Mini-Monkeys: The Odd One Out

Goeldi's monkey

Under the scheme that I have been using over the last year, there are 42 currently recognised species in the marmoset family. Of these, 22 are marmosets, and 19 are either tamarins or lion tamarins. It doesn't require much arithmetic to deduce that there must therefore be one member of the family that's neither.

This is Goeldi's monkey (Callimico goeldii). It is sometimes called "Goeldi's marmoset", or, less commonly, "Goeldi's tamarin", but these names are misleading and inaccurate. It is, undoubtedly, a member of the marmoset family, but it's equally clear that it is quite different from anything else within that family.

First, the similarities. Goeldi's is about the same size, at about 25 cm (10 inches) in body length, as other members of the family, and therefore much smaller than any monkeys outside the family. It has similarly luxuriant fur, in its case almost entirely black, with an almost mane-like ruff around the head. Like members of the marmoset family, but unlike other monkeys, it has claws, rather than nails - something it uses to cling on to rough bark. It also lives in roughly the same area, in the high altitude western margin of the Amazon, from Colombia down to Bolivia and in to western Brazil.

Yet the bodily proportions are slightly off, with longer, stronger, hind-limbs. More significantly, perhaps, one of the key features we often use to distinguish families of mammals are the shape and number of their teeth. There are minor differences in the shape of the teeth in Goeldi's monkey, but there are differences at least as great between marmosets on the one hand and tamarins on the other, so that probably doesn't mean much. However, one of the distinguishing features of the marmoset family is that its species have 32 teeth, not the 36 of other South American monkeys. But Goeldi's monkey really does have 36 teeth, possessing that last molar - what we call the "wisdom tooth" in humans - that all of its relatives lack.

Along with a number of other differences, which I'll get to shortly, this was enough for its relationship to marmosets and tamarins to be seriously questioned for much of the 20th century. As recently as the 1970s, it was being placed in its own family, and thought to be primitive descendant of an ancestral group of South American monkeys, no more related to marmosets than to, say, capuchins, or perhaps even howler monkeys. The argument ran that the apparent similarities were due to the parallel evolution of dwarfism - that there are only so many ways you can make a really small monkey.

Genetic studies over the last 30 years or so have blown that out of the water; the species is unequivocally related to marmosets and tamarins, and is almost certainly closer to the former. In light of that, it's worth looking again at that extra tooth at the back of the mouth. It is, in fact, tiny, and likely too small to be of any use to the monkey. Evidently, as the marmosets and tamarins shrank, and had less room in their small mouths for a full set of teeth, this last tooth shrank away until it vanished altogether. In Goeldi's monkey, for some reason, the process never quite finished, although it got fairly close.

Named for Swiss naturalist Emil Goeldi, Goeldi's monkeys were first described by Oldfield Thomas in 1904, a rather late date for a monkey that is so distinctive. They live in bamboo forest, and in denser jungle close by, and often overlap with regions inhabited by tamarins. At least during the wet season, they sometimes associate with the local tamarins, forming mixed-species groups, as some other members of the family also do. Goeldi's monkeys, however, forage much closer to the ground, rarely travelling more than 5 metres (15 feet) up, except to sleep at night. Here they search for food in the dense understory, and even among leaf litter, perhaps benefiting from the tamarin's knowledge of where fruit can be found, without competing directly with them too much.

Such groups are not especially long-lasting at any time, but they seem to break up entirely during the dry season, when the Goeldi's monkeys head for the bamboo thickets, leaving the tree-dwelling tamarins behind. These bamboo-heavy habitats, and their habit of foraging among the lower tree trunks when in the jungle, help to explain the differences in bodily proportion between these monkeys and tamarins. Goeldi's monkeys spend a lot of time leaping from trunk to trunk using their relatively powerful hind-limbs for propulsion. Even when travelling along the ground, they often do so in a series of long, horizontal leaps. Since they land "hands-first", their shoulders and upper arms are stronger than in other related species, and better able to absorb the shock of impact when they hit a solid tree trunk.

This preference for a slightly different habitat is related to the monkeys' slightly unusual diet. During the wet season, it isn't so different from that of tamarins, with a mix of fruit and insects, especially crickets. But fruit are in short supply among the bamboo, explaining why tamarins tend to avoid such areas. Furthermore, while Goeldi's monkeys do occasionally eat gum, they do so less than just about any other species in their family, and, again, it's something you don't find among bamboo.

