Discussion 105 



behaviour and ecology of the American bison was pubhshed a year 

 ago (McHugh, T. (1958). Zoologica, N.Y., 43, 1), and we are still 

 waiting for a similar work on the European form. The so-called wild 

 European bison are in fact so domesticated that they are not a good 

 example to study. If you want to have samples large enough to be 

 studied, you need to choose rather common species which can live 

 in national parks or some place where human distui'bance is very 

 rare. That is why most of the available information has been drawn 

 from the field of game management or rodent control. 



Chitty: Did you say there was no age-specific mortality rate for 

 small rodents in the wild ? 



Bourliere: I do not know of any study showing such an age- 

 specific mortality rate in wild rodents. 



Chitty: I do not really see how this kind of information could be 

 obtained very easily for wild populations. Such evidence as I have 

 published (1952. Phil. Trans. B., 236, 505) shows that there is a 

 higher mortality rate with increasing age but of course it is exceed- 

 ingly difficult to separate the environmental components from it. 

 There is an increase in mortality rate as the winter goes along, but it 

 is not known whether that is because of changing ecological condi- 

 tions, or because of an increase in average age. 



Kershaw: This may be compared with observations on insects in 

 the wild. There seems to be evidence now that the survival of 

 insects with a rapid population turnover is modified by predators 

 and natural hazards, whereas those with a long and slow population 

 turnover maintain their own intrinsic survival. In mosquitoes it 

 seems likely that the intrinsic survival, having a Gompertz function 

 with a sloping straight line, is altered completely by field conditions, 

 and has a flat Gompertz function. For the last ten years we have 

 been studying the life-cycle of one of the West African flies, Chrysops, 

 which turns over once a year. In the laboratory that fly has a 

 normal rectangular survival, both in those bred from the pupae and 

 in wild-caught flies. We have been following through natural 

 populations of flies coming in to bite man, which of course is a 

 selective, but functionally selective, population. We have found that 

 throughout the year the population is made up of separated succes- 

 sive cohorts, each behaving with its own particular rectangular 

 survival, so that in this particular fly the intrinsic survival is the 

 natural one. I think one has to go back to mosquitoes now that 

 one has biological markers for them, based on parity and so forth, to 

 see whether what is true of Chrysops is also true of mosquitoes. This, 

 of course, is of importance in producing mathematical models. 



