CONTENTS
Preface
Part I:
Introduction --
Ethology, Proximate &
Ultimate, and Sirenians
Part II:
Problem Solving --
Social, Reproductive &
Physical
References
Recommended
Glossary
Acknowledgments
Antillean manatee in Belize 1999 Photo
© Leszek Karczmarski
PREFACE
This Species Brief originated
in WFSC 422, an ethology course taught by my academic advisor, Dr. Jane M.
Packard, at Texas A&M University in 1998.
It was updated this year for distribution to Earthwatch Institute
volunteers. Please consider it a
work in progress – a “draft document” as it is continuously being revised and
updated with better references and new information. Designed to be used by schools, zoos, wildlife
parks, and oceanaria, it makes an excellent starting place for students,
teachers, and others who are interested in learning about animal behavior
and/or sirenians (manatees &
dugongs). But, remember, it is only a
briefing document. Use it to catapult
yourself into the exciting world of animal behavior -- using manatees as an
example. For more details, start with
the Recommended Reading section; scientists and university students are
encouraged to delve into the primary literature listed in the References
section.
PART
I: INTRODUCTION
As we idled
around the corner of Swallow Caye I sighted two manatee noses in the
distance. They were barely visible as
they broke the surface of the clear
Five minutes
passed - how long can these guys stay down?
What are they doing down there? One reason we know so little about these incredibly well adapted
animals is because they spend the majority of their time underwater, regularly
staying submerged 3-5 minutes between breaths.
We heard them before we saw them.
Both noses broke the water with a forceful exhalation at virtually the
same moment. Then they were down
again. I quietly entered the water and
stealthily snorkeled the 50 meters towards their last location. Where did they go? Stop.
Look. Listen. I heard them
breathe again. When they finally came
into view underwater, I thought, "Uh oh...
a mother calf pair -- they are going to run away".
The larger animal was about 3 meters long,
almost twice as big as the smaller.
Ethology
Do manatees breathe
simultaneously? If so, why? Why was the large male manatee nuzzling the
smaller female? Why do manatees
"kiss"? How did they sink to
the bottom and stay - without moving a muscle?
Why did they vocalize during their descent? How do manatees create sound? Do manatees often lie side by side on the
bottom? Are manatees usually found in
pairs, or groups, or alone? Why? How long can manatees stay underwater without
breathing? Why did the smaller animal
surface to breath before the larger animal did? These are just a few of the questions raised
by the brief observation. Some answers
to these "how" and "why" questions are known; other answers
may come through long-term ethological studies.
Ethology is a relatively new,
multi-perspective scientific approach to the study of animal behavior. Made famous by the work of 1973 Nobel Prize
winners, Konrad Lorenz, Karl von Frisch, and Nikolaas Tinbergen (www.nobel.se/medicine/laureates/1973/index.html),
it focuses on animal behavior in a natural setting. By using a Scientific Perspective, it
differs from Folk Psychology, which is often used to explain animal
behavior to the general public. Folk psychology perspectives are
intuitive in nature, usually based on personal experiences and
observations. They are considered anthropomorphic
because they describe and explain behavior in human terms – which are often the
only terms we have to start with! These
perspectives are appropriate and very useful when communicating with
non-scientists, such as audiences in zoos, oceanaria, and wildlife parks. An interpreter will often use folk psychology to describe and explain
animal behaviors based on the “model” that animals have desires, beliefs, and
emotions like humans. Scientists also
use these perspectives in developing hypotheses about specific behaviors. For example, I was using folk psychology when I assumed that the manatees in the anecdote above were a mother-calf
pair. My intuition was based on the size
differential and behavioral patterns typical of mother-calf pairs.
Ethology encourages us to develop additional Scientific Perspectives
in understanding, explaining, and/or describing animal behavior; the classical
ethological perspectives include cause, development, evolution,
and function (Martin & Bateson 1993, Lehner 1996). Modern ethologists agree that the behavior of
an animal is the result of complex interactions between the genetic makeup of
an individual and environmental
factors that act upon the individual
(Alcock 1998). However, many aspects of
an animal's behavior can be explained from two very different perspectives: proximate and ultimate (Martin and Bateson 1993, Lehner 1996). This often results in miscommunication among
observers who are looking at behavior from different perspectives. Proximate
and ultimate comparisons are
equivalent to apple and orange comparisons – i.e. they are both valid fruits,
but they are different things.
