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most computer simulations on biological ageing use the penna model @xcite .
this model , motivated by the accumulation mutation hypothesis , provides a population of bit - strings that exhibits many features known for real senescence @xcite .
recently stauffer @xcite in a general review of the penna model has suggested a simpler alternative with less parameters .
this new attempt is based on the postulate of a minimum reproduction age and a maximal genetic lifespan .
only these two numbers are transmitted from generation to generation , with certain mutations , by asexual reproduction .
this simple model shows the basic features required for senescence .
the age distribution shows an increase of mortality with age .
the model reproduces successfully the catastrophic senescence of salmon @xcite and some specific human aspects like the social structures of technological societies @xcite or a transition between extinction and survival if a minimal population for survival is required @xcite .    in the following we examine the other modification to the basic model .
the privilege conditions are introduced to earlier and then to late reproduction . by doing this , in case of late reproduction
, we achieve the exponential growth of mortality in agreement with the gompertz law .
the increase of fertility with age exists in nature .
some fish , lobsters and trees share this feature @xcite and therefore this life strategy has been considered by the penna model also @xcite .
the population consists of @xmath0 individuals @xmath1 @xmath2 initially .
each individual is characterized by three integers : its age @xmath3 , its minimum reproduction age @xmath4 and its maximal genetic lifespan @xmath5 with @xmath6 .
the maximal lifetime is restricted to 32 time units ( called years ) , the minimum reproduction age may be chosen between zero and @xmath7 . within these constraints
the values of @xmath4 and @xmath5 are randomly mutated for an offspring by @xmath8 , away from the maternal values , and for each child separately .
after an @xmath1th individual has reached its minimum reproduction age , it gives birth to one offspring with probability @xmath9 , chosen as @xmath10 thus during its whole mature life any individual gives one offspring at the cumulated probability equal to 1 .
independently of the genetic death , which happens automatically and unavoidably if @xmath11 , at each time interval an individual can also die `` accidentally '' , with the verhulst probability @xmath12 .
@xmath13 is called the carrying capacity to account for the fact that any given environment can only support populations up to some maximal size @xmath13 .
otherwise the individuals die because of food and space limitations .    in this paper differently than in the standard model we consider populations in which the cumulated birth rate is not uniformly distributed among years of reproduction : @xmath14 .
instead we propose to consider :    1 .
privilege to younger third part of a reproduction period : @xmath15 2 .
privilege to older third part of a reproduction period : @xmath16    in the following figures we present characteristics of populations developed when three ways of reproduction are considered : standard , ( a ) and ( b ) models . to avoid possible divergence
we follow the stauffer recipe @xcite and shift the birth rate accordingly : @xmath17 with @xmath18 . in figure 1
we show the population volumes in percentage of the @xmath13 capacity .
the population where the reproduction of older individuals is privileged , i.e. , model ( b ) , is the smallest one and varies in time by @xmath19 .
figures 2 , 3 compare the distribution of @xmath20 the minimal reproduction age and @xmath21 the maximal genetic lifespan , respectively , in the three models considered . a significant change in the length of life of individuals
is observed when the reproduction at old age is favorable , see figure 4 for the distribution of age @xmath22 in a stationary population .
finally figure 5 shows the mortality in the populations simulated .
the mortality is calculated according to the formula @xmath23 and plotted on log - scale to extract the gompertz law .
in stauffer s new proposition of a model for biological ageing , by considering the privilege conditions for time of reproduction we can easily manipulate the mortality in a population .
when reproduction at old age is preferred then the model provides the mortality in agreement with the gompertz law .    in the simple ageing model discussed here the maximal genetic lifespan for each individual is limited to 32 years
this limitation was introduced to the model to make results better comparable with many penna model results .
but one could ask if discarding this assumption would help in getting a nice gompertz law within the standard model .
this useful suggestion pointed out to us by the referee has motivated us for investigations on the simple ageing model .
specially we were interested in where the model ( b ) would drive a population .
preliminary studies show that this model simulated without the limitation for the maximal genetic lifespan leads to the stationary state such that the distribution of @xmath21 is concentrated around age of 50 .
the mortality in this population is presented in figure 6 .
one can notice that the gompertz law of the exponential increase of mortality holds for adult individuals at age lower than about 35 .
m.  argollo de menezes , a.  racco and t.  j.  p.  penna , physica a * 233 * 221 ( 1996 ) ; + m.  argollo de menezes , a.  racco and t.  j.  p.  penna , int
.  j.  mod .
c * 9 * 787 ( 1998 ) ; + t.  j.  p.  penna , a.  racco and a.  o.  sousa , physica a * 295 * 31 ( 2001 )
it is shown that if the computer model of biological ageing proposed by stauffer is modified such that the late reproduction is privileged then the gompertz law of exponential increase of mortality can be retrieved .    * keywords * : population dynamics , ageing , stauffer s model , monte carlo simulations