Orthocephalization in the postweaning squirrel monkey

Twenty male squirrel monkeys (Saimiri sciureus boliviensis) raised in captivity were allotted to one of the following groups: weanling control (C6) sampled at 6 months of age; young control (C24) fed ad libitum on a control diet and killed at 24 months of age; and malnourished (M24) fed ad libitum on a low-protein diet and sampled at 24 months of age. Cranial points and the lateral semicircular canals were marked. On each skull, a strict lateral teleradiograph was taken, and the lengths of the midsagittal chords and their angles with respect to the vestibular line were measured. Age changed the lengths in about 70% of the chords and more than 50% of the angles. Malnutrition arrested about 50% of the lengths, but the angles were practically not affected. It is concluded that the postweaning Saimiri sciureus undergoes orthocephalization according to a general pattern already observed in rodents and suggested for pongids. Postweaning malnutrition affected growth in size but not shape changes related to the orthocephalization of the Saimiri skull. © 1996 Wiley-Liss, Inc.     

Recognition of order effects in squirrel monkey antiphonal call sequences

Squirrel monkeys exchange chuck vocalizations in antiphonal sequences that suggest a request for information, and a response to that request. Chucks are characterized by measurable acoustic differences, notably in peak frequency, related to their position in a sequence. To determine the cues by which animals might identify a chuck that is a request for information from one that is a response, first-in-sequence (α) and second-in-sequence (β) chucks, differing in peak frequency, were recorded from familiar individuals and strangers. These chucks, and a sham control, were played back, in the temporal context of a first-in-sequence call, to a target group of socially housed captive squirrel monkeys. Animals responded more strongly to α chucks than to β chucks for calls originating from familiar individuals. No distinction was made between the α and β chucks of strangers, and all strangers' chucks got a weaker response than did familiar α chucks. Squirrel monkeys probably use acoustic differences, rather than context, to discriminate the ordinal significance of a chuck, but cannot do so unless they are familiar with the caller. © 1993 Wiley-Liss, Inc.     

Stillbirth of twins in a squirrel monkey (Saimiri boliviensis peruviensis)

This is the first published report of twinning in a squirrel monkey (genus Saimiri). The mother survived but the twins, both male and close to full term, were stillborn     

Genetically determined antigens of squirrel monkey (Saimiri sciureus) IgG

Alloantigens of squirrel monkey (Saimiri sciureus) IgG were studied using alloantisera prepared by cross-immunizations among four subspecies. The results demonstrated the occurrence of IgG antigenic polymorphism. Three distinct Fd fragment allotypes were detected. Greater alloantigenic complexity occurred among Colombian and Peruvian squirrel monkeys, which occupy the more central regions of the geographic range; Bolivian and Guyanan monkeys, which occupy the extremes of the range, expressed less alloantigenic complexity.     

Susceptibility of squirrel monkey (Saimiri sciureus) to infection by mammalian schistosomes.

In a study of host-schistosome relationships, the squirrel monkey (Saimiri sciureus) was exposed to 500 cercariae of Schistosoma bovis (Kenya), S. intercalation (Cameroon), S. mattheei (South Africa), 2 strains of S. mansoni (Puerto Rico and South Africa), 2 strains of S. rodhaini (Uganda and Kenya), and Schistosomatium douthitti (North America). It is apparent that the squirrel monkey can be employed as an experimental host for a broad range of mammalian schistosomes. Based upon cercariae-adult worm ratios, it is a good host for S. intercalatum, S. mattheei, the Puerto Rico and South Africa strains of S. mansoni, and Schistosomatium, but only a fair host for S. bovis and the Uganda and Kenya strains of S. rodhaini. Individuality of host-parasite relationships is borne out by the great ranges recorded for egg deposits in different organs as well as by total body egg counts and eggs/worm pair.     

