22-Agric-A1 Applied Plant, Animal or Human Physiology · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. 04-Agric-A1 Applied Plant, Animal or Human Physiology, National Exams May 2018 — 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, piloerection, endotherm/ectotherm physiology); K. Schmidt-Nielsen, Animal Physiology: Adaptation and Environment, 5th ed. (metabolic body-size scaling, Kleiber's law, thermoconformers, calorimetry); P. McDonald et al., Animal Nutrition, 7th ed. (gross/digestible/metabolizable/net energy, feed-energy partition, growth efficiency); R.L. Curtis, Environmental Management in Animal Agriculture, Iowa State University Press (thermoneutral zone, animal housing microclimate); D.M. Lewis & T.R. Morris, Poultry Lighting: the Theory and Practice (photoperiodism); ASABE Standards (American Society of Agricultural and Biological Engineers), Livestock Energetics and Thermal Environmental Management (design sensible heat production, calorimetry).
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) Lower critical temperature (LCT). The lower critical temperature is the ambient temperature that marks the lower boundary of an endothermic animal's thermoneutral zone. Above the LCT, the animal can hold its core temperature constant using only physical (vasomotor and postural/behavioural) heat-conserving mechanisms — cutaneous vasoconstriction, piloerection, huddling — while its metabolic heat production stays at the basal/resting level. Once ambient temperature falls below the LCT, physical insulation alone can no longer balance the core-to-environment heat loss, so the animal must actively raise metabolic heat production (regulatory or "cold" thermogenesis: shivering and non-shivering thermogenesis) to defend its set-point temperature; below the LCT, metabolic rate therefore rises roughly linearly as ambient temperature falls further.
b) Thermoconformer. A thermoconformer is an organism whose body temperature passively tracks (conforms to) the temperature of its surrounding environment, rather than being held at a stable, internally-defended set point. Most ectotherms (reptiles, amphibians, fish, most invertebrates) are thermoconformers in air or water of varying temperature: as ambient temperature changes, their body temperature — and with it their metabolic rate and physiological performance — changes proportionally. This is the direct opposite of a thermoregulator (essentially all birds and mammals), which spends metabolic energy to hold body temperature nearly constant across a wide range of ambient conditions. The distinction is one of strategy, not ability: many thermoconformers regulate their temperature very precisely through behaviour (basking, burrowing) even though they do not regulate it physiologically.
c) Piloerection. Piloerection is the sympathetically-driven contraction of the tiny arrector pili muscles at the base of each hair (or, in birds, feather) follicle, which makes the hair/feather stand more upright. This thickens the layer of still air trapped within the coat, and because still air is a good thermal insulator, piloerection raises the coat's effective insulating value and reduces convective/radiative heat loss from the skin. It is one of the animal's fast, physical (non-metabolic) cold-defence responses, engaged alongside cutaneous vasoconstriction as ambient temperature falls toward and below the LCT, before the more energetically costly step of raising metabolic heat production is required.
d) Photoperiodism. Photoperiodism is a physiological response of an organism to the relative length of day versus night (the photoperiod), used as a reliable environmental cue to time seasonal biological events. In livestock, photoperiod is sensed via the eye and transduced through the pineal gland's melatonin secretion pattern (a long nightly melatonin pulse signals a short day, a short pulse signals a long day), which in turn drives the hypothalamic-pituitary-gonadal axis. Long-day breeders such as domestic poultry increase egg-laying and reproductive activity as day length lengthens in spring, while short-day breeders such as sheep and goats come into oestrus as day length shortens in autumn; the effect is exploited commercially through supplemental lighting programs in poultry and dairy barns to manipulate laying rate, growth rate, and breeding season.
