time, averaging between 20 and 40 individuals. Langur dominance hierarchies are
linear and tend to be organized into matrilines (due to females being philopatric,
or remaining in their natal group), and stabilized through coalitions between high
ranking females and their subordinates. Within single male breeding troops one male
dominates and mates with all the females within his group. In order to maintain this
privilege, the male must defend his position from outside males. These males come
from bachelor bands that are composed of immature males and adults.
Juvenile males are often forced out of a group by dominant males who are not
their fathers. After being forced from the group, these individuals may spend
several years in all-male bands, forming alliances that can be utilized in the
takeover of harem groups. In contrast to the predominately male and female troops,
multi-male/multi-female bands have a polygamist group dynamic. Group membership among multi-male/multi-female groups is very flexible within these coed
bands, with males constantly joining and leaving the group [1]. Due to the instability of these multi-male/multi-female groups and the complexity of social relationships between them, we model here a single male harem situation.
For our model, longitudinal data was available for a group of provisioned harem
groups in Jodphur, India [2]. Because this group is provisioned and also raids crops,
we did not include any density component in the model.
The Langurs of Jodphur, on which the model is based, show no seasonality in
births, although there is a peak in March and a low in November [2]. Their ability to
produce infants year-round is primarily due to the lack of seasonality in access to
food. In contrast, Langurs found in the Himalayas, for example, show a seasonally
varying birth pattern.
After a dominant male has been replaced in the harem group by a new male, the
incoming male will kill the offspring of the ousted male. This triggers the end of
lactational amennorhea and the female enters estrus sooner than she would have
otherwise. This is the major tenant of the “sexual selection hypothesis” that favors
infanticide as a male reproductive strategy [3]. This hypothesis assumes that males
are deliberately killing unrelated infants in order to shorten the interbirth interval.
Over 80 % of infant deaths occur within the first 9 months.
For infanticide to be an effective male reproductive strategy a male would have to
be able to distinguish his offspring from that of another male. Proof of this can be
seen in the female strategy of paternity confusion were a male will not tend to attack
an infant whose mother he mated with. Furthermore, an infanticidal male would
have to remain around the female long enough to prevent other males from attacking
his infant. Hausfater [4] has shown that a 26.5 month stay is optimal for this task. We
therefore assume that the average tenure of a male to be dominant is 26.5 months.
For the average interbirth interval we assume 16.7 months in the case of no
infanticide, and 12.8 months after the death of an infant. Gestation is 6.5 months,
and the age at which a juvenile female is considered an adult and gives birth for her
fist time is 43 months.
Each month 25 % of the females become pregnant. Since the cycle is around
1 month, any given week one quarter of the females are able to become pregnant.
Pregnancy is known from the literature to be 6.5 months, and the conveyor
120
14 Langur Infanticide and Long-Term Matriline Fitness
linear and tend to be organized into matrilines (due to females being philopatric,
or remaining in their natal group), and stabilized through coalitions between high
ranking females and their subordinates. Within single male breeding troops one male
dominates and mates with all the females within his group. In order to maintain this
privilege, the male must defend his position from outside males. These males come
from bachelor bands that are composed of immature males and adults.
Juvenile males are often forced out of a group by dominant males who are not
their fathers. After being forced from the group, these individuals may spend
several years in all-male bands, forming alliances that can be utilized in the
takeover of harem groups. In contrast to the predominately male and female troops,
multi-male/multi-female bands have a polygamist group dynamic. Group membership among multi-male/multi-female groups is very flexible within these coed
bands, with males constantly joining and leaving the group [1]. Due to the instability of these multi-male/multi-female groups and the complexity of social relationships between them, we model here a single male harem situation.
For our model, longitudinal data was available for a group of provisioned harem
groups in Jodphur, India [2]. Because this group is provisioned and also raids crops,
we did not include any density component in the model.
The Langurs of Jodphur, on which the model is based, show no seasonality in
births, although there is a peak in March and a low in November [2]. Their ability to
produce infants year-round is primarily due to the lack of seasonality in access to
food. In contrast, Langurs found in the Himalayas, for example, show a seasonally
varying birth pattern.
After a dominant male has been replaced in the harem group by a new male, the
incoming male will kill the offspring of the ousted male. This triggers the end of
lactational amennorhea and the female enters estrus sooner than she would have
otherwise. This is the major tenant of the “sexual selection hypothesis” that favors
infanticide as a male reproductive strategy [3]. This hypothesis assumes that males
are deliberately killing unrelated infants in order to shorten the interbirth interval.
Over 80 % of infant deaths occur within the first 9 months.
For infanticide to be an effective male reproductive strategy a male would have to
be able to distinguish his offspring from that of another male. Proof of this can be
seen in the female strategy of paternity confusion were a male will not tend to attack
an infant whose mother he mated with. Furthermore, an infanticidal male would
have to remain around the female long enough to prevent other males from attacking
his infant. Hausfater [4] has shown that a 26.5 month stay is optimal for this task. We
therefore assume that the average tenure of a male to be dominant is 26.5 months.
For the average interbirth interval we assume 16.7 months in the case of no
infanticide, and 12.8 months after the death of an infant. Gestation is 6.5 months,
and the age at which a juvenile female is considered an adult and gives birth for her
fist time is 43 months.
Each month 25 % of the females become pregnant. Since the cycle is around
1 month, any given week one quarter of the females are able to become pregnant.
Pregnancy is known from the literature to be 6.5 months, and the conveyor
120
14 Langur Infanticide and Long-Term Matriline Fitness