Walter Carl Hartwig

Perinatal life history traits in New World monkeys

Gestation length, neonatal and maternal body weight, and neonatal and adult brain weight data were collected for New World monkeys in an attempt to establish typical patterns of perinatal life history. This study attempts to illuminate the most accurate values from the available data, which suggest that gestation length and prenatal growth rate are broadly conserved in relation to maternal size in New World monkeys. Exceptions to the patterns evident in the data point to derivations in life history strategies. In particular, this study suggests that the extended gestation length of callitrichines is a function of minimum viable neonate size and not exclusively energy minimization associated with simultaneous lactation. Cebus is shown to undergo more postnatal brain growth relative to other New World monkeys, but not as much as previously believed. Alouatta is shown to be relatively small brained at birth as well as in adulthood. Saimiri is shown to present the most unusual package of perinatal life history traits, in which precocial neonates are gestated for a relatively long time and at a slightly faster growth rate than is typical for New World monkeys.

Degrees of sexual dimorphism in Cebus and other new world monkeys

Sexual dimorphism in primate species expresses the effects of phylogeny, life history, behavior, and ontogeny. The causes and implications of sexual dimorphism have been studied in several different primates using a variety of morphological databases such as body weight, canine length, and coat color and ornamentation. In addition to these different patterns of dimorphism, the degree to which a species is dimorphic results from a variety of possible causes. In this study we test the general hypothesis that a species highly dimorphic for one size-based index of dimorphism will be equally dimorphic (relative to other species) for other size-based indices. Specifically, the degree and pattern of sexual dimorphism in Cebus and several other New World monkey species is measured using craniometric data as a substitute for the troublesome range of variation in body weight estimates. In general, the rank ordering of species for dimorphism ratios differs considerably across neural vs. non-neural functional domains of the cranium. The relative degree of sexual dimorphism in different functional regions of the cranium is affected by the independent action of natural selection on those regions. Regions of the cranium upon which natural selection is presumed to have acted within a species show greater degrees of dimorphism than do the same regions in closely related taxa. Within Cebus, C. apella is consistently more dimorphic than other Cebus species for facial measurements, but not for neural or body weight measurements. The pattern in C. apella indicates no single best measurement of the degree of dimorphism in a species; rather, the relative degree of dimorphism applies only to the region being measured and may be enhanced by other selective pressures on morphology.

Relative Brain Size, Gut Size, and Evolution in New World Monkeys

The dynamics of brain evolution in New World monkeys are poorly understood. New data on brain weight and body weight from 162 necropsied adult individuals, and a second series on body weight and gut size from 59 individuals, are compared with previously published reports based on smaller samples as well as large databases derived from museum records. We confirm elevated brain sizes for Cebus and Saimiri and also report that Cacajao and Chiropotes have relatively large brains. From more limited data we show that gut size and brain mass have a strongly inverse relationship at the low end of the relative brain size scale but a more diffuse interaction at the upper end, where platyrrhines with relatively high encephalization quotients may have either relatively undifferentiated guts or similar within‐gut proportions to low‐EQ species. Three of the four main platyrrhine clades exhibit a wide range of relative brain sizes, suggesting each may have differentiated while brains were relatively small and a multiplicity of forces acting to maintain or drive encephalization. Alouatta is a likely candidate for de‐encephalization, although its “starting point” is difficult to establish. Factors that may have compelled parallel evolution of relatively large brains in cebids, atelids and pitheciids may involve large social group sizes as well as complex foraging strategies, with both aspects exaggerated in the hyper‐encephalized Cebus. With diet playing an important role selecting for digestive strategies among the seed‐eating pitheciins, comparable in ways to folivores, Chiropotes evolved a relatively larger brain in conjunction with a moderately large and differentiated gut. Anat Rec, 2011.

Fractal analysis of sagittal suture morphology

Quantifying shape is a broad problem in the morphological sciences. Most techniques for numerically describing shape abstract the shape into the most logical ideal Euclidean dimension. The fractional, or fractal, dimension is a simple computation that expresses shape in real, rather than ideal, space. The structured walk technique developed for the fractal analysis of rugged boundaries is applied here to the contour of the human sagittal suture in order to discriminate the separate morphological patterns of interfingering and interlocking. These attributes contribute differentially to the sutures “complexity,” a concept often used in biomechanical hypotheses. Previous techniques for estimating sutural complexity do not isolate small‐scale from large‐scale morphological patterns. Results indicate that despite the visual appearance of great variation, human sagittal sutures are remarkably consistent in the degree of complexity expressed separately by large‐scale interfingering lateral excursions and small‐scale interlocking ruggedness. There is no significant correlation between the absolute or bregma‐lambda chord length of the human sagittal suture and its degree of complexity as determined by the structured walk technique.

Dental variability inSaimiri and the taxonomic status ofNeosaimiri fieldsi, an early squirrel monkey from La Venta, Colombia

Neosaimiri fieldsi, from the South American middle Miocene locality of La Venta, is represented by a relatively complete mandible and dentition that strongly resembles that of extantSaimiri. Comparison with a large sample of mandibles ofSaimiri indicates that this specimen cannot be distinguished from modern populations on the basis of any reportedly diagnostic feature, such as cingulid development, molar length ratio, trigonic/talonid ratio, or mandibular depth. The fossil is best considered an extinct species of the modern genusSaimiri until further material indicates otherwise.

