Are carnivores really brainier than their prey?
(writing in progress)
Most educated lay persons believe that, because predators must outwit their prey, specialised carnivorous mammals are smarter than the herbivorous mammals they eat.
Even experienced mammalogists might agree. Who would doubt that wolves are more intelligent than bisons? Or that the Canada lynx has a proportionately bigger brain than its staple prey, the snowshoe hare?
However, this pattern doesn’t stand up to scrutiny overall – especially when it comes to Australia’s extinct carnivorous marsupials.
Surprisingly, it is not necessarily true that specialised carnivores are brainier than specialised herbivores. Instead, as a general rule mammalian predators and prey tend towards parity in contests of brain size relative to body size. And the particular braininess of exceptional carnivores – such as the African hunting dog and the wolverine – has evolved not as much for killing prey as for avoiding being killed by bigger, competing carnivores such as lions and wolves.
The braininess of a given species can be quantified by the encephalisation quotient (EQ) – as discussed in the article ‘Of bandicoots, beaks and braininess’ on p. 31 of WIldlife Australia Magazine Vol 55 No 2. This number (defined as an average among adult individuals of both sexes) reflects the mass of the brain relative to the mass of the whole body, calculated by means of an exponential formula allowing for the fact that the brain tends to become proportionately smaller as body size increases among species.
The average EQ for mammals is by definition 1.0. Species with superior braininess score greater than 1.0, while species with inferior braininess score less than 1.0. Human EQ averages 5.7, which is about 6-fold brainier than the average mammal. The least brainy mammals have EQ values of only 0.3 – which means that the human species is about 8.5-fold brainier than e.g. tenrecs and the most decephalised rodents or marsupials.
Braininess in carnivorous Australians
In the recent fauna of marsupials in Australia, several species have qualified as carnivores in the sense that the staples killed by them consisted of herbivorous marsupials. These are the thylacine, the Tasmanian devil and the spotted-tail quoll. Although the marsupial lion went extinct too long ago to be eligible here, we can also mention its EQ.
Measurements of the cranial capacities suggest that these carnivorous marsupials were more or less on a par with living and extinct kangaroos and wombats in braininess. Instead of having EQ greater than that of their prey, the thylacine and the marsupial lion had EQ of only about 0.7. The Tasmanian devil (Sarcophilus harrisii) has EQ about 0.36, while the spotted-tail quoll (Dasyurus maculatus) has EQ about 0.47.
However, some doubt may remain about the broader picture, because our knowledge of marsupial carnivores is so limited. What is the pattern among carnivores and herbivores on other continents?
Does hunting require smarts?
Some carnivores are brainier than their staple prey, but it remains dubious
whether this has evolved to boost their success in hunting. It is once the
prey is down and the bigger carnivores arrive as freeloaders that extra
brains are likely to count in favour of certain relatively small carnivores.
And, because the big carnivorous marsupials in prehistoric Australia were
limited to a few, non-gregarious species, there seems to have been no
evolutionary pressure boosting braininess in the case of our Australian
‘underdog’, the thylacine. In order to catch kangaroos and wallabies, all
the thylacine needed was a certain combination of stealth, speed and
bite-power – as opposed to any special brain-power.
The same seems to apply to the relevant co-members of the Dasyuroidea
superfamily of marsupials. Dasyurids smaller than the spotted-tail quoll, which do
not eat herbivorous mammals as staples, are relatively brainy. For
example planigales (EQ = 0.7–0.8), phascogales (EQ = 0.7–0.9), and
antechinuses ( EQ = 0.9–1.0) are encephalised compared with larger species
of quolls and the Tasmanian devil. The numbat (Myrmecobius fasciatus) is
likewise on the brainy side (EQ = 0.8), despite its extremely specialised
diet of termites.
Big cats have more or less average braininess for mammals, with EQ about 1.0. Since antelopes and deer likewise have average braininess for mammals, predator and prey turn out to be approximately evenly matched. One way to explain this parity is that felids hunt hoofed mammals mainly by stealth rather than by any particular trickery.
The spotted hyena often hunts in addition to scavenging, and formerly ranged over a vast range from the southern Africa to Europe and China. EQ values for this carnivore are rather inconsistent but they indicate braininess somewhat superior to that of big cats. Among the usual prey of the spotted hyena the closest match in braininess seems to occur in zebras, which also somewhat outscore the average mammal in EQ (see ‘Not so black and white’ on p.7 of WAM Vol 54 No. 4).
Brawn instead of brains?
