What is dispersal?
There comes a time in the lives of most animals when the apron strings are cut, either voluntarily or forcibly, and youngsters strike out on their own. We humans refer to this as “leaving home”, but in the rest of the animal kingdom the process is called dispersal. When the young remain in their parents’ territory it’s known as philopatry.
Over the years, ethologists have proposed several theories to account for why animals leave their parents – after all, striking out on your own can be dangerous, so it would seem to require a good reason. We lack the detailed understanding of sociality in other animals to know whether, as with humans, some youngsters leave because they just don’t get along with their folks. Instead we turn our attention to more empirical explanations.
In 1970, John Christian at the Albert Einstein Medical Center in Philadelphia published a paper in the journal Science in which he reviewed the literature on mammal sociality and evolution. From the various studies he looked at, Christian suggested a direct correlation between aggression and dispersal in mammal populations. In other words, subordinates dispersed because they were driven out by dominant animals. This seemed a logical situation, with dominant animals wanting to secure resources (food, shelter, mates, etc.) for themselves and future offspring. Furthermore, having your young hang around on the territory not only consumes valuable resources, it also raises the potential for inbreeding (i.e. breeding with close relatives), which can genetically “weaken” the population and is thus selected against.
Nearly five decades on from Christian’s analysis, resource protection and inbreeding avoidance are still considered the main reasons for dispersal in animal populations, but the mechanisms behind the emigration appear more complex, variable and subtle than we initially thought. It’s not all about outright aggression by dominant animals, and foxes illustrate this point exceptionally well.
Should I stay or should I go?
The decision to leave home can be a difficult one to make. In the natural world most of what we and other animals do is driven, whether we recognise it or not, by our genes. Natural selection unconsciously favours those individuals that survive to breed, thereby passing their genes to the next generation. Those that choose unwisely can find themselves filtered out of the gene pool. As such, for a behaviour to persist through generations (i.e. to be selected for) we expect that it is beneficial to the individual in question - or, perhaps more practically, that it is not detrimental to the survival or reproduction of the individual. Given that we see examples of both philopatry and dispersal in foxes, we can reasonably assume that each strategy has its benefits.
Most of us can appreciate that staying at home offers the benefit of familiarity – the fox was born on the territory and knows the configuration well by the time it reaches sexual maturity. This may mean a fox has more time to hone its hunting skills in a familiar and relatively safe environment. It may also increase lifespan, although this may not be a straightforward relationship, even in the same population.
A tracking study conducted in Bristol by Stephen Harris and William Trewhella between 1977 and 1986 reported that, for males, life expectancy was significantly reduced in dispersing animals, particularly those that moved considerable distances, suggesting philopatry was the safer option. Similarly, in a 1990 paper to the Journal of Animal Ecology, Tom Woollard and Harris presented tracking data showing that several of their dispersers died before their first birthday, while all those remaining on their natal territory survived to at least a year old.
Intriguingly, more recent data from Bristol, this time collected by Carl Soulsbury and his team, published in 2008, found that dispersing animals were no more likely to die than philopatric ones, despite being wounded more often (see below). To further complicate the picture, a study of 162 foxes in Illinois and Indiana between 1997 and 2001 by Todd Gosselink and colleagues found that, in general, dispersing foxes actually lived longer than philopatric ones. Taken in concert, these data illustrate how the cost-benefit ratio of a decision can vary in different habitats, and even the same habitat at different times.
Philopatry, nonetheless, seems to offer the potential for a longer life and this may, therefore, increase the chance of breeding. Tracking and genetic data from Bristol suggest, however, that there are several factors affecting whether this is actually the case, with the fox’s gender being the most significant.
To breed or not to breed
Soulsbury and his team observed that males were unlikely to breed in their first year, whether they stayed on the natal territory or not, while females were likely to miss out on a breeding season if they dispersed. This implies that philopatry has fitness benefits for females during their first year of life. More recent analysis looking at the genetic relatedness of the Bristol population suggests the situation is actually more complicated.
