The Effects of Interbreeding on the Morphological and Genetic Attributes of Non-human Primates / Beth Christine Laas
"The Effects of Interbreeding on the Morphological and Genetic Attributes of Non-human Primates"
By: Beth Christine Laas
Anthropology Senior Thesis
Dr. Martin Nickels, Advisor
Spring 2001
Abstract
Primatologists have been observing interbreeding in non-human primate species since the early
1960s. Often this interspecies breeding produces a mixture of morphological and genetic features in
the hybrid offspring. The purpose of this paper is to address the effects this interspecies breeding has
on the morphological, fertility and viability of the offspring. This paper will analyze observational data
and available species studies on interbreeding between Papio hamadryas and Papio anubis in a
natural environment, as well as the mating of a gibbon and siamang and a rhesus macaque and a
baboon in captive situations. It will also examine the different explanations for this interbreeding as
well as what perpetuates it. Through this data collection this paper will show that the different
explanations of how interbreeding is perpetuated are both integral parts of this behavior.
Introduction
Interbreeding between different non-human primate species has always been of interest to primatologists and geneticists alike. The results of interbreeding are offspring that are intermediate between their parents in terms of morphology, genetic traits and behavior. In this paper I will analyze the data from existing journals and species studies of a number of different primate species, including two species of baboons in the wild, as well as the interbreeding of macaque species and a gibbon and siamang in a captive situation, in order to determine the effects of hybridization on the offspring. When examining hybridization between different species, certain questions arise such as whether there is a closer resemblance to one parent or the other, or if there is a possible advantage to being a hybrid rather than a more "pure" form. There are also two prevalent explanations as to why interbreeding occurs in the wild and how it is perpetuated.
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These two ideas are presented, and their appropriateness will be determined. The purpose of this paper is to evaluate these ideas and, in a broader sense, address the overall effects of interbreeding on nonhuman primates in general. I will be looking at several different hybrid crosses in order to analyze the morphological and genetic features found in the offspring of each. Morphological, genetic and behavioral attributes found in hybrid offspring are often shown to be a mix of both parents’ features (Moore, et. al.: 1999:120). This paper will analyze these crosses and evaluate their results in order to determine which species’ are more successful and why. In addition, I will also discuss any effects interbreeding has on the physical well being of the offspring, whether positive or negative. I expect to find that the hybrid offspring produced through interbreeding in the wild will exhibit a mixture of morphological and genetic traits inherited from each parent in the cross.
Literature Review
Interbreeding among nonhuman primate species has been well documented in the wild between two baboon species, Papio hamadryas and P. anubis. Explanations have been proposed as to why these two species keep interbreeding. Suggested explanations why these and other primates interbreed are related to their behavioral and morphological attributes. The observation of these two species interbreeding led to study of further inter-species breeding in captivity, such as the gibbon and siamang or the baboon and rhesus monkey. Initially, the physical attributes were studied, but over time behavioral attributes, social structure, genetic traits, and adaptations were studied also.
Extensive observation of the hybrid offspring of the two baboon species in the Awash Valley in Ethiopia began in the early 1970s, particularly with Nagel's fieldwork. Nagel believed the primary motivation behind the interbreeding was the abduction of anubis females by hamadryas males (Nagel: 1973). This ties into Nagel's other idea: that the hybrid zone was fixed, rather than fluctuating. Nagel did not see environmental factors as playing a part in the groups interbreeding and felt that fixed ecological barriers kept the hybrid zone strictly demarcated. Phillips-Conroy and Jolly contradicted these hypotheses. While female abduction did occur, this in and of itself was "…not frequent enough to account for such rapid changes." (Phillips-Conroy, Jolly: 1986:346). They also found in their observations that "…the hybrid zone was in flux, not static" and that it was environmental factors, such as anubis' vulnerability to drought, that
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As for other interspecies crosses, there can be no question about the motivation behind them because they occurred in a captive situation. The gibbon and siamang mating as well as the many attempts at intergeneric crosses between Macaca and Papio all happened in zoos and captive situations (Hill:
1970:217). Any motivation behind this interbreeding would most likely come from being held in the same area together, or through human intervention.
