Introduction

        As long as men have watched primates and primates have peered obtusely back, there have been many unasked questions in the murky field of behavioral science, and one of the most persistent has always been "Which monkey species plummets the fastest?"  Ever since Plato's Why Falls the Simian?, curious minds have wondered whether the lesser primates actually do plummet more slowly than the great apes, and whether this is due to lesser weight, greater surface area-to-volume ratio, both, or something else entirely.  Plato spent several years pursuing primate studies, and was a great fan of monkeys in general.  It has never been clear whether he actually ever saw an ape, but it seems certain that either a giant pygmy chimpanzee or a gorilla of some sort or other was the animal he referred to as Primator impericus, the 'royal monkey.'
       But there were others besides Plato in the cloudy past who must also be considered among the first true primate pesematologists.  It was the Assyrian idiot savant astronomer Nettichus who first observed plummeting primates while on a camping trip in the neighboring Kingdom of Mitanni, which was heavily forested and occupied by monkeys at the time.  They were next seen by the Carthaginian explorer and pirate Kartha Punic, who wrote in his Historie of the African Continente that "...dropt they like flyes round us [as we] presst through the murke and glume of the forrest."  The Roman general Simias Flavius brought back several thousand individuals of many different species of monkeys from the coastal jungles of Africa to test their plummeting potential by encouraging them to jump from the top of Trajan's column.  Watching displays of free-falling monkeys eventually became a major pastime which distracted the people from dealing with more important issues in declining Rome, and Pope Leo the Shy was forced to lead a protest against the practice.
       In 1502 a Flemish mathematician named Ultrecht Haarlemmermeer announced that primate plummeting rates were related somehow to the curvature of the earth.  In 1503 another Flemish mathematician named Merksem Amersfoort declared that plummeting behavior had something to do with the tidal effects of the moon.  And in 1505 yet another Flemish mathematician named Oss Eindhoven stated in his Encyclopedium of Known World Monkeys that he had almost definite proof that primate plummeting rates either increased or decreased or something during periods of intensified sunspot activity.  By 1510 all three men had been committed to institutions for the mentally unsteady.
       In the late 18th century, the notorious French animal doctor Jean-Marie Basteogne, Vicomte de Bal Une, discovered that all monkeys did not plummet at the same rate.  He accomplished this by dropping them from a hot-air device of his own invention and then timing the impacts.  Unfortunately, his Swiss timepiece malfunctioned and the French Revolution broke out, preventing him from either continuing his research or keeping his head.  From the 18th to the 20th century, little new information was added to what was already known about primate plummeting rates.
       Prior to the 1970's, only Streebs-Watley (1893), Vanhammerfest (1937), and Kiplinger (1940) had published anything at all on the subject.  Before that time it had not been possible to accurately ascertain very many primate plummeting rates in the wild, although several variants of the old stopwatch and sextant technique had been tried with little success.  But the utilization and modification of the baseball speed gun in 1971 gave primate pesematatologists the tool they needed to begin comprehensive studies, and now many workers have undertaken investigations into a number of aspects of plummeting behavior.  Few however have attempted to integrate data from several different species of primates to try to compare and contrast this behavior.
       It has been known for a long time in musty museums and lofty halls of academe that primates evolved first in the bushes.  Although we might not want to admit it, our ancestors were in the bushes for a long time before they were ever in the trees, and an even longer time before they came down from the trees.  So it is to the bushes that we must first look.  But fortunately we need not look too long, since plummeting behavior itself was a product of a more arboreal way of life.  Early primates soon found that plummeting from bushes was a fruitless endeavor, and it may well have been the desire to gain altitude (and fruit) that drove our little shrewlike ancestors into the taller treees in the first place.
       Theory suggests that on an airless earth objects like monkeys and cream pitchers should fall at the same rate, accelerating at a uniform 32ft/sec/sec and continuing to accelerate until impact.  The effect of the earth's atmosphere then is to introduce the factor of air resistance to the equation.  Depending on latitude, temperature, density and a number of other arcane atmospheric variables, the air will provide greater resistance to an object with a large surface area than to one with a small surface area.  On the other hand, air resistance can be overcome by sufficient mass or weight, and will be overcome to a greater extent by a heavier object than by a light one.  Since the mass of the earth is so great in relation to that of any given primate, even a fat one, the difference in weights between individuals of any species is almost inconsequential, in and of itself.  But the larger a primate grows, the larger its mass-to-surface area ratio becomes, and the more easily it overcomes the air resistance acting on its proportionately smaller surface area.
       Falling objects in the air accelerate until the coefficient of friction of the air is great enough to prevent further acceleration.  This then is terminal velocity.  On an airless earth, terminal velocity would be the velocity at impact, but in the atmosphere an object with a large surface area-to-weight ratio will quickly reach terminal velocity, assuming of course a high enough jump-off point.  On the other hand, an object with a small surface area-to-weight ratio will continue to overcome the air's resistance and accelerate for a longer period of time, thus reaching terminal velocity later and at a higher speed.  It should also be understood that a small object has a greater surface area-to-volume ratio than a large object, and since air resistance acts on surface area, a small object is likely to reach terminal velocity first and at a slower speed (Smith, 1985; Ricketts, 1990; Marbelson, 1992).  This is why an insect can fall off the Empire State Building and land with nothing worse than a bruised labrum or a thumped thorax.  With these things in firmly in mind, we may now proceed to the matter of plummeting monkeys.

