The evolutionary consequences of human-wildlife conflict in cities.

Human-wildlife interactions, including human-wildlife conflict, are increasingly common as expanding urbanization worldwide creates more opportunities for people to encounter wildlife. Wildlife-vehicle collisions, zoonotic disease transmission, property damage, and physical attacks to people or their pets have negative consequences for both people and wildlife, underscoring the need for comprehensive strategies that mitigate and prevent conflict altogether. Management techniques often aim to deter, relocate, or remove individual organisms, all of which may present a significant selective force in both urban and nonurban systems. Management-induced selection may significantly affect the adaptive or nonadaptive evolutionary processes of urban populations, yet few studies explicate the links among conflict, wildlife management, and urban evolution. Moreover, the intensity of conflict management can vary considerably by taxon, public perception, policy, religious and cultural beliefs, and geographic region, which underscores the complexity of developing flexible tools to reduce conflict. Here, we present a cross-disciplinary perspective that integrates human-wildlife conflict, wildlife management, and urban evolution to address how social-ecological processes drive wildlife adaptation in cities. We emphasize that variance in implemented management actions shapes the strength and rate of phenotypic and evolutionary change. We also consider how specific management strategies either promote genetic or plastic changes, and how leveraging those biological inferences could help optimize management actions while minimizing conflict. Investigating human-wildlife conflict as an evolutionary phenomenon may provide insights into how conflict arises and how management plays a critical role in shaping urban wildlife phenotypes.

Convergence of human and Old World monkey gut microbiomes demonstrates the importance of human ecology over phylogeny.

BACKGROUND: Comparative data from non-human primates provide insight into the processes that shaped the evolution of the human gut microbiome and highlight microbiome traits that differentiate humans from other primates. Here, in an effort to improve our understanding of the human microbiome, we compare gut microbiome composition and functional potential in 14 populations of humans from ten nations and 18 species of wild, non-human primates. RESULTS: Contrary to expectations from host phylogenetics, we find that human gut microbiome composition and functional potential are more similar to those of cercopithecines, a subfamily of Old World monkey, particularly baboons, than to those of African apes. Additionally, our data reveal more inter-individual variation in gut microbiome functional potential within the human species than across other primate species, suggesting that the human gut microbiome may exhibit more plasticity in response to environmental variation compared to that of other primates. CONCLUSIONS: Given similarities of ancestral human habitats and dietary strategies to those of baboons, these findings suggest that convergent ecologies shaped the gut microbiomes of both humans and cercopithecines, perhaps through environmental exposure to microbes, diet, and/or associated physiological adaptations. Increased inter-individual variation in the human microbiome may be associated with human dietary diversity or the ability of humans to inhabit novel environments. Overall, these findings show that diet, ecology, and physiological adaptations are more important than host-microbe co-diversification in shaping the human microbiome, providing a key foundation for comparative analyses of the role of the microbiome in human biology and health.

Seasonal and habitat effects on the nutritional properties of savanna vegetation: Potential implications for early hominin dietary ecology.

The African savannas that many early hominins occupied likely experienced stark seasonality and contained mosaic habitats (i.e., combinations of woodlands, wetlands, grasslands, etc.). Most would agree that the bulk of dietary calories obtained by taxa such as Australopithecus and Paranthropus came from the consumption of vegetation growing across these landscapes. It is also likely that many early hominins were selective feeders that consumed particular plants/plant parts (e.g., leaves, fruit, storage organs) depending on the habitat and season within which they were foraging. Thus, improving our understanding of how the nutritional properties of potential hominin plant foods growing in modern African savanna ecosystems respond to season and vary by habitat will improve our ability to model early hominin dietary behavior. Here, we present nutritional analyses (crude protein and acid detergent fiber) of plants growing in eastern and southern African savanna habitats across both wet and dry seasons. We find that many assumptions about savanna vegetation are warranted. For instance, plants growing in our woodland habitats have higher average protein/fiber ratios than those growing in our wetland and grassland transects. However, we find that the effects of season and habitat are complex, an example being the unexpectedly higher protein levels we observe in the grasses and sedges growing in our Amboseli wetlands during the dry season. Also, we find significant differences between the vegetation growing in our eastern and southern African field sites, particularly among plants using the C4 photosynthetic pathway. This may have implications for the differences we see between the stable carbon isotope compositions and dental microwear patterns of eastern and southern African Paranthropus species, despite their shared, highly derived craniodental anatomy.

