Lesley Brown & Dr. Samuel H. Gruber
MSc (by Research) Behavioural Ecology
Manchester Metropolitan University, England
Juvenile lemon sharks
schooling in the pen
This study was almost purely observational. The purpose of which was to look at social behaviour, dominance, spatial use and activity periods in the juvenile lemon shark. There was an initial observational period when an ethogram was constructed from 95 hours of observation. The second phase dealt with dominance in the group based on a form of avoidance behaviour called a Give-way. Using this behaviour it was found that there was perfect linearity in the hierarchy and was strongly correlated to size as the largest shark was most dominant and smallest shark was totally subordinate. For the next stage of the study one of the middle ranking male sharks was substituted for a similar sized inexperienced female shark. Similar to the original group there was perfect linearity in this hierarchy and again it was linked to size.
For the next stage the duration between feeds was doubled from once every three days to once every 6 days. This was in an attempt to destabilise the group however aggression in the group did not increase. In addition resting periods and swimming speeds did not alter. Again dominance in the group was recorded and during the 5th and 6th days since fed the linearity of the hierarchy dropped significantly.
During feeding sessions dominance also played a role in the proportion of meal obtained. The largest shark obtained on average the largest meal and the smallest shark on average obtained the smallest proportion of the meal. This lends evidence to Chase's (1980) argument that high status in a hierarchy usually leads to priority of access to a resource.
The observation pen
So it appears that when kept as a group in captivity the juvenile lemon shark forms a strong hierarchy most likely as a means to reduce costly acts of aggression between conspecifics. Interestingly there was a reversal of size order between two sharks during the substitution and starvation stages of the study. However there was no resultant change in dominance and subordination. This lends evidence to an initial assessment period during which dominance roles are cemented.
Finally during all these studies over 1500 instantaneous swimming speeds were recorded. The results of these showed definite periodicity in activity over the period of the day with sharks swimming faster at dusk and at dawn which follows previous studies carried out on juveniles in a laboratory situation (Nixon & Gruber 1988) and a field study on sub-adults (Gruber et al 1988). Resting periods were also recorded but found to show no pattern of periodicity similar to that of swimming speeds.
Relevant papers and References
- Allee, W. C. and Dickinson, J. C. (1954). Dominance and subordination in the Smooth Dogfish Mustelis canis (Mitchill). Physiological Zoology 27:356-364.
- Chase, I. D. (1980). Social process and hierarchy formation in small groups: a comparative perspective. American Sociological Review 45:905-924.
- Gruber, S. H., Nelson, D. R. and Morrissey, J. F. (1988). Patterns of activity and space utilization of Lemon sharks, Negaprion brevirostris, in a shallow Bahamian lagoon. Bulletin of Marine Science 43(1):61-76.
- Klimley, A. P. (1982). Grouping Behaviour in the Scalloped Hammerhead. Oceanus 24:65-71.
- Myrberg, A. A. and Gruber, S. H. (1974). The Behaviour of the Bonnethead Shark, Sphyrna tiburo. Copeia 1974(2):358-373.
- Nixon, A. J. and Gruber, S. H. (1988). Diel Metabolic and Activity Patterns of the Lemon Shark (Negaprion brevirostris). The Journal of Experimental Zoology 248:187-196.
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