put
the Costa Rican population at approximately 200,000 individuals. Today, it is
estimated that approximately 1,000 S.o.citrinellus and 3,000 S.o.oerstedii
remain in the highly fragmented forest along the Pacific coast of Central
America. Fragmentation has occurred due largely to the expansion of the
agricultural frontier, timber extraction, and tourism development. Other
threats include pesticide use on plantations surrounding forested areas, the
pet trade, and disturbance by tourists, to which these primates appear
particularly sensitive.
One interesting obstacle in the efforts to protect this remarkable and
beautiful monkey has been doubts over its origin. For many years, it was
assumed that S. oerstedii was introduced to Central America by
pre-Columbian indigenous peoples and that it was a hybrid of its South American
relatives. This suspected anthropogenic origin of an otherwise wild species of
monkey made it a low priority for primate conservation efforts in Costa Rica.
It was also the basis for a decision to exclude the species from
The neglected status of S. oerstedii during the past 20 years of
Costa Rican conservation history (and its concurrent population crash) brings
up an interesting question for reflection and discussion: Does the possible
anthropogenic origin of a species like S. oerstedii make it less worthy
of conservation efforts? Why or why not?
1 Adapted from Hewlett, James A. 2000. "Trouble
in Paradise: A Case of Speciation." Available on the University at Buffalo
National Center for Case Study Teaching in Science web site at http://ublib.buffalo.edu/libraries/projects/cases/paradise/paradise.html.
2 The scenario presented here is fictional in part.
At the time of this writing, the only known populations of S. oesterdii
in Costa Rica are confined to Manuel Antonio National Park and Corcovado
National Park. No populations have been found near Ballena, although this area
was once part of the monkey's natural range.
3 Costa Rica is divided into 11 regional
conservation areas, each of which is intended to locally integrate the
management of lands assigned to different uses (e.g., forest reserves, national
parks, wildlife refuges, etc.). Manuel Antonio National Park belongs to the Central
Pacific Conservation Area, while Corcovado and Ballena Marine National Parks
belong to the Osa Conservation Area.
- Boinski, Sue. 1999. The social organization of squirrel monkeys:
Implications for ecological models of social evolution. Evolutionary
Anthropology 8(3):101-112.
- Boinski, Sue, and Susan Cropp. 1999. Disparate data sets resolve squirrel monkey (Saimiri)
taxonomy: Implications for behavioral ecology and biomedical usage. Int.
J. Primatol. 20:237-256.
- Boinski, Sue, Katharine Jack, Craig Lamarsh and Jessica A. Coltrane.
1998. Squirrel monkeys in Costa Rica: drifting to extinction. Oryx
32:45-58.
- Boinski, Sue, and Laura Sirot. 1997. Uncertain conservation status of
squirrel monkeys in Costa Rica, Saimiri oerstedii oerstedii and Saimiri
oerstedii citrinellus. Folia Primatol 68:181-193.
- Cropp, Susan, and Sue Boinski. 2000. The Central American Squirrel Monkey (Saimiri oerstedii):
Introduced hybrid or endemic species? Molecular Phylogenetics and
Evolution. 16(3):350-365.
- Janzen, D.H. 1983. Costa Rican
Natural History. Chicago, Illinois: University of Chicago Press. pp
12-65.
- Stiles, F. Gary, and Alexander Skutch. 1989. A Guide to the Birds
of Costa Rica. Ithaca, New York: Comstock Publishing. pp. 4-19.
If you would like to locate more Central American squirrel monkey
references, consult the library database BINABITROP at the OTS web site http://www.ots.ac.cr.
Image Credit: Photograph of
Saimiri oerstedii is provided courtesy of Lawrence Williams and the Squirrel Monkey Breeding and Research Resource, University
of South Alabama. Used with permission and partially funded by NIH grant
P40-RR01254.
Your Assignment
Working in your assigned group, use the information from the case study to
develop and present a model of evolutionary divergence between these two
populations. You may include as many details as you like in your story, but you
must follow some guidelines:
- Your story should account for at least some of the data supplied in
the case study.
- It is acceptable to add more data to your story as long as it fits the
patterns you see in the data provided. Attached to this document is additional information on the squirrel monkey, as
well as references that may prove helpful in
developing your story.
- As part of your story, you may report fictitious experiments that
support or disprove your hypotheses for evolutionary divergence. Any
fictitious experiments must be potentially feasible, given the logistical
and ethical constraints of field biology. Moreover, any fictitious results
presented must be biologically compatible with what we already know about squirrel
monkeys.
- Your story must incorporate the three concepts from the "Concept List" assigned to your
group. If you wish, you may include additional concepts. For each concept
you include, whether assigned or not, you must demonstrate its relevance
to your story.
- The concepts you use may or may not be compatible in a single
evolutionary story. For example, you may present a story that includes
three or more concepts working together in the process of speciation.
Alternatively, you may treat two or more concepts as competing hypotheses
and present evidence for or against each hypothesis.
- Keep in mind that in an evolutionary story you will describe events in
your monkey population(s) that may have occurred over very long time
periods. Current evidence indicates the ancestors of S. oerstedii
arrived in Central America from South America some 500,000 years ago, so
you may decide that geological and climatic events since that time are
relevant to your story. In addition, remember the historical range of S.
oerstedii was continuous along the Pacific Coast of Costa Rica and
western Panama until only 20 years ago. In other words, S. oerstedii
was only very recently confined to the small populations of Manuel Antonio
and Corcovado.
- The presentation of your story should last no more than 10 minutes.
Prior to your presentation, your group must submit a one-page written
summary of your story.
- You are not limited in terms of how you tell your story. Be creative!
- You are free to include graphics and illustrations with your story.
Remember: Your oral presentation must
demonstrate to your classmates how certain evolutionary processes can function
in the real world. Therefore, you must elaborate on how each concept you
incorporate would work in the context of squirrel monkey evolution. For
example, stating that "the animals became two species because of sexual
selection" is not sufficient. You must also explain the process and
implications of sexual selection, how sexual selection can give rise to
speciation, and what evidence might indicate that sexual selection is at work.