Institute of Zoology, Bulgarian Academy of Science, Sofia, Bulgaria
web address: http://www.groundsquirrel.org
Interspecific behaviour of European ground squirrel (Spermophilus citellus) is still poorly studied. During behavioural studies of free-ranging European ground squirrels tree kinds of aggression to other vertebrate species: reptile, bird and mammal (Lacerta trilineata, Corvus frugilegus, Mustela nivalis) have been described. To our knowledge, this is the first field study describing interspecies aggressive behaviour in S. citellus. The description of this behaviour may have important implications for interpreting studies on interspecies competition and interactions, behaviour activity, and predation in this rear and threatened semi-fossorial rodent.
Keywords: Spermophilus citellus, aggressive behaviour, Lacerta trilineata, Corvus frugilegus, Mustela nivalis
European ground squirrel (also know as European souslik) (Spermophilus citellus L. 1766) is an obligate hibernator and typically inhabits steppes and open woodland of the Central Europe and the Balkans from the sea level to an altitude of 2500 m. Because of the negative populations’ trend (2, 7) the species is included as vulnerable in IUCN Red List of Threatened Species and has unfavourable conservation status (2). Aboveground activity patterns of diurnal mammals could be studied in details by simple direct observations. The European ground squirrel is a diurnal rodent species with a pronounced annual cycle of aboveground activity. In spring
and summer, the animals reproduce and subsequently prepare for hibernation in autumn and winter (3). At present, European ground squirrel occurs in loosely structured populations (know as “aggregations” or “colonies”). These colonies demonstrate diverse anti-predator behaviour, including emitting of alarm calls (9). There are some studies on behaviour of free-living European ground squirrels concerning their activity pattern (3, 6, 8, 17), structure and variability of alarm calls (9), home range size (16), tunnel blocking behaviour (5), sympatry with European hamster (Cricetus cricetus) (4, 10), etc. Unfortunately, interspecies behaviour of European ground
squirrels and competition with other species are poorly studied. The aim of the present study is to present some cases of interspecies behaviour of European ground squirrel (S. citellus) observed during behaviour studies in Bulgaria
Materials and Methods
Detailed descriptions of the recording of environmental conditions, capture and scanning methods of the study are presented in Koshev and Kocheva (8). Study areas The study was carried out in 2005-2008 in populations of
free-living European ground squirrels (S. citellus) in two sites in Bulgaria. The first site was in South Bulgaria near Maritza River – Zvanichevo village (Pazardzhik district) (42°12.34`N; 24°15.00`E; 215 m a.s.l.) and the second one was in Northwest Bulgaria near town Knezha (Pleven district) (43°29.92`N; 24°05.89`E; 147 m a.s.l.). The two sites were heavily grazed pastures. An experimental plot of 1ha (100 x 100 m), which could be observed from a single observation position, was traced out on this pasture.
The animals were observed using scanning procedure (1) from one and the same observational point, from which the entire study plot is visible. An automobile was used as a shelter. The study plot was scanned for 5 min every 10 min with binoculars Olympus DSP R 8×40 and telescope Exakta 20-60×70, beginning from the left side towards the right side. All the individuals and their behaviours, observed above ground in these five minutes were recorded. The agonistic behaviour includes fighting, aggression (attack, threat behaviour, defence) and fleeing. This behaviour is interspecific when animals of different species are involved (11, 12).
Results and Discussion
During the behavioural studies of European ground squirrel (S. citellus) tree types of aggressive behaviour to different vertebrate species were observed:
Aggressive behaviour toward a reptile In the study plot near Zvanichevo on 27 March 2005 was observed an aggressive behavioural act of a ground squirrel toward Balkan green lizard (Lacerta trilineata). The environmental conditions were favourable: sunny day, cloudiness 0%, temperature 18° C, humidity 50%, without wind and rain (Beaufort scale). Around 2.00 p.m., at 20-25 cm of the ground squirrel’s hole with fresh mound a lizard was standing. The length of the lizard was about 25 cm. Suddenly, without apparent reason, one ground squirrel
attacked the lizard, and bit him on the neck; the lizard began waving his feet. The entire battle continued a few seconds. The ground squirrel then disappeared into the hole. The lizard remained completely helpless (about 2-3 minutes) with pieces of soil pinched on his heck (Fig. 1). Although there wasn’t any blood, it got visible injuries. When lizard recovered, it runs away. The reason for this particular aggressive act is unclear. There are many data about antipredator behaviour of Spermophilus species to snakes. There are data for predation on a snake (Coluber constructor) by S. tridecemlineatus (18). Straka (14) reported that in stomach content of S. citellus were found parts of European green lizard (Lacerta viridis) and Common vole (Microtus arvalis), without pointing its origin (carcases or prey). He concluded that in spring the ground squirrels needs more animal food like insects and
small vertebrate animals. The aggression toward the lizard was observed in the beginning of the active season, 1-2 weeks after the emergence of ground squirrels. It could represent either competition for shelter or a predation attempt on a lizard by European ground squirrel.