In a fascinating paper to PLOS One in 2011, Helen Whiteside and colleagues presented the results of their analysis of a long-term dataset of the city’s foxes. The researchers compared cub relatedness within the group to its social status and probability of dispersal. How closely related a cub was to the dominant male didn’t affect whether or not it dispersed. Instead, a key factor in explaining dispersal was the cub’s relatedness to the dominant female. Male cubs born to dominant vixens were much more likely to disperse than those born to subordinate ones. The opposite was true for females, with daughters of dominant vixens more likely to be philopatric.
Whiteside and her team suggest that unrelated subordinate males aren’t in direct reproductive competition with the dominant dog; dominant males monopolise mating within the group, forcing subordinates to look for mates outside the territory. As such, low-ranking males are more likely to be tolerated and adopt philopatry. Conversely, it seems that males born to dominant females have greater potential to mate with their mothers and may disperse to prevent inbreeding.
At this point, it is worth mentioning that this may be a situation unique to foxes living in urban areas, where resources are generally plentiful. Other authors, including David Macdonald, working with foxes on Boars Hill in Oxford, have noted how it is rare for there to be more than a single male in a family group, which may indicate that only dominant animals breed in rural areas. Furthermore, Dennis Voigt tracked radio-tagged cubs in a litter in Ontario, Canada between 1974 and 1982 and found evidence that dispersal may not always prevent inbreeding. One October, a male dispersed and set up a territory 10km (6 mi.) away, while a vixen dispersed in a different direction. The following month, the sister ended up in her brother’s territory and they reared cubs together the following year.
The situation for vixens is different and Whiteside and her co-workers suggest that, by retaining her female offspring, a dominant vixen makes it more likely one of them will inherit the territory in the event of her death or becoming barren, increasing the potential for her genes to continue down the generations. By contrast, it’s costly to retain unrelated subordinate females because they compete with her offspring for future dominance and mating opportunities. Carl Soulsbury’s team noted that females were significantly more likely to become dominant if they dispersed than if they stayed with their parents; breeding is less strongly associated with dominance for vixens than for males, however, particularly in high quality habitats, so attaining high social status may be less of a priority for females. In some cases, a son or daughter may “overthrow” their parent(s) and take over the group, but such instances appear the exception rather than the rule.
So, from a cub’s perspective, if we accept the goal is to leave as many offspring as possible to succeed you, for males the costs of philopatry outweigh those of dispersal, even though striking out alone may be risky. Staying at home means you’re very unlikely to breed and even if you do, there’s potential for inbreeding because the females in the group are already relatives. For vixens, by contrast, dispersal makes most sense if you’re subordinate; otherwise, philopatry seems to offer the most potential breeding opportunities with the lowest risk. Furthermore, for cubs of dominant mothers, staying in the family group may also offer the potential to accrue some childcare experience by helping to raise younger brothers and sisters which also carry some of your genes. For dominant adults, the greatest benefit (in terms of the “inclusive fitness” of the group) comes from retaining female offspring and compelling males to disperse. Indeed, this trend for dispersal by males and philopatry in females is what we typically see.
In his study of foxes in Hillingdon, west London, R.J.C Page recorded that 85% of males dispersed, while only 20% of females left their natal ranges. Harris and Trewhella, in their 1988 study, found that, by the end of their second year, 73% of male and 32% of female cubs had dispersed. In North America, Gosselink and colleagues observed that 70% of their male subjects dispersed, while most females (64%) stayed on their home territory. Similarly, recovery of foxes in North Dakota by Steve Allen and Alan Sargeant showed that, after eight years, 97% of males and 52% of females had dispersed from the area in which they were tagged as cubs. Working in Iowa, Robert Phillips and his co-workers found a clear tendency for more one- and two-year-old males than females to leave their natal areas.
Dispersal in adults has been reported. Harris and Trewhella, for example, recorded 35% of adult males and 29% of females dispersing. In his 1980 book Red Fox, Huw Lloyd notes that settled adults in the UK and Europe occasionally disperse and appear more likely to do so in North America. In rare cases dispersal happens while still a cub (i.e. less than 26 weeks old). Harris and Trewhella found a male cub almost 4.5km (2.7 miles) and a female cub 2km (1.3 miles) away from the earth, but 93% of their subjects remained within 700m (~ half a mile) of the earth.
