Whole Body Animal Growth and Development Study Guide

Course Information and Reference Materials

• Lecture Topic: Whole Body Animal Growth.
• Lecture Date: 3-31-26.
• Quiz Date: 4-7-26.
• Primary Reference Materials:     * Principles of Animal Growth and Development, Gerrard and Grant: Chapter 9, pages $149-163$.     * Growth of Farm Animals, Lawrence, Fowler, and Novakofski; 3rd ed. (2012): Chapter 11, pages $213-229$.

The Human Life Cycle

• The progression of human growth and development follows a cyclical path:     * Fertilised egg (Starting point of the cycle).     * Foetus (In utero development).     * Baby (Post-natal start).     * Child.     * Adolescent.     * Adult.     * Old age.     * Death (Conclusion of the biological cycle).

Study Objectives for Animal Growth

1. Understand the various phases and inflection points for whole animal growth described by sigmoidal growth curves.
2. Describe the difference between the Physiological and Chronological age of an animal.
3. Identify how external factors may alter growth patterns.
4. Be able to calculate and plot Cumulative, Absolute, and Relative growth curves.
5. Identify the same phases and inflection points on the different growth curve types.
6. Describe John Hammond’s two important observations about the components of growth.
7. Understand the principles of Allometric growth.
8. Explain the deficiencies in the Allometric model of growth.
9. Understand how Allometric growth curves are described by the formula $Y=ax^b$.
10. Explain how the rate of growth of different tissues/body parts relative to the whole body is described by "b" (the slope of the line).
11. Explain the composition of growth on an Absolute and Percentage basis.
12. Know how the relative contribution of muscle, adipose, and connective tissue change with age.

Cumulative Growth and the Sigmoidal Curve

• Definition: Cumulative growth is the plot of total animal weight over time.
• The Sigmoidal Curve: The curve resulting from cumulative growth data is shaped like the letter "S."
• Expression: Cumulative weight is expressed as the total weight reached at any given time point.
• Sporadic Nature: While the curve is generalized as smooth, the actual shape is "stair-stepped" due to sporadic growth spurts.
• Phases of the Sigmoidal Curve:     * Self-Accelerating Phase: Occurs from conception through birth and early development.     * Point of Inflection: The transition point where growth velocity is at its maximum.     * Self-Inhibiting (Decelerating) Phase: The period following the inflection point as the animal approaches maturity.     * Asymptote: The plateau representing mature weight.

Self-Accelerating Phase of Growth

• Exponential Nature: This phase is characterized by exponential growth rates.
• Cellular Mechanics: Each cell divides into two daughter cells at a constant rate, resulting in rapid growth with relatively little initial complexity (e.g., fetal calf growth).
• Increasing Velocity: The velocity of growth continues to increase because a progressively larger pool of cells becomes available to divide.
• Deceleration within Acceleration: Eventually, the rate at which each individual cell divides begins to slow. This is because it becomes physically difficult to supply every cell with necessary nutrients while efficiently removing harmful wastes.
• Systemic Adaptation: The embryo responds to these limitations by developing increasingly complex transport mechanisms to ensure nutrient supply and waste removal.
• Linear Growth: Growth becomes linear when the force of acceleration is in balance with the counteracting effects of increased complexity and limited available nutrients.

The Point of Inflection

• Maximum Velocity: This is the specific point of maximum growth velocity. After this point, the animal continues to grow, but the rate of growth begins to decrease.
• Greatest Average Daily Gain (ADG): The point of inflection represents the period of the animal's greatest average daily gain.
• Biological Milestones: In many species, the point of inflection occurs during puberty.
• Analogy: It is compared to the time point when the maximum number of construction workers can simultaneously work to expand a house.

