Central Nervous System, Endocrinology, Reproduction and Musculoskeletal Systems

Central Nervous System

It is common knowledge to many, that humans have highly evolved central nervous systems. It is also common knowledge that humans share a common ancestor with modern chimpanzees and bonobos. This being addressed, there are many similarities and differences between the neuroanatomical structures that make up the brains of pygmy chimpanzees (bonobos) and common chimpanzees. Though similarities can be found between all primates, brain size and development is highly variable, with smaller brains occurring in Lemurs, larger brains occurring in monkeys, and the largest brains occurring in apes, including humans (Fleagle, 1987). In humans, for example, the motor cortex differs from apes significantly. Thus, the way humans and other apes go about interacting with objects within their environment differs, as apes are more prone to the use of toes, whereas humans interact more with their fingers (August, 1988). Another distinction can be found in the developmental sophistication and size of the cerebellum, a structure that mediates posture, motor coordination, and motor learning (August, 1988). As one can see, the human brain has evolved more robust features of motor control.

In comparison to apes such as bonobos and chimpanzees, humans possess prodigious cerebral hemispheres, with frontal, parietal, temporal, and occipital lobes that dwarf those possessed by other great apes (Fleagle, 1987). Thus, the overall area of the cerebral cortex is much larger. These areas of the brain serve various functions, with the frontal lobe housing the motor cortex, and playing a part in planning, attention, and language. The parietal lobe is essential tactile sensory processing and subjective spatial awareness, while the temporal lobe takes part in auditory processing, speech (in humans), memory, and emotional processing. Finally, the occipital lobe contains the visual cortex (August, 1989). See figure 1.

A critical distinction between human brains and those of other primates is that of the development of an area local to the frontal lobe known as the Broca's area, and an associated area of the parietal and temporal lobes known as the Wernicke's area (Fleagal, 1987). These areas play an essential role in both the production of language and language comprehension (Pinker, 1994). Hemispherically variable, these areas can manifest themselves on either side of the bilateral brain. Though the left hemisphere controls language in 97% of right-handed individuals, the right hemisphere tends to control language in only about 19% of left-handed individuals, with 68% of individuals having these structures in the left hemisphere or both hemispheres (Pinker, 1994). The evolutionary development of these structures in humans vs. chimpanzees and bonobos is a significant development that further separates Homo sapiens from other great apes. See figure 2.

Other neuroanatomical features that set the great apes (bonobos, chimpanzees, gorilla, and humans) apart from lower primates are olfactory structures. Among diurnal apes, smell serves a lesser advantage than other senses such as vision (Fleagle, 1987). Because of this, one can observe olfactory bulbs of a smaller proportion in higher primates, such as humans and bonobos, with much larger olfactory hardware in monkeys (Fleagle, 1987). Thus, as one can see, by comparison, the visual cortex and its associated occipital lobe are more pronounced in higher apes (see figure 1). Another difference concerns the fact that chimpanzees have been found to have a larger cerebellum than bonobos (Rilling et al., 2012). As one can see, anatomical variation of brain anatomy is ubiquitous amongst primates.

Endocrine

In addition to the brain structures already addressed, endocrine-related organs are controlled and mediated by a neuroglandular component of the hypothalamus, the pituitary gland (Behringer et al., 2014). In mammals, this gland consists of two hemispheres that include forward anterior pituitary lobe and a posterior lobe, both of which serve the function of hormone production (Constantinescu, 2017). Thus, the hypothalamus and it's subsidiary pituitary gland are the primary modulators of the mammalian endocrine system and modulate endocrine function in various organs by the release of hormones (Brown-Borg, 2007). See figure 3.

In mammals, the anterior pituitary lobe synthesizes adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH) (Brown-Borg, 2017) and luteinizing hormone (LH) (Dufau, 1998). Other anterior pituitary hormones ubiquitous among mammals include growth hormone (GH) and prolactin (Brown-Borg, 2007). The posterior lobe of this gland is reserved for the production of vasopressin (antidiuretic hormone, (ADH) and oxytocin (Heller & Pickering, 1960) (Porter & Kaplan, 2019).

The hormones of this gland mediate many other subsystems within the endocrine system. Often, the secretion of one hormone will induce the synthesis and release of different hormones from targeted components of the neuroendocrine structures. As such, the growth of bones and connective tissues is mediated by neuroglandular growth hormone. Breast lactation is regulated by prolactin. Ovarian and testicular function (and their associated hormones estrogen and testosterone) are regulated by luteinizing hormone and follicle-stimulating hormone. Adrenal cortex function and its hormonal products are regulated by adrenocorticotropic hormone, and thyroid function is modulated by thyroid-stimulating hormone (Porter & Kaplan, 2019). Posterior pituitary function is involved in the regulation of kidney function via antidiuretic hormone and plays a part in regulating water retention. Breast and uterus function mediated by oxytocin (Porter & Kaplan, 2019).