"How" questions are usually asked from a proximate perspective; how questions seek explanations about the
physical and chemical mechanisms that trigger an individual animal's behavior at any given point in time. "Why" questions, on the other hand,
are usually based on ultimate
perspectives; answers to these questions attempt to explain why certain
behaviors exist within a population
(or species) of animals. In other words,
what pressures of natural selection led to the existence of a particular
behavior within a population or
species. Ethologists further divide proximate and ultimate into the sub-categories of cause, development, evolution, and function based on the work of Niko Tinbergen (Martin and Bateson
1993, Lehner 1996).
Proximate: Cause and
Development are proximate
perspectives, which look at the behavior of an individual animal. Proximate
Cause perspectives include looking at both internal mechanisms (hormones,
neurotransmitters) and external stimuli (pheromones, photo-period, temperature)
that interact to trigger specific behaviors in a mature animal. Dr. Jane M. Packard explains it using the
analogy of a camera, “Think of Proximate Cause as a snapshot in time
that shows what is causing the behavior at that particular moment.” For
example, in our observation above, what “caused” the manatees to kiss? Was the female was giving off some signal
(vocal, chemical, or behavioral) that attracted the male? Did the tactile stimulation by the male cause
hormone production in the female, which triggered the “kiss”? Most likely, it was is
a complex interaction between both the internal state of each animal and the
resulting external behavioral stimuli. Proximate
Development perspectives look at behavioral changes that occur as an
individual animal matures. Think of Proximate
Development as a "video" that shows how a behavior develops and
changes over time as an individual
animal matures. How might the behavior
of manatees at different ages compare to the interactions we observed?
Ultimate: Evolution and
Function are ultimate perspectives, which look at specific behaviors
present in a population of
animals. These behaviors are thought to
have evolved over time through the process called Natural selection. For Natural selection to act on a
behavioral characteristic, the behavior must meet certain criteria – the same
criteria necessary for natural selection
to act on a physical trait such as coloration:
(1) the trait must vary among individuals
within a population; (2) the
variation must be heritable; (3) if the heritable variation results in
differential fitness (i.e. variations in the trait result in some individuals reproducing more
successfully than others); then (4) we would expect the behavior to become
genetically fixed in the population
as the proportion of individuals
displaying the trait increased (i.e. changes in the proportion of genotype and resulting phenotype).
Ultimate
Evolution perspectives include the comparison of behaviors among
different, closely related species. This
is our “video” perspective. From an Ultimate
Evolution perspective, we hypothesize about how a behavior has changed (or
remained the same) at the population
and/or the species level over many generations.
In my study of Antillean manatees, I will be comparing behavior to
previous observations of behavior in
Ultimate
Function perspectives attempt to explain what the function of a specific behavior is
within a population, (i.e. why
animals that display this behavioral trait are more reproductively successful
than individuals who do not). This is our “snapshot” perspective. If variation exists within the behavior, Ultimate
Function is the perspective used to explain why. Some
Hypotheses about “why” these
behaviors exist in manatees are different from hypotheses about “how” these
behaviors are executed. The “why”
questions are from ultimate perspectives of evolution and function,
the “how” questions are from proximate perspectives of cause and development.
|
TIME/ ANALYSIS |
Pattern-Static “Snapshot” |
Process-Dynamic “Video” |
|
Proximate Perspective Individual Animals “How Questions” |
CAUSE (control) behavioral triggers: internal state/ external stimuli |
DEVELOPMENT (ontogeny) changes in behavior as an animal ages: maturation and/or
learning |
|
Ultimate Perspective Populations/ Species “Why Questions” |
FUNCTION adaptive significance: effect on reproductive fitness |
EVOLUTION (phylogeny) changes in behavior (genotype) as populations/ species diverge |
We can remember the concepts
of ethology with the acronym AB=CDEF (Animal Behavior = Cause,
Development, Evolution, Function).
On the TIME axis, Cause and Function are “snapshot” perspectives
that look at internal state and external stimuli in an individual animal or at reproductive success in a population of animals. Development
and Evolution are “video”
perspectives that look at changes in behaviors over time, either as the individual animal matures or as the population evolves. On the ANALYSIS axis, Cause and Development
are proximate perspectives that
attempt to answer "how" questions at the level of individual animals. Evolution
and Function are ultimate perspectives that attempt to
answer "why" questions at the level of populations and/or species.
