Consideraciones acerca de la regulación nutricional de la reproducción y dinámica poblacional en cérvidos

FLUCK, WT; SMITH-FLUECK, JM

Chile

Congreso; First Int. Semin. Native and Introduced Deer in Chile; 1991

It may be a common occurrence that free-ranging populations of cervids are limited nutritionally. While overt pathologies associated with undernutrition may be rare (except in relation to energy), subclinical deficiencies are difficult to observe, yet may be very important. Undernutrition can affect the reproductive potential of females and, therefore, determine the dynamics of the population. More generally, undernutrition will also influence the condition of individuals such as body size including antler size of males, and the immune system.    Undernutrition may occur due to a relative lack of macro and/or micro elements. Amongst trace elements, deficiencies of Co, Cu, I, Mn, and Se have a pronounced effect on reproduction of ruminants. Se in particular has been shown to affect reproduction of cervids. Subclinical deficiency of Se was shown to lower the probability of survival of young black-tailed deer (Odocoileus hemionus columbianus) during their first few months of life. Although trace element cycles originate from local soil, it is important to recognize the possibility that such cycles are not static, and some trace element cycles may be subject to rapid alterations through anthropogenic influences. In the case of the Se cycle, the most important factors which may lower subsequent Se bioavailability include a) acidification of soils, b) soil and plant contamination with heavy metals, c) use of plant fertilizers, d) rate of biomass removal, and e) changes in plant community composition.    The incidence of undernutrition due to insufficient energy may be frequent amongst cervids, and we believe is occurring in a steadily increasing number of populations. This may be related to a) increased anthropogenic disturbance in natural habitats where top carnivores are being reduced in numbers or exterminated, b) an increasing number of introduced populations concurrent with inadequate predatory systems, and c) the usually inadequate control of populations through recreational hunting. The latter aspect frequently is exacerbated by public opinion opposing the hunting of females and young of the year. For management purposes the female and male populations can be treated separately, while considering the whole population. When population control is necessary, it can only be achieved by including adequate management of the female population. In temperate climate, overpopulation frequently results in mass die-offs during the winter, however, we observed a significant die-off of black-tailed deer during the summer time due to inadequate energy availability. Summer die-offs are more difficult to discern in forested areas because the animals tend to be dispersed.    Lack of energy may further have a strong effect on the reproductive potential of females. Seasonal predictability of forage may have been the driving force resulting in the seasonal breeding strategy found in non-equatorial cervids. The change in photoperiod determines in a general way (melatonin) the timing for physiological adjustments in preparation for ovulation. However, the level of storage fat obtained before the breeding season may ultimately determine the hormonal milieu (estrogen), which is necessary for ovulation to occur. Biochemical mechanisms may regulate the probability of ovulation in such a way that females only ovulate if their body energy stores are adequate to carry a pregnancy to term and lactate without compromising progeny or themselves. Without population regulation such as through predation, free-ranging cervids do not exhibit a strategy providing optimal fat reserves to adult females ensuring optimal reproductive success. Rather they tend to saturate winter and/or summer habitat to the extent that mass die-offs occur periodically. In a population of black-tailed deer lacking sufficient energy  we observed a delay of the breeding season by about an estrus cycle (ca. 22-28 days), as well as unbred females of young age classes.    In man-modified environments, it is becoming increasingly necessary to manage free-ranging populations of cervids. The existence of inadequate energy availability can be determined by evaluation of body condition and reproductive parameters of females. Management should first focus on population control and secondarily, may involve improvement of the forage base. Subclinical deficiencies of trace elements can only be determined with production response trials. Such clinical trials in free-ranging populations of cervids unfortunately are a costly enterprise. Comparisons with known normal tissue concentrations (possibly of other species) may provide approximations which should be used cautiously.