SD 3

Sexual Dimorphism and Vocal Communication in Songbirds

Female and Male Characteristics in Birdsong

Sexual dimorphism is highly pronounced in songbirds, primarily through the production and function of song. In most species, such as the Zebra finch (Taeniopygia guttata) and the Atlantic canary (Serinus canaria), song represents a critical sexual signal.

• Functions of Male Song:     - Acts as a signal to females for mating purposes.     - Communicates species identity and individual identity.     - Indicates the specific location of the singer.     - Demonstrates readiness to mate.     - Serves as a tool to compete with rivals and establish/defend territory.     - Helps distinguish between neighbors and strangers.

Neuroanatomy and Neurogenesis

The brain structures dedicated to song production are sexually dimorphic.

• Vocal Motor Pathway: This pathway consists of motor nuclei that regulate vocalization and respiratory systems. It is significantly reduced in size and complexity in females.

• Robust Nucleus of the Arcopallium (RA): Visible through cresyl violet cell body staining, the RA is a primary song production nucleus. It is significantly larger and contains more dense cell bodies in males compared to females.

• Hormonal Regulation and Seasonal Neurogenesis: The pathway is influenced by a cascade starting with Testosterone, which leads to an increase in Brain-Derived Neurotrophic Factor ($BDNF$). This process triggers seasonal neurogenesis, where new neurons are integrated into the song circuit.

Song Development and Spectrogram Analysis

Birdsong is characterized as long, complex, and composed of distinct syllables.

• Sound Production: Occurs during expiration involving the vibration of the tympanic membrane. Bird pitch and volume modulation are finely tuned.

• Learning Phases:     - Sensory Phase: In the days after hatching, a young bird listens to a "tutor song" and memorizes it.     - Sensorimotor Phase: The immature bird attempts to sing (subsong) and compares its own vocalizations to the memorized tutor song.     - Crystallization: The song takes its mature, finalized form.

Atypical Reproductive Strategies and Hormonal Behavior

Parthenogenesis in the Whiptail Lizard

The lizard species Aspidoscelis uniparens is unique as an all-female ($\text{♀}$) population.

• Reproduction: They reproduce via parthenogenesis, which is the development of an embryo from an unfertilized egg.

• Pre-meiotic Endoreplication: This process results in $2n$ oocytes, maintaining diploidy without sperm.

• Pseudocopulatory Behavior: Despite being all female, courtship behavior is required to stimulate ovulation. This behavior follows a cycle:     - Progesterone: Associates with "male-like" mounting behavior; acts on the Medial Preoptic Area (MPOA)-equivalent in the hypothalamus.     - Estrogen (Oestrogen): Associates with "female-like" receptive behavior; acts on the Ventromedial Hypothalamus (VMH)-equivalent.

Human Sex Determination and Development

Genetic and Hormonal Cascades

Sex determination in humans is governed by the presence or absence of the $SRY$ gene on the Y chromosome.

• Male Pathway (XY): The $SRY$ gene triggers the development of testes, which produce:     - Testosterone: Drives the development of male internal and external structures.     - Müllerian-inhibiting substance (MIS): A $TGFB$ family glycoprotein that prevents the development of female internal organs.

• Female Pathway (XX): In the absence of $SRY$, the individual develops as a female.

• Numerical Data - Testosterone Levels: Fetal testosterone levels peak between weeks $10$ and $20$ post-conception, reaching concentrations between approximately $250\,ng/dl$ and $350\,ng/dl$, before declining toward birth ($38$ weeks).

Anatomical Differentiation Timeline

• 9 Weeks: The fetus possesses a genital tubercle, urogenital fold, urethral groove, and labioscrotal folds.

• 12 Weeks: Differentiation becomes clear.     - In males: Development of the glans penis, urethral orifice, body of the penis, and scrotum (fused labioscrotal folds).     - In females: Development of the glans clitoris, clitoris, urethral orifice, and vagina.

Genetic Disorders Resulting in Intersexuality

Steroid Hormone Mechanisms in the Brain

Enzymatic Pathways and Signaling

Steroid hormones act as powerful modulators of brain structure and function.

• Alpha-5-Reductase: Converts Testosterone ($T$) into Dihydrotestosterone ($DHT$), which is highly active on androgen receptors ($AR$).

• Aromatase: Converts Testosterone ($T$) into 17\text{-}̢β̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢̢β̢̢ḝ}estradiol ($E2$), which acts on estrogen receptors ($ER$) in both males and females.

• Direct Action: Hormones can alter membrane permeability, neurotransmitter synthesis, release, or reuptake.

• Indirect (Nuclear) Action: Hormones bind to receptors that act as transcription factors, promoting or inhibiting gene transcription at the DNA level.

Organizational vs. Activational Effects

• Organization (Sensitive Period): Occurs perinatally. In rodents, the sensitive period is roughly between Embryonic day $18$ ($E18$) and Postnatal day $10$ ($P10$).     - Alpha-Fetoprotein ($\alpha FP$): In female fetuses, $\alpha FP$ binds to maternal estrogen, preventing it from entering the fetal brain and protecting the brain from defeminization.     - Male Defeminization: Testosterone enters the male fetal brain, is converted to estradiol by local aromatase, and promotes the "male configuration" of the brain.

• Activation: Occurs during and after puberty. Circulating hormones activate pre-organized circuits. For example, testosterone activates male-typical behaviors like mounting in adulthood.

Impact of Steroids on Neuronal Structure and Physiology

Growth and Differentiation

• Neurite Growth: Estradiol treatment significantly increases the number and length of neurites in mouse hypothalamus explants compared to controls.

• Dendritic Spines: In the female rat hippocampus ($CA1$ region), dendritic spine density fluctuates with the estrus cycle. It is high during pro-oestrus (peak estrogen) and lower during oestrus.

• Electrophysiology: Estradiol increases the amplitude and frequency of Excitatory Postsynaptic Potentials ($EPSPs$) in the hippocampus, facilitating Long-Term Potentiation ($LTP$).

Dimorphism in the Spinal Cord

• Spinal Nucleus of the Bulbocavernosus (SNB):     - In male rodents, testosterone acts on the bulbocavernosus muscle, providing trophic factors that prevent motor neuron death.     - In female rodents, the absence of testosterone leads to apoptosis of these muscle cells and subsequent degeneration of the SNB neurons.

• Human Equivalent - Onuf’s Nucleus: Contains two groups: the Ventral-lateral ($VL$) and Dorsal-medial ($DM$). Motor neuron counts are significantly higher in males ($\approx 2400$) compared to females ($\approx 90$ in specific nuclei clusters like the SNB equivalent).