PROPOSAL TO INVESTIGATE INDIVIDUALLY DISTINCT VOCALIZATIONS OF ANTILLEAN MANATEES
(Trichechus manatus manatus) WITHIN MOTHER-CALF PAIRS
Caryn Self Sullivan, Department of Wildlife and Fisheries Sciences, Texas A&M University,
College Station, TX 77843.
Committee: William E. Evans,
Ph.D. and Jane M. Packard, Ph.D., CO-CHAIRS;
Robert E. Randall, Ph.D. and Jeff Norris, Ph.D.
This material is based upon work supported under a
National Science Foundation Graduate Fellowship.
Vocal Signatures
Individually distinct vocalizations, sometimes referred to as vocal signatures, have been
identified in both birds and mammals (Dhondt and Lambrechts
1992, Cheney and Seyfarth 1980). Theoretically, the
evolution of vocal signatures is dependent on the increased probability of misdirected parental
care which is more likely to occur on colonial breeding grounds and in species where mother and
offspring are often separated. In colonial species: a Japanese horseshoe bat (Rhinolophus
ferrumequinum nippon) mother is able to distinguish her offspring from others in the colony
by its "attractive" calls (Matsumura 1979);
parent-offspring vocal recognition exists in emperor penguin (Aptenodytes forsteri)
colonies where parents must leave their precocial chicks to forage
(Aubin 1997); in a study of California sea lion
(Zalophus californianus) colonies, mother-pup pairs correctly identified each other
by vocalization 90% of the time (Schusterman et al. 1992
). Alternatively, in solitary species such as kittiwakes (Rissa tridactyla),
where young are confined to the isolated nest for weeks, parents are unable to recognize their
own offspring (Bradbury and Vehrencamp 1998).
Vocal signatures are not limited to colonial breeding mammals. In other mammals where reproduction rates are low and parental care is for extended periods, vocal recognition appears important for parents to be able recognize offspring following temporary separations. In playback experiments, Cheney and Seyfarth (1980) found that vervet monkey (Cercopithecus aethiops) mothers' responses to their own offspring's screams were quicker and longer than they were to non-offspring screams. In marine mammals, bottlenose dolphins (Tursiops truncatus) used an individually distinct call known as a signature whistle (Caldwell et al. 1990). Live captured mother-calf pairs used signature whistles to keep in contact during physical separation (Sayigh et al. 1992). Most vocal signature studies on dolphins have been conducted during live captures or in captivity due to the difficulty of in situ studies (Smolker et al. 1993). When Smolker et al. (1993) studied free-ranging animals in Shark Bay, Western Australia, they found that bottlenose dolphin signature whistles occur primarily when mother-calf pairs are separated and that the probability of occurrence increases with the distance of separation.
Sirenian Studies
Few studies have been conducted on Sirenia (manatees and dugongs) vocalizations
(Bengtson and Fitzgerald 1985). Two captive and
5 wild Florida manatees (Trichechus manatus latirostris) recorded in 1964 represent
the first sirenia vocalization data published (Schevill
and Watkins 1965). Since then, Evans and Herald (1970)
recorded a single captive Amazon manatee (T. inunguis); Sonoda and Takemura
(1973) recorded both the Amazon and the Antillean manatee
(T. m. manatus) in captivity; and Nair and Lal Mohan
(1977) recorded one captive dugong (Dugong dugon) in
air. These early references are limited to physical descriptions of the vocalizations (e.g.
frequency, duration, harmonic structure). Hartman (1979)
conducted the first intensive field studies on Florida manatees in Crystal and Homosassas
Springs, Florida, from 1967 to 1969; he published contextual observations of vocalizations
noting that manatees were relatively quiet, but vocalized when alarmed, during sexual activity,
during play, and while alone. The only predictable vocal exchange he observed was an
"alarm duet" (p. 99) between mother and calf, noting that this exchange rarely attracted
attention from nearby manatees. Reynolds (1981)
reported on Florida manatee vocalization structure and context between 1 mother-calf pair;
and on the quantity of calls as a function of context by animals in Blue Lagoon Lake, Miami,
Florida. Bengtson and Fitzgerald (1985) recorded
manatees opportunistically in Florida. They analyzed the variance in number of calls as a
function of social context and found a significantly higher number of calls during milling
and cavorting than during feeding, resting, or swimming. They did not attempt to differentiate
between calls as a function of context. Steel (1982)
reported on significant differences between calls of Florida manatees as function of sex and
maturity. Thomas O'Shea (pers. comm.) analyzed variances within and among individual Florida
manatee vocalizations recorded from 1980-85, reporting that most of the calls were between
mothers and calves. Most recently, Anderson and Barclay
(1995) reported details on the vocalizations of a
resident group of presumably male dugongs in Shark Bay, Australia; they distinguished between
3 types of calls: chirp-squeaks, barks, and trills. Barks and trills were observed in different
contexts from chirp-squeaks, however, barks and trills were recorded only on 1 occasion and only
from 1 animal.
JUSTIFICATION
Manatees offer an opportunity to study vocalizations of marine mammals using focal animal observation techniques. Compared to cetaceans, they are slower swimmers, inhabit shallower waters, are more solitary, and are more stationary feeders. These factors should make field observation and recording of focal animals possible for extended time periods. Females calf once every 3-5 years and calves remain with mothers for 1-2 years (Hartman 1979). Theoretically, this extensive maternal investment combined with the observance of mother-calf separation, provide for the possibility of vocal signatures. Opportunistic observations of mother-calf pairs by Hartman (1979), Bengtson and Fitzgerald (1985), and Thomas O'Shea (pers. comm.) support the possibility, but the hypothesis remains untested. Steel (1982) found statistically significant variation in Florida manatee vocalizations among age and sex classes, but was unable to correlate specific calls with specific behaviors. My research will quantify and test individual vocalizations as a function of context and analyze the variance within and among individuals to determine if vocal signatures exist within the context of mother-calf contact and/or alarm calls. If the vocal signature hypothesis is supported, this study may lead to further analysis of distinctiveness within and among other individual manatees, and between populations or sub-species. If it becomes possible to identify individuals and/or populations by vocal distinctiveness, this can be extremely useful in terms of identifying the distribution, abundance, and migration of endangered West Indian manatees.