Metagenomic analyses reveal previously unrecognized variation in the diets of sympatric Old World monkey species.

Insectivory, or the consumption of insects and other arthropods, is a significant yet cryptic component of omnivorous primate diets. Here, we used high-throughput DNA sequencing to identify arthropods from fecal DNA and assess variation in insectivory by closely-related sympatric primates. We identified arthropod prey taxa and tested the hypothesis that variation in insectivory facilitates niche differentiation and coexistence among closely-related species with high dietary overlap. We collected 233 fecal samples from redtail (Cercopithecus ascanius; n = 118) and blue monkeys (C. mitis; n = 115) and used a CO1 metabarcoding approach to identify arthropod DNA in each fecal sample. Arthropod DNA was detected in 99% of samples (N = 223 samples), and a total of 68 families (15 orders) were identified. Redtails consumed arthropods from 54 families, of which 12 (21.8%) were absent from blue monkey samples. Blue monkeys consumed arthropods from 56 families, of which 14 (24.6%) were absent from redtail samples. For both species, >97% of taxa present belonged to four orders (Araneae, Diptera, Hymenoptera, Lepidoptera). Redtail samples contained more Lepidoptera taxa (p<0.05), while blue monkey samples contained more Araneae (p<0.05). Blue monkeys consumed a greater diversity of arthropod taxa than redtail monkeys (p<0.05); however, the average number of arthropod families present per fecal sample was greater in the redtail monkey samples (p<0.05). These results indicate that while overlap exists in the arthropod portion of their diets, 20-25% of taxa consumed are unique to each group. Our findings suggest that variation in arthropod intake may help decrease dietary niche overlap and hence facilitate coexistence of closely-related primate species.

Geophagy among nonhuman primates: A systematic review of current knowledge and suggestions for future directions.

OBJECTIVES: Geophagy, the intentional consumption of earth, is widely practiced among humans and other mammals, but its causes are not well understood. Given the growing number of reports of geophagy among nonhuman primates (NHP), we sought to (1) advance and codify our understanding of the patterns and functional and evolutionary significance of geophagy among NHP and (2) provide a research agenda for a more unified approach to its study. METHODS: We systematically reviewed all available literature on NHP geophagy, extracted available data on taxa, geography, climate, diet, sex, age-class, reproductive status, and the characteristics of the earth. We used these data to evaluate three major hypotheses about geophagy, that it is protective, provides mineral supplementation, and is nonadaptive. RESULTS: We identified 287 accounts of geophagy among 136 species, adding 79 new primate species to the list of those considered in prior reviews. Nineteen percent of species were in the suborder Strepsirrhini, while 81% were in the suborder Haplorrhini. There were reports of geophagy from 9 of the 17 families and 39 of the 76 genera currently recognized by the International Union for Conservation of Nature. DISCUSSION: The limited evidence suggests that geophagy is adaptive, and provides protection and mineral supplementation. We specify the behavioral, dietary, and soil data required to more rigorously investigate these hypotheses across representative species of all taxonomic groups, geographical regions, and dietary classification. Given the plausibility of geophagy for maintaining the health of both wild and captive populations, we urge further study and conservation of geophagy sites.

Grass leaves as potential hominin dietary resources.

Discussions about early hominin diets have generally excluded grass leaves as a staple food resource, despite their ubiquity in most early hominin habitats. In particular, stable carbon isotope studies have shown a prevalent C4 component in the diets of most taxa, and grass leaves are the single most abundant C4 resource in African savannas. Grass leaves are typically portrayed as having little nutritional value (e.g., low in protein and high in fiber) for hominins lacking specialized digestive systems. It has also been argued that they present mechanical challenges (i.e., high toughness) for hominins with bunodont dentition. Here, we compare the nutritional and mechanical properties of grass leaves with the plants growing alongside them in African savanna habitats. We also compare grass leaves to the leaves consumed by other hominoids and demonstrate that many, though by no means all, compare favorably with the nutritional and mechanical properties of known primate foods. Our data reveal that grass leaves exhibit tremendous variation and suggest that future reconstructions of hominin dietary ecology take a more nuanced approach when considering grass leaves as a potential hominin dietary resource.