An intriguing pattern is that the biggest hoofed mammals vulnerable as adults to carnivores show reduced braininess for unknown reasons, with EQ less than 0.5 in giraffes, rhinos, hippopotami and big bovines. When the lion hunts mature male giraffes – which happens surprisingly often - the extreme vigilance conferred by a long neck seems to be undermined by below-par braininess in the prey species. And even in outsize relatives of antelopes, such as the water buffalo and the African savanna buffalo, it seems true that carnivore beats herbivore in braininess where big cats or the spotted hyena hunt these hoofed mammals far bigger than themselves. This decephalisation of big bovines remains puzzling. For example, where wolves have learned to hunt bisons, these predators score up to 1.5-fold the average braininess for mammals – compared with only half this average in the prey. This disparity is unexplained by fecundity because big bovines reproduce at the rate predicted for ruminants of their body size. My tentative explanation is that big bovines have an exceptional ability to injure carnivores by gregarious defence.
When lion, leopard, cheetah or spotted hyena attack warthogs, the carnivores have a distinct advantage in terms of braininess.
Two other groups of herbivores are also inferior to carnivores in braininess: warthogs (Phacochoerus spp.) and hares (Lepus spp.). Warthogs are unusual among pigs in eating mainly grass; they have EQ only about 0.6, compared with the 1.0 of antelopes and deer. Perhaps this shortfall in braininess is because pigs survive by breeding rather than by thinking: the litter size of pigs exceeds that of like-size antelopes and deer. Whatever the reason, it is indeed true that, when lion, leopard, cheetah or spotted hyena attack warthogs, the carnivores have an advantage in terms of braininess.
This trade-off of fecundity vs braininess seems equally relevant to hares. The average EQ for this group is 0.7, inferior to the value of 1.0 for small antelopes. Hares can make up for predation by means of rapid reproduction: their gestation lasts less than two months, compared with at least four months in the hare-size smallest antelopes. So it makes sense that the Canada lynx (EQ = 0.95) is brainier than its staple the snowshoe hare (EQ = 0.74). However, there are few ecosystems in which hares – which play bit-parts in food-webs dominated by hoofed mammals - are major prey for big carnivores.
A converse trade-off may explain why those primates specialised for eating greens excel in EQ compared to other herbivores. Herbivorous monkeys are fair game in several ecosystems, such as Indian savannas with their abundant, semi-terrestrial langurs. But an extreme example is the gelada (EQ = 2.9), a grass-eating relative of baboons in the mountains of Ethiopia, which is nearly three-fold brainier than its predator the leopard. The gelada, like most primates, breeds far more slowly than do like-size hoofed mammals; so it makes sense that the gelada compensates with smartness unthinkable for any antelope.
The carnivores mentioned so far have been about average in braininess for mammals. But two groups, namely the hunting dogs and the wolverine, are particularly brainy and demand additional explanation.
The biggest and most social of wild canids, such as African hunting dog, dhole and wolves, tend to depend on hoofed mammals for food. All are brainier than felids or hyenas, with EQ exceeding 1.3. The brainiest, namely the African hunting dog, is nearly twice as brainy as the average mammal, twice as brainy as many antelopes and deer, and considerably more brainy than wolves. Could these species – which collectively occurred over much of the world – prove that some carnivores have evolved to outsmart herbivores in an adaptive ‘arms-race’?
Fighting like cats and dogs
One obvious correlate with the superior braininess of the specialised carnivorous canids is their gregarious hunting. However, both the lion and spotted hyena also often hunt by teamwork despite their modest braininess for mammals. So could it be that the selective pressures shaping the brainiest canids have been exerted not so much by their prey and more by their own ‘predators’: the coexisting carnivores brawnier than them, some of which are gregarious to boot?
Carnivorous species are unlike herbivorous species in that they seek every opportunity to kill each other, even if they do not deign to eat each other. Furthermore, they compete for food far more hectically than herbivores do, including stealing kills from each other. So, in African savannas for example, the various big cats and the African hunting dog are at each other’s throats, with even the lion vulnerable to groups of the spotted hyena. In this carnivore-on-carnivore ‘arms race’, each species needs some combination of force, numbers, evasion, fecundity and deception to survive - as if the killing of herbivores is the easy part of their lives.
The lion relies on force; the spotted hyena, although less fecund than the lion, can outnumber it while deploying fearsome teeth; the leopard relies on climbing; and the cheetah evades with speed but also outbreeds all the other big cats. For its part, the African hunting dog – by far the smallest in this community of killers - arguably relies on experience and learning as well as a fecundity outranking even the cheetah. Although the dhole lived beyond the range of the spotted hyena, it is smaller again (15 kg) than the African hunting dog (25 kg) and thus most vulnerable, particularly to the tiger.
The intermediate braininess of wolves seems to make sense accordingly. Unlike African hunting dog and dhole, wolves did not generally coexist with the biggest cats. When Europeans colonised North America, the biggest specialised carnivore threatening wolves was the puma, which is solitary and hardly bigger than the leopard. The range of wolves did overlap with that of the spotted hyena in parts of Eurasia. However, the prehistoric extermination of the spotted hyena beyond Africa makes it hard to know if there was a segregation by habitat.