The endocrine system plays an essential role in the regulation of sex hormones, something that is of interest to primatologists, as bonobos have a sexual lifestyle that is unique among higher primates and somewhat similar to that of humans (de Waal, 1995). Among the many functions associated with the hypothalamus and its glandular components, neuroendocrine mediation via hormone synthesis and secretion plays a role in sexual behavior (Rilling et al., 2012). As detailed above, associated pituitary function modulates the regulation of reproductive endocrine organs, such as the testes and ovaries. In addition, as mentioned above, oxytocin is also involved in reproductive function, primarily associated with female breast and uterus function. This is important to readdress here, as the hypersexual disposition of bonobos, as well as their female-dominated social structure and egalitarian temperament, is likely related to biochemical processes associated with endocrine function.

As previously stated, bonobos are hypersexual primates. Unlike other apes, with the exception of humans, homosexuality among both sexes is not uncommon, and is frequent (de Waal, 1995). There are also different sexual acts that occur between male and female bonobos that are more human-like than chimpanzee like. Examples of novel sexual behavior among bonobos include face-to-face copulation and psudocopulation, genital rubbing between females, genital contact between males, and sex for food (da Waal, 1995). Much of this behavior, such as same-sex sexual interaction, is rare among chimpanzees (Goodall, 1971). Thus, over the years, primatologists have come up with a plethora of hypotheses concerning the adaptive advantage of this behavior (Moscovice et al., 2019). A rough consensus assumes that this behavior reduces conflict by diverting attention and diffusing social tension (de Wall, 1995). A unique aspect of these sexual interactions is that genital-genital rubbing is usually done in a face-to-face position that involves mutual eye contact, something commonly associated with increases in the pituitary hormone oxytocin (Moscovice et al., 2019). Elevated levels of this hormone is an understood factor in human social attachments, including sexual and nonsexual human relationships, such as mother/infant bonding and romantic partnerships (Moscovice et al., 2019). Thus, as concluded by Moscovice et al. (2019), habitual hypersexuality, same-sex pseudocopulation, and bonded cooperation among female bonobos is likely, in some part, mediated by endocrine function concerning oxytocin (Moscovice et al., 2019).

Expanding on endocrine function, hormones also serve the purpose of maintaining reproductive ability. Testosterone from the male gonads promotes the development of secondary sex characteristics and the production of sperm. In females, estrogen also promotes secondary sex characteristics but also regulates pregnancy and the production of eggs (Porter & Kaplan, 2019).

Reproduction

Bonobos, like all primates, have mammalian reproductive organs in which the female egg is fertilized internally, and in which the embryo develops within the uterus for some months before birth (Fleagle, 1987). Males possess parred testicles contained within a scrotum, as well as a penis. In contrast, females possess paired ovaries and fallopian tubes, which extend laterally toward the ovaries from the uterus, of which the vagina is located below (Fleagle, 1987). The vagina opens onto the perineum. It is here that the external genitals exist, consisting of two sets of labia and the clitoris (Fleagle, 1987). In female bonobos, pink genital swellings indicate a period of heightened fertility, something that is attractive to males and can last up to 13.4 days (Paoli et al. 2006). See figure 5.

In bonobos and other higher primates, the placenta is segregated into one or two disks and is characterized by close proximity between the fetal and menstrual blood supply, providing efficient transfer of nutrients to the fetal material (Fleagle, 1987). This hemochorial placenta is homologous between humans and other great apes. In addition gestation in bonobos equates to an approximate 8 months (Drews et al., 2010). Postnatal development in primates is slow in comparison to other mammals (Fleagle, 1987), with bonobo infancy lasting up to two years, reaching adulthood anatomical commonalities between ages 14 and 16 (San Diego Zoo Global Library, N.D.). As detailed in an information sheet provided by the San Diego Zoo (N.D.), managed/captive life expectancies can reach up 60 years of age, with wild longevity ranging between 40 and 50 years of age. See figure 4.

Musculoskeletal

Bonobos are often confused with chimpanzees, and for obvious reasons. Bonobos are often refereed to as pygmy chimpanzees, or lesser chimpanzee, as they share nearly all of their apparent characteristics with chimpanzees, with the primary physiological differences occurring mostly in size (Zihlman & Cramer, 1978). Compared to chimpanzees, the skeletal system is dwarfed in comparison, exhibiting less pronounced brow and facial structures and a more rounded skull with a reduced mandible protrusion (Zihlman & Cramer, 1978). In addition, the thoracic cage of the bonobo is also reduced in size, further reducing the bodily frame of their appendicular skeleton Coolidge & Shea, 1982). Other skeletal features, such as the length of the femur and and feet also differ in comparison to those of chimpanzees, with these features being more robustly expressed in bonobos by longer length (Diogo et al., 2017). Though there are skeletal differences, to some degree, between these two species of primate, muscular structures are relatively similar, though recent anatomical and phylogenetic assays have shown some muscular features that correlate more with Homo sapiens than with chimpanzees, raising questions as to how closely related humans are anatomically to chimpanzees in comparison with bonobos (Diogo, Molnar & Wood, 2017). An example of these features can bee seen in the fibularis tertius muscle, a muscles associated with human bipedalism, which was found to be common in bonobos (Diogo, Molnar & Wood, 2017).