4 Living in groups
4.1 The advantages
On the basis of LoM and the TV programme, and hearing so much about African hunting dogs and lions, you might be tempted to believe that carnivores generally live in groups.
Question: Do you think this generalisation is true? Can you think of examples from LoM and the programme of carnivores that lead essentially solitary lives?
If you refer back to Table 1, you're likely to be struck by the number of carnivores that are not notably sociable. Think of bears, mustelids and the many solitary cats - leopard, jaguar, snow leopard, the serval, lynx, cheetah, tiger - even the domestic cat! They are all essentially solitary hunters.
It's estimated that only about 10-15% of all carnivore species congregate at some period outside the breeding season - a time when for all carnivores a degree of sociability is essential. But in some groups of carnivores, notably the dog family as DA points out, 'sociability is a characteristic that manifests itself again and again' [p. 128]. So, amongst the carnivore group as a whole, there is a range from those that live an almost solitary existence, through animals that live some of their time in groups, to animals that spend their whole lives within the same group of individuals. And grouping takes various forms:
population groups, which do no more than share a common home range;
feeding groups, with individuals sharing the same resource for a period;
foraging groups, with individuals combining forces to search for food;
breeding groups, where individuals form a reproductive unit.
The nature and permanence of these groupings vary not only between different species, but also within a single species. For example, the clans of spotted hyenas of the Ngorongoro Crater are composed of as many as 55 individuals. The clan divides up into hunting groups of about seven adults, but after prey capture, groups can swell to as many as 19 individuals. A pride of African lions may number 20 or more [p. 151] that share a particular home range, but an individual member of the pride may spend days or weeks on its own or in a small subgroup. Only two or three pride members might hunt at any one time, though many more individuals share the kill.
With such variability, speculating why group living has evolved in some carnivores and not in others is a complex task. We can start by comparing the hunting strategies and habitats of two contrasting 'big cats'.
Question: From LoM and the TV programme, what are the differences between tigers and lions in terms of (a) habitat, and (b) availability of prey, and what are some of the implications of these differences?
(a) The lions you see in the programme live in open terrain, with limited opportunity to use vegetation as cover. The tiger's habitat generally has denser cover, though with eight subspecies recognised, generalisations are difficult. (b) Tigers occupy habitats that have a good many ungulates (deer and wild cattle) but prey is often scattered, so solitary stalking is likely to be more successful; cooperative hunting has not been reported. For lions, prey is generally more concentrated, as the TV sequence (see 29.16) suggests.
This (and a good deal of other) evidence suggests that the likely advantages of foraging in groups, and perhaps the size of any such group, depends both on the availability of prey and density of cover. In lions, for instance, the largest prides are found in the most open terrain, with the most plentiful prey. (Much the same effect is evident in non-carnivores too. For example, baboons - a member of the primate group - are, in the main, herbivorous; they band together in large groups, of 50 or more individuals, only in environments where the food supply is rich.)
By this logic, those carnivores that come together in groups do so because of an increase in their hunting success. Perhaps a group of hunters would have an increased ability to tackle the types of large and dangerous prey that feature in the TV programme and LoM - zebras, wildebeest, bison, for example. This suggestion would imply that in the evolutionary past of such species, there was a strong selection pressure for increasing group size as a consequence of the increased hunting success associated with sociability.
Question: From the evidence of the TV programme and LoM, what species of carnivore do you think this argument would hold up for?
Perhaps African wild dogs [pp. 137-139] and wolves. 'Common-sense' thinking might suggest that where the prey is very sizeable (as with hunting dogs tackling wildebeest) many individuals, acting in a coordinated way, would be needed to ensure success. (Indeed, some members of the dog pack seem to be specialist 'nose-grabbers', others 'tail-grabbers'.)
You might argue that the high 'strike rate' of African wild dogs supports such an argument - 85% in one location [p. 140]. But what about wolves where, according to the programme, only 10% of grey wolf hunts are successful? And group hunters are not obliged to go for such large prey; perhaps hunting individually for smaller prey would be just as productive - may be more so.