Nagel and Phillips-Conroy and Jolly did agree on the calculation of morphological hybrid index (MHI). Using seven different external features to find the MHI, Nagel discovered that there were no "pure" groups; rather there were varying degrees of "hamadryas-like" features and "anubis-like" features (Nagel: 1973). Social structure and behavior can often be affected by interbreeding. Certain innate behaviors of one species can often become confused with those of another species in a hybrid offspring. Sugawara studied the social structure of a group of wild baboon hybrids in the same region as Nagel, Phillips-Conroy and Jolly. What
Sugawara found was that while the social structure in some of the study groups was closer to that of the anubis type, it did exhibit hamadryas characteristics as well, and vice versa (Sugawara: 1979:21-3). This finding was further backed up by Phillips-Conroy, et al. in 1991, when they categorized three different types of groups, all of which displayed a certain amount of admixture (Phillips-Conroy, et. al.: 1991:355-6). There were no absolutely "pure" anubis or hamadryas groups either in a morphological or behavioral sense. As far as individuals, according to Phillips-Conroy and Jolly, there is a correlation between individual behavior and morphology. According to their 1986 article, the extent to which hybrids exhibited hamadryas-like behavior was proportional to the "hamadryas-ness" of their appearance (Phillips-Conroy, Jolly: 1986:337). The morphology and behavior of an intergeneric hybrid was described by Moore et al. (1999). The result of a Macaca mulatta x Papio hamadryas cross, this offspring exhibited external features and behaviors of both parents, as well as a vocal sound that appeared to be a combination of both species. Its head, face
http://www.soa.ilstu.edu/anthropology/theses/laas/index.htm (3 of 15) [11/30/2007 1:20:42 PM] The Effects of Interbreeding on the Morphological and Genetic Attributes of Non-human Primates / Beth Christine Laas and hair color was more like that of a macaque, while the body build resembled that of a baboon (Moore et al.: 1999:126). Behaviors such as the mantle shake and eyebrow raise were the same as in a baboon, but he lacked the teeth grinding and threat display common to this species. He also displayed fewer vocalizations than both of the parent species (Moore et al. 1999:121). This hybrid is the only long-term survivor of this type of cross, and most of the other infants died within 72 hours (Moore et al.: 1999:121). A cross between a gibbon and a siamang in captivity also produced an intergeneric hybrid. In this case, the offspring was also described as intermediate between each parent. The siabon displayed features that were found in both parents as well as a few that were not found in either. Wolkin and Myers suggested that since the siabon had more features in common with the siamang, but lacked the specialized adaptations (such as the laryngeal sac and alter-developing ischial callosities), that her features are more generalized and more closely resemble an ancestral hylobatid (the siamang is considered more generalized than the gibbon [p. 217]).
Hybridization not only occurs between two species in a natural environment. It also has been documented between two different genera: Papio and Macaca. These are often less successful than interspecific crosses, but there is one documented case of a "rheboon" male that survived to reproductive age.
"Baboons and macaques are closely related in phylogenetic terms and are quite genetically similar as well" (Moore, et. al.: 1999:119); however, this "rheboon" was the only survivor out of twenty-six attempted crosses and the individual was found to be sterile (Moore, et. al.: 1999:121-5). These intergeneric crosses often end up this way, as previously described by Hill (1970:216-217). Clearly, interspecific hybrids occurring either in a natural setting or in captivity are more successful in that the offspring more frequently lives to reproductive age and are viable. Aside from these two genera, interbreeding has also been documented between a gibbon and a siamang. The offspring was found to have features that fell in between those of the parents, as well as a few that were found in neither parent. Examination of the external morphological traits shows that this "siabon" has an intermediate appearance, falling between the features of the gibbon and siamang. Wolkin and Myers suggested that it is possible that the siabon inherited genetically based features that are "basic or rudimentary to both species" (221). It was unknown at the time of publication for this study whether or not the siabon was fertile, because she was still a very young animal, and no blood samples or genetic tests were performed. The viability of the offspring in the
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Obviously, in the field observations primatologists continue to see hybrid offspring; therefore, the Papio hybrids must be nearly as successful as each of the parent species. According to their 1991 field observations, Phillips-Conroy and Jolly found the baboon groups that were toward either end of the hamadryas or anubis extreme being replaced by groups of hybrid individuals (363-4). This is not trivial; it means that perhaps there is a selective advantage in being a hybrid than being either a typical anubis or typical hamadryas baboon.
