Dogs: Why canids rule the world

 

Not even close. Until recently, this accolade belonged to the grey wolf. When its numbers plummeted, succumbing to heavy persecution by humans, the crown passed to the red fox, currently found in 83 countries and on five continents.

 

This species continues to spread both northwards, taking advantage of climatic changes, and into deserts and other inhospitable areas where human populations are now increasing.

 

Throughout its range, the red fox capitalises on the waste, habitat changes and farming activities that are associated with human communities. So as our numbers continue to rise inexorably, we can also expect the fox to expand its range.

 

And what do wolves and foxes have in common? They are both canids, members of the Canidae – the dog family, one of the most successful groups of mammals in the world.

 

 

Canids first appeared about 40 million years ago in North America. Early in their history, a key behavioural trait emerged: the pair bond.

 

Males provision their lactating partners, then also provide for the developing young. This support is critical to their success: it enables mothers to rear large litters – typically up to eight pups, though some species can raise double this number.

 

In contrast, cats (felids) don’t have this pair bond: the male does not support the female, who must rear her kittens – generally only two or three at a time – unaided. 

 

Big litters help canids to survive heavy persecution: typically, they can withstand mortality rates of 70 per cent or higher every year without a decline in population. Cats, with fewer offspring, are much more vulnerable to hunting pressures.

 

 

The other great benefit of the pair bond is that it has enabled the canids to develop complex social systems. Even species such as the red fox, which were once thought to be mostly solitary, can form very big social groups: the largest recorded comprised 10 adults.

 

Generally, these extended families still only produce one, or sometimes two, litters of cubs, with the adults all helping to rear them.

 

This allo-parental care takes many forms: all of the group’s adults bring food to the pups to varying extents – females may even lactate spontaneously to help suckle the dominant female’s pups. These behaviours are all believed to contribute to the successful rearing of large litters.

 

 

Unlike wolves and other species that hunt in packs, red foxes still forage alone; they may spend just a few minutes with other group members each day. But like many other canids, their social life is extremely flexible and adapts rapidly to changing circumstances, as well as different habitats.

 

After sarcoptic mange caused numbers to crash in 1994, foxes in Bristol adopted very different social systems, even though their habitat hadn’t changed. When the population was at its lowest, families consisted of single females with pups, or breeding pairs with very large territories.

 

Then, as numbers recovered over the next 15 years, social groups grew to become small associations of often one male and two females, then several adults of both sexes living in small territories: the pre-mange social system.

 

Mating behaviour also changed in tandem with these factors, from monogamous pair bonds in sparse populations to increased rates of intergroup copulations at high densities. The social systems of other successful canids – coyotes, wolves and jackals – show similar flexibility.

 

 

Clearly, greater numbers are beneficial: bigger wolf packs can hunt more substantial prey, and larger groups of African wild dogs are better at defending their kills from hyenas.

 

These flexible, and highly adaptable, social structures call for a complex system of communication. So canids employ a diverse array of other social behaviours including a range of body postures to signal subtle changes in dominance and submission.

 

They use varied vocalisations, too; red foxes have 20 different calls. Scent marks are also incredibly complex: different chemical signatures identify each animal’s sex, health, status and reproductive condition. And we still have a great deal more to learn about the intricacies of their social organisation.

 

 

Canids aren’t fussy eaters – and that’s another factor in their global success. Most are generalists, feeding on a variety of vertebrates, invertebrates and plants; this dietary flexibility has enabled them to colonise a wide range of environments.

 

Smaller species such as the fennec fox (weighing as little as 1kg) can exploit resource-poor habitats such as deserts. Red foxes have colonised more than 140 cities around the world, and coyotes are common in many North American towns.

 

For the same reason, canids are able to live in areas where the types of food available vary widely throughout the year. For instance, in winter Arctic foxes often spend long periods out on the sea ice looking for the remains of polar-bear kills and other dead marine mammals; during the summer they hunt lemmings and birds on land.

 

 

Those species that are more dependent on large prey have been less able to survive change – for instance, the dire wolf, which disappeared 10,000 years ago. Specialists with limited ranges, such as the Ethiopian wolf, island fox and bush dog, are also at risk.

 

The real generalists, both in terms of their diet and the habitats they occupy, are usually extremely common: red foxes, coyotes and black-backed jackals thrive despite widespread persecution.

 

 

A key factor in the canids’ ability to exploit a wide range of food types is their lack of a highly specialised dentition: they have different kinds of teeth to tackle just about any meal.

 

Cats, in contrast, have large canines and carnassials to kill and chew their prey; most other teeth are vestigial or have been lost – they are not required for their more specialist diets. So cats typically have 30 teeth, dogs 42.

 

Differences in prey are also reflected in the jaws and skull: canid species that specialise in hunting small animals have long, narrow jaws adapted to close fast, rather than bite with great power; their skulls are relatively weak.

 

Those that hunt large, dangerous animals have short, broad jaws, the most powerful bites and very strong skulls to enable them to survive struggles with big prey.