Impending extinction crisis of the world's primates: Why primates matter.

Nonhuman primates, our closest biological relatives, play important roles in the livelihoods, cultures, and religions of many societies and offer unique insights into human evolution, biology, behavior, and the threat of emerging diseases. They are an essential component of tropical biodiversity, contributing to forest regeneration and ecosystem health. Current information shows the existence of 504 species in 79 genera distributed in the Neotropics, mainland Africa, Madagascar, and Asia. Alarmingly, ~60% of primate species are now threatened with extinction and ~75% have declining populations. This situation is the result of escalating anthropogenic pressures on primates and their habitats-mainly global and local market demands, leading to extensive habitat loss through the expansion of industrial agriculture, large-scale cattle ranching, logging, oil and gas drilling, mining, dam building, and the construction of new road networks in primate range regions. Other important drivers are increased bushmeat hunting and the illegal trade of primates as pets and primate body parts, along with emerging threats, such as climate change and anthroponotic diseases. Often, these pressures act in synergy, exacerbating primate population declines. Given that primate range regions overlap extensively with a large, and rapidly growing, human population characterized by high levels of poverty, global attention is needed immediately to reverse the looming risk of primate extinctions and to attend to local human needs in sustainable ways. Raising global scientific and public awareness of the plight of the world's primates and the costs of their loss to ecosystem health and human society is imperative.

Sympatric Apes in Sacred Forests: Shared Space and Habitat Use by Humans and Endangered Javan Gibbons (Hylobates moloch).

In this research, we use a combination of ethnographic observation and GIS analysis to explore the use of space by humans and gibbons (Hylobates moloch) to determine areas of potential space competition in the sacred forest and nature reserve Cagar Alam Leuweung Sancang in West Java, Indonesia. More specifically, we test whether gibbons respond to the presence of humans in a manner consistent with predator-avoidance and predicted that the gibbon study subjects would avoid areas visited by humans (Risk-Disturbance Hypothesis). Data were collected August 2010-June 2011. We collected GPS locations and behavioral data on both the humans (6,652 hours) and the gibbons (1,253 hours) in the forest using 10 minute instantaneous sampling. Results indicate that humans preferentially assemble at the most sacred spot in the forest (Cikajayaan waterfall). Two gibbon groups' home ranges encompassed most of the sacred areas. Group B avoided areas of high human use, as high human use areas and high gibbon use areas did not overlap. Group C, though, continued to use areas that were heavily visited by humans. We thus found partial support for the Risk-Disturbance Hypothesis, although the variation in gibbon response to human disturbance indicates behavioral flexibility. We suggest that understanding the effects of shared space on wildlife is necessary for informing conservation policy in human-visited forests.

The Evolution of Stomach Acidity and Its Relevance to the Human Microbiome.

Gastric acidity is likely a key factor shaping the diversity and composition of microbial communities found in the vertebrate gut. We conducted a systematic review to test the hypothesis that a key role of the vertebrate stomach is to maintain the gut microbial community by filtering out novel microbial taxa before they pass into the intestines. We propose that species feeding either on carrion or on organisms that are close phylogenetic relatives should require the most restrictive filter (measured as high stomach acidity) as protection from foreign microbes. Conversely, species feeding on a lower trophic level or on food that is distantly related to them (e.g. herbivores) should require the least restrictive filter, as the risk of pathogen exposure is lower. Comparisons of stomach acidity across trophic groups in mammal and bird taxa show that scavengers and carnivores have significantly higher stomach acidities compared to herbivores or carnivores feeding on phylogenetically distant prey such as insects or fish. In addition, we find when stomach acidity varies within species either naturally (with age) or in treatments such as bariatric surgery, the effects on gut bacterial pathogens and communities are in line with our hypothesis that the stomach acts as an ecological filter. Together these results highlight the importance of including measurements of gastric pH when investigating gut microbial dynamics within and across species.

Fecal microbial diversity and putative function in captive western lowland gorillas (Gorilla gorilla gorilla), common chimpanzees (Pan troglodytes), Hamadryas baboons (Papio hamadryas) and binturongs (Arctictis binturong).

