Friday, 7 March 2014

Floating monkeys

In a recent blogpost Ian Fraser referred to the monkeys of South America having originated in Africa and floated on vegetation across to South America.  In an exchange of comments he has explained that this was facilitated by the direction of the currents at the time.  The concept interested me - suspecting that my initial mental image of a couple of gibbons clinging to a log was not quite right!  This seemed to get a bit long for a comment on Ian's blog so I have created this post (and some might think it a bit long for a blogpost).

(Of course, if Australia's current Government had been in power in South America then, all the monkeys would have been stuck in camps as soon as they landed which would have stopped all that naughty evolution in its tracks.  Some might say that the level of cranial activity of our current Government  is such that they haven't evolved greatly in the last few million years.)  

According to a wikipage on oceanic dispersal the movement of primates happened in the Oligocene period, about 23-34 million years ago.  That would seem to give plenty of time for a lot of genetic diversity to appear, with the results shown in Ian's post.  The questions that interested me were more along the lines of:

  • How many monkeys would need to get to South America to form the basic breeding stock?
  • How big a raft would be needed? 
  • How long for the trip?
  • Are there any current examples of big rafts?

How many monkeys are needed?

This seems to head for the concept of Minimum Viable Population (MVP).  One of the major issues seems to be inbreeding depression in which 'bad' genes build up leading to reduced breeding success.  The linked article doesn't deliver a firm answer on MVP (most likely, because it varies between species) but suggests that an average of about 4,000 individuals is required.  I have significant trouble with the thought of a group of 4,000 primates rafting across a proto-Atlantic.

However there seem to be a few examples of bird species recovering from a far lower number of individuals.  (The images which follow in this section are all from the linked wikis.)

Bird examples

in 1941 there were 21 wild and 2 captive birds.  In 2011 there were 437 wild and 165+ in captivity.  

In 1912 they were down to 7 adults and 5 juveniles.  By 2004 the population was up to 576.

1987 there were 22 condors, all in captivity.  By 2011 there were 394 individuals with ~205 in the wild.

So, on this evidence, with a lot of assistance from scientists (and without interference from other interests - who mentioned the Shooters Party?) these species appears to be able to recover with a population of about 10 -20 individuals.

Primate examples

I am aware of an easy example of a primate species which is continuing, under heavy threat.  This is the Tanzanian Red Colobus (my snap).
There are about 2000 of them with about half living in Jozani National Park.  The troops we saw were about 40 strong.  (The Park Rules say you mustn't get within 3m of the monkeys to prevent disease transmission.  The monkeys haven't read these rules and one frequently has to duck as a monkey sails over ones head.)

A second example could be the chimpanzees of Gombe National Park.  Some of the families in that reserve are 6 individuals so one could hypothesise that 12 Chimps could with a few positive mutations, and ther good luck, be a viable seed population.

How big a raft is needed?

A first point is that the raft could be a lot smaller if there was no need to escape predators.  Fortunately, in the Oligocene, guns hadn't been invented and thus neither had the NRA nor the Shooters Party. Presumably there would have been reptiles but so there are in Jozani today and a balance can be achieved in a reasonably small area.

So taking the case of Jozani it seems that ~1000 moderate sized monkeys can exist for at least a medium term in 25sq km.  Possibly 1 sq km could accommodate 40 monkeys, with a bit of luck including good food resources.

As another example, the chimpanzees of Gombe Stream National Park number about 80 in an area of 52 km sq.  Assuming two families are needed for an MVP about 12 sq km would seem  enough (but for the time periods suggested in the next section it could be quite a bit smaller even for such large animals.

How long for the trip?

This is obviously a difficult question because I have no knowledge of the winds or currents in the Oligocene Atlantic.  From Ian's response to my comment on his blogpost it seems that others do, at least to the point of concluding the trip is feasible.  So I will take as an example the achievements of the Kon-Tiki expedition.  They sailed about 6900 km in 101 days (or about 65km per day).  

Assuming favourable currents and that some (natural) elements of the raft (see below) formed something resembling sails it seems reasonable to assume that a natural raft could manage half the speed of Kon-Tiki.  Thus to cover 2000km would only take a couple of months.

Any modern examples of big rafts?

When I started off on this I was "thinking" of the size of the ice floes that break off Antarctica.  In looking for information on this I came across a blog about other ice floes.  The author states that several of these are 15+km across, so things that size do float.  However they are a bit sparse in the primate department.

I have been aware of the concept of floating islands in the Amazon but have had some difficulty locating much information about them.  (TV documentaries have shown folk living on these and raising crops.)   In this blog it is suggested that some of the meadows exceed a square mile (~2.25 sqkm) and sustain capybaras, which are way bigger than Colobus.  Although they are called meadows the TV doco included images of small trees, which could function as sails and assist the process (if the headwinds weren't too bad).

So it is possible that in Oligocene Africa similar things existed and could get washed out to sea.

Bringing it all together

I hope the preceding babble has suggested that it is quite feasible that a genetically viable group of primates could have rafted across the proto-Atlantic and set up home in proto-South America.

Frances commented that thousands of rafts could set out but only one needed to arrive.  Thinking on this,  if there were (hypothetical numbers will follow) 10 rafts breaking off the land each year over 100 years and the currents etc are favorable it is quite possible that one of them will get across.  Extend that to 100,000 years years and one arrives at:
  • well above MVP numbers (if  100 rafts arrive with ~40 primates per raft one comes to the average of 4,000) of primates arriving, and
  • the later arrivals being genetically distinct, but
  • possibly capable of hybridising with the first settlers.
So I conclude it all hangs together as feasible, using modern examples let alone the evidence of the fossil record and the arcana of genetic sequencing..


Ian Fraser said...

Firstly, thank you Martin for doing the hard work that I skipped! I greatly appreciate it, as I'm sure do others. A couple of obs, triggered by your excellent analysis. Firstly, my guess (and that's all) would be that the proto-Kon Tiki-ists could well have been small animals, reducing the resources required. Secondly I have read of vast rafts of tangled vegetation - many trees and other material locked together - being found well out to sea after major floods in PNG. Third, I think that Frances makes a key point - which you picked up of course - that we needn't postulate just one such event. After 30+ million years we can't say that all SAm monkeys derive from one ancestor (or group), given that they all came from the same place and the same ancestor ultimately. We can only say when they began to diverge from African relations. And lastly, it's pertinent that many modern SAm (and African) monkeys are leaf eaters; it would be hard to argue that fruit would have been available throughout such a voyage, but less difficult to imagine a leaf supply if the raft was big enough, even though the supplies must have been pretty tricky by the end. But I did say that it must have been a horror voyage. Thanks again for taking this further.

Flabmeister said...

Thanks for the comment Ian. In terms of fruit, I agree it is a bit difficult to imagine (say) a mango tree on a floating island. Its a lot less difficult (mentally) to accommodate a good crop of (say) ancestor-cranberries or possibly hawthorns.

Luck would be a big part of this, as I suspect is the case with most evolution.