We have successfully applied micro-accelerometer based sensing of flipper strokes to our work on phocid seals. To achieve this, we are using ultra-low power, monolithic capacitive micro-accelerometer sensors, glued to the base of the tail of seals. The sensor in its present configuration has a sensitivity of ±2g, and produces an output voltage proportional to lateral tail acceleration. Data is stored in two different ways: we either record the output voltage as a frequency-modulated signal over a carrier frequency of 1 kHz onto one of the audio tracks of the animal-borne video recording system, or we digitize the output voltage at a rate of 16 Hz, and store the values in the memory of a digital data recorder. This latter setup is shown in the image below.

The image below shows a modular recorder in black in the lower left. This base module of the recorder contains the batteries, a swim speed sensor (see section on Velocity Sensors), a heartbeat (ECG) sensing circuit, and the accelerometer digitizing circuitry. The three little knobs to the left are LEDs that communicate with the separate data recording module. This unit - shown here above the base unit - is detachable. It contains a highly accurate piezo-resistive depth sensor, and several memory modules with 8 Mb of SRAM. The module records pressure at a 1 Hz rate (12-bit resolution), hear trate and swim speed at a 4 Hz rate, and accelerometer output at a 16 Hz rate. Data transfer to a PC is via special optical interface box. Connected to the base unit via waterproof connectors is the accelerometer sensor itself, here cast in white electrical resin, as well as a set of twin intravascular ECG electrodes integrated into a coiled white catheter (French 7, with lumen suitable for injection / sampling).

This image below shows a modular data recorder. The base module in black is in the lower left. This base module contains the batteries, a swimspeed sensor and circuitry. Three LEDs can be seen on this base module. These LEDs are used to communicate with the data logging module that for deployments is attached to the base module. The data recording module is shown here above the base unit. Connected to the base unit via a waterproof connector and a long cable is the accelerometer sensor itself, here cast in white electrical resin. Also conneted via another waterproof connector is a set of twin intravascular ECG electrodes integrated into a coiled white catheter, that also has an attachment for syringes for injection or withdrawal of samples.

A sample of the raw, un-calibrated data from such a device is shown in the image below, which shows a short excerpt from a weddell seal dive. The seal performs what is called burst-and-glide type swimming. Several strokes of the tail flippers are recognized by the alternating accelerometer data. Simultaneously, the seal accelerates during these bursts of stroking, and then decelerates during the intermittent glide periods.

swimspeed data in meters per second, and relative tail acceleration in counts / 16th of a second, for a dive segment of about three minutes. The tailstroke data shows the animal doing three to five vigorous strokes, during which times it typically accelerates from less than two to almost three meters per second swimspeed over maybe 5 to 8 seconds. This stroking is alternated with periods of non-stroking during which the speed gradually reduces back to less than two meters per second.

The following image shows a slightly expanded view of the previous image. Click on either image to enlarge.

swimspeed data in meters per second, and relative tail accelleration in counts / 16th of a second, for a dive segment of about 90 seconds. The tailstroke data shows the animal doing three or four vigorous strokes, during which times it accelerates from less than two to almost three meters per second swimspeed over maybe 5 to 8 seconds. This stroking is alternated with periods of non-stroking during which the speed gradually reduces back to less than two meters per second.

Why is flipper stroke sensing so important? For a long time we have simply recorded swimspeed to monitor the locomotor and associated energetic effort of diving animals. The image below illustrates the importance of stroke recording (click on image to enlarge). On this single, deep dive, the weddell seal performed continuous stroking for the initial few seconds of the dive, followed by burst and glide swimming up to a depth of about 150 m. From that point onward, the seal became negatively buoyant, enabling a very cost-saving descent to nearly 600m without a single stroke. The ascent was performed under constant stroking.

This graph shows three data sets during one complete deep dive of a weddell seal lasting about 30 minutes. The depth trace shows the animal diving to a maximum depth of nearly 600 meters. The tail acceleration data shows various patterns of stroking or gliding. Initially, the animal strokes vigorously for a few seconds to gain initial speed. Then it performs the burst-and-glide type stroke pattern illustrated in the preceeding two graphs. Once it reaches a depth of 150 meters, the animal stops stroking and glides to a depth of nearly 600 meters without a single stroke. During ascent the animal strokes constantly. During stroking, the speed varies greatly as illustrated during the preceeding graphs. During the glide phase of dive descent, the speed is much more constant, with a very gradual accelleration resulting solely from negative buoyancy, from less than two to about 2.8 meters per second.

You can find more detailed information on such cost-saving stroking patterns in these publications:

Williams TM, Davis RW, Fuiman LA, Francis J, Le Boeuf BJ, Horning M, Calambokidis J, Croll DA (2000) Sink or Swim: Strategies for Cost-Efficient Diving by Marine Mammals. Science 288: 133-136.

Davis RW, Fuiman LA, Williams TM, Collier SO, Hagey WP, Kanatous SB, Kohin S, Horning M (1999) Hunting Behavior of a Marine Mammal Beneath the Antarctic Fast Ice. Science 283: 993-996.