A risky business
We’ve already seen that, in some cases at least, dispersing animals have shorter life expectancies than philopatric individuals. Some of this mortality occurs just because the animals are moving around more, often over unfamiliar ground, which puts them at greater risk of being run over, killed by hunters, or succumbing to some other misadventure. An additional danger comes from the fact that foxes are territorial and a dispersing animal will often pass through already occupied territories while looking for a place to settle, putting them in conflict with the current owner.
In their 2008 paper to Behavioural Ecology & Sociobiology, Carl Soulsbury and colleagues described how dispersing foxes suffered more bite wounds than those remaining on their natal territory. Most injuries were located on the muzzle and were minor, suggesting they were the result of ritualised fighting. Soulsbury and his team did also record some more serious injuries and, although comparatively rare, they were more common in dispersing animals. While these injuries didn’t seem to affect the individual’s survival, they do illustrate how dispersers put themselves in conflict with neighbouring territory holders and that dispersal holds the potential for increased risk.
How dangerous dispersal is for an individual varies according to the density of foxes and how neighbouring groups operate their territory. As we’ve seen previously, not all foxes approach territoriality in the same way, with some defending only core areas or feeding sites and rarely visiting the boundaries. Furthermore, we know from the meticulous observations of David Macdonald that territoriality is often more important to the dominant dog and that they tend to tolerate females trespassing while males are violently expelled. As such, a dispersing vixen may be at less risk of violent conflict than a dispersing male. This perhaps helps explain observations in Bristol that male cubs are almost one-fifth heavier than female cubs and that bigger foxes tend to be bolder, travel further and more likely to secure a territory for themselves.
The final straw – triggers for dispersal
We’ve seen that the decision to disperse or not is probably based around the potential to breed, but it’s unlikely fox cubs know from the outset whether they’re likely to produce young of their own. Instead, the desire to stick around or leave is likely triggered by other behavioural interactions and the data collected to date suggest there are two main drivers: food and social contact.
Food for thought
David Macdonald’s studies on the foxes in Oxford led him to the conclusion that not all group members had equal access to the richest feeding grounds because they were harassed by more dominant animals. As such, these individuals often found themselves restricted to feeding in a small, less productive part of the territory that more dominant members of the group rarely used. Similarly, on his study site in rural Sweden, Torbjorn von Schantz observed that only females with access to hunt at the rabbit warrens on the territory successfully reared cubs and that this area was the preserve of the dominant animals in the group.
The restriction of access to resources seems the most plausible driver for dispersal in many populations and probably all rural fox groups, where resources are likely to be reduced in comparison to urban areas. Indeed, in North America, Gosselink and his team observed that urban foxes were more likely to show philopatry (60% staying at home), while rural animals were more likely to disperse (only 37% remaining). Similarly, in their Bristol study, Harris and Trewhella found that foxes in low quality habitat were more likely to disperse than those in high quality areas. They also observed that animals of both sexes were more likely to disperse from large litters than smaller ones, and this presumably reflects the greater potential for resource conflict among large families.
In areas where resources are plentiful, such as in many of our cities, exclusion from the best feeding sites is unlikely to be a significant driver of dispersal. Indeed, in their 1990 paper to the Journal of Animal Ecology, Tom Woollard and Stephen Harris noted:
“… competition for food is unlikely to initiate dispersal in Bristol, since food does not appear to be a particularly limiting resource. In this study, exploratory trips outside the natal home range were not made to specific food sources such as rubbish tips or particularly productive gardens, and on exploratory trips the foxes spent little time foraging. Instead, they appeared to be simply exploring new areas and in the process were covering as much new ground as possible.”
Indeed, Carl Soulsbury’s Bristol study found that dispersing individuals weren’t more nutritionally stressed than philopatric foxes. In such situations, it appears that social contact affects whether an individual remains or disperses. This is not a new idea and in a paper to the American Naturalist in 1977, dog behaviourist Marc Bekoff proposed the “social cohesion hypothesis”, whereby individuals with fewer social interactions within the group are more prone to dispersing. Data from Bristol suggest this is the case for the city’s foxes.
