The global population of domestic dogs Canis familiaris
has been conservatively estimated to be 700 million and growing [1
]. Most animals are owned, and in countries or regions with lower levels of socio-economic development, many owned dogs are free-ranging. This reflects a lower ‘Dog Development Index’, which is characterized by limited dog husbandry and investment in their welfare due to owners’ lack of resources and capacity [2
]. Free-ranging dogs pose a threat to human health as vectors of zoonoses, most notably rabies [3
], and other pathogens [4
]. They also threaten wildlife conservation [5
] as competitors with other carnivores for resources [6
], and as reservoirs and vectors of disease [8
], particularly if preyed upon by wild carnivores [2
]. The understanding of dog–human relationships and demographics is important for improved management [10
], and this is most essential in remote and less developed regions where these problems are most prevalent [11
Crucial to the persistence of free-ranging dog populations commensal with humans is the availability of anthropogenic food [5
]. Dogs are efficient scavengers of waste, frequently outcompeting wild species [7
], and even specialists such as vultures [6
]. In rural areas of less developed countries, free-ranging dogs subsist on waste in and around villages, including coprophagy of human faeces [6
]. It has been suggested that as a result of the dispersed and inert nature of this food, free-ranging dogs have little incentive to form cooperative groups [15
]. The nature of human waste may have also played a role in the domestication of wolves Canis lupus
; the ‘self-domestication hypothesis’ proposes that early proto-dogs abandoned the conspecific group hunting of wolves due to the alternative availability of inert human waste [17
This paper is part of a broader study of the ecology, demography, rabies epidemiology, and wildlife interactions of free-ranging dogs in Zimbabwe’s rural communal lands [2
]. Communal lands cover 42% of the country’s land area and are broadly representative of many rural areas in sub-Saharan Africa in terms of dog ecology and dog–human relationships [19
]. In 1990 it was estimated that 71% of Zimbabwe’s dog population lived in communal lands [21
], but this proportion is now likely to be higher due to rapid population growth [19
]. A key question is how the free-ranging dog population continues to grow despite high mortality rates. One untested hypothesis is that a consistent supply of anthropogenic food subsidies enables adult females to reproduce successfully throughout the year, offsetting high mortality [19
The objective of this study was to analyse the diet of free-ranging dogs in a remote area in which the livelihoods of the human population are based on traditional small-scale agriculture. We assessed the supply of food to free-ranging dogs, paying particular attention to the previously unquantified dietary contribution of human faeces. We also assessed possible dietary effects on dog condition and social ecology. Finally, we considered the implications of our results for the management of dog populations in regions where the Dog Development Index is low.
In common with the majority of free-ranging dog populations worldwide [5
], dogs in the study area were predominantly scavengers of human waste: 88% of the dogs’ observed diet consisted of human-derived food, consisting of a wide range of vegetables, fruits, domestic animals, poultry, plus human faeces. With the exception of insects, all food items were inert and none were killed independently by the focal animals, although there were rare cases of predation by other dogs [9
]. Furthermore, 87% of the observed diet was gained independently, and only 13% was deliberately fed by people. This is largely a consequence of the waste disposal practices of the people in the study area. Leftover sadza and other human foods such as cooked vegetables and fruit were usually thrown into pits on the homestead perimeter and were thus easily accessible to dogs. Furthermore, 59% of domestic animal fatalities were left in situ and provided freely accessible dog food.
Human faeces was also readily available because people typically defaecated in the open away from the homestead. Along with carcasses, dogs easily located this food source in the hinterland around homesteads by smell. Human faeces occurred in 56.2% of scats and contributed 20.5% by mass to the observed diet, ranking faeces as the fourth and third most important food item recorded, respectively.
Previous studies have recorded free-ranging dogs exhibiting coprophagy in India [13
] and Ethiopia [14
], but none have quantified the nutritional value of human faeces to the diet, or its supply. Nutritional analyses indicated that fresh human faeces was surprisingly nutritious, yielding 18.7% crude protein and 18.7 KJ/kg gross energy, which exceeded the results for sadza, the human staple (8.3% and 18.5 KJ/kg, respectively). Relative to mammal remains, which was the second most important food item in scats (81.3%) and the most important by mass in the observed diet (48.8%), human faeces was comparable to the lower range for crude protein, but near the maximum for gross energy. In general, up to 75% of the organic component of human faeces consists of undigested fat, protein, fibre, and carbohydrates [29
], explaining its nutritional and energetic value.
By comparison to sadza and human faeces, mammal remains occurred erratically in the diet, perhaps due to their stochastic supply as slaughter offal and mortalities caused by disease and predation [6
]. Vegetables and fruit were highly seasonal in the diet, corresponding to their fruiting and harvesting months. Insects were also seasonal, peaking in occurrence during the wet season. These patterns suggest that dogs consume food types relative to their availability. Hence the consistent occurrence of human faeces and sadza indicates that they are regularly available throughout the year. Based on estimates of faeces production in other communal lands [22
], we calculate that with 92% of households not possessing a latrine or toilet, over 18 months there was potentially 64,800 kg available for consumption by dogs at a density of 1964 kg/km2
, and at a rate of 109 kg/km2
/month. This density is almost three times that for carcass remains in the GCL study area (696 kg/km2
), and four times the density in the adjacent SWRA (484 kg/km2
). Clearly, even based on these crude calculations, human faeces provides a consistent and substantial source of protein and energy-rich food for dogs. The value of this food source is augmented by its inert nature, requiring little effort to secure it, and its ease of location by smell.
