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Note: This paper was prepared for the proceedings of the European Cetacean Society Conference in 1992, and represents a sample of the scientific work ongoing at the Whale Center of New England.
Mason T. Weinrich, Mark R. Schilling, and Cynthia R. Belt Whale Center of New England, P.O. Box 159, Gloucester MA 01930, U.S.A.
Since 1980, we have studied the behavior and ecology of humpback whales (Megaptera novaeangliae) from commercial whale watching and dedicated vessels out of Gloucester Massachusetts, U.S.A. Our studies are centered around the northern end of Stellwagen Bank, a sandy glacial deposit off the Massachusetts coast where humpbacks congregate to feed on sand lance (Ammodytes americanus) between April and December of each year. Individuals were identified by photographs of the pigment pattern on the ventral side of the flukes and the shape and scarring of the dorsal fin (Katona and Whitehead 1981).
During sightings the sequence and timing of behaviors from an ethogram of 64 different behavior types were recorded on focal animals. Behaviors were grouped by category: aerial behaviors (including flipper slaps, belly-up flipper slaps, lobtails, belly-up lobtails, tail breaches, and five types of full body breaches), social/inquisitive behaviors (including trumpet blows, rolls, head-ups (high and low), belly-ups, tail stands, and single bubbles), and surface feeding behaviors (feeding lunges, bubble clouds, bubble rows, bubble nets). The classification of behavior types was based on the authors' experience observing each behavior in particular contexts.
To examine changes in behavioral rates of maturing animals, we used only data collected from individuals first photographed as calves. By doing this, we insured that behaviors were correctly assigned to the proper age class. The resultant data set consisted of behavioral observations on 76 animals, although sample size decreased as age increased. This is due to (1) observations of individuals who were not resighted after any given age; and (2) increased numbers of calves identified in the second half of the study period (Clapham and Mayo 1990). In order to make the data on all ages comparable, behavioral frequencies were transformed into rates by dividing the total number of occurrences of a behavior by the total number of observation hours for that year class.
Aerial behaviors were found to peak in known age animals at ages 2-3, and decrease dramatically at 5-6 years of age to levels below that seen at other ages (Fig. 1). This was due more to the number of times behaviors were displayed given any aerial activity occurring, as opposed to the overall number of observations in which any aerial activity was recorded. Social/inquisitive behaviors were consistent throughout the period, and showed a slight increase at ages 5-6 (Fig. 1). Finally, feeding behaviors gradually increased from very low rates among nursing calves to much higher levels in 6 year old animals.
While each behavioral trend is interesting in and of itself, it is only in relation to each other that the shifts in time budgets allow insight into the developmental process (Fig. 1). Clearly, whales showed a trend of declining aerial activity as their body grew larger, and they moved towards maturity at 5-6 years of age (Matthews 1937; Chittleborough 1965; Clapham and Mayo 1990). Younger whale may have had surplus energy available for aerial activity; they are not constrained by the energetics of breeding, and it was important that "play" behaviors be used to perfect social actions that will be used extensively after maturity (Whitehead 1985). Further, the smaller body size presumably required less energetic cost per aerial behavior. Social behaviors remained constant through development, as the young animal was increasingly a part of the adult social organization as it approached maturation (Weinrich and Kuhlberg 1991). Surface feeding, however, increased in importance in relation to the other behaviors so that six year old animals spent more time feeding than engaged in social or aerial activity. This was expected; the amount of stored energy plays an determining role in breeding success of both males and females (Lockyer 1981; Darling 1983; Tyack and Whitehead 1983; Weinrich and Kuhlberg 1991).
The lack of surface feeding among one and two year old humpback whales likely reflected two factors. Young animals appeared to spend much of their feeding time engaged in sub-surface feeding (Weinrich et al. 1985; Belt et al. 1991); because of the supposition inherently involved in assigning sub-surface feeding, these periods were not included in calculations of feeding behavior display rates. Additionally, bubble feeding appeared to develop through practice when the whale was two years old. Figure 2 shows that the rate of 'single bubbles' ("bls" - patches of light effervescence) increased dramatically when the whale was two, then reduced again at three. When one further examines the rate of 'bubble clouds' ("blc" - dense patches of effervescence, 7-10 m across, followed by a feeding lunge) and 'bubble cloud behaviors' ("cld" - dense patches of effervescence similar to that of a bubble cloud, but not followed by a feeding lunge), a similar trend between the two behaviors was seen until age 3, at which point bubble clouds became far more common. We hypothesize that two-year old whales were using single bubbles to strengthen their abilities in bubble feeding, which was successfully and consistently used in prey capture starting at age 3.
Further evidence for the two-year point being a critical age for learning feeding behavior comes from documentation of a novel feeding behavior, 'lobtail feeding,' which has spread through the population in the past 10 years (Fig. 3). This behavior involves the whale slapping its tail on the surface before diving and completing a bubble cloud sequence. We hypothesize that the tail slap either stunned and momentarily stopped the prey, or marked a prey location for the whale as it dove. The incidence of whales using this behavior has increased gradually from 0% in 1980 to over 50% in 1989 (Fig. 3).
What is more striking is a comparison of which whales displayed this novel feeding behavior. We split the whales observed surface feeding into two groups - those first photographed, and therefore alive, before 1982 (either in our sighting records, or those found in Mayo et al. (1985)), and those first photographed after 1982. The latter group included primarily calves and whales first photographed as small juvenile whales, also likely born after 1982. In the former group, only 13 out of 104 animals (12.5%) used lobtail feeding; among those animals first photographed after 1982, 82 of 146 animals (56.2%) used the lobtail feeding technique. Among those animals of known maternal descent who used lobtail feeding, only 3 out of 20 whales had mothers who also employed lobtail feeding. Hence, much of the learning had to be culturally, and not matrilineally, based (Weinrich et al. in press).
Seven different humpback whales were seen engaged in apparent incompetent or practice lobtail feeding when two years old. These animals have been repeatedly seen slapping their tails down, and blowing bubble patches, but did not appear to be feeding. In one case animal "Porcupine," a two-year old female, was seen engaged in this activity on at least seven separate occasions over a two-month period in 1991. In cases where these animals have been seen surface feeding in subsequent years, they have all been seen to use lobtail feeding. This further indicates that when the animal was two years old it went through a critical learning period where surface feeding behavior was developed. Since that is over a year after typical separation from the mother (Baker et al. 1987; Clapham and Mayo 1990; Baraff et al. in prep), it further implicates the importance of cultural learning in humpback whales.
In conclusion, our data suggests that development in humpback whales, like that of many other mammals, involves a complex combination of play (or practice) behavior and shifting energetic needs. Further, development appears to be influenced by cultural learning in addition to the more traditional matrilineal learning previously documented in humpback whales (Clapham and Mayo 1987, 1990; Baker et al. 1990).
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