Social Structure

Male and Female Characteristics

Mantled howlers form multi-male/multi-female groups with a linear hierarchy in both the females and the males, yet the alpha male still has dominance over the entire troop. Females and males of howler groups are really compatible with one another. They act in parallel as they feed and rarely come into conflict. Although females sometime exert control over the group by picking direction they need to travel in, the male is the leader of the group. They form this multi-male/multi-female groups for many reasons. One is that by having a lot of males around, the female has better protection for herself and her offspring. The males are unsure of who the father is, so they all assume they are the father, and are willing to protect the females and her offspring on this assumption. Another reason is that females and males are free to have sexual interactions whenever they feel like it, except during certain periods of estrus, so there is less of a fight for who gets to mate in the group. This allows all the males to feel they are spreading their seed and producing offspring, which in turn creates less infanticide for the group because there is less aggression and competition. These multi-male groups are also successful at preventing outside males from coming into the group and committing infanticide. This may have been one reason they formed into groups like these.

When traveling, females carrying young are positioned toward the rear, so the leading male(s) can ward off any danger. The younger, more reproductive female is the alpha female, while the others spend most of their day feeding and resting. Males are larger and different colors, and are more likely to roar. In defense, males will act together. Usually, after one male establishes a bond with a female in the group, other males in the group do not compete for that female.

Females form the stable social units and rarely leave the group once they are established within the group, whereas, the males will often emigrate to other howler groups (cited in Schoville, 1999). The younger females and males in the troop are usually the higher-ranking individuals in the group. The females form this hierarchy because the youngest female is usually the best reproducer in the group. Howler females are ranked according to their reproductive ability. The youngest female in the group is often ranked the highest (she is the alpha). Yet, the females rank is not stable, so the female in estrus at a particular time who is the youngest (because of her better chances at reproduction) is considered the alpha female. This alpha role for female switches fluidly, with the younger females losing all of their rank once they are pregnant and having to fight back for that rank after they have birth. There are some females that never reproduce because they cannot maintain any kind of dominance role within the group, therefore they remain foragers for their entire lives. Older females and lower-ranking females spend most of their time foraging for food, while the higher-ranking females invest considerably more time in caring for their infants. This is a way for the higher-ranking females to conserve reproductive and competitive energy (cited Schoville, 1999). These females show that kin selection plays a significant part in their behavior because they are finding food for the reproducing females, which are directly related to the foraging females. So these females are spreading some of their genes through helping the reproducing females produce related offspring. Yet, this can also be attributed to cooperative behavior because the female foragers participate with other males in locating food sources for the entire group, which benefits both the group and the foragers by increasing health and strength of group and offering the female foragers protection. This can also be cooperative behavior in a sense because the female foragers are helping reproductive females to reproduce offspring that will offer protection for the group or draw attention away from the female foragers when attacked by predators. From this veiw point, it is a selfish strategy.

Male alouatta palliata form similar hierarchies, where the youngest, strongest male usually assumes the role of alpha male (Crockett and Eisenberg, 1987). The alpha male has a short-lived tenure, usually only lasting about 2.5 - 3 years). Crockett & Eisenberg (1987) report that many studies theorize that the multi-male groups may be a result of the maturation of natal groups over a period of time. Since the alpha male's tenure is so short, the problem of incest by the alpha male with one of his female offspring does not usually arise. The alpha male maintains this hierarchy through subtle interactions and rarely involves direct aggression.

 

Adult male howler resting on tree: photo credited to Estrada et al.

 

 

Although some fighting occurs, it is minimal and usually does not result in serious injuries (cited in Broekema, 1999). However, Crockett & Eisenberg's study claims that howler males are much more aggressive than previously thought, especially in relation to when a new alpha male is taking over the group. They assert that howlers will often fight to the death and perform horrible acts of infanticide.

Emigration

Howler groups are not really closed unit. They emigrate to other groups randomly and emerge back into old groups later. Both male and female howlers emigrate as juvenile or sub adults. This dispersal helps protect against inbreeding and social instability.

The male will often experience aggression from the other males in the troop if the social stability of that group is in jeopardy. However, if a male is needed in that group, some natal males will have the option of remaining in that group, but these are usually rare occurrences (Young, 1981). Yet, in most cases the juvenile male is usually forced to leave the troop and become a solitary male in search of another troop he can enter. Males can remain solitary for up to four years in some cases, whereas females usually only remain solitary for one year. Once a female is established in a troop, she is less likely to leave that troop during her lifetime than a male established in the same troop (cited in Schoville, 1999).

Group Size

The average group size for the mantled howler monkey varies significantly. The size of the group is not necessarily linked with factors such as- limited food resources, predators, intra-specific aggression, or disease- but simply a result of howler monkey preferring a particular group size at that moment in time (Baldwin and Baldwin, 1972). In Baldwin & Baldwin's (1972) study, the troop size ranged from 18.2 to 28 individuals, and they reported some instances of two or three groups merging together to form a "supergroup" comprised of sixty or more members. This occurred during the rain seasons, when food was plentiful and predators may have been high. These supergroups may have been an old trait of howler monkey's that has been selected against, but occurred in rare instances. Another study reports similar ranges for group size, with an overall range of 2 - 45 members (DeVore, 1965). When groups grow too large for stability, where males are competing over females too often or resources are becoming limited, they divide into subgroups that may or may not combine back together later on. Usually, howlers like to maintain smaller, more stable social units with an average of 15-20 individuals (1965).

Home Range

Alouatta palliata are non-territorial animals and have significant overlap in their home ranges with other groups of howlers. They are, however, aggressive toward conspecifics not of their own group. Howlers do not defend territories, but defend the places they are momentarily located. Their defense is usually associated with the roaring or a stand off when approached by an intruder. This type of home range has probably evolved because howlers need a lot of vegetation for their diets. In order to get such a wide range and amount of vegetation, howlers have to travel around many different sites. If they were to simply stay at one site, they would deplete this site of resources too quickly and no longer have a source of food (Mittermeier, 1973; Baldwin and Baldwin, 1972; cited in Broekema, 1999).

In addition, they need to travel around in the rainforest so they can search for healthier food and plentiful resources. Most of their food is low in nutritional value, so they have to continually travel around to look for better quality food. Howlers do not defend a territory because it does not provide: the adequate amount of resources need for their diet, they can nest anywhere, and the sections of the rainforest do not produce do not produce predictive resources. They also do not defend territories because it requires to much energy and they are extreme energy conservationists.

The home range sizes vary from 8.0 acres to 17.2 acres, with an average of 12.1 acres per group (Baldwin and Baldwin, 1972). Home range overlap varies from 20 % with Mittermeier's (1973) study to 100 % with Baldwin & Baldwin's (1972) study, yet more studies report higher overlap percentages rather than lower overlap percentages (Carpenter, 1932; DeVore, 1965).

Day Range

Most mantled howlers home ranges can be completely traveled in 3 to 10 days. Daily movement by howler was recorded at about 200 meters per day, but there was a lot of variation in this movement depending on the weather. During the dry season, howlers travel less and less about their home ranges to conserve energy in the hot climate. Howlers usually travel more during the wet season, depending on the resources in a particular area (DeVore, 1965).

Sleeping Sites

Howlers sleep in horizontal branches or convenient resting places formed into the tree. They usually try to locate these resting sites near an area with abundant resources so they can begin foraging as soon as they wake up in the mourning, which is one of the peak feeding times. This also allows howlers to forage in bad weather, because normally they would not travel in bad weather, but if the resource is near by, they are more likely to attempt foraging (Mittermeier, 1973).

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