Bio-Economics of Large-Scale Eradication of Feral Goats From Santiago Island,Galápagos

ABSTRACT Invasive mammals are premier drivers of extinction and ecosystem change, particularly on islands. In the 1960s, conservation practitioners started developing techniques to eradicate invasive mammal populations from islands. Larger and more biologically complex islands are being targeted for restoration worldwide. We conducted a feral goat (Capra hircus) eradication campaign on Santiago Island in the Galápagos archipelago, which was an unprecedented advance in the ability to reverse biodiversity impacts by invasive species. We removed >79,000 goats from Santiago Island (58,465 ha) in <4.5 years, at an approximate cost of US$6.1 million. An eradication ethic combined with a suite of techniques and technologies made eradication possible. A field-based Geographic Information System facilitated an adaptive management strategy, including adjustment and integration of hunting methods. Specialized ground hunting techniques with dogs removed most of the goat population. Aerial hunting by helicopter and Judas goat techniques were also critical. Mata Hari goats, sterilized female Judas goats induced into a long-term estrus, removed males from the remnant feral population at an elevated rate, which likely decreased the length and cost of the eradication campaign. The last 1,000 goats cost US$2.0 million to remove; we spent an additional US$467,064 on monitoring to confirm eradication. Aerial hunting is cost-effective even in countries where labor is inexpensive. Local sociopolitical environments and best practices emerging from large-scale, fast-paced eradications should drive future strategies. For nonnative ungulate eradications, island size is arguably no longer the limiting factor. Future challenges will involve removing invasive mammals from large inhabited islands while increasing cost-effectiveness of removing low-density populations and confirming eradication. Those challenges will require leveraging technology and applying theory from other disciplines, along with conservation practitioners working alongside sociologists and educators.     

Feral donkey (<Emphasis Type="Italic">Equus asinus</Emphasis>) eradications in the Galápagos

Introduced herbivores are major drivers of ecosystem change and biodiversity loss, particularly on islands. Tools and techniques now exist to routinely remove introduced herbivores from islands, providing a powerful conservation tool. Here, we summarize the few documented feral donkey removals on islands worldwide, and report on the removal of populations from the Galápagos archipelago, Ecuador. After decades of sporadic control programs on Santiago Island and Alcedo Volcano, Isabela Island, donkey populations were removed from both areas, concurrent with a goat eradication program. Both ground and aerial hunting programs were utilized. The latter method was highly efficient; donkeys were removed from Santiago Island with less than 80 h of aerial hunting. Given the clear impacts of introduced herbivores on islands worldwide, feral donkey populations should be routinely removed from islands.     

Archipelago-wide island restoration in the Galápagos Islands: reducing costs of invasive mammal eradication programs and reinvasion risk

Invasive alien mammals are the major driver of biodiversity loss and ecosystem degradation on islands. Over the past three decades, invasive mammal eradication from islands has become one of society's most powerful tools for preventing extinction of insular endemics and restoring insular ecosystems. As practitioners tackle larger islands for restoration, three factors will heavily influence success and outcomes: the degree of local support, the ability to mitigate for non-target impacts, and the ability to eradicate non-native species more cost-effectively. Investments in removing invasive species, however, must be weighed against the risk of reintroduction. One way to reduce reintroduction risks is to eradicate the target invasive species from an entire archipelago, and thus eliminate readily available sources. We illustrate the costs and benefits of this approach with the efforts to remove invasive goats from the Galápagos Islands. Project Isabela, the world's largest island restoration effort to date, removed >140,000 goats from >500,000 ha for a cost of US$10.5 million. Leveraging the capacity built during Project Isabela, and given that goat reintroductions have been common over the past decade, we implemented an archipelago-wide goat eradication strategy. Feral goats remain on three islands in the archipelago, and removal efforts are underway. Efforts on the Galápagos Islands demonstrate that for some species, island size is no longer the limiting factor with respect to eradication. Rather, bureaucratic processes, financing, political will, and stakeholder approval appear to be the new challenges. Eradication efforts have delivered a suite of biodiversity benefits that are in the process of revealing themselves. The costs of rectifying intentional reintroductions are high in terms of financial and human resources. Reducing the archipelago-wide goat density to low levels is a technical approach to reducing reintroduction risk in the short-term, and is being complemented with a longer-term social approach focused on education and governance.     