In the far North where wolves hold sway to this day, the wolverine – although not a canid – is the ‘underdog’. This relative of weasels turns out to be brainier than wolves with EQ about 1.6. Although the wolverine is brawny relative to other members of its family, its body mass of about 15 kg limits its force in any contest with a group of wolves.
Evolutionary pressures on braininess
So what is my verdict on the semi-popular assumption that mammalian carnivores tend to be smarter than their prey? Well, the cerebral advantage of the predators – however intuitive it may seem - hardly stands up to scrutiny overall. It holds for the few situations where particularly fecund herbivores are the main prey. However, it is easy to overlook converse situations featuring herbivorous primates – which would outsmart even the brainiest canids.
Antoni still needs to sort this out: Where wolves have learned to hunt bisons, the predator makes up for its limited body size by scoring up to 1.5-fold the average for mammals – compared to the bovine which scores only half this average. Another situation is where hares are the main herbivorous prey. However, these are not necessarily more common than situations where herbivorous primates – which can be brainier than even the brainiest canids - – occur. Greens-eating monkeys are prey in several ecosystems, such as Indian savannas with their abundant, semi-terrestrial langurs (Semnopithecus spp.).
In those situations where carnivore seems superior to herbivore in braininess, what deserves further scrutiny is the evolutionary pressure for encephalisation in the carnivores. I suggest that even the Canada lynx, with its hare-brained prey, needs to outsmart puma and wolf, which have murderous intent even if in spite rather than in hunger.
Main reference:
see Boddy et al. (2012) http://onlinelibrary.wiley.com/doi/10.1111/j.1420-9101.2012.02491.x/full and the supplementary data in its Table 2
READING ?Boddy AM, et al. 2012. Comparative analysis of encephalization in mammals reveals relaxed constraints on anthropoid primate and cetacean brain scaling. Journal of Evolutionary Biology, 25(5): 981–994. ?Ashwell KWS. 2008. Encephalization of Australian and New Guinean marsupials. Brain, Behavior and Evolution, 71(3): 181–199. ?Wroe S. 2003. Australian marsupial carnivores: Recent advances in palaeontology in Predators with Pouches: The Biology of Marsupial Carnivores, (M. Jones, C. Dickman & M. Archer eds), CSIRO Publishing, Collingwood.
Antoni still needs to sort this out:
In summary, some carnivores are brainier than their staple prey, but it remains dubious whether this has evolved to boost their success in hunting. It is once the prey is down and the bigger carnivores arrive as freeloaders that extra brains are likely to count in favour of certain relatively small carnivores. And, because the community of big carnivores in prehistoric Australia consisted of just two, non-gregarious species, there seems to have been no evolutionary pressure boosting braininess in the case of our Australian ‘underdog’, the thylacine. In order to catch kangaroos and wallabies, all the thylacine needed was a certain combination of stealth, speed and bite-power – as opposed to any special brain-power.
I don’t think we can truly call Thylacoleo and the thylacine the largest without clarification. Thylacoleo was the largest of its time, but the modern Thylacine wasn't the largest within its genus.
What about the carnivorous kangaroo (Propleopus oscillans)? and Whollydooleya tomnpatrichorum? http://www.nationalgeographic.com.au/animals/aussie-scientists-discover-new-carnivorous-marsupial.aspx Best to stipulate that this is the case during the Pleistocene but not necessarily earlier.
There are seven extinct species of Thylacinidae, with the largest, Thylacinus potens, growing to around wolf size, but most died out in the Micoene, so if we stick with the Pleistocene we should be safe.
This statement might be a bit broad, Antoni, and probably needs some clarification. Prehistoric Australia also included carnivorous kangaroos (Balbaroo fangaroo) as well as Megalania etc.
I don’t think we can truly call Thylacoleo and the thylacine the largest without clarification. Thylacoleo was the largest of its time, but the modern Thylacine wasn't the largest within its genus.What about the carnivorous kangaroo (Propleopus oscillans)? and Whollydooleya tomnpatrichorum? http://www.nationalgeographic.com.au/animals/aussie-scientists-discover-new-carnivorous-marsupial.aspx Best to stipulate that this is the case during the Pleistocene but not necessarily earlier.
There are seven extinct species of Thylacinidae, with the largest, Thylacinus potens, growing to around wolf size, but most died out in the Micoene, so if we stick with the Pleistocene we should be safe.
This statement might be a bit broad, Antoni, and probably needs some clarification. Prehistoric Australia also included carnivorous kangaroos (Balbaroo fangaroo) as well as Megalania etc.
(writing in progress)
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