For more information on Ethology, I encourage you to visit Dr. Packard’s
website: http://www.tamu.edu/ethology/. The four basic concepts of ethology can be
arranged in a 2 x 2 table comparing TIME FRAME and ANALYSIS PERSPECTIVES (Table
© Jane M. Packard).
Sirenians
So what are manatees anyway, and why should we study their
behavior? Manatees belong to the order Sirenia of which there are only 4 extant species in 2 families,
Trichechidae and Dugongidae. Although scientists often lump sirenians together with the order Cetacea (whales and dolphins) as
totally aquatic marine mammals, manatees and the dugong are actually more
closely related to elephants, hyraxes, and aardvarks than to any other marine
mammal (Fischer 1990, Maluf 1995, Springer et al.
1997, Gaeth et al. 1999). Until a few years ago, very few people had
ever heard of manatees, dugongs, or sea cows.
But, as we learn more about these elusive and highly specialized creatures
that share our coastal habitats, they are becoming more and more popular among
both scientists and conservationists.
The West Indian manatee (Trichechus manatus), the West African
manatee (Trichechus senegalensis), and the Amazonian manatee (Trichechus
inunguis) are members of the family Trichechidae. The dugong (Dugong
dugon) is the only surviving member of the family Dugongidae (Reynolds and
Odell 1991). Steller's sea cow (Hydrodamalis
gigas) is usually included when we talk about modern sirenians; it was in the family Dugongidae (Reynolds and Odell
1991), but the species was extirpated
by humans in 1768, just 27 years after it was discovered by Russian explorers
in 1741 (Stejneger 1887). Today, local
and international laws protect all four living species, but they are also either
threatened or endangered by humans wherever they exist.
Antillean
Manatees: Even before Russian sailors were exploiting Steller’s sea cow meat
in the North Pacific, European explorers were provisioning their ships with
Antillean manatee meat from the
PART
II: PROBLEM-SOLVING
One way to interpret animal
behavior is as a method of problem solving.
Over geological time, animals have evolved behaviors that enable them to
solve problems. Some of these behaviors
are identical from one individual to
another. We describe such a behavior as
a Fixed Action Pattern (FAP), because the individual’s
genes control the trait. In other words,
the genetic trait has become fixed in the population and all individuals perform the behavior in
exactly the same way because they inherited the trait from their
ancestors. At the other end of the
behavioral scale, we find behaviors that vary a great deal among individuals. We describe such a behavior as a Variable
Action Pattern (VAP), because the environment controls the trait. In other words, each individual performs the behavior differently due to different
environmental factors during development.
Additionally, any individual
may perform the behavior differently at different times, depending its internal
state (hormones, chemicals, neurons) and on external stimuli
(environment). Of course FAP and VAP are
not specific categories, but are the end points along a continuum. If a behavior falls near the middle of this
continuum, we describe it as a Modal Action Pattern (MAP). In other words, the behavior is controlled in
part by genetics and in part by the environment.
We can divide problem solving
into three major categories: reproductive, physical, and social. Manatees have
evolved some interesting behaviors to overcome reproductive and physical
problems. But, they are not considered to
be very social animals. Although they tend to aggregate on resources,
they do not appear to live in social
groups and significant social
behaviors have not been observed outside of reproductive activities. This species brief will use the concepts of
ethology to introduce you to the behavioral methods manatees use to solve some
of their reproductive and physical problems.
We will examine specific behaviors using the different perspectives of proximate cause, proximate development, ultimate evolution, and ultimate function to answer a few questions
regarding "how" and "why" manatees behave as the do.
Reproductive
Problem Solving
We idled
into one of my favorite coves at the end of Bogue C in the Drowned Cayes
near Belize City. I had taken volunteer
researchers to this spot on previous occasions and ALWAYS, there had been a
manatee resting in the manatee hole
on the far side of the cove. As if on
cue, before we could even cut the engine and anchor the boat, a single manatee
surfaced in the vicinity of the manatee
hole. Within minutes a second
manatee surfaced in the middle of the cove.
Two minutes later, a third animal entered the cove. "Gee...” I thought, "This must be a
popular resting cove!" But, the two animals in the center of the cove were
too active to be resting. I didn’t think
they could be feeding, either, because previous habitat snorkels had found NO
vegetation on the bottom. Another four
minutes passed and two more manatees swam under the boat to join the active
pair in the middle of the cove.
