© 1998 Martin Hallmen

Introduction

The Common Garter Snake Thamnophis sirtalis is described as a food generalist preying on a wide variety of food items (SWEENEY, 1992; MUTSCHMANN, 1995; ROSSMAN et al., 1996). DRUMMOND (1983) notes that Thamnophis sirtalis as a generalist is less efficient in handling prey than a specialist snake. This premise was modified by HALLOY & BURGHARDT (1990). They showed that experience in handling prey could make a generalist nearly as effective as a specialist. But that also means that newborns of Thamnophis sirtalis as a generalist could not handle prey as well or as quickly as adult snakes of the same species. In specialized snake species newborns handle their prey nearly as well as adult snakes of their own species. To put it another way: Thamnophis sirtalis has to learn to eat for example a prey fish head-first (figure) (ROSSMAN et al., 1996). In a specialist the same behavioural patterns are more dependent on a stereotyped genetical program.

I would like to make some observations about the behaviour of swallowing a prey fish in the melanistic Common Garter Snake Thamnophis sirtalis sitralis.

Material and methods

The observations were made on 2,2 melanistic garter snakes of the species Thamnophis sirtalis sirtalis. At the beginning of the observations all of them were 3 months old. Information about the descent of the animals and the keeping conditions are published in HALLMEN (1997a).

The animals were fed with total smelts (Osmerus eperlanus) every 2-3 days (HALLMEN, 1998). The time of day mostly was in the evening. In nearly every feeding time more smelts were offered than could be eaten by the snakes. Sometimes the smelts were dusted with a vitamin powder. The behaviour of the snakes was observed until all fish were eaten or the snakes were no longer hungry (about 20-30 minutes).

The observation time was from Oct 29^th.1997 until Feb 17^th.1998. Each snake was observed individually (HALLMEN, 1997b). It was marked if the prey fish was swallowed head-first, tail-first or snapped in the middle. The observations were finished because the size of the prey fish seemed to become too small in relation to the size especially of female snakes.

Results

The behaviour of swallowing a prey fish was observed 332 times. 104 smelts were swallowed by the male snakes and 228 by the females. At the beginning of the observation period male and female snakes ate nearly the same number of fish. With the increasing age of the snakes the female began to eat more than the males. The male garter snakes ate 3 fish at one feeding time at maximum. The females ate up to 11 fish per feeding.

At the age of 5 ½ months the female snakes began with an increasing number to snap some prey fish in the middle and swallow it this way. This was never observed with the male snakes. When 6 months old the females began to swallow 2 smelts at the same time.

The results of the addition of all smelts together at one feeding time show cyclic waves (schedule). The distance between the minima was about 11-20 days.

The results of the observations show no significant difference in the behaviour of swallowing a prey fish between the age of the 3^rd and 6^th months. The number of smelts swallowed head-first or tail-first varied greatly from one feeding to the other. Neither was there any significant difference in the behaviour of males and females.

Discussion

The 4 specimens of melanistic Common Garter Snakes showed a normal increase of food capture when growing older. The males appetite only grew very slowly but the females ate more and more fish corresponding to their body weight. The cyclic decreases in the amount of swallowed prey fish directly correlate with the phases of shedding.

The results show no effect of learning in swallowing the prey fish. The nearly newborns ate the offered smelts in the same way as they did 3 months later. These results are in opposite to HALLOY & BURGHARDT (1990). One conclusion might be, that melanistic specimens of the Common Garter Snake do not learn handling and swallowing a prey fish like other subspecies of Thamnophis sirtalis. But concerning the observations in this work it is surely more a result of the methods used. So the size of the Smelts was not kept constant in relation to the size of the snake as it was practiced by HALLOY & BURGHARDT (1990). Because of that the influence of the fish scales direction to the swallowing of the prey, which is said to be the reason for learning to swallow it head-first, might decrease during the period in which the snake should learn the correct swallowing technique for a prey fish. No reason for learning to eat fish head-first will lead to no visuable learning effect. It also may had a negative effect, that the scales of the smelts are normally very soft (MUUS & DAHLSTRÖM, 1968).

