Mating System/Behavior

Mantled howler monkeys form hierarchical promiscuity mating systems, in which the alpha male has mating rights to all the females in the group. Yet, the subordinate males can also mate with all the other females when the alpha male is not interested.

A female is able to engage in many copulation's with different males so she can ensure her pregnancy. The howler females have to mate with the alpha male when the alpha male notices that the female is in the prime point of her estrus cycle. Yet, during the rest of the cycle the other males are free to mate with the other females. When the dominant male finds a female in prime cycle, he removes her from the group to ensure that other males do not mate with her and spoil his seed. This guarding helps the dominant male better his chances of being the father of the majority of the offspring of the troop, which is his ultimate goal.

Hierarchical promiscuity is effective because it enables the males to maintain their basic mating strategy. They have the opportunity to try and impregnate as many females as possible and hopefully produce a large amount of offspring. And since there are too many females in the troop for one male to control all the time, promiscuous relationships form more easily. Alpha howler males are unwilling to expend so much energy on trying to suppress other males from mating with females. They cannot waste energy on that. Another reason hierarchical promiscuity is possible is because females can take care of the infant without the aid of the male if the situation arises.

A study performed by Kenneth Glander (1980) gives a detailed description of the howler monkey mating process. He reports that females experience skin changes, consisting of swelling and color changing, when in her cycle. This is the

 

 

 

period when conception occurs. One thing that Glander really helped clarify was the difference between primiparous (females having produced one infant) and multiparous (females having produced more than one infant). He reported that the mother became less aggressive just prior (about 1 week) to the birth of an infant, even the alpha female. This caused the female to lose her rank and not regain it again, if regain it at all, for at least two months after the birth of an infant. An interesting finding involves how primiparous females were more likely to engage in dominance competition after their first birth, whereas multiparous females were not. He also observed that infants rode on their mother's back until about three months old, for predator and safety reasons. In postpartum acyclicity, he observed that a female did not solicit or accept sexual advances for three to four months following the birth of her infants. Yet, if the infant died, the female would begin to solicit mating within three weeks of the infants' death and become pregnant almost immediately (1980).

Glander also observed that adult females experience a regular estrus cycle averaging 16.3 days and participated in multiple copulation's before becoming pregnant. Gestation averaged 186 days. The average interval between births was 22.5 months. Sexual maturity occurred at approximately 36 to 42 months for females and males. The adult female was an average 3.5 years of age at her first birth. He noticed that since adult females were only receptive to copulation's for two or four days of her cycle, the alpha male would monitor her urine and only copulate with her during these days to ensure success. This finding is also prevalent in literature about baboons and shows some similar mating styles. Yet, the beta or lower males would copulate on any given opportunity. He found that genitalia sniffing and licking are common greeting ceremony's during sexual encounters. His study also found that males had a higher mortality rate than females in infancy (1980).

Surprisingly, Glander found that the alpha females infants had the least chances of survival and usually died. Whereas the middle ranked females experienced the greatest reproductive success. He accounted for this by saying that since the alpha male is under more stress and energy requirements when maintaining her dominance, her reproductive success would be low because infants might be hurt or malnourished. Glander proposed that the benefits for the alpha female must occur when the alpha female is reduced to middle rank, because the only way to get to the middle is to come from the top. He finally talks about the fact that both male and female emigrate from group so most of group is unrelated (helps prevent inbreeding since they are extremely susceptible to disease) and that all neighboring troops are probably somewhat related. This fact might offer the possibility of kin recognition and selection, because juveniles may be coming back to help their mothers during her pregnancy. But he notes that more studying needs to be done. Yet, he notes that most strangers or emigrants in the group stay for around two years or the length of time between a females births (1980).

Another study performed by Glander & Nisbett (1996) gives a detailed account of the actual birth process by a howler female. They claim that her first signs of discomfort (a contraction) occurred 47 minutes before the second contraction. After the second contraction, the following contractions occurred in shorter time intervals, eventually only two minutes apart. They observed many similarities with past studies on the howler monkey birth process (all performed by the female)- smelling and licking the hands, restlessness manifested in walking-standing-sitting position behavior sequences, raising the hindquarters/arching the back, licking the neonate, eating the placenta, chewing on the umbilical cord, scant attention from the rest of the group during and shortly after birth, an intensely attentive adult alpha female, and a longer period of actual expulsion compared with red howler monkeys. Nisbett and Glander reported that 78.5 minutes passed between the times that the female howler monkey first showed signs of impending discomfort to the actual expulsion of neonate, which lasted for 6.5 minutes. This was considerably longer than previous studies, which reported an average of 66 minutes, but they accounted for this difference from the small size of the female howlers in this population of howlers. Their smaller size inhibits them from giving a quicker birth, because it is more of a strain on the female giving birth.

Broekema (1999) introduces something new by claiming that males reportedly mother infants by carrying them, and the that infanticide is the cause of 40 % of infant mortality. Infants are mostly killed after a new male takes over the group, but other females attracted to an infant may often prevent a new mother from feeding and sleeping, causing stress for the new mother in hopes that aggressor can take the infant as its own (1999). IN multi-male/multi-female groups of howlers, the father of the infant is mostly unknown, so when the outside male attempts to commit infanticide, all the males in the group will band together to protect what they all think is their offspring. This allows the females and infants to be defended by all the males of the group and show a reason why females enjoy these groups. The infanticide for howlers is also a reproductive strategy for males. Once they kill the other males offspring, the female will automatically go back into estrus and thus is able to reproduce with the new male. This male has now immediately established his genes in that troop. And the female will rely on the previous males to defend her, but even if they get killed, she will still benefit because she will now have stronger genes for her offspring.

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