Review of negative effects of introduced rodents on small mammals on islands

In this first comprehensive review of negative effects of introduced rodents on insular small mammals, the focal species Rattus rattus, R. norvegicus, R. exulans and Mus musculus are implicated in at least 11 extinctions. Furthermore, removal experiments, eradication campaigns and control programmes provide evidence for negative effects on extant populations. While data are currently insufficient for meaningful generalisation with regard to the most threatening rodents, the most threatened small mammals, and the true extent of the problem, it is interesting that R. rattus is implicated in the majority of impacts. This may be explained by its extensive distribution and ecological plasticity. I conclude with methodological recommendations to guide data collection for impact quantification and the study of impact mechanism. This information should facilitate the prioritisation and justification of eradication campaigns, control programmes and biosecurity measures while ensuring that much-needed attention is paid to the conservation of insular small mammals.     

A review on the impacts of feral cats (Felis silvestris catus) in the Canary Islands: implications for the conservation of its endangered fauna

Feral cats have been directly responsible for the extinction of numerous species on islands worldwide, including endemic species of mammals, birds and reptiles. The diet of feral cats in the main habitats of the Canary Islands, as generally occurred on oceanic islands, is mainly composed of introduced mammals, and native species of birds, reptiles and insects. The impact of feral cat upon the endangered species was assessed by evaluating their relative abundance in the cats’ diet and by considering their current conservation status. A total of 68 different preys were identified at species level in all studies carried out in the Canary Islands (5 mammals, 16 birds, 15 reptiles and 32 invertebrates). From all the species preyed by feral cats in the Canary Islands, only four of them are considered threatened by the IUCN Red List of Threatened Species: one endemic bird Saxicola dacotiae and three endemic giant lizards, Gallotia simonyi, Gallotia intermedia, and Gallotia gomerana. Although some efforts on management control have been carried out, it is necessary to enforce these conservation activities on those areas of Tenerife, La Gomera and El Hierro where giant lizards are still present. Furthermore some local areas where endangered bird species are highly predated should be protected. Nevertheless, it is important to take into account the presence of other introduced species such as rats, mice or rabbits in order to avoid problems derived from the hyperpredation process and mesopredator release effect.     

Restoration of Insular Ecosystems: Control of Feral Goats on Aldabra Atoll, Republic of Seychelles

The control of feral goats (Capra hircus) was studied on Aldabra Atoll, Republic of Seychelles, from October 1993 to May 1994 and November 1994 to May 1995. A total of 832 goats was killed on Aldabra using the Judas goat technique and traditional hunting methods. Twenty-eight goats equipped with radio transmitters (Judas goats) were used to locate and kill other feral goats. The remnant goat populations on Ile Picard (n=13) and Ile Malabar (n=19) were eradicated during the first season. On Grande Terre, a total of 798 (374 M : 424 F) goats were killed. Mean group size was 3.2 with a range of 1–20. Judas goat hunting became increasingly important over time with 18.0% (n=85) of goats killed in the presence of Judas goats in the first season compared to 42.3% (n=126) of goats killed during the second season. The overall kill rate for the project was almost 2 times greater for Judas goat hunting (0.61 goats killed/h) than traditional hunting (0.32 goats killed/h). The home range size of each Judas goat and the number of goats killed in association with it was significantly related. Using the Leslie–Davis removal method of population estimation, 84 goats were estimated to remain on all of Grande Terre at the end of thebreak project. This revised version was published online in July 2006 with corrections to the Cover Date.     