Bridge to your heart
In a paper to the journal Animal Behaviour in 1992, Stephen Harris and Pirian White presented their data on social interactions among fox family groups based on the amount of chewing recorded on ear tags used to individually mark the animals for study. Overall, females received more grooming than males, indicating that they were more closely bonded to the social group, and resident adults were groomed to a greater degree than dispersing ones by other adults, suggesting adults with looser social bonds with the rest of the group were more likely to disperse. Cubs groomed adult females more than the adult males in the group and all their future non-dispersing littermates more than those that dispersed – this was consistent only for male cubs. In the paper, Harris and White explain:
“Hence there was a strong relationship between affiliative behaviour between littermates in the first 13 weeks of life and future dispersal behaviour of male foxes, with the animals that disperse having the least social bonding with their littermates.”
The observation that this strong negative relationship between grooming behaviour with littermates during the first 13 weeks of life and future dispersal was true only for males implies that social factors acting on dispersing vixens happen later in life.
Gosselink’s study in east-central Illinois and western Indiana observed a phenomenon similar to that described by Harris and White. In their 2010 paper to the Journal of Wildlife Management, they wrote:
“Philopatry was associated with higher home range overlap among family members, and closer location distances among family members throughout the summer relative to foxes that dispersed, suggesting that family members located closer together dispersed less frequently.”
One final aspect to touch on is the division of territories, which may result in an individual acquiring their own range without either dispersing or inheriting. In a 1998 paper in the journal Animal Behaviour, a team of Bristol University biologists led by Phil Baker described how two vixens in their study population attained dominant status when their natal territory was divided; their parents using one half and the daughter using the other. It is unknown how often such division happens, but it’s thought to be a response to a significant increase in resources (i.e. food) within the territory. In this particular case, the territories split when the available food exceeded 40 MJ per group per week.
Dispersal behaviour in foxes
Humans approach leaving home in different ways according to their personality and situation. Some just pack a bag and get on the next train or bus, while others may plan more carefully, visiting several places and weighing up the pros and cons before choosing where to settle. Radio-tracking studies of foxes suggest something similar and in his book, Red Fox, Huw Lloyd states:
“It’s clear that some males and females disperse suddenly and settle quickly, whereas others precede dispersal by reconnaissance extending over periods of weeks before the final move away is made.”
Most dispersal appears to take place between September and February, although there are reports of it starting as early as July in the rare cases where cubs leave the group. Dispersal can be a protracted process, varying with habitat and between years. Gosselink and his colleagues working in Illinois and Indiana, for example, found that littermates dispersed at about the same time, but went off in different directions. Conversely, in North Dakota, Steve Allen and Alan Sargeant found varying dispersal times, but with a tendency for littermates to disperse in the same direction. Similarly, Harris and Trewhella observed also observed a tendency for littermates to head off in the same general direction. Regardless of timing and direction, males generally leave before females.
In their 1990 study, Tom Woollard and Stephen Harris radio-tracked 21 subadult male foxes to investigate their dispersal behaviour and found at least four distinct dispersal patterns. Three foxes dispersed by making several exploratory trips and then oscillating between new area and their old home range. Two individuals moved into a temporary range before settling elsewhere, while another two made one-off movements with no exploratory trips directly to a new range. The final two simply shifted their ranges to neighbouring areas. For most of the time, the speed at which the foxes moved each night was not different between dispersers and non-dispersers, except during exploratory trips, when dispersers moved faster than non-dispersers on a normal night's activity. Overall, the main difference in the behaviour of dispersers vs. non-dispersers was that the former spent more time lying up outside of their natal range, while the latter seldom, if ever, left their ranges, and spent most of their time lying up in core areas.
In his 1987 book Running with the Fox, David Macdonald described similar behaviour to that observed by Woollard and Harris, noting how some foxes upped sticks and left suddenly; some left after a series of excursions; some oscillated between two ranges before settling on one; some wove an erratic trail across the countryside; others travelled fast and far in roughly straight lines. What causes these different types of dispersal is unknown, but it could be down to a combination of the distribution of resources, local fox density and the personality of the individual. Whatever the reason, it probably explains the observations of Gosselink and his co-workers that the average time between leaving the natal range and settling down was between two and 114 days (almost four months). Urban animals, it appears, took longer to settle than rural ones, which perhaps reflects availability of suitable unoccupied habitat. The researchers also witnessed similar behaviour to that described by Macdonald and Woollard and Harris, writing:
“Our study revealed a host of behavioral patterns during dispersal, ranging from foxes with rapid, erratic movements over short periods, to foxes that dispersed over long periods with several resting periods between movements.”
