Field observations help us to determine
reproductive success in both males and females. It is definitely
easier and more accurate to determine female reproductive success in the
field than it is to determine male reproductive success.
Female reproductive success is determined by whether
or not a female successfully weans her pup (the pup must live until the age
of weaning). Each day during the breeding season, observations are made to determine
if females have given birth, are nursing their pup, or if they have weaned their
pup. Mothers and pups are marked with the same dye mark soon after the mother
gives birth. This facilitates observations and enables researchers to know whether
or not females are nursing their pups (sometimes switching does happen). We
then keep track daily whether or not each female is seen nursing her pup or
if the pup has died. We know that pups are weaned after approximately one month
of nursing. We determine that pups have been weaned successfully by observing
their mother depart the beach for her foraging trip (or noting that she is no
longer on the beach) and seeing that the approximately one month old pup is
still alive at the harem.
Male reproductive success is a little harder
to determine because an observation of copulation does not necessarily
mean that the male successfully impregnated the female. Usually,
the alpha males are assumed to have higher reproductive success, but there
are many factors that affect reproductive success. Contributing factors
include rank in harem, time in harem, number of matings, and duration and
timing of copulations.
In order to verify that field observations
can accurately estimate male reproductive success, paternity studies have
to be done concurrently. In order to test paternity, all males that copulate
or are regular harem attendees must be tissue sampled. The following year,
returning mothers and their pups are tissue sampled. Paternity analyses
are then conducted using microsatellite DNA markers (a type of DNA test
to look for relatedness among individuals). This paternity analysis
has only been done comprehensively on the southern species in Argentina
(Wainstein et al. 1999).
The bottom line is that number of matings
is a good indicator of how well males do relative to each other. But if
you really want to know how any particular male will do, you need to know
quite a bit of detail about the actual matings, like their duration and
their timing. There is some baseline evidence for northern elephant
seals that alpha males sometimes do well, sometimes not so well, but a
real in-depth comparison of those measures relative to mating behavior
is currently in the works
(hopefully in a couple of months Michelle Wainsteinwill have finished
the data on northern elephant seals).
Recruitment
Recruitment estimates are made during
census counts during the breeding season. We count all pups that
are born and successfully weaned at Año Nuevo to obtain recruitment
number (number of new members in the population) each year.
References for field methods:
Le Boeuf B.J., Morris P., and Reiter J.
(1994) Juvenile survivorship of northern elephant seals. In:
Le Boeuf B.J. and Laws R.M. (eds.) Elephant seals: population ecology,
behavior, and physiology. University of California Press, Berkeley,
pp. 121-136
Wainstein M, Le Boeuf B, Campagna C, Amos
B, Gemmell N, and Ortiz L (1999) Paternity in southern elephant
seals (Mirounga leonina). In: The 13th Biennial Conference
on the Biology of Marine Mammals Abstracts. The Society for Marine
Mammalogy, pp. 195
Module I Glossary
Adaptive
logic - behaviors often have an adaptive logic; that is, their actions
make sense when viewed in terms of evolutionary principles, such as the
drive to carry on genes to the next generation.
Alpha
males - the biggest, fiercest male adults who are able to fight off
all comers and have their own harem of up to100 females. Alpha bulls, or
beachmasters, arrive early and leave late from the breeding rookery.
Census
- Elephant seals are censused each year. Tags attached to their flippers
when they are pups are read and recorded every year.
Delayed implantation - In
elephant seals, after the egg is fertilized, the blastocyte (a fertilized
egg that has undergone several cell divisions) remains free in the uterus
for approximately 4 months. After 4 1/2 months, the egg implants into the
uterus and active fetal growth occurs for or approximately 7 1/2 months.
Gestation
- length of time from impregnation until birth.
Mortality
rate - how many elephant seals die each year, as expressed in
a percentage. Aggression as a reproductive strategy, both males and females
are aggressive during breeding season.
Phenotypic
sexual maturity - ability to father offspring and possessing
all secondary sex characteristics.
Polygynous
- polygyny means that only the males have more than one partner (polygamy
means that both sexes have more than one partner.
Recruitment
- elephant seals from other rookeries instead of returning to the beach
where they were born will move to another site—most often males who are
looking for a chance with some new females (at Año Nuevo, they usually
come from southern rookeries, most likely those on the Channel Islands).
