Ravens and wolves

Issue 26, April 2003

Scavengers are animals that take food from the kills of others: this food stealing is called interspecific kleptoparasitism. The scavengers gain by getting the chance to feed on often large, high quality food. Ravens (Corvus corax) scavenge on the kills of wolves (Canis lupus), see Figures 1 & 2. Wolves prey mainly on elk (Cervus elaphus).

Figure 1. A raven.

Figure 2. A wolf.

The association between these two animals was recently studied by scientists* who followed 3 packs of wolves. Some of the wolves in each pack had been given radio-collars so that the animals could be tracked and found. The researchers observed all these animals using telescopes. When a wolf pack was spotted they recorded:

• the number of wolves present;
• their behaviour;
• the duration of the behaviour;
• if ravens were present;
• how many ravens were present;
• the length of time the ravens were with the wolves;
• if the ravens were < 1 m from the wolves, or not.

The researchers also took elk kills (carcasses) from road incidents with vehicles and put the carcasses in the same areas where wolves had been seen. The carcasses were cut open, placed on the snow and blood was sprinkled around. The scientists recorded the behaviour of ravens around the kill for the next 60 minutes. They also recorded whether ravens were present before and after putting out the carcass.

Stahler, D., Heinrich, B. & Smith, D. 2002. Common ravens, Corvus corax, preferentially associate with grey wolves, Canis lupus, as a foraging strategy in winter. Animal Behaviour, 64, 283 - 290.

1. State one advantage and one disadvantage of kleptoparasitism. ( 2 )
2. Ravens are carrion eaters. What is carrion? ( 1 )
3. Why do ravens need wolves to make a kill before they can eat elk? ( 1 )
4. The scientists recorded behaviour in three packs of wolves. The range of animals in each pack varied, the numbers being 13 - 22, 7 - 11, and 8 - 14. Suggest one reason why wolf packs vary in size? ( 1 )
5. The researchers recorded the activities of the wolves, which included travelling, resting, chasing prey and feeding. They also recorded the associations between wolf activity and raven activity. Some of their data are in Table 1.

   Table 1. Percentage of time the ravens were observed in association with wolves.

   Activities of wolves	% time ravens were seen with when they were of wolves engaged in that particular activity
   Travelling	37.7
   Resting	27.5
   Chasing Prey	62.9
   Feeding	99.7

   Comment on these data. ( 2 )

6. The researchers put out experimental carcasses and measured the time it took the ravens to discover a carcass. Their data are illustrated in Figure 3. Comment on these data. ( 2 )
   

   Figure 3. Discovery time of wolf-killed (N = 29) and experimental (N = 9) carcasses by ravens.

7. For the 29 kills the scientists observed, 24 always had ravens present at the time of the kill. The other 5 kills were discovered by ravens within 4 minutes. They provided the following pieces of data for all 29 kills: mean = 0.50 minutes, median = 0.00 minutes, range 0.00 - 4.00 minutes. Explain what the mean, median and range of a set of data are and say why you think the researchers gave all three pieces of information rather than just one, say the mean. ( 4 )
8. The number of ravens found at wolf kills were graphed (see Figure 4). Comment on these data. ( 2 )
   

   Figure 4. Number of ravens at wolf-killed (N = 29) and experimental carcasses (N = 9) at 15 minute intervals after they first appear, for the first 60 minutes that the carcass was present.

9. Ravens seem to be neophobic when they discover a carcass that is not already attended by wolves.
   1. What is neophobia? ( 1 )
   2. Why might neophobia be advantageous to ravens? ( 2 )
10. Why do you think ravens don`t follow elk around: after all, elk are the main prey of wolves? ( 2 )

[We are grateful to Elsevier Science for permission to use Figure 1 page 287 and Figure 2 page 287 from the article by D. Stahler et al. in Animal Behaviour, (2002), 64, 283 - 290.]