Microbial populations in the gastrointestinal tract contribute to host health and nutrition. Although gut microbial ecology is well studied in livestock and domestic animals, little is known of the endogenous populations inhabiting primates or carnivora. We characterized microbial populations in fecal cultures from gorillas (Gorilla gorilla gorilla), common chimpanzees (Pan troglodytes), Hamadryas baboons (Papio hamadryas) and binturongs (Arctictis binturong) to compare the microbiomes associated with different gastrointestinal morphologies and different omnivorous feeding strategies. Each species was fed a distinct standardized diet for 2 weeks prior to fecal collection. All diets were formulated to reflect the species' feeding strategies in situ. Fresh fecal samples were pooled within species and used to inoculate in vitro batch cultures. Acetate, propionate, butyrate and valerate were measured after 24 h of incubation. Eubacterial DNA was extracted from individual fecal samples, pooled, and the cpn60 gene region was amplified and then sequenced to identify the major eubacterial constituents associated with each host species. Short chain fatty acids (P < 0.001) and methane (P < 0.001) were significantly different across species. Eubacterial profiles were consistent with fermentation data and suggest an increase in diversity with dietary fiber.

Binturong (Arctictis binturong) and Kinkajou (Potos flavus) digestive strategy: implications for interpreting frugivory in Carnivora and primates.

Exclusive frugivory is rare. As a food resource, fruit is temporally and spatially patchy, low in protein, and variable in terms of energy yield from different carbohydrate types. Here, we evaluate the digestive physiology of two frugivorous Carnivora species (Potos flavus, Arctictis binturong) that converge with primates in a diversity of ecological and anatomical traits related to fruit consumption. We conducted feeding trials to determine mean digestive retention times (MRT) on captive animals at the Carnivore Preservation Trust (now Carolina Tiger Rescue), Pittsboro, NC. Fecal samples were collected on study subjects for in vitro analysis to determine methane, pH, and short chain fatty acid profiles; fiber was assayed using standard neutral detergent (NDF) and acid detergent (ADF) fiber methods. Results indicate that both carnivoran species have rapid digestive passage for mammals that consume a predominantly plant-based diet: A. binturong MRT = 6.5 hrs (0.3); P. flavus MRT = 2.5 hrs (1.6). In vitro experiments revealed no fermentation of structural polysaccharides--methane levels did not shift from 0 h to either 24 or 48 hours and no short chain fatty acids were detected. In both species, however, pH declined from one incubation period to another suggesting acidification and bacterial activity of microbes using soluble carbohydrates. A comparison with primates indicates that the study species are most similar in digestive retention times to Ateles--the most frugivorous anthropoid primate taxon.

Meta-analysis of the effects of human disturbance on seed dispersal by animals.

Animal-mediated seed dispersal is important for sustaining biological diversity in forest ecosystems, particularly in the tropics. Forest fragmentation, hunting, and selective logging modify forests in myriad ways and their effects on animal-mediated seed dispersal have been examined in many case studies. However, the overall effects of different types of human disturbance on animal-mediated seed dispersal are still unknown. We identified 35 articles that provided 83 comparisons of animal-mediated seed dispersal between disturbed and undisturbed forests; all comparisons except one were conducted in tropical or subtropical ecosystems. We assessed the effects of forest fragmentation, hunting, and selective logging on seed dispersal of fleshy-fruited tree species. We carried out a meta-analysis to test whether forest fragmentation, hunting, and selective logging affected 3 components of animal-mediated seed dispersal: frugivore visitation rate, number of seeds removed, and distance of seed dispersal. Forest fragmentation, hunting, and selective logging did not affect visitation rate and were marginally associated with a reduction in seed-dispersal distance. Hunting and selective logging, but not fragmentation, were associated with a large reduction in the number of seeds removed. Fewer seeds of large-seeded than of small-seeded tree species were removed in hunted or selectively logged forests. A plausible explanation for the consistently negative effects of hunting and selective logging on large-seeded plant species is that large frugivores, as the predominant seed dispersers for large-seeded plant species, are the first animals to be extirpated from hunted or logged forests. The reduction in forest area after fragmentation appeared to have weaker effects on frugivore communities and animal-mediated seed dispersal than hunting and selective logging. The differential effects of hunting and selective logging on large- and small-seeded tree species underpinned case studies that showed disrupted plant-frugivore interactions could trigger a homogenization of seed traits in tree communities in hunted or logged tropical forests.

Digestive retention times for Allen's swamp monkey and L'Hoest's monkey: data with implications for the evolution of cercopithecine digestive strategy.

