104 Discussion 



maximum lifespan and the ability to lower the body temperature 

 and the basal metabolic rate for more or less prolonged periods. 



Danielli: Have experiments been done in which groups of animals 

 which normally hibernate have been prevented from doing so ? 



Bourliere: At the present time I know of no good observations 



* which have been made on mammals. The difficulty is not only to 



house a large number of animals during 30 or 40 years, but also to 



secure an investigator who could study such a problem for three 



decades. 



Danielli: Even if one did, of course, you would still be up against 

 some difficulty in interpreting the facts, because it seems to me that 

 the advantage which is gained may either be that the "biological 

 time-scale" is altered by hibernating, or alternatively the hibernat- 

 ing animal may be protected from all accidents and so forth. One 

 does not know which of these two alternatives is involved. 



Comfort: Many small birds presumably die in winter. If a species 

 hibernates it has not got to search for food, and so it is not so liable 

 to die from lack of it. 



Bourliere: In swifts there is no true hibernation during winter, 

 because they migrate to tropical Africa at that time of the year, but 

 their temperature control is nevertheless very different from that of 

 passerines. The studies on swifts which have been made in Oxford 

 and Switzerland by D. Lack and E. Weitnauer have shown (see Lack, 

 1956) that during bad weather, especially in early spring when the 

 parent swifts are unable to obtain enough flying insects to feed their 

 young, the young then undergo a kind of pseudo-hibernation or 

 torpid stage instead of dying as other species do. J. Huxley, C. S. 

 Webb and A. T. Best (1939. Nature (Lond.), 143, 683) have des- 

 cribed the same feature in adult humming-birds at night, and they 

 are also long-lived animals. One humming-bird lived in captivity in 

 the Copenhagen Zoo for more than eight years, which is quite a 

 record for a bird so difficult to keep in captivity. 



Danielli: Humming-birds might be the right material, as they have 

 this diurnal "hibernation". 



Maynard Smith: In the terns there was a very low mortality in the 

 second and third years of life. Is this associated with the fact that 

 this species does not breed until its fourth year ? 



Bourliere: Yes, common terns do not return to breed in quantity 

 until their fourth summer. 



Sacher: Have the life-tables of animal populations in wild-life 

 preserves, such as the European and American bison, been studied ? 



Bourliere : As far as I know, no such studies have ever been made 

 on bison in America or in Europe. The first good study on the 