Preliminary observations on squirrel monkey (Saimiri oerstedi) vocalizations in Costa Rica

This study was an initial attempt to analyze quantitatively vocalizations from the Central American squirrel monkey, Saimiri oerstedi. Vocalizations were recorded in the wild at P. N. Corcovado (S. o. oerstedi). and at P. N. Manuel Antonio (S. o. citrinellus). Additional recordings were made from captive individuals of both subspecies. The acoustic structure of analyzed vocalizations resembled most closely the vocal repertoire of South American Saimiri with the “gothic arch” phenotype, in agreement with the prominent gothic arch facial pattern of S. oerstedi. New structural subtypes of the twitter (“dog-tooth twitter”) and the chuck (“bent-mast chuck”) not previously found in an extensive library of South American Saimiri sound spectrograms were documented. Calls used by older infants when socially separated and when approaching an adult male were essentially identical (sharing typical “isolation peep” structural features) but had significantly different duration and peak frequency. Analysis of recordings from an escaped captive female calling outside her compound over a 3-day period indicated the presence of numerous structural intermediates between typical chuck, twitter, and peep calls.     

Effect of life history on the squirrel monkey (Platyrrhini,Saimiri) cranium

Among primates, squirrel monkeys uniquely possess an interorbital fenestra, in which the midline bony orbitosphenoid septum is largely absent and the soft tissues of the orbits are separated only by a thin membrane. Neural development may contribute to the approximation of the orbits to the midline in Saimiri, insofar as other platyrrhines with relatively large brains also have relatively narrow interorbital spaces compared to their near relatives. In Saimiri the narrow spacing of the orbits is further exacerbated by intense predation pressure on infants that may select for precocial neonates. The result is a large-headed neonate that is subject to unusual parturition constraints. These parturition constraints apply to the size and dolichocephalic shape of the squirrel monkey head in general, and to the relatively large eyes and approximated orbits in particular. The unique interorbital condition in Saimiri is an example of the effects of life history on skeletal morphology. © 1995 Wiley-Liss, Inc.     

Taste preference thresholds for food-associated sugars in the squirrel monkey (Saimiri sciureus)

The purpose of this study was to determine taste preference thresholds for four different food-associated sugars inSaimiri sciureus. Using a two-bottle test three male squirrel monkeys were found to significantly prefer 10 mM sucrose, 40 mM fructose, 90 mM glucose, and 100 mM lactose over tap water. Presentation of suprathreshold sugar solutions led to marked concentration-dependent and monotonically increasing polydipsia. The results showed the squirrel monkey to prefer lower sugar concentrations compared to other non-human primates and thus support the assumption that this species may use sweetness as a criterion for food selection.     

Meiotic characterization and sex determination system of neotropical primates: Bolivian squirrel monkey Saimiri boliviensis (primates: Cebidae)

The chromosomal sex determination system differs among platyrrhine monkeys more than any other group of primates. Although a number of studies have investigated mitotic chromosomes across platyrrhine species, the meiotic chromosomes of many genera have not yet been described. The goal of this study was to characterize the sex determination system of Saimiri boliviensis. We described for the first time the meiotic cycle, confirming the sexual system in germ cells from testicular biopsies of four adult male S. boliviensis. All specimens were weighed and testicular volume was measured. We observed 22 bivalents corresponding to 2N = 44, and a "human-like" XY bivalent was found in diakinesis/metaphase I. In addition, mitotic studies from blood samples of both sexes were performed and G- and C-banding patterns agreed with previously reported karylogy of S. boliviensis boliviensis. Further meiotic studies should be performed in New World primates based on the great value of those studies for systematic evolutionary biology and conservation programs.     

Sociophysiology of squirrel monkeys

Primate sociophysiology is an emerging discipline concerned with understanding the proximate mechanisms that contribute to the generation and maintenance of species-typical social systems. Studies of squirrel monkeys illustrate that sociophysiological processes are more varied than is commonly assumed with respect to both the effective social dimensions that influence physiological function and the nature of concomitant physiological effects. For adult squirrel monkeys, a major consequence of social stimulation is altered regulation of pituitary-adrenal, pituitary-gonadal, and autonomic function. In females, socially-induced physiological changes appear to be independent of specific interindividual relationships with other adults, whereas in males, many sociophysiological effects can be understood only when specific relational attributes are considered. In all instances, adult relationships are associated with distinctly different sociophysiological profiles than have been found for the mother-infant relationship and do not conform to the stress/buffering model within which they are often interpreted.     

