Observational and
Experimental Studies
Experimental Studies v. Observational
Studies

In experimental studies, a treatment (the
independent variable) is applied.

In observational studies, you do not have
influence over the independent variables.
 Experimental
studies are preferred but it is
often not feasible to experiment.
 e.g.
cannot exchange giraffe’s necks with
other animal
 Behavior
is hard to experiment because the
experiment often alters natural behavior.
Zonosemata:
a fly genus
(group of species)
Observations?
Questions?
Zonosemata:
a fly genus with distinct
wing markings, as well as
wing-waving behavior.
Distinct dark bands on wings
 Holds wings up and waves them
 Display seems to mimic threat display of
jumping spiders

Hypotheses?
Experimental Study:
to find the function of wing markings
and wavings of Zonosemata

Scientific Question:


Do wing markings and waving behavior of Zonosemata mimic
threat displays of jumping spiders and deter predation?
Possible hypotheses:

Flies do not mimic jumping spiders


Display may instead be used in courtship

Flies mimic jumping spiders to deter non-spider predators

Flies mimic jumping spiders to deter jumping spiders
Experimental procedure

Created five experimental groups to test hypotheses
Experiment (Test):
Created five
experimental groups
to test hypotheses
Predictions:
Expectations for
each trial relating to
each hypothesis.
Data (Results):

Other
predators
(anole lizards)
killed and ate
all the test fly
types equally.

Clipping and
gluing did not
affect flying
or displaying
Analysis and Conclusion:

Results consistent with hypothesis 3, but not 1 or 2

Support for hypothesis that Zonosemata deters its
predators by acting like one.

Important experimental design


Testing control groups

All treatments handled identically

Randomization of order of treatments

Replication of treatments
Study was successful because many variables were
tested, but each was tested independently.
 Question:

Do snakes make adaptive choices of
where to sleep at night?


Ray Huey implanted garter snakes with thermo-radio
transmitters
Background Research:

Preferred body temperature of garter snakes
is 28– 32°C

They move to keep body temperature near
preferred during day and night

Exposed to the sun or under rocks/ground
Data Observations:
Observational Study
Night Retreats of
Garter Snakes

Experimental Question/Problem:


Hypothesis:


How do garter snakes choose good retreats at night?
Garter snakes make retreat decisions based on the
thickness of the rocks
Background Research:

Thickness of rock determines microhabitat temperature

Thin rocks heat a lot during day and cool a lot during night

Thick rocks heat and cool slowly

Medium rocks heat and cool just enough
Observational Study
Night Retreats of
Garter Snakes


Experimental Procedure:

Huey placed snake models under rocks of different
thickness (thick, medium, thin) and in burrows

Tested temperature fluctuations at the sites
Data Analysis:


Found that snakes choose medium rocks to heat and cool
near their preferred temperature range
Conclusion:

Garter snakes retreating under rocks at night show a
strong preference for rocks of medium thickness