Self-Decelerating Phase of Growth

• Approaching Maturity: The animal approaches its mature weight at a decreasing rate of growth.
• Food Intake and Maintenance: There is a gradual diminution in the increase of food intake until the energy consumed eventually approaches the energy required for maintenance.
• Signals for Reduction: The reduced growth rate is controlled by various biological signals, notably the secretion of Somatostatin by the hypothalamus.
• Analogy: This represents the stage where additions are still being made to the house, but at a significantly slower rate.

General Growth Hormone (GH) and IGF-1 Feedback Loop

• Hypothalamus: Secretes GHRH (Growth Hormone Releasing Hormone) to stimulate growth and GHIH (Growth Hormone Inhibiting Hormone, also known as Somatostatin) to inhibit growth.
• Pituitary Gland: Releases GH (Growth Hormone).
• Liver: Receives GH and produces IGF-1 (Insulin-like Growth Factor 1).
• Systemic Circulation: IGF-1 enters the blood to reach various tissues.
• Tissue Effects: Produces anabolic and catabolic effects on various tissues. IGF-1 provides feedback to the hypothalamus and pituitary.

Asymptote and Mature Body Weight

• Maintenance Equilibrium: The asymptote is the point where food intake exactly matches the maintenance requirement.
• Definition: This is regarded as the mature body weight of the animal.
• Fluctuations: Mature body weight is not static; it fluctuates throughout life based on:     * Available food supply.     * The reproductive cycle.     * Seasonal changes.
• Analogy: "Our house is finished!"

Senescence and Death

• Genetic Program: Senescence (biological aging) is an intentional part of the genetic program.
• System Failure: The failure of vital systems eventually results in the loss of body condition and subsequent death.
• Purpose: This usually occurs after the reproductive phase of life to ensure space and resources are available for the next generation.

Chronological vs. Physiological Age

• Chronological Age:     * Age measured in absolute time units (days, months, years).     * Limitations: Animals within the same species, breed, or sex do not grow, develop, or fatten at the same rate. They also do not reached maturity at the same chronological age.     * Influencing Factors: Nutrition, Disease, Stress, and Activity Level.     * Example: Two pigs from the same litter can be fed differently. A "Fast" fed pig reaches maturity sooner than a "Slow" fed pig on a restricted diet, even though both eventually reach the same physical and chemical composition.
• Physiological Age:     * Used to overcome the drawbacks of chronological age comparisons.     * Refers to specific physical or chemical stages of maturity, such as height, weight, composition, or puberty.     * Interspecies Comparison: An elephant reaches sexual maturity at approximately $50\,\text{years}$, whereas a mouse reaches it at $9\,\text{weeks}$. Both can be compared at these points as being of the same physiological age regarding reproductive maturity.

Quantitative Growth Calculations

• Absolute Growth Rate Curve:     * Plots gain per unit of time against time (e.g., ADG).     * Formula for ADG: $ADG = \frac{Y_2 - Y_1}{t_2 - t_1}$ where $Y$ is the measure of size and $t$ is time.     * Specific Example: A calf growing from $65\,\text{lbs}$ to $700\,\text{lbs}$ over $84\,\text{days}$.     * Calculated ADG: $\frac{700 - 65}{84} = 7.56\,\text{lbs/day}$.     * Limitation: This only represents the "average" over the whole period. ADG changes constantly. Measuring at weekly intervals provides more accuracy (e.g., weekly gains might vary from $6.43\,\text{lbs/day}$ to $9.29\,\text{lbs/day}$).
• Relative Growth Rate (RGR) Curve:     * Describes growth in relation to total weight (growth rate per unit weight plotted against time).     * Relative growth is greatest during initial development because the animal is small.     * Formula for RGR: $RGR = \frac{\ln(Y_2) - \text{ln}(Y_1)}{t_2 - t_1}$ where $\ln$ is the natural log (base $e \approx 2.71828$).     * Specific Example (84 days): $\frac{\ln(700) - \ln(65)}{84} = \frac{6.55 - 4.17}{84} = 2.8\% /\text{day}$.     * Specific Example (First 7 days): $\frac{\ln(110) - \ln(65)}{7} = \frac{4.70 - 4.17}{7} = 7.6\% /\text{day}$.

