Odd colour morphs in leaf monkeys

Madura De Silva and colleagues and I recently, independently, published papers on odd colour morphs in leaf monkeys. They documented a white phase of the Sri Lankan langur Semnopithecus vetulus (they are usually grey) and I documented a red phase of silvered lutung (or leaf monkey), Trachypithecus cristatus. (see references at end of post).

This is most interesting and it doesn’t happen very often. Unlike other taxa such as birds, which often have different color morphs, primates rarely do. I suppose this is because in mammals, when there is a mutation that causes a colour morph, either natural selection or sexual selection quickly eliminates it. One can imagine, for example, that a white morph of S. vetulus might be more visible to eagles, and so predation pressures might eliminate them. But De Silva's photos and mine also show that some white S. vetulus and red T. cristatus have babies, and so negative sexual selection must not be much of a factor. It is even possible that sexual selection could be a positive force, meaning that white potential mates are attractive because of their differences. As we say, “Variety is the spice of life!”.

In the case of the red T. cristatus, since they were documented some 70 years ago and still persist, indeed seem to have spread far upriver, one can speculate that there must be something about their environment that either doesn’t discriminate against red ones (for example, lack of avian predators), or actually encourages them. The only big eagle we saw was the Crested Serpent-eagle, and I presume that this is a snake/lizard specialist. Large snakes and wild cats of all sizes up to Clouded Leopard are present in Kinabatangan River habitat of T. cristatus, but I don’t know anything about their colour vision. It might be that red is just as invisible to these predators as black, maybe even more so. The black ones were certainly more visible to me, everywhere I have seen them (several places in Borneo and also in Peninsular Malaysia). Also, since there is a red species of leaf monkey, Presbytis rubicunda, that lives in the same area, there is obviously no environmental factor that would negatively affect T. cristatus.
About S. vetulus: it would be interesting to know how long they have persisted. Since De Silva et al mentioned a pale specimen from 1923, it seems possible that some variation was present 90 years ago, and may have increased in proportion and also in whiteness since then. If predation is a factor in coat color, would it be reasonable to speculate that since 1923, predation pressure might have slackened? Perhaps some of the langur predators have declined in the region where the white morphs persist?

References

SILVA, M. A. D., N. C. HAPUARACHCHI, AND P. A. R. KRISHANTHA. 2011. A new colour morph of Southern purple faced leaf langur (Semnopithecus vetulus vetulus) from the rain forests of southwestern Sri Lanka. Wildlife Conservation Society – Galle: 16.

HARDING, L. E. 2011. Red morph of silvered lutung (Trachypithecus cristatus) rediscovered in Borneo, Malaysia, Taprobanica 3:47-48.