Progress Towards the Eradication of Amblyomma variegatum from the Caribbean

Progress in the Caribbean Amblyomma Program (CAP) is reviewed since its inception in 1995 when regional eradication activities for Amblyomma variegatum ticks were initiated using Bayticol pour-on. Technical achievements in the various islands were slow initially, and showed a wide diversity in attaining eradication targets. St. Kitts, considered as a model program, eliminated the tropical Bont tick (TBT) from most of the island in less than 3 years. However, the elimination of very low numbers of residual adult TBT that persisted in three ‘hot-spots’ took another 3 years. A similar problem was faced in St. Lucia, although the approach there was to cull the wild cattle in the last remaining hot-spot. Both islands were certified as ‘Provisionally free from TBT’ in November 2001. In 2002, Anguilla and Montserrat attained the same status, and Barbados and Dominica also qualify for certification. Certification is based on strict, three-monthly, surveillance criteria: essentially, two successive TBT-free surveillance rounds on a statistically acceptable sample of the livestock population. Within the responsibility of the CAP, three islands remain TBT-infested, Antigua, Nevis, and St. Martin/St. Maarten. Nevis is of concern because it has been operational since late 1995, and appears unable to resolve the problem of treating stray small ruminants. Current funding should be adequate to complete eradication on Nevis and in St. Martin/St. Maarten. The major outstanding challenge is Antigua. In comparison to other islands they have the largest number of livestock, and have much larger populations than originally reported: sheep and goats are 4–5-fold higher, and cattle are 1.35-fold higher. The cost for Bayticol is, therefore, almost double and an additional US$ 1.5 million is required for that commodity alone. This revised version was published online in July 2006 with corrections to the Cover Date.     

Population Response of Giant Galápagos Tortoises to Feral Goat Removal

Efforts to eradicate nonnative mammals to restore oceanic island ecosystems have become increasingly successful but parallel tracking of response by native species for which control efforts are intended to benefit has been rare. A major campaign to eradicate nonnative goats and burros was initiated in 1995 on Alcedo Volcano in the Galápagos Archipelago that ultimately removed 62,868 goats and eliminated them by 2006. Planners of the eradication program had the foresight to invest in intensive monitoring of the status of the volcano's giant tortoise (Chelonoidis nigra) population whose welfare was a primary motivation for the eradication effort. Monitoring revealed an increase in the proportion of juveniles among all tortoises as well as increased growth rates of individual tortoises on Alcedo Volcano from earlier to later phases of the eradication campaign. Over the same time frame in a control population on nearby Santa Cruz Island (where goats and donkeys were not removed) juvenile fraction and individual growth rates remained unchanged. Although goat removal coincided with occurrence of a rare climatic event that simultaneously boosted forage availability for tortoises, failure to observe a comparable improvement in the control population implies that removal of goats and burros was the primary causative factor of improving population status of tortoises on Alcedo Volcano.     

Historical bird and terrestrial mammal extinction rates and causes

Aim Conservation of species is an ongoing concern. Location Worldwide. Methods We examined historical extinction rates for birds and mammals and contrasted island and continental extinctions. Australia was included as an island because of its isolation. Results Only six continental birds and three continental mammals were recorded in standard databases as going extinct since 1500 compared to 123 bird species and 58 mammal species on islands. Of the extinctions, 95% were on islands. On a per unit area basis, the extinction rate on islands was 177 times higher for mammals and 187 times higher for birds than on continents. The continental mammal extinction rate was between 0.89 and 7.4 times the background rate, whereas the island mammal extinction rate was between 82 and 702 times background. The continental bird extinction rate was between 0.69 and 5.9 times the background rate, whereas for islands it was between 98 and 844 times the background rate. Undocumented prehistoric extinctions, particularly on islands, amplify these trends. Island extinction rates are much higher than continental rates largely because of introductions of alien predators (including man) and diseases. Main conclusions Our analysis suggests that conservation strategies for birds and mammals on continents should not be based on island extinction rates and that on islands the key factor to enhance conservation is to alleviate pressures from uncontrolled hunting and predation.     