The Study

        It is not always clear just what ambition any given primate is attempting to realize by plummeting in this fashion, what inhuman emotion drives him to risk his life and limb this way, or what warped evolutionary pressure created this odd behavioral pattern in the first place, but there seems little question that it is widespread across the primate order, and possibly present also in species that are now extinct, such as Robertson's pink-nosed guenon and the Alpine wheezing macaque (Shaw, 1979; Axelrod-Abernathy, 1982; Winklehurter, 1990). While it is true that there may be specific non-evolutionary reasons for plummeting, such as escaping predation by upper canopy hunters, trying to beat an opponent to a terrestrial food source, or simply losing one's balance and plummeting inadvertantly, the ubiquitous nature of primate plummeting in most forest environments in both the Old and New Worlds demonstrates that it is a behavioral pattern that has been around for a long time.
       The idea for this study was originally proposed in 1963 at a meeting of the Society for the Prevention of Falling Mammals, whose members later somewhat inexplicably voted 427 to 2 against having anything to do with it.  Then, in 1970, the Massachusetts Primatological Association and the USSR Academy of Primatology, Minsk, joined hands to fund a long-term project to observe and record primate plummeting behavior in Africa.  Unfortunately, they decided to award the grant to the peripatetic Icelandic primatologist-adventurers Leif Englanberg and Olaf Petersen, and they still haven't discovered anything.  Our project began in October, 1972, and ran until June, 1989, when the Natural Geographic Society found out exactly what it was that they were supporting and abruptly withdrew their funding.  Still, we did manage to collect data on ten of the eighty-six primates we originally planned to study.
       We set out with the intention of scrupulously balancing our data to provide a rigorous comparison between New and Old World lesser primates, chiefly cercopithecines, colobines, callitrichids and cebids.  We were interested especially in great blue marmosets, flowery pottos, gray paladins, rubberneck guenons, bleary-eyed baboons, stinky galagos, and winking martindales, seven of the best plummeters in the primate order, but our assistants couldn't find enough of them to study.  We also wanted baseline information on great and lesser apes, such as the giant pygmy chimpanzee and the great horned gibbon.  Regrettably, much of this further information will perforce have to wait upon the eventual resumption of rational thinking at our major funding agencies.  For now, the data collected from a variety of sources must suffice as the first true indicator of comparative primate plummeting rates.
       The question of just which species to include in our study was one that my erstwhile co-author and I agonized over for nine long years.  It was such an arduous and stressful task that we frequently despaired of ever accomplishing it.  Three times we ran out of fuel and once the refrigerator stopped working.  Our subscription to Primate Nooz was summarily suspended for lack of payment, and we were asked to leave the Library of Congress for fighting in the stacks and had to find another place to shower.  The American Primatological Association announced that they were investigating our data-collecting methods, and then all funding for gnathic research was abruptly cancelled by Gabon, Togobogo and Great Britain.  Just when we thought we had finally made some right choices, something else came up and we had to start all over again.  Ultimately however, a selection was made and the real work of the project began.  The following table shows the ten species we were able to research thoroughly.

