BEHAVIOURAL SLEEP IN A NORTHERN FUR SEAL CALLORHINUS URSINUS

INTRODUCTION Comparitive Sleep Research is one important way to understand the functions of sleep in different animals and their adaptations to different ecological situations (Webbs 1979, Allison and Chichetti 1976, Zepelin and Rechtshaffen 1974 and Tobler 1984). The classical sleep stages, which were originally defined in humans (Berger 1930) using Electroencephalography (EEG), are in principle also present in all other mammalian and avian species (Tobler 1984).

The study of sleep in aquatic mammals led to some very surprising results: 1.) Dolphins, Porpoises and Otariidae-Seals showed unihemispheric Slow Wave Sleep (Quiet Sleep or "Deep Sleep), this means, that each brainhemisphere sleeps alternately, when the other one is awake or in a shallow sleep-stage (Mukhametov 1984, Mukhametov et al. 1988 ). 2.) With the exception of the most primitive mammals, the egg-laying Monotremata, Dolphins and Porpoises are the only warmblooded animals, that never show any sign of Active Sleep (also called Paradoxical Sleep, REM(Rapid Eye Movements)-Sleep or "Dream-Sleep"). These practically unique features within the homoiothermic Vertebrates might be caused by the return of the primary terrestrial ancestors of the modern whales and seals to an aquatic environment (Mukhametov 1984).

The aim of this study was to develope and test behavioural categories for seals in order to destinguish different stages of sleep and wakefulness and to compare these findings with EEG results.

METHODS Observations were made in a roofed open-air basin of approximately 3x3x1,5m at the Black Sea Station of the Severtsov-Institute. The subject of this study, an approximately 7 year old female Northern Fur Seal Callorhinus ursinus named "Mathilda", lived in different dolphinaria for more than 6 years. Before measurement started, she got about one week to adapt to the new surroundings. The observers were situated and hidden from the animal in a hut which borders the basin. They looked out of a small window, which is above the basin's wall and allows the observers to look down at the animal from a distance of about 2,5 m while sitting at a table. Registration of time and duration of main stages, such as Active Wakefulness (AW)- see below - and substages like swimming, grooming etc. (not included in this paper) was done with the help of a computer program (actogram), which was developed by the seniorauthor (Oleg Lyamin) of this study. Observation during night were only possible through using a 100 W bulb, which hung appr. 2,5 m above the middle of the basin. Eye Configuration and AS-phenomenons were observed using a field glass.

The experiment was subdevided into 3 parts:

Stage I: Low waterlevel (approx. 0,25m) and offer of a platform for resting (days 1-4).

Stage II: High waterlevel (approx. 1,5m) without platform (days 5-34).

Stage III: Same conditions as Stage I (days 35 and 36).

About 20 24h non-stop observation sessions were done alternately by 3-4 different observers, which were all well-introduced in the application of the behavioural categories. The adaptation time of about one week was used to modify the behavioural categories which were developed by Ursin and Sterman 1981 in adult cats, for studying sleep in seals.

Main stages:

Active Wakefulness (AW):

-General State (GS): typically movement (small episodes of quiescence possible).

-Body Position (BP): sitting, standing, walking or swimming.

-Head Position (HP): head up(right)

-Eye Configuration (EC): eyes permanent open.

Quiet Wakefulness (QW):

-GS: typically quiescence (short episodes of movement possible).

-BP: sitting or lying.

-HP: typically head down (if head up, only slightly moving).

-EC: eyes permanent open - periodically closed (not longer than 29 sec).

Changes between AW and QW were only registrated, if the new stage is kept for more than 10 sec without coming back to the old one.

Quiet Sleep (QS):

-GS: quiescence.

-BP: lying or sitting.

-HP: typically head down (if head up, not moving).

-EC: eyes permanent closed for more than 30 sec.

Active Sleep (AS):

-GS: quiescence with frequent jerky body movements, most prominent at the head (neck), extremities, vibrisses, ears and nostrils.

-BP: lying (muscle hypotonia !).