A review of mammal eradications on Mediterranean islands

1. Impacts of alien invasive species on island communities and ecosystems may be even more detrimental than on the mainland. Therefore, since the 1950s, hundreds of restoration projects have been implemented worldwide, with the aim of controlling or eradicating alien species from islands. To date, no review has been focused on eradication on Mediterranean islands. To fill the gap, I reviewed the available information concerning mammal eradications so far carried out on Mediterranean islands, examining the details of several aspects of project implementation and monitoring.
2. I obtained data for 139 attempted eradications on 107 Mediterranean islands in eight countries, with Greece, Italy, and Spain accounting for the highest number. Eradication projects targeted 13 mammal species. The black rat Rattus rattus was the target of over 75% of the known attempted eradications in the Mediterranean Basin; other species targeted were feral goat Capra hircus, house mouse Mus musculus, European rabbit Oryctolagus cuniculus, and domestic cat Felis catus. The most widely adopted technique was poisoning (77% of all eradications), followed by trapping (15%) and hunting (4%). However, techniques were largely target-specific.
3. The average failure rate was about 11%. However, this percentage varied according to the specific mammalian order, and eradications of Carnivora failed more often than those of other mammals. Among rodents, house mouse eradication attained a very high failure rate (75%). Reinvasion occurred after 15% of successful eradications.
4. A better understanding of the motivations of animal rights activists may improve the chance of success when eradicating charismatic or domesticated species. Furthermore, it is crucial to collect data and case studies about reinvasions, in order to strengthen biosecurity programmes following eradication. As in other parts of the world, the next frontier in alien mammal management on Mediterranean islands concerns the eradication of invasive species from inhabited islands.

     

Review of impacts of the introduced house mouse on islands in the Southern Ocean: are mice equivalent to rats?

Research on the impacts of house mice Mus musculus introduced to islands is patchy across most of the species’ global range, except on islands of the Southern Ocean. Here we review mouse impacts on Southern Ocean islands’ plants, invertebrates, land birds and seabirds, and describe the kinds of effects that can be expected elsewhere. A key finding is that where mice occur as part of a complex of invasive mammals, especially other rodents, their densities appear to be suppressed and rat-like impacts have not been reported. Where mice are the only introduced mammal, a greater range of native biota is impacted and the impacts are most severe, and include the only examples of predation on seabird eggs and chicks. Thus mice can have devastating, irreversible and ecosystem-changing effects on islands, impacts typically associated with introduced rats Rattus spp. Island restoration projects should routinely include mouse eradication or manage mouse impacts.     

Genetic Conservation of Insular Populations of Monterey Pine (Pinus Radiata D. Don)

Monterey pine (Pinus radiata D. Don) has only five extant native populations: three disjunct populations along the coast of California, USA and two on Mexican islands. All populations have been influenced by human activity, but the island populations in particular have been affected by introduced biota. On Guadalupe Island, the pine population has suffered drastically from overgrazing by introduced goats. We visited both island populations and described their status, took measurements, and made seed collections. We counted approximately 200 mature pine trees and virtually no seedlings on Guadalupe Island: a reduction of approximately half the population in the last 50 years. The trees are all large (mean diameter of 144 cm) –considerably larger than trees from the other four populations – and arguably near the end of their natural lifespan. The population on Cedros Island is much more robust, with thousands of trees. None sampled were as large as those on Guadalupe Island (mean diameter of 20 cm) and many groves were young and even-aged – presumably the consequence of natural regeneration after a recent fire. Tissue samples from trees on both islands did not show evidence of infection from the pitch canker pathogen, Fusarium circinatum, that has caused significant mortality in the three mainland populations. Caution is recommended in any restoration activity for the Guadalupe Island pines. Inbreeding levels could indicate the need for some planting or seeding intervention but there are also risks associated with this. Natural regeneration – after goat removal – is preferred.     