Data
In order to examine the effects hybridization has on the offspring, the morphological features must be discussed. In the early 1970s observers of hamadryas-anubis interbreeding developed a method for determining the morphological hybrid index (MHI) of the hybrid offspring. Seven morphological features were examined and described, and then a score from zero to fourteen (zero being "pure hamadryas" and fourteen being "pure anubis"). These scores were then averaged to find the hybrid index of the group as a whole (GHI) (Phillips-Conroy, Jolly: 1986:340). The description of the seven external traits for each of hamadryas and anubis are noted in table one. It was found that there were no "pure" phenotypes, that each
Table 1: Key to hamadryas and anubis character states* Feature Hamadryas Condition Anubis Condition (A) Facial Skin Bright pink to red Dark brown-black (B) Cheek tufts Silvery white As rest of coat (C) Cheek tuft shape Long, projecting Circular, not projecting laterally beyond facial outline (D) Mane length To elbows, contrasts Shorter, blends with with back back fur (E) Tail shape Gently arched distal Sharply bent, tip hangs part, carried ± vertically horizontally (F) Rump skin color Bright pink to red Dark plum color (G) Rump patch shape Large, heart-shaped, Narrow, scarcely visible laterally visible laterally *In each case, the “hybrid” condition is intermediate between those of the two parent states. Modified from Nagel (1973). From: J.E. Phillips-Conroy and C.J. Jolly, 1991. male hybrid in the sampling was morphologically distinctive, and that all the groups in the sample were to
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1991:355-6). Morphological attributes for the hybrid offspring of a gibbon and a siamang are noted in in table two. It is noted that the "siabon" has features of both the parents, giving in an
Table 2: Morphological Differences Among S. syndactylus, H. lar and Hybrids¹ __S. syndactylus______H. lar______Hybrids____ Feature Typical Mother ² Typical Father² I² II² Fur Density Sparser X Very dense X Sparser X
Interdigital Present X Absent Present Present X Webbing
Hair Color Black X Black, buff, Gray Black Brown grey
Ischial callosities Late -- Early -- Early Late appear (in infancy
Laryngeal sac Present X Absent Present Present X
Brachial index Low X High X Low X
Face ring Absent X Present X Present Absent
¹From: Wolkin and Myers (adapted from Groves, 1972 [51-53]) pg. 218-19 (some content edited).
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²X indicates the presence of the characteristic in that animal; -- indicates measurements currently unavailable or not known. intermediate look (See figure one). This same type of intermediate appearance is
[ Insert Figure 1 ] found in the "rheboon" (rhesus macaque x hamadryas baboon). This hybrid has a weight that falls in between that of a male baboon and a male rhesus macaque. His tail shape is more like that of the macaque, his body shape and size are like that of a baboon, but his maxilla is shortened which creates an underbite (See figure two). He also exhibits behavior of both parents, as well as at least one
[ Insert Figure 2 ] vocalization that is a combination of baboon and macaque sounds (Moore et al.: 1999:121).
The second factor that must be discussed is the genetic attributes of the nonhuman primate hybrids. Table three shows some of the proteins examined in the blood sampling of a particular group of hybrids outside the Awash Valley.