Components of Whole Body Growth

• Tissue Development Rates: Whole body growth curves are the sum of growth for many tissues (muscle, fat, bone, organs) that develop at different rates.
• John Hammond (1889-1964): A pioneer at Cambridge University who dissected farm animals from early life to market weight to determine composition under typical and malnutrition conditions.
• Hammond's First Observation: Body components that are physiologically more important develop first.     * Priority Sequence: $\text{Nerve} > \text{Bone} > \text{Muscle} > \text{Fat}$.
• Hammond's Second Observation: Extremities complete development first; development occurs from the outside in.     * Priority Sequence: Head, Hands, Feet > Neck, Tibia, Ulna > Thorax, Femur, Humerus > Loin, Pelvis.

Allometric Growth and the Huxley Model

• Definition: Allometric growth is the study of how body parts (tissues, organs, components) change in size relative to the development of the whole animal.
• Julian Huxley (1887-1975): Oxford University zoologist who proposed that proportions are determined by overall size. He studied the fighting claw of the fiddler crab.
• Allometric Formula: $Y = ax^b$     * $Y$ = log weight of carcass or animal.     * $a$ = constant.     * $x$ = log weight of tissue or component.     * $b$ = growth coefficient (slope of the line).
• Interpretation of "b":     * $b > 1$ (Late developing): Tissue grows faster than the whole body; high growth impetus (e.g., total fat).     * $b < 1$ (Early developing): Tissue grows slower than the whole body; low growth impetus (e.g., total bone).     * $b = 1$ (Isometric growth): Tissue grows at a similar rate to the whole body; average growth impetus (e.g., total muscle).
• Deficiencies in Allometry: Hammond found that nutrition can disrupt these proportions. For example, the same amount of fat may be found in a large lean pig or a small fat pig; thus, fat is not always proportional to body size.

Growth Patterns of Standard Muscle Groups (Steers)

• Early developing muscles ($b < 1$): Distal aspects of limbs; proximal thoracic and hind limbs.
• Late developing muscles ($b > 1$): Abdominal muscles (linked to rumen functional demand); Neck to thoracic limb (linked to weight bearing and secondary sex characteristics).
• Isometric growth ($b = 1$): Muscles surrounding the spinal column.

Whole Body Composition Changes

• Absolute Basis: Cumulative weight gain over time. Peak growth timing follows the sequence: $\text{Bone} > \text{Muscle} > \text{Fat}$.
• Percentage Basis: Proportion of total body weight summing to $100\%$.     * Fat becomes a larger proportion of the body with age.     * Muscle and bone make up a smaller proportion of the whole body as the animal ages.
• Chemical Proportions:     * Fat becomes a larger proportion with age.     * Water and protein make up a smaller proportion with age.

Tissue-Specific Chemical Changes

• Muscle:     * Water content is $68$ to $85\%$. Water is highest early in life and decreases with age.     * Absolute and percentage protein (sarcoplasmic and myofibrillar) increase with age.
• Adipose (Fat):     * Water content of fat cells is high early and decreases with age.     * Protein is highest early and decreases later.     * Depot Maturity Sequence: $\text{Kidney} > \text{Mesenteric} > \text{Subcutaneous} > \text{Intermuscular} > \text{Intramuscular}$.
• Bone:     * Water content decreases with age; protein content decreases later in life.     * Absolute mineral content increases with age.     * Fat content in bone increases (e.g., bone marrow fat in a $24\text{-month-old}$ rat is significantly higher than in a $4\text{-month-old}$ rat).
• Connective Tissue:     * Absolute amount increases with age (associated with muscles/tendons).     * Collagen and elastin increase significantly with muscle development, but decrease as a percentage due to the "dilution effect."     * If muscle atrophies with age (Senescence), the percentage of connective tissue increases proportionally.