Primates access energy from plant fiber via bacterial fermentation in either a modified forestomach ('foregut'), a caecocolic ('hindgut') chamber of the large intestine, or both. Longer digestive retention times allow for more complete fermentation; as such, primates that consume an herbivorous diet high in fiber are expected to have both relatively and absolutely longer retention times than those mammals that rely on more readily digestible plant foods, such as fruit. We used particulate markers to measure the digestive retention times of captive Allen's swamp monkeys [Allenopithecus nigroviridis (Pocock, 1907)] (n= 3) and L'Hoest's monkey (Cercopithecus lhoesti P. Sclater, 1899) (n= 2). Results indicate mean retention times of 23.2-29.4 h and 23.2-24.0 h for C. lhoesti and A. nigroviridus, respectively. Results from this study, in combination with previously published data on digestive retention times in other primate species, indicate that cercopithecines differ from other primate taxa by having lengthier retention times that can be predicted by body mass alone. These data are consistent with the hypothesis that relatively lengthy retention times are a primitive trait for Cercopithecinae.

Frugivores and seed dispersal: mechanisms and consequences for biodiversity of a key ecological interaction.

The 5th Symposium on Frugivores and Seed Dispersal, held in Montpellier (France), 13-18 June 2010, brought together more than 220 researchers exemplifying a wide diversity of approaches to the study of frugivory and dispersal of seeds. Following Ted Fleming and Alejandro Estrada's initiative in 1985, this event was a celebration of the 25th anniversary of the first meeting in Veracruz, Mexico. Frugivory and seed dispersal are active research areas that have diversified in multiple directions since 1985 to include evolution (e.g. phylogenetic diversity and dispersal adaptations), physiology (e.g. sensory cues and digestion), landscape ecology (movement patterns), molecular ecology (e.g. gene flow, genetic diversity and structure), community ecology (e.g. mutualistic interaction networks) and conservation biology (effects of hunting, fragmentation, invasion and extinction), among others. This meeting provided an opportunity to assess conceptual and methodological progress, to present ever more sophisticated insights into frugivory in animals and dispersal patterns in plants, and to report the advances made in examining the mechanisms and consequences of seed dispersal for plants and frugivores.

Primate seed dispersers as umbrella species: a case study from Kibale National Park, Uganda, with implications for Afrotropical forest conservation.

Almost half of the world's extant primate species are of conservation concern [IUCN, International Union for the Conservation of Nature Red List of Threatened Species, 2008]. Primates are also effective seed dispersers. The implications of and interactions between these two facts are increasingly understood, and data demonstrating the consequences of losing primates for forest ecology are now available from throughout the tropics. However, a reality is that not all species-and the mutualisms among them-can be protected. Conservation managers must make difficult decisions and use shortcuts in the implementation of conservation tactics. Using taxa as "umbrellas" is one such shortcut, although a lack of an operational definition of what an umbrella species is and how to choose one has made implementing this tactic difficult. In this study, I discuss primates as umbrellas by defining a selection index in terms of richness/co-occurrence, rarity, and sensitivity to anthropogenic disturbance. I evaluate the anthropoid assemblage of Kibale National Park, Uganda, in light of the selection index and determine that Cercopithecus is the genus best fitting the criteria for umbrella status. I then evaluate the functional significance-in terms of seed dispersal-of using Cercopithecus monkeys (guenons) as umbrellas. Results from 1,047 hr of observation of focal fruiting trees in Kibale indicate that Cercopithecus ascanius was the most commonly observed frugivore visitor (July 2001-June 2002). These data corroborate earlier data collected in Kibale demonstrating that guenons are highly effective seed dispersers. Patterns of richness/co-occurrence, rarity, and sensitivity observed in Kibale are reflected in Afrotropical forests more generally, with the genus Cercopithecus tending to exhibit greatest richness/co-occurrence with taxonomically similar species, to be neither extremely rare nor ubiquitous, and also to be moderately sensitive to human disturbance. Moreover, in all available evaluations of frugivory in Afrotropical forests, guenons emerge as among the most important seed dispersers relative to other taxa.

Summary to the symposium issue: Primate fallback strategies as adaptive phenotypic plasticity--scale, pattern, and process.