Reproductive
Success - measured in males by successful mating encounters, in females
by success in raising pup to state of weaning.
Sexual
maturity - for females, the ability to get pregnant and
have a pup. This can occur after the first year of life, but usually females
give birth for the first time at an average age of 3-4 years. Males are
mature by age 5, and can father offspring at this age but rarely do as
they are not large enough to successfully compete for females.
Sexually
dimorphic - the adult males and females look very different from each
other; in elephant seals, males are from 3 to 7.5 times larger than females
and can weigh over 5,000 pounds. Females can weigh up to 1500 pounds.
Subadult
- males who show secondary sexual characters (neck shield and proboscis)
but who are not yet adults. They are usually from 4 to 7 years old.
Tag Record
- flipper tags are attached to all elephant seal pups born at Año
Nuevo. These tags are read each year, and is a record is kept on
the animal until it dies or no longer returns to Año Nuevo.
IV. Activities for Module I
Activity 1: What are your chances?
The social organization and mating behavior
of elephant seals revolves around fierce and bloody encounters between
males in a dominance hierarchy at each breeding area. The males fight with
each other to establish who is the top, or alpha, male in the breeding
area. The dominance hierarchy works like this: the alpha male dominates
all other males, the second-ranked male keeps all other males away from
females but defers to the alpha male, and so it goes with decreasing rank.
Only the highest-ranking males have access to the females. The females
group together in harems, seeking each other out for safety from the aggressive
males.
Objective for Student Learning
In these two activities, students will
calculate what their chances are of reaching reproductive
success if they were born an elephant seal instead of a human. As a
result of this activity, they will gain insight into what forces shape
elephant seal aggression, the dynamics that form an elephant seal population,
and the dangers that face young seals. Most likely students will decide
they are glad they were born human and not elephant seal.
Activity Data
The first challenge to achieving reproductive
success is to be able to simply survive. Dead elephant seals don’t mate!
One study at Año Nuevo showed the
highest mortality occurred within the first year, and mortality
rate decreased as the animals got older. Those animals that survive
the first four years (the juvenile period) have a good chance of reaching
maturity. There is no sex difference in juvenile survivorship.
There was a study that took place at Año
Nuevo over a 7-year period from 1971-1978 (Le Boeuf et al 1994). In this
study, only 16% of the pups survived to age 4, the earliest that a male
can possibly mate (though virtually none do at this age; they have to wait
until they get much larger, at around 8 years of age). Most females first
mate at 2-4 years of age.
Activity 1 (Easier activity):
Life is not guaranteed!
Objective for Student Learning
Students participate in an exercise that
graphically illustrates how few elephant seal pups survive to reproductive
maturity.
Materials and preparation
2 days before:
Download DS1.1,
which includes instruction for group activity.
Download TM1.1-8 from web site. Copy onto
overhead transparencies for whole class discussion, or copy onto paper
for small group discussions.
Day 1:
Conduct the activity as shown on DS1.1.
Students in the classroom are newborn
elephant seal pups. Using the survival percentages as stated above, do
this activity to see how many of these pups will survive to adulthood.
a. Using a standard deck of
cards, distribute them face down, one to each student. Discard the cards
set aside.
b. At the end of the activity,
how many elephant seals are left? These are the only ones who can potentially
reproduce. How many males and females are there? Review with students the
many ways to die if you are an elephant seal.
Day 2:
Take one hour to go over TM1.1-8 as a
group discussion, or divide into 8 4-person groups to discuss each data
sheet separately. Have each group present what they learned from their
data sheet. You can project the transparencies as the students present
their answers to the discussion questions.
Assessment
Final discussion: Discuss the hazards
of being an elephant seal, and the pros and cons of being a female or male.
Discuss as a whole class what you think
are the reasons for so few animals returning to the rookery, and why the
numbers get better as the animals get older.
Exploring the web site
Count the number of pups you can see on
the beach during February and March. How many of these pups will survive
to maturity (calculate out what 16% is of the pups seen)?
Activity 2: I survived! Can I mate
now?
Seeing the odds of a male ever successfully
becoming an alpha bull, the aggressive fighting for rank among males becomes
more understandable.