Morphometric analysis of the zona pellucida and ooplasm of squirrel monkey (Saimiri sciureus) eggs

The objective of this study was to correlate the thickness of the zona pellucida (zona) with egg (oocyte) maturity determined through a widely-used method of assessing oocyte maturation namely, the evidence for cumulus and corona cells expansion. Measurements of the zona of cultured oocytes were recorded at 0, 3, 6, 24, and 48 hr after laparoscopic oocyte retrieval from hormonallystimulated squirrel monkeys. The results indicated that at the time of oocyte retrieval, oocytes that were classified as mature had thicker zonae compared with immature oocytes. The zona layer of the mature oocyte expanded to a maximum after 6 hr of incubation while the zona layer of the immature oocyte became compressed. The diameter of the mature oocyte (minus the zona and perivitelline space) became smaller with time while the immature oocyte diameter remained relatively unchanged. The correlation between the maturational state of the oocyte and the thickness of the zona layer suggest the possible application of zona morphometric evaluations as an indicator of oocyte maturation.     

Wild squirrel monkey (Saimiri sciureus) “caregiver” calls: Contexts and acoustic structure

A field study of the vocal behavior of 22 wild adult female squirrel monkeys (Saimiri sciureus) in Parque Nacional del Manu, Peru, found that 21% of vocalizations were “caregiver” calls. Caregiver calls are brief, low frequency calls, often with numerous harmonics, that are addressed by caregivers to their own infants in three contexts: 1) prenurse, signalling the caregiver's location and willingness to nurse; 2) nurse, while nursing; and 3) end nurse, indicating the end of the nursing bout. Three measures (start, end, and peak frequency) of the acoustic structure of the fundamental frequency of the caregiver calls significantly differed across the contexts. Duration of caregiver calls, however, was not distinguished by context. Compared to other primate taxa, the specificity and importance of caregiver calls in squirrel monkey vocal behavior appears unusual, if not unique. That S. sciureus caregiver calls are highly developed and employed so extensively probably follows from an unusual combination of ecological and life history factors. These factors include delayed weaning and large infant body size, high levels of indirect foraging competition which encourages spatial separation, susceptibility to predation, and specialization on a densely foliated, branch-end microhabitat in which visual contact is often impeded. © 1995 Wiley-Liss, Inc.     

Allelic variation in the squirrel monkey x-linked color vision gene: biogeographical and behavioral correlates

Most Neotropical primate species possess a polymorphic X-linked and a monomorphic autosomal color vision gene. Consequently, populations are composed of both dichromatics and trichromatics. Most theories on the maintenance of this genetic system revolve around possible advantages for foraging ecology. To examine the issue from a different angle, we compared the numbers and relative frequencies of alleles at the X-linked locus among three species of Saimiri representing a wide range of geographical and behavioral variation in the genus. Exons 3, 4, and 5 of the X-linked opsin gene were sequenced for a large number of X chromosomes for all three species. Several synonymous mutations were detected in exons 4 and 5 for the originally reported alleles but only a single nonsynonymous change was detected. Two alleles were found that appeared to be the result of recombination events. The low occurrence of recombinant alleles and absence of mutations in the amino acids critical for spectral tuning indicates that stabilizing selection acts to maintain the combinations of critical sites specific to each allele. Allele frequencies were approximately the same for all Saimiri species, with a slight but significant difference between S. boliviensis and S. oerstedii. No apparent correlation exists between allele frequencies and behavioral or biogeographical differences between species, casting doubt on the speculation that the spectral sensitivities of the alleles have been maintained because they are specifically well-tuned to Saimiri visual ecology. Rather, the spectral tuning peaks might have been maintained because they are as widely spaced as possible within the limited range of middlewave to longwave spectra useful to all primates. This arrangement creates a balance between maximizing the distance between spectral tuning peaks (allowing the color opponency of the visual system to distinguish between peaks) and maximizing the number of alleles within a limited range (yielding the greatest possible frequency of heterozygotes).     

Human-type ABO blood groups as genetic markers for the management of a squirrel monkey (Saimiri sciureus) breeding colony

The human-type ABO blood groups were determined for 94 families of the squirrel monkey which included 151 animals. Four phenotypes of ABO blood groups (A, B, AB, and O) were detected. Family analysis revealed that the human-type ABO blood groups in this species were governed by three alleles, codominantA andB and silentO. There were intraspecific differences in the distribution of phenotypes and gene frequency among three populations imported by different routes at different times. The usefulness of ABO blood groups for defining the genetic variability of a squirrel monkey breeding colony through successive generations is discussed on the basis of the difference in distribution of ABO blood groups between wild-originated parental and its first colony-born populations.     