Question: To be convinced by the argument that increased hunting success is the main driver for increased group size in carnivores, what clinching evidence would you seek?
Perhaps some of the prey animals could be removed from their group, to see if isolation had any effect on the predators' hunting strategies. But this intervention might affect the behaviour and vulnerability of the remaining individuals, and alter more factors than group size alone. A less intrusive approach would be to compare hunting success in different carnivores living in different-sized groups. But comparing 'like with like' would be difficult; there could be other variables that would confound the comparison, perhaps differences in habitat, or body size, or their own vulnerability to prey, or the availability of different prey options.
A good approach is to study a carnivore species that displays variation in group size - the coyote is a good candidate. But in doing so, no simple correlations are evident. For example, coyotes that feed mainly on smaller prey, rodents, have a larger than anticipated group size. As you'll appreciate from LoM pp. 152-154, which discusses lions, there does indeed seem to be more to group living than simply promoting hunting.
Question: With lions in mind, identify some other possible advantages of living in groups. Rereading LoM pp. 152-154 will help.
DA speculates [p. 154] that camaraderie amongst females is helpful to successful reproduction and that bonding in males may reflect the difficulty of gaining entry as a single individual. To some degree, lionesses share the tasks of parenting, including suckling. Lionesses within a pride (and African hunting dog females in a pack) are likely to be related, which means that mutual help is entirely consistent with the rules of natural selection. You might have learned in course S182_3 that the notion of inclusive fitness encompasses the reproductive success of close relatives.
The same benefit of group living is evident in marmots, as detailed in S182_3. Perhaps yet more advantages of group living can be identified, especially if we spread our attention to the herbivores that live in herds, as discussed in S182_4, so if you have worked through the course you should be aware of some of the arguments.
Question: It's been claimed that groups are better able than isolated individuals to identify predators. Do you find this proposal convincing, and can you think of examples from LoM and the 'Plant Predators' programme?
Yes, this is the 'many pairs of eyes' argument alluded to in S182_4. (You'll see a wonderful example of joint alertness of mammals 'on the lookout' - in this instance, for predators such as yellow cobras - if you watch the opening sequences of the TV programme 'Life in the Trees', which features a group of meerkats.)
Carnivores too have predators, especially those that live in open habitat. For African hunting dogs, lions are a big problem; smaller species (like some mongooses) can fall victim to birds of prey. Incidentally, Thomson's gazelles differ in the degree of vigilance that individuals display in the presence of predators. Approaching cheetahs chase the least vigilant of the nearest individuals, more often than would be expected by chance. Thomson's gazelles don't give an alarm call when they spot a predator - they display a 'stare posture' and show their conspicuous white tail. The more alert individuals are more likely to spot these signs in others, so looking around attentively (and in other species listening for alarm calls) brings greater advantages than simply spotting predators directly. What also emerges from this example is that effective hunters are likely to be very expert observers of animal behaviour. They are able to detect vulnerabilities in prey that might well be undetectable by the human eye. Very often, predators selectively attack the infirm and sick, as research on the animals killed by grey wolves clearly shows, and such individuals no doubt inadvertently advertise their susceptibility.
It may be that a predator is less able to wreak havoc once it gets within a group of prey - it may take one individual, while the remainder escape. The combined vigilance of a group may be better at spotting competitors, not just predators. Lions, for example, are said to be at their most cooperative when defending their territory against invaders. (There was a glimpse of their cooperation in hunting at 31.26 in the 'Meat Eaters' programme.)
Other advantages have been claimed for group living - such as improving the chances of locating prey (the 'many eyes' argument again), guarding captured prey from attackers, gaining access to a wider variety of prey. Other claimed benefits include gaining reproductive access to members of the opposite sex, enhancing learning in youngsters, for example teaching them to hunt, or being better able to resist harsh environmental conditions. The list of benefits seems impressively long.