Individual study: The effectiveness of four different multi-capture live-traps in the capture of brown rats Rattus norvegicus; an observation room study, Surrey, England
Taylor K.D., Hammond L.E. & Quy R.J. (1974) The reactions of common rats to four types of live-capture trap. Journal of Applied Ecology, 11, 453-459
Brown rats Rattus norvegicus are comparatively difficult to trap, and in capture-recapture studies some individuals subsequently show a pronounced tendency to avoid traps. The frequent failure to capture more than a proportion of rats in a population may be due to 'trap-shyness', which can be either inherent (i.e. traps avoided when first put in place as unfamiliar with them) or could be learned. This study examined trapping efficiency, in terms of the number of rats caught per trap and the proportion of a population caught, of four designs of 'multi-capture' traps.
Consideration the trap design was given for: provision of a relatively large amount of bait to facilitate pre-baiting; a holding cage in which rats caught were kept away from the entrance; and a door which remained in the same position as when the trap was set after pre-baiting. These features were incorporated into two new trap designs:
Pitfall trap: This incorporated a treadle supported by a small magnetic catch, so that a rat entering was well past the balance point before its weight caused the treadle to gave way. As an added precaution against escape, the treadle when depressed, closed the trap entrance . The trap was constructed of sheet metal.
Repeater trap: The action of a captured rat passing through to the holding cage, reset the trap for the next rat. It was constructed of welded wire mesh with a sheet metal bait hopper.
Other traps: Two other traps were used: a welded wire mesh cage (60 x 90 x 25 cm) with a single 8 cm square entrance 8 cm above ground level. A 15 cm-long sheet-metal tunnel with a door on the lower side led into the entrance, when the door opened (downwards) rats could enter. A counter-balance held the door in the closed position. The other was the 'wonder' trap, employing a similar non-return door.
The four trap designs were tested using wild rats (12 to 24 individuals) kept in an observation room. The cage trap was used continuously, but other traps were put in singly for varying periods. Each test period consisted of pre-baiting (4-31 days) and trapping (1-4 days). At the end of each trapping day any captured rats were released into the room, thus becoming available for recapture.
Pitfall trap: During the first day that the pitfall trap was set only three rats approached but only one entered and was caught. Over 5.5 h observation no more approached; during the pre-baiting period they approached or entered the trap many times over a similar time period. The noise made by the trapped rat appeared to alarm most other rats, but during a period when it was quiet, another rat entered.
Repeater trap: The noise made by the mechanism when the first rat was caught caused other rats alarm, and again noise made by captured rats scared others away. Two rats were caught significantly less often than the others and earlier observations showed that both were shy of entering the trap when unset, even when it contained the only source of food.
Cage trap: Most rats entering the pre-baited cage trap approached without hesitation and the few that turned away usually did so as a result of interaction with other rats. When the trap was set, however, many that approached the entrance were wary of the now closed door; a rat would run to the entrance then stop and run away without pausing, or sniff at the door briefly before running off. Subsequent approaches by the same rat were very cautious, the individual creeping towards the door with head and body out-stretched, and then running off. Several rats approached a few times in quick succession but then remained under cover for the 6.5 h that observations were made. Two rats were seen escaping from the trap by pulling down the door with their forepaws. A further three in the trap at the end of the day's observation had escaped by the following morning.
Wonder trap: Setting the wonder trap produced similar reactions to that of the cage trap. Rats appeared to escape more readily than from the cage trap. Escape was facilitated by others depressing the door from outside, and on one occasion, a small rat was seen to hesitate on the door of the trap, allowing six large rats inside to escape.
Conclusions: The repeater trap was best (95% efficiency; 19/20 rats caught over 4-days). The other traps often caught less than 50% of the rats. The pitfall trap was particularly ineffective; over one 4-day trapping period in which the only food available was within the trap, only 12 of 19 rats were caught; on the third day a few grains of wheat became lodged under the door, holding it slightly open and the magnetic catch did not engage. The wonder trap was also relatively inefficient when set for 1 day, catching only 17% of rats whereas the repeater caught nearly 50%.