Objective for student learning
Students will use survival rates gathered
by scientists to predict what the survival statistics might be for a different
year. The survival rate of the first 4 years is incredibly low, and has
an affect on the numbers of animals who eventually get to mate, or in the
males’ case, the opportunity to compete to mate.
Materials and preparation
Day before:
Download Module
1 Reproduction Background. Make copies for small group discussions (one
copy per group, 4 or 8 groups)
Download DS1.2.
Make copies for small group discussion (one copy per group, 4 or 8 groups).
Day of:
Follow activity as described on DS1.2. Optional:
as a group, answer Extensions questions.
Extensions/ Topics for Further Discussion
(male dominance ranking system in elephant seals)
Seeing the odds of a male ever successfully
becoming an alpha bull, the aggressive fighting for rank among males becomes
more understandable.
1. List the evolutionary advantages of
a male dominance ranking system like the elephant seals have.
(Stronger, bigger males perpetuate
their genes, reducing the potential of a population based on the genes
of smaller, weaker individuals.)
2. List the disadvantages of a male dominance
ranking system like the elephant seals have.
(In this system, size is everything.
What happens to the system if the food runs out?
Pup, weaner, and female injuries
due to aggressive males may affect the population.)
3. List how the elephant seal mating system
differs from humans.
(Male humans in their prime breeding
years have more sex than the majority of male elephant seals. Female elephant
seals are probably about the same as humans.
The fight over who gets to mate
is between males.
There is a lot more diversity in
human populations because there are more fathers.
Male humans do not normally mate
with their mothers, siblings, or offspring.)
4. List how the elephant seal mating system
is similar to humans.
(Male aggression also occurs in
humans.
There is a high parental investment
in pups by the mother, because they have just one pup per year.
Rape occurs, and females learn how
to avoid harm.)
5. Seals have separate foraging and breeding
areas, and males and females are only in contact for 1-2 months each year.
In this situation, polygyny would be more advantageous (or necessary) for
increasing reproductive success than the "human system" would be. Polygyny,
with a dominance hierarchy, is less of an advantage and harder to maintain
in a system where females do not have a synchronized, brief estrus period
(similar to humans). Given this perspective, which mating system do you
think leads to higher reproductive success for the survival of a species
in each situation: the dominance ranking system of the elephant seals,
or the "system" of humans?
Activity 3: How to get a mate, and
is it worth the effort?
(moderately difficult; students make x-y
graphs from tabled data)
The reproductive strategies that male elephant
seals use select for larger, more aggressive, older males. Age, size, and
reproductive effort (RE, as measured by percentage of mass lost over the
breeding season, and represents the energetic costs of reproduction) add
up to mating success for males. This pattern of sexual selection has apparently
been strongly influenced by the mating system and the intensity of male-male
competition for mates (Deutsch et al., 1994).
Objective for student learning
Students take a closer look at what increases
the odds of a male elephant seal being successful in the breeding rookery,
and how it affects their life outside of mating. They will read a data
table, extract information, and chart the data on a xy graph to visually
present their results.
Materials and preparation
Day before:
Download DS1.3.
Make copies for students to read individually, or make a transparency for whole
class discussion.
Have students bring in four colored pencils
for next day’s activity.
Purchase graph paper.
Day of:
See TM1.3 to either show on an overhead
projector or hand out to students. Students can use a sheet of lined paper
to make the graph, and it is helpful to have four colored pencils or pens.
Exploring the web site
Look at photos from late December. Can
you pick out which animals may become alpha bulls? How many are there?
Can you detect subadult males in the group? Where are they in relation
to the adult males? Look at photos from late January and early February.
How many bulls are there now? Are there any subadult males around? What
are the difference in male numbers and classes of males present during
these two time periods?
Extensions
1. Why would a system for fasting and
breeding evolve this way?
(Foraging adult seals
go to different areas based on gender. Males go to the Alaska continental
shelf and females to the international date-line in the open Pacific Ocean.