-HP: head down (muscle hypotonia !).

-EC: generally closed or half opened. REM.

A chance from QS to AS was only registrated, if the AS phenomenons had a longer duration than 30 sec.. In water Fur Seals show a typical sleeping posture, the so called: "Ring". Distinction between QW and QS only through the frequency of movements and the time of eye-closure. Distinction between QS and AS due to the presence of jerky body movements or REM and the sinking of the head and nostrils under the water surface because of muscle hypotonia.

RESULTS

Stage I: The amount of both sleep stages QS and AS increased from the first day to the fourth day of the experiment (7,1-19,8 % vs. 0,9-4,4 % of 24 h observationtime respectively). In the same time, there was a decrease of both wakefulness-stages, most of all AW from 39,0-27,8 %. The animal slept exclusively on the platform (Fig. 1).

Stage II: The first 24 h after rising of the waterlevel and removing of the platform at the fifth day of the experiment, "Mathilda" was in AW all the time, mostly swimming. Within the 30 days of this stage in the experiment, she never showed less than 78,9 % AW. QW developes from zero to 10,8 %; QS from zero up to 15,5 %. AS was not detected (Fig.1).

Stage III: After reducing the waterlevel to Stage I conditions and offering again a platform for resting, the animal showed higher levels for QW (59,6 vs. 50,8 %) and lower levels for AW: 22,6 vs 39,0 % compared with Stage I. The amount of both sleep stages QS and AS were slightly lower than in stage I: QS (19,1 vs. 19,8 %) and AS (3,0 vs. 4,4 %) (Fig.1).

DISCUSSION Although the waterlevel in Stage I of the experiment was very shallow, "Mathilda" never slept in water, but exclusively on the platform. Callorhinus ursinus seems to prefers to sleep on land if it is available.

During the first 4 days, the increase of the amount of sleep on one hand and the decrease of wakefulness on the other , reflects an increasing habituation of the animal to her new basin. We do not know, wether this process had already reached its final end, because we were forced to interrupt this stage at day 5 in order to have enough time for the main part of the experiment: the High Waterlevel Conditions. These are in a way similar to open-sea conditions, in which Callorhinus ursinus live for more than two months every year, when they migrate from the high Northern Pacific Ocean to more southern latitudes during autumn and on their way back in spring. During this time the species must be able to sleep in the open sea and to survive with a smaller amount of sleep. Our data show a reduction of total sleep time (both QS and AS) from 24,2 % to zero at the first day of stage II and to a maximum of 15,5 % at day 32. The absence of AS for at least one month might be due to difficulties in detecting short and less prominent episodes of AS during sleep in water. During electrophysiological research on our species. Mukhametov et al. 1988 found a clear reduction of the amount of AS in water to about 0,3 %, compared with 3,6 % on land (Table 1). A comparison of our results with these data (Table 1) show a high degree of conformity. Although it is not possible to detect such surprising results like the presence of unihemisperic sleep in some marine mammals, it is possible to achieve interesting knowledges about sleep in animals, using behavioural technics only.

We do not understand so far, how Northern Fur Seals are able to survive long periods of at least one month with half the amount of sleep and less than 10 % of AS compared with sleep on land.

If our results prove to be represantative for this species (Oleg Lyamin will repeat the experiment with another two individuals of Callorhinus ursinus), sleep rebound is not used to compansate sleep defiency (see the results of Stage III - there was even a reduction of total sleep time as well as both QS and AS compared with Stage I !). So it might be possible that Otariidae - Seals are able to reduce their demand for sleep, even under physically exhausting situations, to a very surprising low level for higher mammals. This would be very interesting for all hypothesis on the functions of sleep (Webbs 1979, Karmanova 1982 and Meddis 1975), especially AS ( Fishbein and Gutwein 1977 and Vogel 1979).

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Tab.1: Comparison between our results and data obtained after MUKHAMETOV et al. 1988 following EEG-investigations (Mean values for 4 individual seals). There is a general agreement between these two data sets.


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