In this discussion, I evaluate our understanding of fallback foods in primate and hominin ecology and evolution with reference to the challenges of nomenclature, scale, and of linking individual responses to food availability and properties (process) to species traits (pattern). I use these challenges to form the framework of my discussion and ultimately conclude that we situate the discussion of primate fallback strategy into a broader, "synthetic" framework of animal form and the evolutionary significance of phenotypic plasticity.

Measuring physical traits of primates remotely: the use of parallel lasers.

Physical traits, such as body size, and processes like growth can be used as indices of primate health and can add to our understanding of life history and behavior. Accurately measuring physical traits in the wild can be challenging because capture is difficult, disrupts animals, and may cause injury. To measure physical traits of arboreal primates remotely, we adapted a parallel laser technique that has been used with terrestrial and marine mammals. Two parallel lasers separated by a known distance (4 cm) and mounted onto a digital camera are projected onto an animal. When a photograph is taken, the laser projections on the target provide a scale bar. We validated the technique for measuring the physical traits of identifiable red colobus monkeys (Procolobus rufomitratus) in Kibale National Park, Uganda. First, we photographed the tails of monkeys with laser projections and compared these with measurements previously obtained when the animals were captured. Second, we manually measured the distance between two markers placed on tree branches at similar heights to those used by monkeys, and compared them with the measurements obtained through digital photographs of the markers with parallel laser projections. The mean tail length of the monkeys via manual measurements was 63.3+/-4.4 cm, and via remote measurements was 63.0+/-4.1 cm. The mean distance between the markers on tree branches via manual measurements was 13.8+/-3.59 cm, and via remote measurements was 13.9+/-3.58 cm. The mean error using parallel lasers was 1.7% in both cases. Although the needed precision will depend on the question asked, our results suggest that sufficiently precise measurements of physical traits or substrates of arboreal primates can be obtained remotely using parallel lasers.

Competition, predation, and the evolutionary significance of the cercopithecine cheek pouch: the case of Cercopithecus and Lophocebus.

Reports of cercopithecine cheek pouch use and functional significance are largely anecdotal, and to date there have been no investigations into its use by species living in closed forest habitats. Here, I report on cheek pouch use in Cercopithecus ascanius and Lophocebus albigena in the Kibale National Park, Uganda, between July-October 1997. Two hypotheses were evaluated: this feature was selected for because of its role in 1) increasing feeding efficiency via a reduction in potential feeding competition, and/or 2) reducing vulnerability to predation. Results indicate that both species were more likely to use their cheek pouches when feeding on contestable foods and, after filling cheek pouches, retreated to more densely vegetated ("safer") positions for processing food. There was no influence of age and sex on L. albigena cheek pouch use. Subadult C. ascanius cheek-pouched less frequently than adults, although there were no differences between adult males and females. There was no relationship between feeding-patch size and number of plant food items cheek-pouched in either species. However, the diameter of breast height (dbh; a measure of patch size) of trees in which C. ascanius used their cheek pouches was significantly larger than the dbh of trees in which they did not. Both species were more likely to use their cheek pouches in the presence of greater numbers of conspecifics. These data provide insight into the relationship(s) among oral anatomy and feeding efficiency, and facilitate understanding into the selection for this important oral feature in stem cercopithecines.

Hardness of cercopithecine foods: implications for the critical function of enamel thickness in exploiting fallback foods.

We evaluate the hardness of foods consumed by sympatric Cercopithecus ascanius (redtail guenons) and Lophocebus albigena (grey-cheeked mangabeys), and consider how selection might operate to influence foraging adaptations. Since L. albigena has among the thickest dental enamel in extant primates and is commonly referred to as a hard-object consumer, we predicted that their diet would be harder than that of the guenon. Data on diet and food hardness (as measured by resistance to puncture and crushing) were collected between June-October of 1997 at Kibale National Park, Uganda, and were compared to similar data collected in Kibale between 1991-1994. Contrary to what we predicted, there was no difference in dietary hardness when the puncture resistance of all fruit consumed by the two species was compared (31 tree species in both study periods). However, in June-October 1997, L. albigena exploited a diet more resistant to puncture and crushing than C. ascanius. This difference is largely explained by the higher percentage of bark and seeds consumed by the L. albigena during this period. We suggest that it is the difference in the mechanical properties of fallback foods during critical periods that may have served as the selective pressure for thick enamel in L. albigena.