Instead, they eat fungi. Now, most members of the marmoset family will do this from time to time, and some more than others, but for Goeldi's monkeys, fungus is a staple part of the diet, comprising at least 40%, and possibly as much as 60% of their food supply at certain times of the year. The fungi eaten include Auricularia and Ascopolyporus, and are high in protein, although it's unclear how much of that the monkeys can really digest, so they may be more nutritionally poor than appears at first glance - requiring the monkeys to spend much of their day foraging.

Goeldi's monkeys live, much like other members of the family, in small family groups, rarely containing more than about six individuals. However, there are key differences from their relatives. For one, they don't bother to patrol the borders of their home territory, simply tending to stay wherever the best food is. However, the structure of the groups is also different. Unlike many other members of the family, it is quite common for there to be two breeding females within the same group; there's evidently no reproductive suppression as occurs in some other species in the family.

With one or two breeding males as well, it isn't clear quite what the mating arrangements are. In smaller groups, monogamy is likely the norm, if only by default, but the potential is there, not just for the polyandry that's common in related species, but for polygyny, too. It's also unclear whether or not mated females are typically related to one another (most likely as mother and daughter), or whether they tend to come from different birth groups. Clearly they can be related without there being a problem, because we've seen that in captivity, but whether that's what they'd normally do in the wild is another question entirely.

After a pregnancy lasting 151 days, one of the more dramatic differences between Goeldi's monkey and its relatives becomes apparent: they give birth to single offspring. Every other species in the family gives birth to twins by default, and so it has been assumed that this is how tamarins and marmosets have always given birth. Yet Goeldi's monkey doesn't. Which means that either twinning has evolved twice in the family (once each for marmosets and tamarins), or that the ancestors of Goeldi's species used to give birth to twins, but then stopped, and went back to the more usual primate pattern. On the balance of probabilities, the latter seems more likely... but why it should have happened remains a mystery.

Numbers represent estimated age, in millions of years

Within the marmoset family, it is common for all members of the group to start caring for the young from a very early age, literally within a day or two of the birth. This may relate to the difficulty of caring for twins, which might be why it's all so different in Goeldi's monkey. In this species, the mother carries her infant about for three or four weeks, doing all of the caring herself - as most other mammals do. Eventually, though, she hands the infant on to the father, and from that point on, everyone, particularly the males, helps in rearing and feeding. Indeed, the mother's contact with her child drastically declines after this handover, being largely restricted to nursing until such time as it is fully weaned. 

Fathers do, however, take interest in their children right from the day of birth. It's just that the females won't let them help out until they're ready for the handover, something that seems to be related to how heavy the child they're carrying about has become, and that can happen earlier if the female feels a need to be more active - for example, because there's a predator about. 

Growing more rapidly than other members of the family, possibly because they get all their mother's milk to themselves, young Goeldi's monkeys are fully weaned by about five months. They reach sexual maturity at around a year, which is lower than the average among their relatives, but not unique, and typically give birth about twice a year when they can.

We don't have enough fossils to determine quite when, let alone how, Goeldi's monkey set off on this slightly divergent path from its relatives. Using genetic analysis, however, we can get a fairly good idea of how the family as a whole holds together, and the picture here is becoming quite clear. Perhaps the most important split within the group is that between the 'true' tamarins and everything else, which happened around 15 million years ago, in the mid Miocene epoch. While the 'true' tamarins remained relatively unchanged, the other lineage became ever more specialised, resulting in the evolution of Goeldi's monkey, the gum-feeding marmosets, and ultimately, the smallest of all monkeys, the highly derived pygmy marmoset. 

The family as a whole, however, originated around 20 million years ago. The diagram above shows their closest living relatives as being the relatively obscure night monkeys, but it may just as well be the capuchins and squirrel monkeys. For that matter, as I indicated in my original post in this series, those two groups might be equally close relatives - the differences are apparently hard to tease apart at that level. At any rate, we don't know exactly what drove the ancestral marmosets and tamarins to become so small and, in some cases, to develop such unusual diets, but today there are a great array of species, many of them colourful and extravagant. And, for all we know, there may be more as yet undiscovered, somewhere in the depths of the Amazon.

[Photo by Malene Thyssen, from Wikimedia Commons. Cladogram adapted from Perelman et al, 2001.]