Table 3: List of Blood Proteins Examined
Abbreviation Name of Blood ProteinReference PA-1 Plasma prealbumin-1 Barnicot et al. (1965) PA-2 Plasma prealbumin-2 ------et al. (1965) Tf Plasma transferring Barnicot et al. (1965) Pi Plasma protease Omoto et al. (1970) inhibitor ± Plasma slow ± globulinBarnicot et al. (1965) Cat Plasma catalase Shaw & Prasad (1970) Ch-Es Plasma cholineesterase Barnicot et al. (1965) Plasma Es Plasma esterase ------et al. (1965) Hb- ± Hemoglobin- ± Ishimoto, Kuwata & Shotake (1975) Hb- ² Hemoglobin- ² ------, ------& ------(1975) PHI Cellphosphohexose Shotake, Ohkura & ismerase Ishimoto (1977) 6PGD Cell 6- Ishimoto (1972) phosphogluconate dehydrogenase
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PGM-I Cell Shotake, Hopkinson & phosphoglucomutase-I Harris (1968) PCM-II Cell ------, ------& ------phosphoglucomutase-II (1968) CA-I Cell carbonic Shotake & Okhura anhydrase-I (1975) CA-II Cell carbonic Tashian et al. (1971) anhydrase-II MDH Cell malate Shotake & Nozawa dehydrogenase (1974) LDH-A Cell lactate ------(1974) dehydrogenase-A LDH-B Cell lactate ------(1974) dehydrogenase-B TO Cell tetrazolium oxidase Baur & Schorr (1969) IDH Cell isocitrate Ishimoto, Kuwata & dehydrogenase Shotake (1973) EsD Cell esterase D Hopkinson et al. (1973) Cell Es Cell esterase Kitajima et al. (1975) From: T. Shotake, 1981. (some content edited)
Through this genetic sampling, it has been suggested that the hybrid zone between the two Papio species is not as narrow as it was once thought to be. It seems also that gene flow around the borders between hamadryas and anubis groups could possibly be caused by the kidnapping of anubis females; however, the fact that a group formed of almost pure anubis that consisted of almost pure anubis genes only by one-way gene flow suggests something else. It is likely that the hybridization of anubis troops by the hamadryas was accomplished over time by hybrid baboons who could adapt to the anubis environment and social structure easily (Shotake: 1981:302-3). Genetic studies of the "rheboon," show that the banding patterns of the hybrid are fairly similar to those of macaque and the baboon. This is illustrated in figure three. Fertility studies are an important
[ Insert Figure 3 ] part of understanding the hybridization process. While intergeneric breeding between macaques and baboons has been relatively unsuccessful, interbreeding between species of macaques has proven to be more successful. This is illustrated in in table four, which shows survival and fertility rates of certain
Macaca hybrids. If hybrid offspring are sterile, there is no way for this to be
Table 4: Macaque Hybrids Produced and Hybrid Fertility Demonstrations
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Mother Father Sex Birthdate Survival Reproduction Survival
Macaca Macaca &@ 25.Aug.64 Alive 18.9.70, paternal Stillbirth nemestrina fascicularis backcross
Macaca Macaca &B 11.Nov.64 Alive Two confirmed -- nemestrina fascicularis with hybrid female
Macaca Macaca. &@ 15.Nov.67 Stillbirth -- -- nemestrina niger
Macaca Macaca &@ 22.May.71 10 weeks -- -- nemestrina fascicularis
Macaca nigerMacaca &@ 17.Sept.68 Alive &@ 4.7.73 10 weeks nemestrina [email protected] Stillbirth
[email protected] Stillbirth
[email protected] Alive
Macaca Macaca ? 6.Nov.1970 Aborted -- -- hecki nemestrina
Macaca Macaca &B 5.Apr.72 Alive -- -- mulatta fascicularis
Macaca Macaca &@ 11.Apr.67 Alive &B24.5.73, with Alive nemestrina mulatta hybrid
&B24.7.74, with hybrid 6 weeks
&B4.6.75, paternal backcross Alive
From: Donald G. Lindburg: 1980:128-9. perpetuated. Since there are so many varied groups of hybrid offspring in and around the Awash Valley, it seems safe to assume that the hybrids are successful; that is, they survive to reproductive age, and pass on their hybridized traits to successive generations, although they may or may not be as successful as each of the parent groups. Studies indicate that this is much more likely in member of the same genus, as opposed to intergeneric crosses. This is illustrated in the fertility study of a "rheboon" cross. In this situation,
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[ Insert Figure 4 ] hybrid’s biopsy, there was a lack of any mature spermatozoa, supporting tissue and there were very few Leydig cells found (Moore et al.: 1999:121-2). The testosterone secreted by the Leydig cells is responsible for the development of secondary sex characteristics and it promotes spermatogenesis.