The effect of relative humidity on osmoregulation in the squirrel monkey (Saimiri sciureus)

Osmoregulatory balance was studied in four young, tamed squirrel monkeys (Saimiri sciureus, Columbia) after acclimatization to relative humidities (rh) commonly used in laboratories (30% and 50%) and to higher humidities representative of the dry and wet seasons in their natural environment (75% and 95%). The temperature was constant at 25°C and the light-dark cycle was 12:12 hours. The animals were maintained in large metabolism cages and were free moving. Water consumption and urine flow rates increased at each higher humidity from 30% rh to 75% rh and then decreased at 95% rh (p<0.05). Fecal water loss was greater with higher humidity (p<0.05). Evaporative water loss remained relatively constant until a break between 75% and 95% rh, at which point it decreased dramatically (p<0.05). Expressed relative to total water intake, evaporative water loss demonstrated a progressive decrease with increasing humidity: 65%, 56%, 51%, and 42%, at 30%, 50%, 75%, and 95% rh. This indicates that as the humidity approaches maximum saturation, not only is the evaporative water loss component necessarily diminished, but also the apparent precipitation of alternate strategies of thermoregulation leads to a reduction in the overall water requirements of the animals. Potassium/sodium excretion ratios were relatively constant at 30%, 50%, and 95% rh but decreased significantly at 75% rh (p<0.05). As a reflection of adrenocorticosteroid activity, this suggests that at 25°C, 75% rh is an optimal humidity in the maintenance of squirrel monkeys. The significant osmoregulatory alteration occurring between 75% and 95% rh provides further evidence that relative humidity may be an important factor in the seasonal physiological cycles of the squirrel monkey. In conducting the research described in this report the investigators adhered to the “Guide for Laboratory Animal Facilities and Care,” as promulgated by the Committee on the Guide for Laboratory Animal National Academy of Sciences, National Research Council.     

Neuronal cytoplasmic inclusions in the dorsal horn and supraoptic nucleus of the squirrel monkey,Saimiri sciureus

In the squirrel monkey (Saimiri sciureus) two types of cytoplasmic inclusion bodies have been observed sporadically in neurons of both the dorsal horn (Rexed's laminae I-III in the lumbosacral region) and the supraoptic nucleus. One of these, designated here the "vesicular body", is a round inclusion which measures up to 1.4 mu in diameter. It occurs only in perikarya and is composed of vesicular-like chambers 300-400 A in diameter. We have not found previous references to this structure in the literature, but its 50 A substructural particles are similar in size to those described in nematosomes. The other inclusion, a "filamentous whorl", is found in nerve cell bodies and dendrites and it is structurally similar to the Hirano body. The structure measures up to 2.2 mu in diameter and is composed of circularly arrayed filaments which vary in configuration and size depending on the plane of section. There are no indications that the vesicular body and the filamentous whorl are in any way related to each other; and usually both are not found in the same cell profiles.     

The social organizations of squirrel monkeys: Implications for ecological models of social evolution

“Squirrel-monkeys occur in a considerable number of slightly different forms, but all are built upon a similar body plan and have a basic color scheme. One of the larger races—from inner Perú—is also the most colorful and one of the brightest colored of all mammals. It may be taken as a point of departure. The top of the head and the upper and outer parts of the body and the upper side of the basal half of the tail are a vivid green, with a pepper and salt effect of yellow and gray. The face is pure white except for black spectacles, muzzle and chin; the throat, chest, underside, insides of limbs, and the underside of the basal half of the tail are brilliant daffodil yellow. The terminal half of the tail is jet black and rather bushy. The flesh of the hands is pale pink. Other races vary in the intensity of the green and yellow, so that some may be olive brown above and white below, and in the amount and arrangement of the black areas on the face and the tip of the tail. Some have almost naked ears, others have these organs clothed in short fur, and still others bear thereupon long tufts or fringes. All these variations seem to blend into the other geographically . . . some of these pure color variations may constitute valid regional subspecies or even species. . . .” I. T. Sanderson¹ (p. 77).