The study area’s dog population was highly skewed towards juveniles, indicating high mortality rates. This mirrored the national communal land dog population, where 41% of dogs were aged ≤1 year, with 72% mortality in the first year and a mean life expectancy of 1.1 years [19
]. Despite this high mortality rate, the dog population was estimated to be growing at 6.52%/annum [19
]. One suggested explanation is that the consistent quantity and quality of human-derived food enables adult females to reproduce successfully throughout the year, offsetting the high mortality rate [19
]. Dog population growth is probably also maintained by an expanding food supply, since in 1992 the national human population was growing at 3.13%/annum [19
Most (64.2%) females were in good condition (scores 5 or 4) every month, and adult males fared even better, with 74% in these categories. The slight difference in condition should be expected from the physiological demands of pregnancy and lactation, which are up to four times greater than maintenance levels [30
]. We did not collect detailed information on the nutritional status of the observed adult females, and its influence on their fertility and population demographics. However, the consistently good condition scores of females suggest that the quantity and quality of their food supply may indeed be sufficient to enable high fertility rates, and human faeces makes an important contribution.
The characteristics of the dogs’ food supply could also have influenced their social ecology. The majority (59.9%) of meals were small and consumed in less than 1 min, and largely consisted of sadza, human faeces, and mammal remains. For a larger majority of meals (67.7%) the feeding dog was alone. This suggests that the inert, dispersed, freely accessible, and small size of most food items did not necessitate cooperative foraging, as also suggested for free-ranging dogs in India [16
]. This solitary behaviour is consistent with a plentiful food supply, which obviates the need for group behaviour or territoriality. Although domestic dogs are subject to artificial selection pressures which may modify their social ecology [15
], the commensal relationship between humans and their ‘indigenous’ dogs in remote areas of Africa such as the GCL has probably remained unchanged for millennia [6
]. Hence our study illustrates the conditions that may have encouraged wolves to abandon conspecific group behaviour, due to the availability of inert human waste, elucidating the self-domestication hypothesis [17
The analysis of the dog diet was complicated by the need to quantify scavenged versus potentially predated food, human-fed versus independently-gained items, and the contribution of human faeces. To tackle this we used focal animal observations to estimate the mass of food ingested, which confirmed that scat analysis over-estimated the importance of small items. Scat analysis may also have mis-represented the occurrence of human faeces in the diet, due to our assumption that faeces was present only if sadza and relish remains occurred simultaneously in a scat. It is feasible that these items were directly eaten independently of each other by the dog, resulting in an over-representation of faeces; conversely, faeces may have been consumed that had contained only one or other food item, resulting in an under-representation. Nonetheless, this error was consistent throughout the study, and therefore the a-seasonal pattern of human faeces occurrence in the diet was credible.
Scat analysis showed a wider range of food items than found in the observed diet. For example, 18 wild mammal species were found in scats compared to only five in the observed diet. A possible explanation is that some owners hunted small game such as springhares with dogs, and encroached into the SWRA with and without dogs to set snares [9
], but with the advanced notice of our observation sessions they may have avoided hunting. Hence the scat analysis may give a more accurate quantification of the range of food items eaten by dogs and their owners, while observations provide a better assessment of ingested diet, suggesting that both methods are complementary and necessary when studying free-ranging dogs.
The substantial anthropogenic subsidy to the diet of the study area’s dog population was partly due to the lack of latrines and toilets; only 8% of households possessed one or the other. In a national survey of Zimbabwean communal lands, ownership of sanitation facilities ranged from 10% to 91% of households, with the lowest percentages in remote and under-developed communities [19
]. The contribution of human faeces to dog diet is thus likely to recede in areas experiencing greater socio-economic development. However, this seems unlikely to limit the dog population, given that adult dog condition remains good in communal lands with higher levels of development, indicating a still plentiful and high quality food supply [18
]. In addition, an increased production of edible waste would likely offset the reduced availability of human faeces in more developed areas. Ultimately, a transition may occur when socio-economic development improves the Dog Development Index, enabling owners to increase their investment in dog health and reproductive management, resulting in lower fertility rates [32
The context of our study and its results are probably typical of many rural areas of developing countries where the Dog Development Index is low. In these situations, dogs pose varied threats to wildlife conservation and human health. For example, in Zimbabwean communal lands, dogs compete with vultures for carcasses [6
] and act as reservoirs and vectors of canid disease to humans and wildlife [8
]. However, as illustrated by this study, in these contexts they also provide an important but undervalued service as waste recyclers [34
]. By consuming human faeces, dogs remove potentially harmful bacteria and excrete viruses in their own faeces that maintain immunity amongst local people [35
]. This illustrates the inherent benefits and costs of free-ranging and feral dogs [5
]. Nonetheless, improved sanitation seems unlikely to reduce fertility of free-ranging dogs, because they can switch opportunistically to other anthropogenic food subsidies. Instead, effective vaccination campaigns and reproductive control remain the most appropriate tools for addressing problems associated with free-ranging dog populations [10