Aggressive behaviour toward birds
Another aggressive behaviour act of S. citellus, involving a rook (Corvus frugilegus) was observed in the study plot near Knezha town on 15 June 2008. The environmental conditions were: cloudy day, cloudiness 100%, temperature 19.1° C, humidity 91%, without wind and rain (Beaufort scale). Around 8.00 a.m. an adult ground squirrel was feeding quietly when flock of rooks (about 7 individuals) perched alight on the studied plot. The ground squirrel was frightened of rooks and ran aggressively to one of them. The rooks flew and didn’t come into contact. Yensen and Sherman (19) include ravens (Corvus corax) as a predator for North American Spermophilus species, but the rooks are not mentioned in literature as specific predators of ground squirrels. These two species generally have not specific interactions. But when young ground squirrels emerge on the surface for the first time, they are very inexperienced and helpless. Probably in this sensitive period they can become prey of the rook (Corvus frugilegus). However, there weren’t any young ground squirrels in our study plot at this time, and this aggressive act perhaps was an incident case.
Aggressive behaviour toward mammal
Petar Y. Pelovski (hunter with high expertise and qualification) reported (in pers. comm.) for interspecific aggression toward weasel (Mustela nivalis L.) near the study plot – Knezha town. The concrete date and environmental conditions are unknown. A weasel was seen to pass the pasture holding one young ground squirrel in its mouth. Several ground squirrels elicited alarm calls repeatedly at high frequencies. Then one ground squirrel rushed to the weasel. Under the aggression of ground squirrel, the weasel left the victim and began to defend from the ground squirrel. When the ground squirrel saw the weasel defend behaviour, it escaped into the hole. Caught young ground squirrel (the victim) was inactive; apparently it was dead. After the scuffle the weasel took again the dead ground squirrel and disappeared.
Unfortunately, this observation was not made by a qualified biologist with the necessary equipment and methodology. But such interspecific behavioural events are very rare and random in the nature. The case of coincidence of such behavioural event and qualified biologist to observe it at the same time would be great fortune. For these reasons we consider this behavioural act of aggression as unconfirmed interesting observation.
Many authors have reported weasel as a predator of ground squirrels (5). Its body mass and sizes (Length of body = 110-260 mm; Weight = 40-170 g) are smaller than the average of the adult ground squirrels (Length of body = 180- 230 mm; Weight = 200-350 g). The aggression among mammals is a frequent phenomenon (12, 13). The aggressive behaviour, like a part of predator defence was well studied in North American ground squirrels (19). Active ground squirrels are vulnerable to numerous predators, and their anti-predator behaviours are well developed. Harassment behaviours include tail flagging to attract the snake’s attention, throwing dirt in the face of the snake with the forepaws, remaining outside of striking range, and pouncing on the snake and biting it (15, 19).
Three types of interspecific aggressive behaviour of European ground squirrel (Spermophilus citellus) toward
different vertebrates – reptile (Balkan green lizard Lacerta trilineata), bird (rook Corvus frugilegus) and mammal
(weasel Mustela nivalis) were observed. The agonistic event toward Balkan green lizard probably was competition for shelter or act of predation by S. citellus. The case with aggression to the rook probably was an accidental event,
while toward weasel may be was a part of protective behaviour from predator. To our knowledge, this is the first
field study describing interspecies aggressive behaviour of S. citellus. The description of this behaviour may have important implications for interpreting studies on interspecies competition and interactions, behaviour activity, and predation in this rear and threatened semi-fossorial rodent.
The present study was supported by BG051PO001-3.3.04/41 (European Social Fund through the Ministry of Education, Youth and Science of Bulgaria), and partly supported by Rufford Small Grants for Nature Conservation and BG/KNIP05/04 Small Nature Management Project (Matra/KNIP) Royal Netherlands Embassy. I am very grateful to my relatives of families Toshovi and Pelovski (from Knezha) and family Koshevi (from Zvanichevo) for essential help and hospitality.
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