Because of the insufficient methods it is not possible to draw a real conclusion on learning how to swallow a prey fish in melanistic specimens of Thamnophis sirtalis sirtalis. Other experiments with refined methods should follow.

Acknowledgements

I thank my colleague Karin BERGMANN for correcting my English.

Literature

DRUMMOND, H.M. (1983): Aquatic foraging in garter snakes: a comparison of specialists and generalists. - Behaviour, 86: 1-30.

HALLMEN, M. (1997a): Some observations on stages of shedding in juvenile melanistic Common Garter Snakes Thamnophis sirtalis sirtalis. - The Garter Snake, 4/97: 19-23.

HALLMEN, M. (1997b): A blank form for recording distinctive marks in the melanistic Common Garter Snake Thamnophis sirtlalis sirtalis. - The Garter Snake, 4/97: 13-19.

HALLMEN, M. (1998): The smelt Osmerus erperlanus as a prey fish for Garter Snakes of the genus Thamnophis in the terrarium. - The Garter Snake: in print.

HALLOY, M. & BURGHARDT, G.M. (1990): Ontogeny of fish capture and ingestion in four species of Garter Snakes (Thamnophis). - Behaviour, 112(3/4): 299-318.

MUTSCHMANN, F. (1995): Die Strumpfbandnattern - Biologie, Verbreitung, Haltung. - Westarp Wissenschaftien: 172 pp. Magdeburg.

MUUS, B.J. & DAHLSTRÖM, P. (1968): Süßwasserfische Europas - in Farben abgebildet und beschrieben. Biologie, Fang, wirtschaftliche Bedeutung. - BLV: 224 pp. München.

ROSSMAN, D.A. / FORD, N.B. & SEIGEL, R.A. (1996): The Garter Snakes - Evolution and Ecology. - University of Oklahoma Press: 332 pp. Norman / London.

SWEENEY, R. (1992): Garter Snakes - Their Natural History and Care in Captivity. - Blandford: 128 pp. London.

Zusammenfassung

Über einen Zeitraum von 3 Monaten wurde das Verhalten beim Verschlingen von Stinten (Osmerus eperlanus) bei 2,2 Exemplaren der Gewöhnlichen Strumpfbandnatter Thamnophis sirtalis sirtalis beobachtet. 332 Einzelbeobachtungen wurden ausgewertet. Ihrem Wachstum entsprechend fraßen die weiblichen Schlangen mit zunehmendem Alter immer mehr Stinte, wo hingegen die Anzahl der verschlungenen Fische nahezu konstant blieb. Bei keinem der 4 Tiere konnte ein signifikanter Lerneffekt festgestellt werden. Über den gesamten Beobachtungszeitraum fraßen alle 4 Schlangen die Futterfische abwechselnd mit den Kopf oder mit dem Schwanz voran. Mit zunehmender Größe knickten vor allem die weiblichen Tiere die Stinte in der Mitte und verschlangen sie so. Immer häufiger wurden auch mehrere Stinte auf ein mal verschlungen.

Die Tatsache, dass die 4 melanistischen Exemplare von Thamnophis sirtalis sirtalis im Laufe des Beobachtungszeitraumes nicht lernten, die Futterfische mit dem Kopf voran zu verschlingen, steht im Gegensatz zu Angaben in der Literatur. Die Deutung, dass melanistische Tiere der Gewöhnlichen Strumpfbandnatter im Lernverhalten von anderen Unterarten von Thamnophis sirtalis abweichen, ist aufgrund der angewandten Methodik jedoch nicht möglich. Die Stinte wurden im Verhältnis zu den heranwachsenden Schlangen immer kleiner, so dass für die Tiere kaum eine Notwendigkeit bestand, die Technik des Schluckvorganges zu optimieren. Außerdem verfügt Osmerus eperlanus über sehr weiche Schuppen, die als Reiz zur Richtungserkennung beim Schlingakt möglicher Weise nicht ausreichen.