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The interrelationship of growth, sexual development, feed intake and plasma lipids in the black-tailed deer (Odocoileus hemionus columbianus), and the wistar rat. Stewart, Sheila Frances

Abstract

Based upon the premise that growth (weight change) is of a seasonally cyclical nature in the deer (Odocoileus), and that this change is primarily in the fat compartments of the body, a study of the plasma lipids is undertaken to ascertain if they reflect the seasonal changes. The feed intake is measured throughout the experimental period. Preliminary studies are carried out on the Wistar rat to develop a technique and facilitate a quick prognosis of the assumption. In this study, it is observed that because of the different sex effects plasma cholesterol levels must be determined. The prepubertal growth in the male and female rat is characterized by a phasic type of growth upon which is imposed another pattern of weight accretion also of a phasic, but more frequent nature. The larger pattern of growth is referred to as the Main phasic period of growth, and the more frequent one as the Interim phasic period of growth. The weight, feed and lipid relationships of the female Main phases are similar to those of the male Interim phases, and both are believed to result from the energy balance required for weight accretion. The same relationships, in the female Interim phases are similar to those of the male Main phases, and to these are attributed the occurrence of sexual development. Similar observations on the sexually mature animals indicate, in the female, that the Interim phasic type of growth is synchronous with the estrus cycle. Whereas, in the male no definite conclusions are made. In the deer study six, year old, animals of the species Odocoileus hemionus columbianus (Vancouver Island genotype) are observed for a period of twelve months. In both sexes weight increases from approximately March to October when a decrease in feed intake and weight gain occurs. In the earlier period of weight gain (March - August) both sexes increase their feed intake, but the plasma lipids remain the same, that is a high plasma 'fat' and a low plasma cholesterol. In the latter part of the weight gain period, August to October, both sexes decrease their feed intake accompanied by a rise in plasma 'fat', but no change occurs in the level of plasma cholesterol. At the point of maximum gain, in October, a difference between the sexes occurs. The males decrease their feed intake further and the plasma 'fats' fall. Whereas the females increase their feed intake and the plasma "fats' rise. Both sexes show a marked increase in the plasma cholesterol level. After October, the rut period ensues and is characterized by a loss of weight and a low feed intake in both sexes. The latter regardless of an adequate supply. In the males, the period of weight loss extends to December, and an expected rise in the plasma 'fat' is seen in only one of the three animals, the others being missed because bleeding is not carried out between October and December. In the latter month the plasma cholesterol is significantly lower than in October. In the females, the period of weight loss extends to March, and is never as extensive as in the males. The plasma 'fat' levels rise in December and fall in March when the nadir of the growth curve is reached. In this period the plasma cholesterol levels are varied and appear to depend upon the estrus phase of the animal. By March, the feed intake is increased in both sexes, but i t is not until afterwards that weight accretion begins. The plasma lipid levels are low in all the animals. It is concluded that weight and plasma lipid changes are closely allied to the yearly recrudence and regression of gonadal activity. The plasma 'fat' levels rise to augment the caloric requirements of the animal if the feed intake is not adequate, both when weight is being gained and lost. Whereas the plasma cholesterol appears to reflect the state of sexual activity. That is the level is raised in the 'rut’ period, otherwise it is low.

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