Sunday, August 25, 2013

How Many Bones are in the Mara River?

In both 2011 and 2012, approximately 8,000 wildebeest carcasses per year went into the river due to mass drowning events that occurred during river crossings. Thus far in 2013, we have already had about 2,300 carcasses go into the river, and we have about 2 months of the migration left. So what happens to all these carcasses and how do they affect the river ecosystem? A lot of our research is aimed at answering these questions.

Last year, to quantify how long it took a carcass to decompose in the river, we put different sections of the carcass (skin, muscle, intestine and bone) in small mesh bags, and measured the change in mass over a month period. By the end of the month, almost everything was gone... except the bones, which had lost only about 10% of their mass.

The skeleton makes up about 20% of a wildebeest carcass, which has an average mass of about 150 kg, so this is about 30 kg of bones per carcass. With 8,000 carcasses per year, that means there are about 240,000 kg of bones going into the river each year...and 90% is still remaining after one month?!

Because we will be in the field until May this year, we have the opportunity to measure bone decomposition over a longer period of time. This year, we made mesh bags of only bones we had retrieved from one of the carcasses in the river. Here is a picture of Paul Geemi, our field assistant who is helping me out while Chris is away in Tanzania, holding up part of our pretty bad smelling experiment.

We learned last year that if you just put mesh bags with carcass material in the river, the crocs and Nile monitors consider these little goody bags that they happily pluck off the string... hence the tough wire mesh cage we had welded in Narok to house our little bags. Here are Geemi and Charles, the Warden at Purungat Bridge who was our armed ranger for the day, with the highly scientific-looking unit we're about to install in the river.

I'm thinking we should take bets on how long it will take these bones to decompose. We put in sections of leg, rib, vertebrae and scapula, with two different mesh sizes-- one smaller mesh to only allow microbial decomposition, and one slightly larger mesh to allow aquatic insects and small fish. We will measure these bones each month until the end of April. I'm betting there will still be significant amounts of bone left after 8 months in the river. If this is true, think how many bones may be remaining in sections of the river where a lot of drownings have occurred! How long do you think it will take these bones to decompose?
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