Species we were able to research thoroughly.
______________________________________________

New World:

Old World:

Ateles olympica(jumping spider monkey)
Cajuru imperator (purple wannaby)
Procebus croesusii (Croesus monkey)
Andradus blanca (white-cheeked muscatel)
Saguinus rex (blue-blooded tamarin)

Cercopithecus subterraneus (bluetail guenon)
Nasalis nasalis (hairy-nosed proboscis monkey)
Nycticebus slowpokeii (really-slow-loris)
Tarsius irritatus (sulky tarsier)
Presbytis oro (gold leaf monkey)

The jump-off

        Before we can discuss actual plummeting rates and the reasons why those rates vary so widely, we need to look at the jump-off, because the jump-off is usually the most crucial phase of the plummet.  I have defined the jump-off in my textbook Primate Pesematology as the first 1.5 seconds of the plummet.  It is during the jump-off that the primate customarily chooses its plummeting orientation, thereby setting the stage for the plummet to follow.  Most primates lack the felid's easy ability to change its orientation during a fall, thus they are quite keen on getting the right jump-off angle at the start, and each primate species that engages in this behavior has its own characteristic pre-plummet stance.
       There is some evidence also that pre-plummet postures reflect the state of mind of the particular primate in question.  The purple wannaby acts unobtrusively, the Croesus monkey challenging, and the really-slow-loris cautious to the point of hesitancy.  The bluetail guenon is as fearful as the white-cheeked muscatel is ostentatious. The gold leaf monkey appears only mildly unconcerned, while the hairy-nosed proboscis is often downright reckless (and we have noticed a reduction in the population of this species perhaps as a result).  Both the jumping spider monkey and the blue-blooded tamarin exhibit a true joie de vivre that is characteristic of few other mammals whether anthropoid or not, while the sulky tarsier seems sometimes sunk in a depression that only it can appreciate.
       We can be fairly certain that the behaviors we have witnessed are somewhat representative of the wide range of the primate order, even though plummeting is a noctural activity for 69% of anthropoids (Putney-Swopes, 1994) and 87% of prosimians, and, unfortunately, we always had to return to camp at dusk.  Thus we were not able to record any nighttime plummeting directly (having a searchlight focused on them definitely caused most primates of any species to abort the plummet). Anecdotal evidence and veterinary records suggest, however, that nocturnal plummeting rates in primates vary discrepantly with any size or weight parameters, and that injuries are significantly higher at night, perhaps due to the reduced ability of diurnal mammals to see in the dark.
       One primate whose pre-plummet posture and the jump-off itself have been closely observed is the Australian pouched langur, Avunculus australis.  The plummeting of these peculiar anthropoids has become quite a source of irritation to many since they reside in the basements of people's homes, and plummeting under these conditions, among boxes of mismatched nails, stacks of newspapers and old cans of paint, is not a quiet activity.  The height of a typical pouched langur plummet is necessarily limited by the heights of those basements' ceilings, and none of the basements surveyed were of sufficient height to allow the attainment of terminal velocity.
       Fig. 1 shows the typical jump-off posture of the purple wannaby.  Note the manner in which the chest is inflated, the neck tucked in, the tail extended and the feet widely positioned.  This is the most energy-efficient way to launch

into the type of plummet utilized by the purple wannaby.  Compare this to the very different pre-plummet posture of Procebus croesusii (Fig. 2), with its head thrust defiantly forward, its arms extended outward and upward, and its feet positioned uncomfortably close together.
       Tarsiers prefer to jump off quietly and unobtrusively, almost sadly, from a vertical clinging position, while Andradus stands quadrupedally on a branch and rotates flamboyantly downward with its tail in the air as the plummet begins, letting go first with the hands, and then with the feet.  Gold leaf monkeys have been observed standing bipedally and jumping off to plummet downward feet first, and they also demonstrated a marked preference for plummeting from the crowns of the giant emergent trees Landacia iticulata, Caudatia spoelii, and Provergatus lacchus. The really-slow-loris is slow and deliberate, backing and filling on one branch after another, stopping for long moments to gaze around and then moving again, and generally delaying the jump-off as long as possible.  This is