"This is great,” I told the volunteers, "we'll be able to see
how long it takes them to settle into a resting pattern". But they didn't settle. For the next hour we watched the four
manatees in the middle of the cove breathe, roll, dive, and kiss while the
first animal appeared oblivious to all the activity less than 50 meters away. Were we observing a mating herd?
Mating System: One parameter of the West Indian manatee mating system is known as the mating
herd. A mating system is the species-typical
pattern of problem solving that includes how an individual finds a mate, how long it remains with the mate, and how
much energy it invests in its offspring (Drickamer, et al. 1996).
The West Indian manatee mating
system can be broadly defined as promiscuous with the estrous
female exhibiting polyandrous behavior and the male exhibiting polygynous
behavior (Hartman 1979). A manatee-mating herd consists of a group of
males in pursuit of an estrous
female. The group is ephemeral,
lasting only from a week to a month (Hartman 1979) and consisting of up to 20
males. The group does not remain
together afterwards. Males will
participate in multiple mating herds
and attempt to copulate with many estrous
females; similarly, females will copulate with multiple males among those in
the herd. When we discuss why this mating system exists in manatees, we
are using the ultimate function perspective.
From the perspective of proximate
cause, we do not know exactly what external signal stimulates males to
aggregate around and attempt to copulate with the estrous female. Females must
produce some sort of signal, possibly a chemical or acoustical signal, which
stimulates an internal hormonal mechanism in males causing them to pursue
her. Likewise, the male’s internal state
must be such that he responds to the signal.
Proximate development perspectives would look at how the males’
reaction to such a signal might differ at other stages of maturity.
Daniel S. Hartman, one of the first scientists to make
long-term observations of manatee behavior in the wild, found similarities
between manatee mating herds and
elephant mating, noting that female elephants are also polyandrous - often mating with several males over a period of
several hours. When Hartman (1979)
compares the mating behavior of manatees to that of elephants, he is writing
from an ultimate evolution perspective.
In other words, he is hypothesizing that this aspect of the mating system evolved millions of years
ago in an ancestor shared by both the manatee and the elephant. Since sirenians
and proboscideans are two of only four extant
orders that share a common ancestor among them, manatees are often compared to
elephants using the ultimate evolution perspective
Timing: Let's assume that our
observation was of a mating herd. That is, the group of manatees in the center
of the cove consisted of 1 estrous
female and 3 males.
Why was the first manatee, the one originally sighted in the resting
hole, not involved in the mating herd? Looking at the situation from a proximate perspective, there are
several possibilities, and all involve timing. Suppose the resting manatee was a female. If she was sexually mature, but not in estrous, the mating herd would have no interest as she would not be producing an
estrous signal. An estrous
signal is the proximate cause of the mating herd behavior. It's
"how" the males know the female is ready to conceive. Similarly, if the female were sexually
immature, she could not be in estrous
and therefore would not be sending a signal.
Scientists have only recently answered the question of when a female
manatee becomes sexually mature, thanks to the development of a new aging
technique by Miriam Marmontel, et al.
(1990). Since
manatees continuously regenerate new teeth throughout their lives (Domning and
Hayek 1984), they cannot be aged by their dentition like many other marine
mammals. But, by looking at growth
layers in manatee ear bones, we are now reasonably confident that female
manatees in Florida reach sexual maturity between the age of 3 and 4 years -
most giving birth to their first calf at age 4 (Marmontel 1995). Questions of "how" the behavior of
signaling develops in females as they mature fall under the proximate
development perspective.
On the other hand, if the
resting manatee was a male, why wasn't he attracted to the estrous female in the middle of the cove? He could have been either sexually immature
or sexually inactive. Using the presence
or absence of sperm in the testes as an indicator, Hernandez et al. (1995)
found that sexual maturity (proximate development) varied among Florida
male manatees with both size and age with some males becoming physiologically
mature as young as 2 years and as small as 237 cm. But, from a proximate cause
perspective, they also found that the reproductive system varied in
functionality among mature male manatees depending on season in
Whether the resting manatee was inactive or immature, his timing would have been out of sync with the female and the estrous signal would have no effect on his behavior. From an ultimate function perspective, we say that those manatees whose sexual behavior is triggered at the appropriate time (i.e. when both the male and female are sexually mature, active, and receptive) are more reproductively successful than manatees that waste energy on futile sexual encounters. From an ultimate evolution perspective, there have been some behaviors observed in Antillean manatees that might be associated with seas