Howlers are very heavy-bodied animals, and are not very agile, especially compared to their close relatives the Spider monkeys. Males are much larger than females, with an average mass of 6 to 8 kg, with females usually around 5kg. Rather than hanging from slender branches and reaching out to pick fruit, howlers are specialist in feeding on leaves. As these take far more digestion to extract nutrients, howlers give the impression of being more sluggish, and sleep up top 70% of each day digesting their food. As a plus however, because they do not need to travel long distances in search of ripe fruit, they can survive in small forest areas at very high densities, with home ranges as small as 0.1 sq.km for an average sized troop. They sometimes descend to the ground, but most of their time is spent high in the canopy.
Howler groups vary in size, but typically comprise 1-3 adult males, a similar number of females, plus offspring. Usually only a single adult male breeds, and the subordinate males may in fact be older juveniles who have not yet dispersed to found groups of their own. Eventually almost all males and a majority of females will emigrate from the natal group. Juvenile males may band together to form bachelor groups until they can either attract females or take over an existing group by defeating the resident alpha male. As in many other animals with a similar breeding system, take-overs are usually followed by infanticide of small infants. In one study of Red howlers more than 44% of infant mortality was due to infanticidal attacks.
The group defends their territory by the dawn chorus of their howls, which can carry several kilometres. Both males and females will join in the group howls, but males are much louder as a result of an enlarged hyoid bone, which supports the resonating chamber in their throats. When not howling, they are surprisingly silent creatures, and do not spend a lot of time in socialising even within the group.
|Harpy eagle in flight|
One very odd feature of some species of howler monkeys is that their sex chromosomes have a different pattern to those found in all other mammals. As a standard pattern, mammals have a standard arrangement where females have an XX and males an XY pair of chromosomes in addition to the autosomes which carry the bulk of the genetic material. In A.fusca there is a much more complicated arrangement where there are two different types of X chromosomes, X1 and X2, and there is variation in the diploid number, with the result there are at least four different possible karyotypes, XY/XX; X1X2Y/,X 1X1X2X2, X1X2Y. Some other species of howler monkey have similar variations. There may also be differences in the number of autosomes, so the diploid number of an adult howler can vary extensively. Whether the variation has anything to do with the odd sex-linked colour vision variation in New World primates I talked about in an earlier post is not clear.
In captivity howlers are not often seen compared to many other monkeys. Their status as slow-moving prey animals means they tend to be quite nervous, especially when they cannot retreat as high as possible, and the Black and Gold howler is the only one with a regularly breeding captive population, with around 250 animals in zoos worldwide. This population took a long time to establish, as wild-caught howler monkeys are very prone to stress-related diseases and seldom breed. The captive population of Black and Gold howler has enabled some elucidation of best practise in husbandry, especially housing and diet and the species is now managed genetically – in fact Bristol holds the EAZA stud book for this species. Even though we bred then successfully for many years, as soon as their new enclosure was built they took to spending as much time at the top of the enclosure as possible, and hardly ever come lower than 2m off the ground. This is especially obvious when the zoo is busy and there are crowds around their enclosure. They have in some zoos been kept with other primate species from their range, but seem easily intimidated, even by much smaller species such as tamarins.
The diet of howlers also requires special consideration. In the wild they can eat many hundreds of different plants, especially the leaves although they take some fruit. The captive diet is a specially formulated high-fibre biscuit, supplemented with browse (cut branches and leaves of various trees and shrubs), plus various leafy vegetables and some other green vegetables such as beans or okra. Even with fruit-eating primates, recent thinking in zoos is to avoid commercial fruit in their diet, the reason being that fruits bred for human consumption are far too high in sugar compared to wild fruits, and are a cause of diabetes and tooth decay in many species.
As a result of these difficulties with maintaining captive howlers, there is no co-ordinated captive breeding programme for the endangered howler species, and protection in the wild is likely to be far more effective and easier to carry out than ex-situ conservation breeding. Nonetheless, studies on the captive population is extremely useful for throwing light on the social organization and exact requirements of wild howlers, which could be vital in the future. Although zoos tend to headline captive breeding and release as the main goal for their keeping various animals, research is at least as important, as we cannot conserve a species in the wild unless we know what it actually needs, and a captive study population is often the easiest way to obtain vital information.
Next week, one of the many medium-sized monkeys of South America, the Red Titi.
(Images from wikipedia)