2012 COURSE DATES: AUGUST 4 – 17, 2012 - Sirenian International

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 [NOTE: The other two orders contain the
hyraxes and the aardvarks, which are rarely compared to
manatees in the literature]. Another promiscuous aspect of
the manatee mating system is scramble-polygyny, where
multiple males attempt to mate with the estrous female, but
- without overt competition. Although males aggregate on
the estrous female and jockey for the best position - they
exhibit little agonistic behavior. Interestingly, male
dugongs appear to be more agonistic during mating events.
They set up territories and exhibit lek mating behaviors
(Anderson 1997). What perspective would we use to
compare mating strategies between manatees and dugongs?
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
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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 Florida, with little evidence of spermatogenesis
present during winter months (December – February). In
many mammals, reproductive activity varies seasonally
with photoperiod, or the number of light hours per day.
The pineal gland is usually the organ associated with
behavioral changes affected by photoperiod. However, no
pineal gland has ever been found in manatees or dugongs
(Ralph et al. 1985, W. Welker personal communication
2000). From an ultimate evolution perspective, it is
interesting that the literature is unclear regarding the
existence of a pineal gland in elephants (Ralph et al. 1985).
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 seasonal
spermatogenesis (G. Smith unpublished data). More
studies are necessary before we can determine if seasonality
affects the reproductive behavior of manatees in Belize.
Parental Care: The final aspect of reproductive
problem solving discussed here is parental care. Like many
mammals, female manatees invest considerable time and
energy into a relatively small number of offspring as their
reproductive strategy. From an ultimate function
perspective, data collected by scientists in Florida suggest
that those females that invest 2 years of parental care in
each offspring prior to becoming pregnant again are more
successful than other females (Marmontel 1995). Although
calves begin eating on their own within 3 months of birth,
they continue to nurse periodically (Hartman 1979) as they
grow and learn migration routes from their mothers (R.
Bonde, personal communication 1999). When we ask
"why" this behavior exists, we are asking ultimate function
questions. Perhaps calves need the extra protein and fat
provided by mother's milk during developmental years. Or,
perhaps it takes calves almost two years to learn the routes
to warm water effluents and good foraging grounds
necessary for survival through the temperate winters in
northern and central Florida.
In an ongoing study of Antillean manatees in the
Southern Lagoon of Belize, Buddy Powell is also seeing
mother-calf pairs remain together for long periods of time
(www.wesave.org/manatee/). When we compare this
behavior between the Florida and Antillean subspecies, we
are using the ultimate evolutionary perspective. On the
other hand, "how" the mother-calf pair remains together