One of the many mysteries we have about the sloth site is why we have so many more bones from the right side than left side of the adult.  (The photo (below) is misleading—it comes from an outreach program where we had to spread the bones out so that viewers on the “bad” side had something to look at.  The real difference isn’t absolute, but it’s striking.) There’s no intrinsic reason why the bones on one side of the sloth should have fossilized better than the other, so it must indicate something about the conditions near the time of death. (photo borrowed from)

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Even in the frenzy of the blood and the pain, the snarling and dying at a kill site there is a choreography that has evolved to share the spoils and reduce conflict. The patterns that predators leave behind often provide clues pointing to their identity–even if the site is 12,000 years old and absent distinctive tooth marks. Gary Hanes has spent years studying the kill sites of various predators and his research provides a general picture of their different patterns.

According to Haynes (1988) every wolf pack has its hierarchy and once the prey is dead, and often even before, the process of staking out claims on preferred cuts and dividing the carcass begins. The dominant wolf will take the choice position. The blood and internal organs inside the abdomen are a favorite first pick. Sternal elements and ribs are usually damaged in the process. Other high ranking wolves will claim the rump and upper legs where there are large masses of flesh. If there are more animals in the pack than can comfortably situate themselves around the carcass to eat without invading each other’s space, they’ll start disarticulating the limbs, causing distinctive damage on the proximal ends of the femora and humeri , and their anchoring points on the pelvis and scapulae. The prizes will be carried a short distance away to be gnawed on in relative solitude. Lower ranking wolves will tear off smaller less desirable parts (e.g. ears, tail, jaw/tongue) and carry them further away. These satellite consumption spots will be randomly distributed around the carcass and about 20 feet apart (Haynes, ibid.). Tooth marks and distance will be correlated as wolves lower in the pecking order tend to invest more time gnawing on their meager rations instead of trying to muscle in and steal another portion.

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If predators killed the sloths, and the site hasn’t been disturbed too much (e.g. by scavengers, trampling, weathering, transport, etc.), the killers’ fingerprints will still be present. The signs of predation versus mere scavenging, according to Haynes, are in the evidence left behind after the meal—the kind of damage to specific bones, the pattern of disarticulation, and the arrangement of the bones around the kill site (Haynes, 1980a).  Different predators have different MO’s. Those vary with the specific prey species, the season, environmental conditions, how hungry the predators are, how much meat is available, and how many individuals there are in the pack or pride (Haynes, 1983). The patterns are so regular that one can reliably look for causes other than predation when deviations from the norm are observed.

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Did predators killed the sloths?  Last week Holmes and I were looking at a rib from the adult that’s currently  on display in the lobby when we noticed a large puncture and some adjacent gnaw marks. The wounds are partially obstructed by the case and easy to overlook.  They were obviously caused by  large sharp teeth and indicate a carnivore was present close to the time of death.  Carnivores don’t gnaw on bones to sharpen their teeth like rodents.  They may mouth an old dry bone they happen across, but nothing more.  If a carnivore bit into this bone, there was meat on it.

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Why is it that only 6 Megalonyx skeletons of any consequence have ever been discovered  and no direct adult-juvenile association at all, but we’re lousy with sloths? What’s so unique about the site or the circumstances surrounding our sloths’ deaths?  Much progress has been made in understanding the processes of fossilization and decomposition, but the essential mystery remains–why do just a few thousandths of 1% of all bones become fossilized, absent embalming and burial? (Gill-King, 1997)  How did we beat the odds?

External conditions (i.e. soil chemistry, microbes, etc.) are generally believed to determine the fate of bones, but Bell et al. (1996) suggest internal factors are more important. The bodies of all animals contain a host of microorganisms in their guts ready to escape after the death of their host and use its vascular and lymphatic systems to invade the major body organs. The microbes could be getting into the bone marrow by the Haversian canals that maintain the bones in life.  The actions of predators,  and their manner of killing–specifically, disemboweling prey and disrupting the microbial escape routes may be critical for explaining bone preservation.  I love the irony–fall to predators but live forever as a fossil.  Cool. Continue reading →

Can you explain the deaths of three sloths by anything other than some kind of catastrophe?  In a drought herbivores congregate around the remaining water sources, and soon exhaust all the good forage near by.  If the drought continues, eventually they die of malnutrition. Predators have plenty of meat available so they leave most of the carcasses undisturbed.  Even scavengers that normally consume bones switch to soft tissue.

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Heaven knows we’ve almost lost some students and one Bobcat operator to the muck at the site, but how reasonable is it to think a giant sloth could die that way? How about three sloths? Haynes (1988) has studied hundreds of elephant deaths and reports it’s actually not an uncommon event. Healthy adult elephants never have a problem even in the deepest thickest mud but very young animals and those who are ill or weak some times get stuck.  He has also observed impala, Cape buffalo and black rhinos dying in this manner.

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Why was it that only certain bones, and not others, got separated from the main concentration, and how did these end up here?  If we understood that, would it help us find the missing bones?

Under normal conditions bones don’t start moving until they are completely disarticulated (Hill, 1975).  A lot of researchers have tracked the decomposition of individual carcasses over time, but Hill picked out a large area in East Africa to study and recorded the status of disarticulation for every Topi, Damaliscus korrigum, a common medium-sized antelope, in his 475,000 square mile study area.  He found a surprisingly consistent pattern.

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I’ve been studying an old photo Bob Athen took in 2002 of the bones he and Sonya had collected, spread out in their upstairs hallway and arranged as they originally found them. Like a fortuneteller staring at the leaves on the bottom of  a teacup, I’m trying to figure out what, if anything, it means about the conditions at the time of death and where we should dig next.

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