Making tracks. Do foxes use railway lines to disperse?
Generally speaking, animals don’t waste energy if they can avoid it and this means they tend to use the same paths on successive occasions – taking the “path of least resistance” so to speak. Indeed, in Running with the Fox, Macdonald tells how when he drove his vehicle across grassy fields, foxes would use the resulting “tramlines” of the tyres to move across it afterwards, rather than blazing their own trail. Indeed, in my experience, foxes will often use human, deer and badger tracks through woodland and farmland, and there is an oft-cited statement that foxes will move along railway cuttings as an easy way to transverse the environment. Again, I have observed foxes near railway lines hunting rabbits, but the jury is still out as to how significant they are as “fox highways”.
During the late 1970s and early 80s, R.J.C Page and Hugh Kolb, working in west London and Scotland respectively, reported that railway lines affected how far and in what direction dog foxes dispersed in the cities, proposing that foxes used them as dispersal corridors. In Edinburgh, Kolb observed that itinerant foxes (those without a territory) in particular used railway lines to move around. This behaviour does not, however, appear universally applicable, as William Trewhella and Stephen Harris discussed in a paper to the Journal of Zoology in 1990.
Trewhella and Harris looked at capture-mark-recapture data from 1,387 foxes tagged in Bristol and found no difference between the population density in the “railway” and “non-railway” parts of the city. There were also no significant differences between the proportion of foxes dispersing, the mean dispersal distance, or the mean length of long distance dispersal movements (i.e. those greater than 5 km/3 mi.) of foxes first captured in railway and non-railway areas. Finally, the biologists failed to find any significant difference in dispersal direction in railway vs. non-railway areas, which we’d expect if dispersal was facilitated by tracks radiating out from the city. Overall, Trewhella and Harris concluded:
“Obviously, individual foxes may use lines for dispersal, but it is equally true that dispersing foxes will pass freely along roads, through gardens and across open spaces in urban areas, without the need for any linearly orientated habitat feature.”
Escape to the country?
It is not uncommon to hear people talk of “urban” and “rural” foxes as separate entities. Many times I have heard it said that urban foxes are city slickers no longer able to find food for themselves and, hence, unable to survive in the countryside. Indeed, I have even come across the suggestion that urban and rural foxes are separate species. This is simply not the case, and tracking studies clearly show that foxes move between urban and rural environments, both on a nightly basis to feed/rest and when dispersing.
In Running with the Fox, David Macdonald wrote:
“Indeed, several of the cubs that we ear-tagged in the town eventually set up home after dispersing to the countryside. Other foxes spent their days on farmland and commuted into town at night, or vice versa.”
In Illinois and Indiana, Todd Gosselink and his co-workers observed 77% of urban foxes disperse into rural areas, while 4% of rural foxes moved into the urban areas. Similarly, genetic analysis by Peter Wandeler and colleagues, published in 2003, revealed higher than expected urban-rural geneflow during their study of foxes living in the city of Zurich in Switzerland. Their dataset show urban-rural geneflow at a similar level to rural-rural geneflow, indicating that town and country foxes were intermixing to about the same degree as country foxes mixed with other rural foxes. As Carl Soulsbury and co-authors put it in their chapter on the Red fox in Urban Carnivores:
“… the widely purported belief popularized in the national press in Britain that the “town” or “city” fox is different from its rural counterpart is a fallacy.”
It should be noted that, despite geneflow between urban and rural populations, urban areas are typically richer, meaning they can support more fox territories than rural locations and, as such, dispersing urban foxes may not need to move as far (see below). In addition, heavily industrialised areas are broadly unsuitable for foxes and, as Nick Robinson and Clive Marks noted in their tracking study of foxes in Melbourne, areas of dense housing and/or industry may act as a barrier to dispersal.
