Webb, P.M., D.E. Crocker, S.B. Blackwell, D.P. Costa and B.J. Le Boeuf. 1998. Effects Of Buoyancy On The Diving Behavior Of Northern Elephant Seals, Mirounga angustirostris. J. Exp. Biol. 201:2349-2358.
(Presentation by: Heather Vukelic and T.J. Harper)
Introduction
Elephant seals’ lives are affected by different variables:
adipose tissue
buoyancy
physiological changes
Adipose tissue
A specialized tissue that is a major storage site for fats
the build up of fat is what creates buoyancy.
it also allows for energy storage.
There are two common types of adipose tissue:
brown adipose tissue is directly used to produce heat for non-shivering thermogenesis organisms.
white adipose tissue acts mainly as mechanical cushion and a source of energy.
Buoyancy
Archimedes Principle
"Any object, wholly or partly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object."
Negatively buoyant organisms, will at rest, sink.
Positively buoyant organisms, will at rest, float.
Neutrally buoyant organisms, will at rest, neither float nor sink.
Physiological Changes
Female elephant seals
After having sex, the female elephant seal leaves the rookery with 25% of her body mass being adipose tissue.
After 8 months of feeding the female returns for the breeding season. Her percentage of adipose tissue is now up to 39%.
Over the next 26 days the female does not eat, but is still feeding her pup(s), this reduces her adipose down to 24%.
Male elephant seals
Males may lose up to 56% of their adipose tissue throughout the breeding season.
Elephant seal pups
have an adipose percentage of 48% at the end of nursing.
this is do to the first time exposure to the sea, and now they are feeding for themselves.
Physiology and Buoyancy
The large changes of the amount of adipose tissue throughout the year for an elephant seal creates a change in their relative buoyancy.
These buoyancy changes should directly affect the diving behavior.
The Objective of the Study
"To determine experimentally the relationship between buoyancy and diving behavior in elephant seals".
The experiment design was to interpret diving behaviors on translocated elephant seals, that had buoyancy altering devices attached.
Procedures
13 juvenile northern elephant seals were obtained from Ano Nuevo State Reserve, CA.
7 female seals
6 male seals
Age range was 1.4 to 2.4 years
Methods of measurements
the seals were sedated with tiletamine hydrochloride and zolazepam hydrochloride.
18 ultrasound measurements were taken to determine adipose thickness.
8 length and girth measurements were also taken.
then the seals were modeled in a computer as truncated cones to establish the presence of adipose throughout the body.
Buoyancy Groups
4 seals were made positively buoyant (B+)
5 seals were made negatively buoyant (B-)
4 seals were unaltered and used as a control group (Bc)
- Buoyancy Altering Devices
Polyvinyl chloride tubes (diameter 3in. length 44cm.)
Both ends were capped, but one end had holes in it to allow water to drain in or out.
The positively buoyant group (B+) had foam in the tubes.
The negatively buoyant group (B-)had lead in the tubes.
The control group (Bc)had empty tubes.
The buoyancy alterations were kept with realistic ranges.
Date recorded by attaching time and depth recorders to the seals.
The seals were divided up and released from 3 different locations:
Hopkin’s Marine Station
Pt. Lobos
from ships in Monterey Bay
Results
12 seals returned to Ano Nuevo in a range of 1.9 to 9.2 days
1 seal returned to Gordo, about 120 km south of Nuevo.
this seal was excluded from the data set.
There was no observed pattern between buoyancy treatment groups and return rates.
Dive Types
"A" type dives are shallow water dives.
"E" type dives are transit dives.
"C" type dives are drift dives.
There was no correlation found between dive characteristics vs. buoyancy treatments.
There was no observable pattern between buoyancy treatments and depth, duration, and surface intervals.
There was however a pattern between buoyancy and descent rates.
Descent rates were faster for seals in the B- group.
Discussion
The main correlation came from "C" type dives.
normal "C" dive involves the seal drifting down and actively swimming to the surface.
The B+ group "C" dives showed them swimming down, then drifting to the surface.
The drift portion of the "C" dives could be a form of energy and oxygen conservation.
It is thought that buoyancy does not affect the seal’s speed, the seal increases its energy budget to compensate.
There was drag created and it affected the whole study group.
this is shown by an overall reduction of speed when compared to un-altered seals.
The increased drag of the buoyancy devices was kept constant among all the seals.
is study shows that northern elephant seals are unable to control their buoyancy to
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