In order to further explore the explanations behind nonhuman primate interbreeding in the wild, it is necessary to evaluate the social behavior of the animals as well as the environmental conditions when it occurs. As stated earlier, the kidnapping of anubis females was originally thought to be the most important factor in the perpetuation of natural interbreeding between Papio hamadryas and P. anubis. Early studies by Nagel (1973) showed that anubis groups living in regions above the Falls were dominant over the hamadryas populations inhabiting the region downstream, and could expand into hamadryas territory if there was significant population pressure. In this setting, hamadryas males would kidnap anubis females and would in turn hybridize the hamadryas groups (Phillips-Conroy & Jolly: 1986:337). Continuous crossing between anubis females and their offspring with hamadryas individuals produced “variable but predominantly hamadryaslike populations” (Phillips-Conroy & Jolly: 1986:337). The earliest model constructed by Nagel in 1973 worked with the hypothesis that the hybrid zone strictly followed ecological boundaries and that this hybridization was primarily caused by the kidnapping behavior of the hamadryas males. This would result in a “one-way gene flow from the anubis to the hamadryas side” (Sugawara 1979:22). Both Kummer (1972) and Nagel (1973) observed hamadryas males with anubis females and drew the conclusion that an abduction was in progress.
It has also been suggested that environmental factors play an important role in interbreeding. The hamadryas baboons are more accustomed to living in dry, semi-desert scrub regions while the anubis live in savanna and woodland-type environments. The anubis baboons are vulnerable to periods of drought, and they suffer more than the hamadryas, in that they may experience a much higher rate of dental
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table 5 the Awash Valley. As the amount of rainfall decreases, the zone expands into the drier regions, and it could be predicted that the zone would expand conversely during periods of higher productivity; either way, hybridization could result (Phillips-Conroy, Jolly: 1986:347).
Analysis
The question of what motivates and perpetuates natural hybridization has been answered by two different theories. The first hypothesis drew its support from the kidnapping behavior of hamadryas males as well as the separation of the two species in two very different environments. Nagel believed that the hybrid zone was fixed, rather than in flux due to environmental and population pressures. However, subsequent observations have not seen this kidnapping behavior between hamadryas males and anubis females conclusively demonstrated; that is, it was assumed that it occurred, not actually seen (Phillips-Conroy & Jolly 1986:346). Research seems to indicate that although kidnapping does occur, interbreeding more likely comes as a result of cross-migration and ecological factors. Table six illustrates the fact that individuals that migrate into different troops reside there for rather extended periods of time. Had
Table 6: Age and Duration of Residence of Cross-migrants¹ ID 1stTR Indate Outdate Res TA YOB Inage Outage Fate 10603 08/83 12/73- 08/83- 2-114 17 66 8-17 17-18 Unkn. 16/83 06/84 10612 08/83 12/73- 08/83- 2-114 17 66 8-17 17-18 Unkn. 06/83 06/84 10617 08/83 01/77- 09/84- 16-108 13 70 7-13² 14-15 Unkn. 06/83³ 12/85
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10642 08/84 09/83- 09/86- 27-52 17 67 16-17 19-20 Unkn. 06/84 01/88 10700 06/86 09/84- --³ 55-70+ 14 72 13-14 -- -- 12/85 10720 07/86 09/84- 09/86- 10-40 12 74 10-12 12-14 Unkn. 12/85 01/88 10737 01/88 08/86- --³ 32-49+ 13 75 12-13 -- -- 12/87 10802 01/89 Pre-07/88 --³ 25+ 14 75 14 -- -- 10808 01/89 Pre-01/89 --³ 19+ 12 77 12 -- -- "Humpy" -- Pre-07/88 01/90- 17+ 20+4 694 Unkn. 20+4 -- 05/90 "Brownie" -- Pre-07/88 11/89 16+ 124 774 124 134 Died5 ¹1stTR, date (month/year) of first trapping; Indate, earliest and latest possible date of entry into this Type I group; Outdate, earliest and latest possible date of exit from this group; Res, minimum and maximum possible residence length (number of months during at least part of which animal was resident); TA, estimated age at trapping; YOB, estimated year of birth; Inage, youngest and oldest possible ages at entry; Outage, youngest and oldest possible ages at exit; Fate, how animal exited. ²Earliest dates of entry and youngest age assuming it was an adult. ³Still in residence, July 1990. 4 Age approximate, determined by observation. 5 Killed by predator, or shot. *From: Jane E. Phillips-Conroy, et al.: 1991:361. the primary cause for hybridization been kidnapping, there would probably be little or no evidence of migrant males residing in "foreign" troops. However, this is not to say that the kidnapping behavior had no impact in hybridization; it probably did. It seems unlikely though, that is was the primary motivation.