Are ecological compensation areas attractive hunting sites for common kestrels (Falco tinnunculus) and long-eared owls (Asio otus)?

Common kestrels (Falco tinnunculus) and long-eared owls (Asio otus) in intensively farmed areas in Switzerland decreased markedly as a result of declining vole (Microtus spp.) populations. In order to counteract the loss of biodiversity in intensively farmed areas, the Swiss agri-environment scheme stipulates several types of ecological compensation areas, which together should take up 7% of the farmland. Among them are wild flower and herbaceous strips, which are not mown every year and which in summer support up to 8 times more small mammals than ordinary fields and grassland. This study investigates whether kestrels and long-eared owls preferentially hunt on ecological compensation areas and whether preferred hunting areas are related to the density of small mammals or to the density and height of the vegetation. Both kestrels and long-eared owls mainly hunted on freshly mown low-intensity meadows and artificial grassland, despite low densities of small mammals. Therefore, vegetation structure was more important for the selection of hunting sites than prey abundance. However, both predators preferred to hunt on freshly mown grassland and meadows bordering a wild flower or herbaceous strip. Voles from these strips probably invaded the adjacent freshly mown grassland and became an easy prey for kestrels and owls. In intensively farmed regions, ecological compensation areas, particularly those not mown each year, are an important refuge for small mammals, although in summer the small mammals are not directly accessible to hunting birds. Hence, a mosaic of different habitat types with grassland mown at different times of the year together with undisturbed strips is best suited to provide a year-round supply of accessible food for vole hunters.     

Behavioural responses of invasive American mink Neovison vison to an eradication campaign, revealed by stable isotope analysis

1.  The detrimental impacts of invasive, non-native species on islands are widely acknowledged and it is often best to act rapidly against such species, even where uncertainty exists over the best way to proceed. If management actions are evaluated and refined, using information learnt from the biology of culled animals, this uncertainty can be gradually reduced, increasing the likelihood of a successful outcome.
2.  American mink Neovison vison carcasses were collected as part of an eradication campaign on several islands of the Outer Hebrides, Scotland, and stable isotope analysis was used to describe ecological variation in this invasive non-native predator.
3.  Isotope profiles from individual mink whiskers demonstrated how behaviour at a population level changed markedly over time. As the eradication campaign progressed, mink increased their reliance on marine food sources and focused their activity on the coastline. Stable isotope analyses also demonstrated sex-related changes in foraging and ranging behaviour in relation to food resource availability on the two main island complexes.
4.   Synthesis and applications. Our findings contribute to the refinement of a campaign to extend the successful eradication of mink from Uist and Harris, to the whole of the Outer Hebrides archipelago, UK. They also highlight the potential for stable isotope approaches to provide more detailed postmortem information that can inform adaptive management of wildlife populations for conservation objectives.     

Beetle responses to artificial gaps in an oceanic island forest: implications for invasive tree management to conserve endemic species diversity

Natural forests are often replaced by invasive alien trees on isolated oceanic islands. Adequate eradication of invasive trees should be conducted with the goal of biodiversity conservation, because islands support many endemic organisms that depend on native forests. An invasive alien tree, Bischofia javanica Blume (Euphorbiaceae), has invaded and replaced natural forests on the oceanic Ogasawara (Bonin) Islands, Japan, in the northwestern Pacific Ocean. To determine how the removal of B. javanica trees affects insect diversity, we examined flying beetles captured using Malaise traps in B. javanica forests on Hahajima. The abundance, species density, and species composition of wood-boring beetles (Coleoptera: Cerambycidae, Elateridae, Mordellidae, and Scolytidae) were compared between closed-canopy sites and gaps created by girdling B. javanica trees in alien forests during two seasons (June–July and October–November 2005). Of the collected beetles, 75.8, 87.5, 90.0, and 0.0% of cerambycid, elaterid, mordellid, and scolytid beetle species, respectively, were endemic to the Ogasawara Islands. More cerambycid, elaterid, and mordellid individuals were captured in June–July than in October–November; the number of scolytid individuals did not differ between seasons. More cerambycid, elaterid, and scolytid individuals were captured in artificial gaps than on the closed-canopy forest floor. Although fewer mordellid individuals were captured in gaps, more endemic mordellids were captured in gaps. More cerambycid and scolytid species were captured in artificial gaps than in closed-canopy areas. The positive responses of beetles to artificial gaps suggest that the removal of B. javanica increases beetle diversity and the abundance of endemic beetles.     

