22-Agric-A1 Applied Plant, Animal or Human Physiology · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. 04-Agric-A1 Animal or Human Physiology, National Exams May 2017 — a three-hour closed-book examination; one of two approved calculator models (Casio or Sharp) is permitted. The rubric states that five (5) questions constitute a complete exam paper and that the first five questions appearing in the answer book are marked (worth 20 marks each, 100 marks total); all six (6) printed questions are worked here as a complete study resource.
Reference texts. M.K. Yousef (ed.), Stress Physiology in Livestock, Vol. I — Basic Principles, CRC Press (thermoregulation, thermoneutral zone, endotherm/ectotherm physiology, external stressors); J.A. DeShazer (ed.) and ASABE Standards (American Society of Agricultural and Biological Engineers), Livestock Energetics and Thermal Environmental Management (sensible heat production, metabolic body-size scaling, animal housing design); P. McDonald et al., Animal Nutrition, 7th ed. (gross/digestible/metabolizable/net energy, feed-energy partition); K. Schmidt-Nielsen, Animal Physiology: Adaptation and Environment, 5th ed. (Bergmann's rule, comparative thermal biology, calorimetry); R.L. Curtis, Environmental Management in Animal Agriculture, Iowa State University Press (animal housing microclimate).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
a) Homeostasis. Homeostasis is the maintenance of a relatively stable internal environment (body temperature, blood pH, osmolarity, glucose concentration, and similar variables) within a narrow tolerable range, despite continuous changes in the external environment or in the animal's own activity. It is achieved through negative-feedback control systems: a sensor detects a deviation of some regulated variable from its set point, an integrating centre compares the sensed value against the set point, and effectors are driven to correct the deviation, so the corrective response itself reduces the error signal that triggered it. Homeostatic control is never perfect or instantaneous — the regulated variable oscillates in a narrow band around the set point rather than sitting exactly on it.
b) Ectothermic. An ectothermic ("cold-blooded") animal is one whose body temperature is determined predominantly by the temperature of its external environment rather than by internally generated metabolic heat; reptiles, amphibians, fish, and most invertebrates are ectotherms. Ectotherms have a comparatively low mass-specific resting metabolic rate (they do not spend energy generating heat) and instead regulate their body temperature behaviourally — basking in the sun, seeking shade, burrowing, or moving between microhabitats of different temperature — so their physiological performance (locomotion, digestion, nerve conduction) rises and falls with ambient temperature.
c) Thermoneutral zone (TNZ). The thermoneutral zone is the range of ambient temperature over which an endothermic animal can maintain its core body temperature using only physical (vasomotor and behavioural/postural) mechanisms of heat exchange — vasoconstriction/vasodilation, piloerection, posture change — without having to raise its metabolic heat production above the minimal resting (basal) level. Within the TNZ, metabolic rate is essentially constant and at its minimum; below the TNZ's lower critical temperature (LCT) the animal must increase metabolic heat production (shivering, non-shivering thermogenesis) to defend core temperature, and above the upper critical temperature (UCT) it must engage active evaporative cooling (sweating, panting), which itself has a metabolic and water cost.
d) Photoperiodism. Photoperiodism is a physiological response of an organism to the relative length of day versus night (the photoperiod), used as an environmental cue to time seasonal biological events. In livestock, photoperiod (sensed via the eye → pineal gland → melatonin pathway) drives seasonal reproduction — e.g. long-day breeders such as domestic poultry increase egg-laying and reproductive activity as day length increases in spring, while short-day breeders such as sheep and goats come into oestrus as day length shortens in autumn — and it is exploited commercially through supplemental lighting programs in poultry and dairy barns to manipulate laying rate and growth.
