Poor Little Guy: ACIN's necropsy

Anyone who has read this blog has probably seen that after spending so much time with these animals, we researchers can’t help but get quite attached to them. We get to know every hyena as an individual, which makes it very hard when we lose them. I have been at Fisi camp long enough to have seen many animals go missing, and several cubs die (and be eaten by other hyenas) at the dens, but last month I did my first necropsy.

So first, an introduction to ACIN:

ACIN was one of the first hyenas I learned to identify at Happy Zebra den. When I arrived in the Mara he was six months old and had beautiful spots.

ACIN, showing off his spots

ACIN was very outgoing and bold; he wandered farther than other cubs and was always approaching and exploring the car. He was also a little troublemaker, and was one of the stars of Emily's blog post, The Car is not Food.

Emily, who named him, sent me this after she heard that he had died:

“ACIN, was a wonderful and naughty cub that will be dearly missed.


He was named after a family of toxic proteins called astacin metalleoproteases. They are proteins found in many types of animals but were characterized in the spider venom I was researching in undergrad. We were able to characterize the toxic proteins in many species of haplogyne spiders, woo hoo! Astacins act by eating away at tissue (yuck) so the spider's venom can travel more effectively through the tissue. We injected crickets with different concentrations of venom to determine how nasty the venom actually was and how lethal. At the high concentrations it basically turned the crickets insides into soup. Lovely, I know. 

Super naughty but super adorable, that is how I will always remember that little guy.” 

He was a particular favorite of Sarah's, whose research involved feeding cubs milk powder to try to get them to fight, because he was always the first cub to find the milk powder.

ACIN's mom, SILK, was the lowest ranking female in Happy Zebra, which made him one of the lowest ranking natal animals in the clan, higher only than his older brother, Furadan. He was raised in a communal den where PIKE (the matriarch of Happy Zebra), along with EREM, ARBA, and COEL, all high ranking members of the royal family, were also raising their cubs. This gave ACIN a little bit of a rough start to life; his mom was often chased away from the den by the higher ranking females.

Since that blog from Emily, ACIN had graduated from the den and we saw him several times exploring by himself, looking quite healthy and happy. It had been about a month since I had seen him, and several months since I had seen SILK, when I found him looking very sick on the night of December 18th.

I was the only researcher in camp, out on obs by myself, when I found him sacked out (lying down) near a watering hole. I noticed right away that he wasn’t acting normally; he looked extremely skinny, and he didn’t react at all when I drove up to him. I hung out with him for about 20 minutes and during the whole time he didn’t do much more than lift his head. When I came back to check on him the following morning, he had moved about 150m, but was acting even more lethargic, barely able to pick up his head.

ACIN on the morning of December 19th, about 15 hours before he died

That afternoon, Kenna and Eli arrived in Serena camp and we went back to check on ACIN. It had rained that day, and his fur was wet and matted, making it clear that he was even skinnier than I had first thought. He had moved about 100m, away from a track that a lot of tour vehicles use, but by that point he could not open his eyes and was moving his head feverishly. We came back around 8pm and he had stopped moving entirely, though he was still breathing. The next morning he was dead. We estimated the time of death to be about 10pm on the night of the 19th.

ACIN on the evening of December 19th, about 5 hours before he died.

On the morning of December 20th, about 8 hours after he died

ACIN died 3 days before his first birthday. Hyena cubs are typically weaned between 12 to 18 months, which means ACIN was probably still relying very heavily on his mother for milk. While it is not unusual to go a couple of weeks without seeing a low ranking animal, the fact that we hadn't seen SILK in several months, combined with ACIN's body condition, makes us think that the most likely explanation for his death is that SILK was killed, and ACIN slowly starved to death.

Cub starvation after the death of a mother causes 18% of all hyena deaths. Humans cause the same amount of deaths (18%), exceeded only by lions, who cause 27% of all hyena deaths (Watts & Holekamp, 2009).

Spotted hyenas have an average life span of 12 years, but the first two years of life are by far the most dangerous. In a study conducted in Talek, 48% of hyenas died before they reached one year of age, and 63% of all hyenas died before they turned two (Watts & Holekamp, 2009). Changes in life stages are particularly dangerous. Many cubs are killed by other hyenas right after they are moved from their natal den to a communal den, at about a month old (White, 2005). There is also high mortality when the cub starts to graduate from the den, and right after they are weaned and begin to visit kills (Watts et al. 2009).

On top of those not great odds, ACIN was low ranking, which made him even more likely than other cubs to die before reaching sexual maturity (Watts et al. 2009).

Despite knowing that the odds aren't great for each of these little guys, it was still somehow surprising to find him dead. After photographing ACIN's body, we brought him back to camp, and started the necropsy. The first step was to weigh him, and take body and teeth measurements.

A timelapse of Kenna, Eli, and me measuring ACIN

ACIN weighed 18.45 kg, while the average weight for cubs at 1 year old is 29 kg. He was more than 23 pounds underweight for his age.

You can see here how skinny he was

The skulls of hyenas at a) 7.5 months, b) 14.4 months, and c) 22.2 months old
(Watts et al. 2009)

ACIN's skull

You can see on his skull that ACIN's sagittal crest (the peak on the top of the skull), and his teeth were not fully developed, as is normal for a cub of that age. The steep sagittal crest of the hyena provides a large surface area for jaw muscles to attach, giving them their incredibly strong bite. However, this structure takes a long time to develop, and cubs are pretty bad competitors when it comes to carcasses, making them very reliant on their mothers. You can also see that all of ACIN's teeth are sharp, where on an older hyena they would be worn down from crunching bone. All of this makes it clear why ACIN would have had a tough time surviving on his own without a mom to nurse him or to get him access to carcasses.

After that we proceeded with the necropsy, which involved taking tissue samples from 16 different organs; everything from his heart and lungs to his gallbladder and pancreas. The final step was to remove and flense his head, so we could preserve the skull.

A timelapse of the dissection

ACIN's internal organs

Though I don't have a lot of experience looking at heyna guts, all of ACIN's internal organs seemed healthy and normal looking. His stomach, however, was completely full of hair that had formed several hard hairballs. It seems likely that he was feeding on undesirable carcass scraps before he died, but that they weren't enough to keep him alive.

The contents of ACIN's stomach

While the whole experience was incredibly emotionally draining, it actually was much easier doing the dissection than it had been to watch him slowly die over the course of a few days.

After finishing the necropsy, we took his body out the Breakfast Plains, which are right in front of camp, in our North hyena clan territory. Over the next couple of days we saw both Rocket Scientist and Hey Jude, sub-adults from North clan, visit ACIN’s decomposing carcass to roll in it, which pretty much completes the spotted hyena version of the circle of life.

Watts H.E. Holekamp K.E. 2009 Ecological determinants of survival and reproduction in the                    spotted hyena. Journal of Mammology, 90, 461-471.

Watts H.E Tanner J.B. Lundrigan B.L. Holekamp K.E. 2009 Post-weaning maternal effects and the                          evolution of female dominance in the spotted hyena. Proc R Soc B 276 (1665): 2291-2298

White P.A 2005 Maternal rank is not correlated with cub survival in the spotted hyena, Crocuta                     crocuta. Behav. Ecol. 16, 606–613.