Integrating invasive mammal eradications and biodiversity offsets for fisheries bycatch: conservation opportunities and challenges for seabirds and sea turtles

The removal of invasive mammals from islands is one of society’s most powerful tools for preventing extinctions and restoring ecosystems. Given the demonstrable high conservation impact and return on investment of eradications, new networks are needed to fully leverage invasive mammal eradications programs for biodiversity conservation at-large. There have been over 800 invasive mammal eradications from islands, and emerging innovations in technology and techniques suggest that island area will soon no longer be the limiting factor for removing invasive mammals from islands. Rather, securing the necessary social and economic capital will be one main challenge as practitioners target larger and more biologically complex islands. With a new alliance between conservation practitioners and the fisheries sector, biodiversity offsets may be a promising source of capital. A suite of incentives exists for fisheries, NGOs, and governments to embrace a framework that includes fishery bycatch offsets for seabirds and sea turtles. A bycatch management framework based on the hierarchy of “avoid, minimize, and offset” from the Convention on Biological Diversity would result in cost-effective conservation gains for many threatened seabirds and sea turtles affected by fisheries. Those involved with island conservation and fisheries management are presented with unprecedented opportunities and challenges to operationalize a scheme that will allow for the verifiable offset of fisheries impacts to seabirds and sea turtles, which would likely result in unparalleled marine conservation gains and novel cross-sector alliances.     

Late stage dynamics of a successful feral goat eradication from the UNESCO World Heritage site of Aldabra Atoll,Seychelles

Feral goats Capra hircus, considered among the world’s most destructive invasive mammals, were introduced to Aldabra Atoll, a UNESCO World Heritage site in the Seychelles, before 1878. An eradication programme to remove goats from Aldabra was initiated in 1987, after severe ecological impacts were recorded. Eradication and control efforts continued intermittently for the next 20 years, and a final campaign was launched in 2007 using the Judas goat method. We present the methods, eradication dynamics, outcomes and financial costs of the final eradication campaign between 2007 and 2012. This effort was divided into three phases; (1) establishment of Judas goats and intensive hunting (4 months); (2) monitoring of Judas goats (4 years); and (3) Judas goat elimination and verification of success (8 months). In the focal 5-year period, 227 goats were culled (of 2297 across the entire 25-year period); 202 in phase 1, 21 in phase 2, and four remaining Judas goats in phase 3. The eradication was completed and confirmed successful in August 2012, following the use of multiple measures to confirm the absence of goats. The total cost of the eradication was US$ 185,105, an average of US$ 815/goat, or US$ 31/ha. The eradication, although ultimately successful, posed a unique combination of challenges. We discuss key lessons learned and put the project in context of other major island goat eradications. The financial details, context and lessons are expected to be of value to future practitioners.     

Impacts by feral goats on critically endangered Crepidiastrum grandicollum (Compositae) endemic to the Ogasawara Islands

Oceanic islands are biodiversity hotspots with highly endemic ecosystems that are vulnerable to invasive alien species. Understanding the status of endangered species and identifying threats have the highest priority for insular biodiversity conservation. The two remaining populations of endangered Crepidiastrum grandicollum on Chichi-jima Island were studied for 5 years to describe their status and evaluate the impacts of feral goats (Capra hircus). The main cause of population decline was browsing by goats. The populations protected by the exclosures were stable but declined after an exclosure was removed. Even in the protected population, regeneration was limited outside the exclosures and in 1 year of the survey, a high proportion of feeding damage by moth larvae was observed in one population. These facts indicate that exclosures are not a sufficient conservation measure, and eradication of goats and population restoration in novel habitats are necessary to reduce the extinction risk of C. grandicollum.     

