The difference between killing and eating: ecological shortcomings of puma energetic models

1. What they did

Scientists often use energetic models to estimate how much prey a hidden, hard-to-watch carnivore must kill. These models work from the predator’s body weight and how active they are. The authors tested two common assumptions behind such models: that body weight and activity alone are enough to predict kill rates, and that predators eat everything they kill.

They compared puma kill rates measured in the field with the rates predicted by two published puma energetic models (Ackerman et al. 1986 and Laundré 2005). The work covered three very different study areas: Chilean Patagonia, western Colorado and northern California.

• Capturing pumas. Pumas were mostly caught with hounds, which chased them up trees or onto rocks; box traps and foot snares were also used. Each puma was fitted with a Global Positioning System (GPS) collar that took a location every 2 hours.
• Finding kills. The team visited GPS clusters (groups of locations close together) that included at least one location at night or at dusk or dawn. They searched each site for prey remains (hair, skin, rumen and bone fragments). From the state of the remains, bite marks and which body parts were eaten, they decided whether the puma had killed the animal or was scavenging.
• Measuring kill and eating rates. They calculated kill rates in kilograms of prey per day and in ungulates (hoofed animals) per week, for monitoring periods of at least four weeks. They estimated how much each puma ate from the number of days they stayed at a kill, using feeding trials with captive pumas, and added kittens’ share for mothers.
• Testing the models. They ran both energetic models for each puma and each monitoring period, and tested whether puma weight and daily travel distance predicted the field kill rates.

2. What they found

The pumas and their kills

• 26 pumas (10 males, 16 females) were followed over 45 intensive monitoring periods of 26 days to about a year.
• Patagonia: 694 GPS clusters checked, 433 kills and 6 scavenging events. Prey included 302 guanacos, 41 domestic sheep, 7 huemul and 83 smaller animals.
• Colorado: 1001 clusters checked, 437 kills and 8 scavenging events. Prey included 266 mule deer, 71 elk and 106 smaller animals.
• California: 396 clusters checked, 215 kills and 4 scavenging events. Prey included 176 black-tailed deer, 1 black bear and 38 smaller animals.

Models versus the field

• Field kill rates averaged 10.10 kg of prey per day. The Ackerman model predicted 4.04 kg per day and the Laundré model 2.44 kg per day. All three were statistically different.
• Estimated consumption (what pumas actually ate) averaged 3.35 kg per day in the field, 2.75 kg per day from the Ackerman model and 1.66 kg per day from the Laundré model. The models came much closer to what pumas ate than to what they killed.
• On average, pumas killed about three times more meat than they ate.
• Neither puma weight nor daily travel distance, the very variables the models use, explained differences in kill rates.

Kill rates across places and pumas

• Kill rates in kilograms per day did not differ between the three study areas or between males and females, even though the areas differed in climate and had one to three ungulate prey species.
• Only females with kittens over 3 months old killed more than females without kittens.
• Kill rates in ungulates per week did differ between areas: higher in California than in Colorado. The authors link this to the big differences in prey size, from deer and sheep of about 40 kg to bull elk of up to 385 kg.
• Handling time at carcasses and the amount of meat abandoned did not differ significantly between areas.
• These kill rates were the highest reported for pumas at the time.

3. What they concluded

• Energetic models based only on body size and activity badly underestimate puma kill rates, though they may estimate how much meat a puma needs to eat reasonably well.
• Kill rates in kilograms per day are better for comparing pumas across different places; kill rates in ungulates per week are better for modelling predators and prey within one place.
• Why pumas kill so much more than they eat is still unclear. They may leave carcasses once staying no longer pays, or lose kills to dominant competitors. In Patagonia, scavenging condors had already been shown to raise puma kill rates.
• Rather than refining energetic models further, future research should study the behaviour behind kill rates and the meat pumas abandon.

4. How tracking was or was not used

GPS clusters showed where pumas spent time, and tracks and signs at those sites showed what had happened there. Observers searched kill sites for prey remains (hair, skin, rumen and bones) and used the state of the remains, the presence and position of bite marks, and which body parts had been eaten to tell a kill from scavenging. They also recorded the species, age and sex of the prey, ageing ungulates from their teeth. Kittens with mothers were counted partly from tracks in snow, as well as from cameras at kill sites and direct observation. Pumas were captured with hounds, but the paper does not describe people following tracks.

5. Tracker or observer certification

Ninety-three percent of all field investigations of GPS clusters were done by CyberTracker-certified observers (citing Evans et al. 2009 and Elbroch et al. 2011). The authors say this ensured a consistent field effort and expertise across the three study areas. The certification level is not given.

6. Other technologies, skills or innovations

• GPS collars (Lotek 7000SAW or Lotek Iridium 2D) sending data by Argos or Iridium satellite links.
• ArcGIS mapping software to measure distances, and handheld GPS units to guide observers to clusters.
• Remote cameras at active kill sites.
• Prey weights estimated by age and sex from tooth eruption and wear and from published growth rates (appendix).

7. Implications for conservation (from the paper)

• Underestimating predator kill rates could contribute to declines of prey such as bighorn sheep and mule deer.
• For game species, underestimated kill rates may lead to hunting quotas that are set too high.
• Estimates of livestock losses to pumas depend on accurate kill rates.
• Underestimated kill rates would lead to overestimates of how many predators a prey population can support.
• Pumas leave large amounts of meat that feed many scavengers, so models that include this would better show their role in ecological communities.