Observations following Nagel have resulted in the construction of a very different model to explain natural interbreeding. Phillips-Conroy and Jolly developed this ecological model, of which the starting point was the idea that hamadryas and anubis baboons possess distinct adaptations to different environments
(1986). The social structure of each species is well adapted to its respective environment. From this point, it is clear that ecological stresses can force two species together, and in this scenario, result in hybridization. Periods of drought, as illustrated in table five, are more stressful to the anubis baboons. This species has adapted to life in forests and savannahs, not semi-desert regions. Drought causes a rapid decrease in vegetation, even along the riverain strip, which the anubis use as a base as well as a resource (Phillips-Conroy, Jolly: 1986:347). This vulnerability puts the anubis at a definite disadvantage and it becomes easier for the hamadryas to enter into their groups and mate with the anubis females. It is also reasonable to predict that during times of higher productivity, the anubis and anubislike hybrids would move into the hamadryas regions.
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While these two hypotheses differ in their use of the kidnapping behavior and ecological factors as theories of motivation for hybridization it seems that, rather than using either one or the other for an explanation, both should be used. Behavior and environment each have significant effects on these baboon species. Ecological pressures may force two species into contact with one another, but hamadryas males kidnapping anubis females still occurs. Therefore, it cannot be ruled out as one of the causes for interbreeding.
Aside from the causes for interbreeding, effects on individual hybrid offspring were also discussed. The hybridization that occurs in the Awash Valley is between two different species, unlike the "rheboon" and "siabon" which were the products of two different genera. This research supports the notion that crosses between separate species are often times more successful than intergeneric crosses. While the "rheboon" and "siabon" are survivors of this type of cross, they are not necessarily successful in terms of their own reproduction. This is illustrated by the fact that the "rheboon" as well as other intergeneric hybrids are often found to be sterile (this information is not known for the "siabon"). Interspecies breeding produces more viable offspring, as shown in table four in contrast to the "rheboon" which was the only survivor of twenty- six attempted crosses (Moore et al.: 1999:121). Therefore, it seems safe to assume that interspecies crosses are usually more successful than intergeneric crosses.
Conclusion
This research shows that, while there are different ideas as to why interspecies breeding occurs in the wild, neither one is entirely correct. Both behavior and environment must be taken into consideration in order to accurately evaluate natural hybridization. However, there are limitations to this research project. Not being able to directly observe natural hybridization limited the data to only previously published studies. The only interpretations that could be offered and evaluated were of those who had witnessed this behavior directly. More work should be done in this area in order to expand the data to females of the species, as well as to determine whether or not there is any selective advantage to being a hybrid rather than a purer form.
Since hybrid groups are observed in the Awash Valley, perhaps further study could demonstrate whether or not these groups are becoming more successful or dominant over purer groups. Also, the nonhuman primates are our closest evolutionary cousins. Further observations of interbreeding in a natural setting
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Hill, W.C. Osman. 1970 Primates: Comparative Anatomy and Taxonomy vol. 8. New York: Wiley- Interscience.
Jolly, Clifford J., et al. 1997 "Intergenetic Hybrid Baboons." International Journal of Primatology 18: 597-627.
Kummer, Hans. 1968 Social Organization of Hamadryas Baboons: A Field Study. Chicago: The University of Chicago Press.
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