Introduced house mice as a conservation concern on Gough Island

House mice have previously been identified as a significant threat to both species and ecosystems on Southern Ocean islands. To date, these impacts have been quantified on several sub-Antarctic islands, but the role of house mice on more temperate islands is poorly known. On South Atlantic Gough Island, non-commensal house mice (Mus musculus L.) were probably introduced in the early 19th century and are now extremely abundant. To assess the likely impacts of mice on the fauna and flora at Gough Island we examined the diet of this population from September 1999 to July 2000 using conventional snap trapping techniques. The population has a single breeding season from September to March and mean body mass is notable in being amongst the largest reported for non-laboratory M. musculus. At low elevations (<250 m above sea level [a.s.l.]), avian carrion was the most prevalent dietary item during September and October. From November to February, plant material constituted the bulk of stomach contents and from March to July lumbricid worms were the most common food item. Indigenous invertebrate matter contributed little to mouse diet, independent of season. At altitudes greater than 500 m a.s.l., larvae of endemic brachypterous moths made up a significant proportion of stomach contents. In light of studies elsewhere, these data suggest that mouse predation may pose a significant threat to these species. However, it is not clear whether conservation action, such as an eradication attempt, is warranted. Further assessments of the impacts of mice are required, and in the interim every effort should be made to prevent introductions of other potentially harmful invasive species.     

Predation on mammals by the chimpanzee (Pan troglodytes)

With respect to prey selectivity and predation frequency, chimpanzees (Pan troglodytes) show local differences as well as diachronic variability within the same population. When data on predation from three long-term studies at Mahale, Gombe, and Tai are compared, some differences and similarities emerge; Mahale is more like Gombe than Tai in regard of prey selection but features of hunting at Tai with respect to predation frequency are not conspicuous. The most responsible factor for diversity in prey selectivity is a distinct “prey image” maintained by chimpanzees of different populations, although it is necessary to clarify in future studies why and how such tradition develops. Relative body size of chimpanzees to prey species and/or the degree of cooperation among members of a hunting party may explain the variability in prey size selected at each site, the latter influencing the frequency of successful hunts at the same time. Although various degrees of habituation and different sampling methods including artificial feeding might have obscured the real differences, recent data from the three populations do not seem to be biased greatly by such factors. Nevertheless, it is still difficult to make strict comparisons due to the lack of sufficient standardized data across the three populations on the frequency of hunting and predation. It is suggested that the size or demographic trend of a chimpanzee unit-group, especially the number of adult males included, necessarily influences its hunting frequency as well as its prey profile. It is also suggested that factors which bring these males together into a party (e.g. fruit abundance, swollen females, conflict between unit-groups etc.) strongly affect theactual hunting and kill rates. Other possible factors responsible for the local differences are forest structure (e.g. tree height), skilful “hero” chimpanzees, and competition with sympatric carnivorous animals. A total of at least 32 species have been recorded as prey mammals of chimpanzees from 12 study sites and the most common prey mammals are primates (18 species), of which 13 species are forest monkeys. Forest monkeys, colobine species in particular, are often the most common victims of the predation by chimpanzees at each site. We may point out a tendency toward selective hunting for the forest monkeys in terms of the selectivity of prey fauna among all three subspecies of chimpanzees, including populations living in drier environment. The mode of chimpanzee hunting seems to correspond to the highest available biomass of gregarious, arboreal monkeys in the forest, colobine species in particular. In contrast, bonobos (P. paniscus) are less carnivorous than chimpanzees, only rarely preying on a few species of small mammals. The sharp contrast of the two allied species in their predatory tendencies appears to have something to do with the differences in the structure of primary production between their habitats.