Coming back to land ensures that males and females can mix and reproduction
can occur. Males take advantage of females naturally grouping together
on land. Females have to give birth and nurse their pups on land. Pups
can’t swim or catch fish. Males go to where the females are. They control
the areas the females are in. Females group together on land for protection
(California used to have bears on the beaches, so the seals in the past
only hauled out on islands. Now the bears are gone and it’s safer for the
pups, but occasionally a coyote will take a pup.) Fasting for up
to three months while breeding and birthing, animals have to be strong
to start out. Fasting allows them to stay out of the water where predators
lurk so they can give 100% attention to the job of reproduction. It is
an endurance race, much like a triathlon. Only the strong survive to carry
on their genes. Bigger males can fast longer, fight longer, and consequently
stay on the breeding ground longer.)
Activity 4: The Successful Mother
(moderate level activity, interpreting
and integrating table data)
Female elephant seals are also aggressive
during the breeding season on land, though it is not over males. Why would
female elephant seals show aggressive behavior?
Learning objective
Students will take a closer look at the
grim aspects of life in the rookery, gain a better understanding of how
populations are shaped by circumstances, and the strategies that define
a good elephant seal mother. They aren’t always pretty.
Materials and Preparation
Day before:
Download DS1.4
and TM1.4. Make copies of DS1.4 for student background reading (optional). Make
a transparency of TM1.4
Day of:
Conduct activity as described in TM1.4.
If possible, have students read DS1.4.
Conduct whole class discussion on Extensions questions.
Exploring the web site
Look at a photo of a group of females
with pups. Where are the pups located in relation to the females? Where
are they located in relation to the males? Do you see any off by
themselves? Can you tell the difference between a newborn and a weaner?
Extensions
1. If you were a young pregnant elephant
seal, where would you most likely give birth?
2. What would your pup’s chances of survival
be if you gave birth before age 3?
3. What would you learn by experience?
What would you do differently next time you arrived at the beach pregnant?
4. What do female humans do to assure reproductive
success?
Activity 5: Mating Disasters
(two day minimum)
Sexually aggressive male elephant seals
increase the cost of mating for females (Mesnick and LeBoeuf, 1991). Therefore,
selection should favor the evolution of female responses that minimize
the probability of being injured. Different species have different responses
to sexual harassment and potentially dangerous mating encounters with males.
Elephant seal females have learned a few strategies to surviving the mating
years.
Learning Objective
Students will evaluate the success of
the behavioral strategies female elephant seals use to avoid death by over-aggressive
males. They will prepare a map of Año Nuevo Island based on the
one available through the web site, and map the safe haven sites for females.
They will write a letter home to younger sisters giving them tips they
learned on how to survive what awaits females as they attempt to leave
the safety of the harem.
Materials and preparation
Day One
Download DS1.5
background material.
Download TM1.5.
Prepare copies for small group discussion
(one copy each group, or one copy each person). Students will read background
material (10 minutes)
Conduct Step One and Step Two activities
from TM1.5. (40 minutes plus)
Day Two
Conduct Step Three and Four activities
from TM1.5. (60 minutes)
Exploring the web site
Look at photos from January to March.
Do you see any females trying to leave the harem? Are there any males waiting
in the water for them? What time of the day was the photo taken?
Check the live web site if it is during
peak breeding time (January to February), or if it is off-season check
the archival photos. Take a close look for clues on how the harems are
set up. The camera does a good pretty job of showing the entire harem area
of Bight Beach North, so this would be a good example of a representative
harem.
a. Count the number of fully mature adult
males compared to adult females in the area.
b. How many males are in the picture?
c. How many females?
d. There are roughly an equal number of
males and females born each year, and their survival rate to maturity is
roughly equal. Why are there so few males in this picture? Where have they
all gone?
Extensions
What were the strategies the females used to avoid
harm from aggressive males when they were in the harem? (When a male other than
the alpha male tried to mount them, they would make loud noises to alert the
alpha bull who would then challenge the interloper and chase him away.)
What were the strategies the females used
to avoid harm from aggressive males when they left the harem? (The experienced,
older females were receptive to peripheral males when they attempted to
mount them. The younger ones were less receptive.)
Why do you think they used two different strategies
when they were in the harem and outside of it? (They had just fasted for over
a month and were too tired to fight and just wanted to hurry up and get out
to sea.)
Which strategies were more successful, and why
do you think they were successful? (Leaving at high tide mainly, means shorter
distance to the water where they are more buoyant and less possibility of getting
squished.)
Can you draw any parallels to human mating practices?
(Females evaluate dangerous situations and make decisions based on least harm.
Experience influences behavior.)