Far enough? Dispersal distances in foxes
We’ve seen that dogs are more prone to dispersal than vixens and not only do they disperse earlier, they also tend to move further, with females typically moving much shorter distances than males. In Bristol, for example, Helen Whiteside and her colleagues found that when vixens disperse they often move into neighbouring groups, resulting in high levels of intergroup relatedness across adjacent territories. Similarly, on Boars Hill in Oxford, David Macdonald observed that vixens within a group often shared unusual morphological traits, such as white toes, suggesting such groups are composed at least partly of related animals. Macdonald did, nonetheless, note that vixens didn’t appear to integrate into territories where another female was resident; instead they seemed to pair with a lone dog who held a territory. Again, this illustrates the variable and sometimes complex nature of fox society.
Making a bee-line
The longest straight-line dispersal distances have been reported in North American foxes. The current record is held by a five-month-old dog fox tagged as a cub in Wisconsin during August 1962 that was shot by a hunter in Montgomery County in Indiana during May the following year, having travelled a straight-line distance of 394km (245 miles) – this is the UK equivalent of travelling from London to Newcastle. Given that it’s unlikely the fox took a direct route, it seems probable the young male walked many more kilometres. In roughly the same time, another male cub from the same litter was recaptured 274m (300 yds) away from the earth.
During their study of foxes in North Dakota, Allen and Sargeant recorded straight-line dispersal distances of between zero (i.e. no or very little dispersal) and 302km (187 mi.). Interestingly, although males moved further on average, the three longest dispersals (246km/153 mi., 274km/170 mi. and 302km) were all made by vixens. Meanwhile, Robert Phillips and his colleagues observed average dispersal distances for males and females of 29.6km (18.4 mi.) and 10km (6.2 mi.), respectively, in Iowa and Illinois. These researchers found 14 juvenile males and one juvenile female more than 80km (50 miles) from their natal range.
Radio-tracking 14 fox cubs around Melbourne in Australia, Nick Robinson and Clive Marks found less of a difference between the sexes, with average straight-line distances of 3.5km (2.2 mi.) for males and 2km (1.2 mi.) for females and the longest dispersal being just over 6km (4 mi.) made by a male. Their genetic data supported the tracking, indicating that foxes found it difficult to disperse within urban Melbourne.
In a paper to Acta Zoologica Fennica in 1984, Dennis Voigt and David Macdonald compared the dispersal distances of foxes in Ontario with those in Oxfordshire and noted:
“... while most males in Ontario dispersed straight-line distances of about 30 km [18.6 mi.] during 15 days, no male in Oxfordshire was followed that far.”
Indeed, although some considerable dispersals have been recorded in the UK (in Ireland, James Fairley recorded trips of up to 37km/23 mi., for example) most urban animals move 1-5km (0.6-3 mi.), while rural animals move 10-25km (6.2-15.5 mi.). This distance is dependent primarily on two factors: resource availability and fox density. Hence, in the Welsh hills, Huw Lloyd found the average straight-line distance travelled by dogs and vixens was 13.7km (8.5 mi.) and 2.3km (1.4 mi.), respectively, while in Welsh farmland the figures were 6km (3.8 mi.) and 2km (1.2 mi.), respectively.
Fewer foxes, more dispersal?
It is perhaps relatively easy to understand how resource availability affects dispersal distance. For example, the more food around, the smaller a given territory needs to be, the more territories that can fit into a given area and thus the shorter the distance individuals need to move to settle down. The observation that dispersal probability is negatively correlated with population density, in other words that foxes are more likely to disperse when there are fewer other foxes around, is perhaps contrary to what we might expect, though. It’s worth remembering, however, that dispersal is not all about resource availability and is an inherently risky activity. As such, dispersing when the population density is low presumably increases the chance of being able to secure a territory without having to fight another fox for it.
In his 1984 book Urban Foxes, Stephen Harris noted that dispersal distance was also negatively correlated with population density (the more foxes in the population, the shorter the dispersal), although Allen and Sargeant didn’t observe this in their North Dakota population. In such cases, presumably low density reflects poorer quality habitat, which necessitates larger territories and correspondingly larger dispersal distances, and vice versa.
So, the social system of the red fox is complex and dispersal is flexible in response to the conditions that individuals experience in a given habitat at a given time. This makes accurate generalisation difficult. Nonetheless, we can conclude that, broadly, field studies suggest most males disperse, while most females are philopatric; that most foxes disperse during the first two years of their life; and that males tend to move further than females.