Spider Monkeys: Morphology and evolution of the spider monkey, genus Ateles

Introduction Spider monkeys cast a distinct morphological silhouette – long scrawny arms and a snaky prehensile tail arching from a narrow pot-belly torso, topped by a small round head and blunt face. The commitment of this relatively large-bodied platyrrhine to a large-tree, upper canopy milieu and to ripe fruit foraging is seen throughout its skeletal and craniodental morphology. Spider monkeys are the signature New World suspensory-postured brachiators. Bodily, they are the closest thing to a gibbon that has evolved anywhere else within the Order Primates. Less obvious may be the fact that they are also gibbonesque craniodentally. But in the context of the adaptive array of Latin Americas four ateline genera, Alouatta , Lagothrix , Brachyteles and Ateles , spider monkeys are not simply the polar end of an adaptive morphocline, standing opposite howlers or even opposite Lagothrix if we draw our comparison more narrowly, to encompass only atelins. Spider monkeys are different by far . For example, as close as Brachyteles is to the visage of a spider monkey with its ungainly limbs and shortness of face, it does not match Ateles in the high-energy lifestyle that goes along with eating quickly metabolized fruit and little else. Nor can Brachyteles deftly fly and lope through the trees as if gravity and substrate did not matter and hands, feet and tail were octopus tentacles. How ironic that Geoffroy Saint-Hilaire was so impressed with the spider monkeys lone anatomical “deficiency,” its missing thumb, that in 1806 he named the genus Ateles , meaning imperfect.

Implications of molecular and morphological data for understanding ateline phylogeny

The ateline monkeys constitute as certain a monophyletic group as there is among primates. The group is intriguing because while their adaptations are well-documented and their monophyly as a group is unquestioned, their phylogenetic interrelationships are controversial. Molecular data indicate a phylogeny at odds with their morphology. Traditional morphological comparisons isolate Alouatta from the atelins, and link Ateles and Brachyteles as a sister group to the exclusion of Lagothrix. In contrast, several recent molecular studies point to a closer relationship between Brachyteles and Lagothrix than between Brachyteles and Ateles. At the heart of the problem lie the assumptions we make about the validity of data and the homology of observed traits. The fossil record further confounds the issue. We must account for the fossil record because it is positive evidence. But we cannot control how much of it there is or how much of it ever will be known. At this point in time, the ateline molecular and fossil record provoke us to examine critically our morphological approach to phylogenetic modeling.

Fossil Alouattines and the Origins of Alouatta: Craniodental Diversity and Interrelationships

The howler monkey clade includes species of Alouatta and four extinct genera, Stirtonia, Paralouatta, Protopithecus, and probably Solimoea as well. Contrary to expectations, this radiation may have originated as a largely frugivorous group; advanced, Alouatta-like leaf-eating is a novelty well-developed in the Alouatta-Stirtonia sublineage only. Revised body mass estimates place Stirtonia and Paralouatta within the size range exhibited by the living forms and confirm the place of Protopithecus in a larger, baboon-like size range. While their dentitions are more primitive than the Alouatta-Stirtonia pattern, the cranial anatomy of Protopithecus and Paralouatta is distinctly similar to living howler monkeys in highly derived features relating to enlargement of the subbasal space in the neck and in head carriage, suggesting that ancestral alouattines may have had an enlarged hyolaryngeal apparatus. All alouattines also have relatively small brains, including Protopithecus, a genus that was probably quite frugivorous. The successful origins of the alouattine clade may owe more to key adaptations involving communication and energetics than dental or locomotor breakthroughs. While the fossil record confirms aspects of previous character-analysis reconstructions based on the living forms, alouattines experienced a complexity of adaptive shifts whose history cannot be recoverable without a more complete fossil record.*

Skin: A Natural History Nina G. Jablonski Berkeley: University of California Press. 2006. 266 pp.,

Nina Jablonski has been thinking about skin for a long time, and from many different perspectives. Her attention and scholarship show through in this book, a concise and enjoyable survey of our outermost selves. Skin is styled for a broad readership. Academic references are cited in footnotes, which themselves are listed at the end of the book. This enables you to actually read the book without parenthetical pauses, which I found very appealing. The footnotes preserve the scholarly level of the text and in many cases are intriguing. The same goes for the well-placed illustrations, which include a mid-book section of color plates. Skin’s organization reflects Jablonski’s training as an anatomist and anthropologist. It begins with basic histology of mammalian skin with frequent reference to similarities and differences found in human skin. Jablonski respects the role that sweating has played in the comparative evolution of skin as she sets the stage for the central theme and strength of the book—the function and evolution of human skin pigmentation. Jablonski’s exposition in the middle chapters on “Skin and Sun”, “Skin’s Dark Secret”, and “Color” is clear and compelling. The relationships among clinal levels of ultraviolet radiation intensity, melanocyte physiology, vitamin D synthesis, and neural tube defects weave together seamlessly in the 40 pages on these topics. These chapters can stand alone as a seminar reading in dermatology, evolutionary medicine, population biology, etc. It is to Jablonski’s credit that she goes beyond reiterating her already published work on skin color (which is represented very modestly in this book) to include a wide variety of complementary research. The relationship of skin to concepts of race, social or otherwise, must be addressed in a book such as this, and Jablonski skillfully avoids polemic in the process. In less than two pages she acknowledges skin color has played a role in “the development of ideas about ‘race’ and ‘ethnicity’” (p. 95), and then deftly keeps the focus on the proximate mechanisms of pigmentation, pigmentation intensity, and depigmentation. She shrewdly details the relationship of ultraviolet radiation to skin without mentioning race in the preceding 30 pages, so by the time she does mention race the basis for discounting its dermal imprint has already been established.