The plummet

        For the purposes of the immediate study, the plummet shall be defined as the period between the jump-off and the landing.  To begin with, the typical primate plummets in one of only two possible modes.  He or she either strikes or is struck by intervening vegetation, or falls straight down unimpeded.  It should be obvious then even to pre-seventh graders that the bioarchitechtural character of the forest has much to do with plummeting rates.  Further, different workers in the field have interpreted plummeting rates very differently, choosing for instance to emphasize duration of fall rather than velocity.  Marshall et. al.'s classic monograph on vegetational morphologies and plummeting rates in the first issue of the Darwin Society Journal established clearly that hitting branches and other biotic impediments both slowed the speed of the fall and increased its duration.  Striking intervening vegetative strata also has possible medical consequences.
       It is abundantly clear from the data presented in appendix E that primate plummeting rates vary across taxonomic lines.  This after all can be determined by speed gun analysis and monovariate vector sampling, and is in itself unsurprising.  What is significant is not that anthropoids of different masses accelerate at different rates to reach different terminal velocities, but that through complex behavioral adaptations they are able to affect their plummeting rates and even their chances of survival, and Appendix D demonstrates to nice effect the degree of variance to the Tinkler-Yarmouth Norm that is present in even such a small sample of animals as ours.
       Primates employ an unexpected number of different strategies in attempting to achieve their longterm goals vis-a-vis plummeting.  Some species, such as C. subterraneus and Presbytis oro, seek to increase their surface area in order to reduce their acceleration and lengthen their plummeting time.  Others, of which the nasalids are a good example, strive to present as small an area as possible to be acted upon by wind resistance, and this along with greater weight maximizes their acceleration and allows them to plummet the vertical distances involved in as short a time as physically possible.
       Some methods of slowing the fall that have been utilized with good result in several species are plummeting while holding on to large quantities of vegetation, plummeting horizontally with arms and legs outstretched, and plummeting with an old hornbill nest as a parachute (Fleiglehaus, 1985).  Andradus blanca and Ateles olympica (Fig. 3) both have been observed using the double plummet, either with offspring, or in rarer cases with younger siblings.  Cajuru imperator arcs downward gracefully, using a very small energy budget to adjust its trajectory after the initial jump-off.
       Workers in the field, most notable Dr. Oondóué M. Boué, have reported that bluetail guenons spend much of their fairly limited time outside the burrow in plummeting activities, climbing reluctantly up into the uneven and thorny crowns of nearby Macacia suprensis trees, then carefully grasping a large leaf in each hand and foot, and jumping off

The landing

        The landing is defined as the final second of the plummet.  It is really not all that important to the subject of primate plummeting rates because, after all, at the point of landing the plummet is over, and at that point plummeting rates of all species tend be zero.  The only exception to this is the great blue marmoset, an animal for which data was not sufficient to include in this study, which often chooses to land on a steep slope, prolonging the plummet as it rolls toe over toothcomb down through typical Amazonian hillside vegetation, and giving it by far the longest plummeting duration times recorded to date by western scientists.

The analysis

        Having described in some detail several of the gross behavioral features of our sample of primate plummeters, we can now turn to a statistical analysis of the data so painfully gathered over the course of the last two hundred months.  Appendix E shows rates of acceleration and terminal velocities for the ten species in our study.  It seems clear to us at least that there is no apparent significant correlation between weights, body sizes, and plummeting rates, but of course we could be wrong.

Conclusions

        After nine years of laborious planning and eight years of burdensome field work, we have determined almost categorically and without very much doubt that primates do not plummet at exactly the same rate.  We tried to use quantum point analysis, and made a brief stab at differential time-distance equations, followed by a bit of isopropic calculus and game theorums (Shaw, 1982), and we utilized to somewhat good effect Professor Ashram Suleiman's fractional data scattering codes as proposed for jumping rates of Turkish viverrids.  Our conjectures may be questioned by some, but no one can deny our heartfelt and unfeigned approach.  We expect that there may be some negative reactions in the primatological journals, but we plan to press ahead and not allow the naysayers to deter us from our final goal, the publication of which we are planning to be coincident with the 200th anniversary of the War of 1812.

