represents this. Lactation length can vary, but the average interbirth interval is
16.7 months (without infanticide), thus minus the 6.5 months of gestation, weaning
would be 10.2 months. After the death of an infant, the interbirth interval is
shortened to 12.8 months. Subtracting from the shortened interbirth interval inus
the gestation period of 6.5 months defines 6.3 months as the period after which
females become fertile again.
The model of Fig. 14.1 represents a single matriline of related females at various
ages in the life cycle as they are born, reproduce, and die. It is essential that we are able
to distinguish individuals within the cohorts of infants, juveniles, pregnant, and
lactating females by the time at which they enter a stock and when they leave it—
rather than lumping all of them together with their respective peers. We can therefore
not use STELLA’s default stock—the reservoir—which, like a water reservoir,
receives input and allows for output, without distinguishing what came in first or
last. Instead, we specify the stocks infants, juveniles, pregnant, and lactating females
as conveyors, which work analogous to a conveyor belt that has slats on which entities
are placed, and after some transit time on the conveyor these entities are released in the
order in which they entered the conveyor. Simply click and hold the stock tool on the
toolbar (Fig. 14.2). Going down the list of options in Fig. 14.2 you find
• The reservoir, which lumps all the entries together,
• The conveyor, which works much like a conveyor belt, taking on elements and
releasing them after a set time,
• The queue, which keeps elements queued up until some specified conditions are
met, and
• The oven, which has a given capacity, keeps elements for a defined duration but
does not distinguish its contents by their order of arrival and simply releases
them all after their “cook time.”
Choose the stocks for infants, juveniles, lactating, and pregnant individuals as
conveyors. Specify the transit time for infants as 9 (months) and that for juveniles as
34 (months) because the age at first birth is 43 months (43 À 9 ¼ 34). Set the initial
conditions of each of these conveyors at 5 (individuals). Specify the transit time for
pregnant females as 6.5 (months) and that for lactating females as 10 (months).
Assume we start with ten pregnant and ten lactating individuals.
Note that with the specification of the transit times, the outflows from each stock are
pre-defined: The individuals, who come into the stock first also leave first, after their
specified duration in the stock. Any additional flow out of the conveyors that you draw
will be interpreted as a leak—such as the “leakage” of infants from natural causes and
infanticide. For the leakage, you can specify when it should occur along the conveyor,
such as at its very beginning or anywhere during the transit time. For infant deaths, for
example, we allow the loss of babies to occur as early as at the beginning of their
cohort, and as late as at the very end of the 9 months in their cohort (Fig. 14.3).
We must also select the type of leakage these flow represents—linear or exponential. If you select the linear option, the total amount to leak is equal to the
inflow * DT * leakage_fraction and this amount is distributed evenly across the leakage zone of the conveyer (each slat leaks this amount divided by the number of
slats). In contrast, if you select the exponential option, the amount that leaks from
14.1 Langur Infanticide Model
121
16.7 months (without infanticide), thus minus the 6.5 months of gestation, weaning
would be 10.2 months. After the death of an infant, the interbirth interval is
shortened to 12.8 months. Subtracting from the shortened interbirth interval inus
the gestation period of 6.5 months defines 6.3 months as the period after which
females become fertile again.
The model of Fig. 14.1 represents a single matriline of related females at various
ages in the life cycle as they are born, reproduce, and die. It is essential that we are able
to distinguish individuals within the cohorts of infants, juveniles, pregnant, and
lactating females by the time at which they enter a stock and when they leave it—
rather than lumping all of them together with their respective peers. We can therefore
not use STELLA’s default stock—the reservoir—which, like a water reservoir,
receives input and allows for output, without distinguishing what came in first or
last. Instead, we specify the stocks infants, juveniles, pregnant, and lactating females
as conveyors, which work analogous to a conveyor belt that has slats on which entities
are placed, and after some transit time on the conveyor these entities are released in the
order in which they entered the conveyor. Simply click and hold the stock tool on the
toolbar (Fig. 14.2). Going down the list of options in Fig. 14.2 you find
• The reservoir, which lumps all the entries together,
• The conveyor, which works much like a conveyor belt, taking on elements and
releasing them after a set time,
• The queue, which keeps elements queued up until some specified conditions are
met, and
• The oven, which has a given capacity, keeps elements for a defined duration but
does not distinguish its contents by their order of arrival and simply releases
them all after their “cook time.”
Choose the stocks for infants, juveniles, lactating, and pregnant individuals as
conveyors. Specify the transit time for infants as 9 (months) and that for juveniles as
34 (months) because the age at first birth is 43 months (43 À 9 ¼ 34). Set the initial
conditions of each of these conveyors at 5 (individuals). Specify the transit time for
pregnant females as 6.5 (months) and that for lactating females as 10 (months).
Assume we start with ten pregnant and ten lactating individuals.
Note that with the specification of the transit times, the outflows from each stock are
pre-defined: The individuals, who come into the stock first also leave first, after their
specified duration in the stock. Any additional flow out of the conveyors that you draw
will be interpreted as a leak—such as the “leakage” of infants from natural causes and
infanticide. For the leakage, you can specify when it should occur along the conveyor,
such as at its very beginning or anywhere during the transit time. For infant deaths, for
example, we allow the loss of babies to occur as early as at the beginning of their
cohort, and as late as at the very end of the 9 months in their cohort (Fig. 14.3).
We must also select the type of leakage these flow represents—linear or exponential. If you select the linear option, the total amount to leak is equal to the
inflow * DT * leakage_fraction and this amount is distributed evenly across the leakage zone of the conveyer (each slat leaks this amount divided by the number of
slats). In contrast, if you select the exponential option, the amount that leaks from
14.1 Langur Infanticide Model
121