492 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1964 



though not all, a large insect is dragged home by a group of ants, 

 some of which seem to be pulling together while others seem to be 

 pushing. Some naturalists, struck by the ants' success in moving 

 large prey, have concluded that this group transport shows a high 

 degree of cooperation. Others, who noticed that some ants pull 

 against one another and others simply ride on the prey while their 

 comrades pull, thought that cooperation was absent. 



In the wood ants, which occur in many parts of Britain, I have 

 found evidence which seems to support both sides of the question. 

 A few minutes after offering a large insect to these ants, a group of 

 5 to 10 ants forms round it. Many of these do not pull the prey at all 

 and those that do, pull in different directions (fig. 1). There is a 

 deadlock, and what movement there is, is often reversed and cancelled 

 out in the next minute. At first, it seems that the ants are incapable 

 of cooperation and that the more of them there are, the worse the con- 

 fusion gets. But after 10 or 15 minutes, movement toward the nest 

 starts and short of accidents goes on at a good rate. The group of 

 ants is now usually small (pi. 2, fig. 1) — two pullmg and one pushing 

 is a common combination — and the ants' bodies are more closely alined 

 with one another than in deadlock groups. Transporting groups seem 

 to arise from deadlock groups when some ants leave the prey and others 

 rearrange themselves so that their efforts are not opposed. The push- 

 ing ants are probably acquiescing rather than helping. At this stage 

 the ants seem to be showing a fair degree of cooperation. 



I have shown that changes which result in formation of a transport- 

 ing group from a deadlock stem from behavior which can be seen 

 equally well when a single ant is moving prey. The changes are 

 basically part of the ant's method of coping wath the difficulties it 

 meets in moving prey. Perhaps the most obvious of these is a change 

 in the mode of transport from carrying used for light prey, when 

 the ant walks head foremost to the nest, to dragging, when it walks 

 backward trailing a heavier insect behind it. This change seems to 

 occur when the prey is about three times the weight of the ant. 



The decision to carry or to drag is not, however, made once and 

 for all at the start of transport. The ant changes from one to the other 

 according to the gradient and smoothness of the surface, which affect 

 the resistance the ant feels in pulling. This probably explains the 

 existence of pushers and pullers in groups. Although the prey may be 

 10 times the weight of an ant, an ant pushing feels only a fraction of 

 this and behaves as though it was carrying light prey. Actually the 

 motive power is almost all supplied by the pulling ants, just as gravity 

 supplies the power when a single ant carries prey down a deep slope. 



Other remedies for difficulty in moving prey are not so well defined. 

 Wlien the prey an ant is dragging gets snagged on an obstacle, the ant 

 swings itself through an angle of between 20° and 80° to pull at 