Acknowledgements

        The authors wish to gratefully acknowledge the significant contributions, unstinting support, and just darn good cheer and unquenchable high spirits of the following individuals and institutions:  Mrs. J. Muffet Gloverhouse, the Icelandic Primate Rehabilitation Project, everyone at the Mahatutu Injured Leaf Monkey Shelter, Dr. Heckelen Jeckel, the Tremblay-Waxenhauser National Center for Primate Disorders, the Russian national primatology newspaper Primatsiya Primatsiya, the Alaska School of Orthopedic Veterinary Medicine, the Antigua and Barbuda College of Caribbean Primatology, Dr. Jerry Archbibble and the Hellmouth Municipal Zoo and Exotic Animal Crematorium, the Natural Geographic Society, the Cheesequake Municipal Man and Mammal Museum, the National Primate Bowel Clinic, Dr. Oondóué M. Boué, Dr. Watanabe Kibombo, Dr. Ambato Ambilobe, and Dr. Miedzyrzecz von Czechowice-Dziedzice.  We especially want to recognize the work of the Free Fall Club of Jujube and Togobogo, the French Monkey Academy Precision Flag Plummeting Team, Senhor Teófilo Afonso Rosario Sobradinho, Piet Mons Apeldoorn, the Rochefort-Chateauroux Institute for Simian Science, Drs. P. Rudyard Kiplinger and Buffy Rockefeller, the Hellmouth Human Diseases and Primate Testing Facility, Sir Ian Spotswood Allenby Crofford-Wiggles, the Bluetail Foundation and the Udon Sawan Lesser Ape Sanctuary.  We also wish to recognize the contributions to our work of the International Plummeting Recovery Center and the Arizona Science Foundation.  We are forever in the debt of Dr. Francois Quimper Bonnetable Rochefort-Chateauroux, Dr. Mawbanna Waddamana, Dr. Oscar Simon Bolivar Bolivar-Fuentes, Mr. George Jefferson, and Dr. Poon Sandandtundra.  Finally, we must thank Professor Rolf Sigurd Vanhammerfest, the Baseball Commission, Sigsbee Junior Night College, Primate Nooz and PRIMATE LIFE, the Society for the Prevention of Falling Mammals, the Swedish Primatological Association, the Antique Sulky Tarsier League, Professor Mitsuo Ohhohoho (wherever he is), the people of Afghanistan, and Mr. Chris Shaw.

Appendix A

Comparative weights of selected species:

Appendix B

Comparative dimensions of selected species:

Appendix C

Locations where each species was studied:

Appendix D

Coefficients of drag (M and L) and mean frequency parameters
for primate plummeters (P1 and X2):

Ateles olympica
Cajuru imperator
Procebus croesusii
Andradus blanca
Saguinus rex
Cercopithecus subterraneus
Nasalis nasalis
Nycticebus slowpokeii
Tarsius irritatus
Presbytis oro

    0.032
    0.016
    0.029
    0.007
    0.044
    0.058
    0.018
    0.025
    0.009
    0.030

      2.2+1
      1.9+1
      4.0+1
      3.9+1
      1.2(  )
      2.3-1
      2.8-1
      1.5-2
      3.6-2
      2.5-3

Appendix E

Mean weights (MW) and terminal velocities (TV) of selected species:

Species

House cats*

Tarsius irritatus
Saguinus rex
Andradus blanca
Nycticebus slowpokeii
Cajuru imperator
Cercopithecus subterraneus
Presbytis oro
Ateles olympica
Procebus croesusii
Nasalis nasalis

Homo *

MW:
(in gms)

4536

135
495
955
1096
2280
4619
4850
5120
5125
8473

68040

Speed at TV:
(in feet/sec)

88.12

82.09
84.71
92.92
90.19
93.05
96.42
91.68
96.20
105.47
103.90

176

Reach TV at:
(in feet from jump-
off point)

60

63
45
57
61
72
81
94
89
112
127

300

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