Influence on Reproduction 323 



summer the bees force air through the hive passages with their wings, 

 thus increasing evaporation and lowering the inside temperature. 

 Owing to this primitive "air-conditioning" temperature inside the hive 

 fluctuates only slightly in spite of extreme external conditions ( Uvarov, 

 1931). 



A group of animals is often protected against its predators by the 

 simple efi^ect of numbers. A flock of small birds does not attempt to 

 fight off an attack by a hawk and probably could not do so; hawks 

 generally obtain their prey by picking off stray individuals. Perhaps 

 the hawk is less likely to be successful in catching his prey from a 

 flock because of the confusion of numbers, just as a tennis player is 

 distracted by having his opponent throw three balls to him at once, 

 with the result that he does not succeed in catching any one of them. 



A favorable effect on survival of an optimal degree of crowding 

 has been found in laboratory cultures of Drosophila and of Daphnia 

 and in natural populations of the sessile rotifer, Floscularia. At popu- 

 lation densities of 5 and 10 per 50 cc of culture medium Daphnia lives 

 considerably longer than at densities of either 1 or 25 per culture 

 unit in tests at 25°C. At 18°C mortality appears to be least at a 

 density of about 75. Lower metabolic rate or better control of harm- 

 ful bacterial contaminants by the supraminimal populations are sug- 

 gested as possible explanations (Pratt, 1943). The mean length of 

 life oi . Floscularia is practically doubled if the young rotifer, instead 

 of living a solitary existence on a water plant, attaches to the tube of 

 an older Floscularia and thereby establishes a small colony. No ex- 

 planation of this striking fact has come to light ( Edmondson, 1945 ) . 



Influence on Reproduction. Since most animals and plants re- 

 produce by sexual methods, it is obvious that too great a scarcity of 

 individuals of the opposite sex will reduce the rate of breeding. In 

 an earlier chapter we have called attention to the probability that 

 this constitutes a serious limitation for deep sea fish and perhaps 

 also for whales and other aquatic animals of low population density. 

 Luminescence and underwater sound have been suggested as possibly 

 serving as aids for males and females to find each other in the huge 

 expanses of the ocean. The same limitation affects land animals and 

 plants that are very sparsely distributed. 



A similar situation on a much smaller scale is presented by flour 

 beetles dispersed in a large uniform volume of flour. Investigations 

 with Tribolium confusum have shown that when only two individuals 

 are present in a "microcosm" of 32 g of flour, the number of young 

 produced per female is considerably less than when four individuals 

 inhabit the same volume. The explanation is that fecundity is in- 



