Heurística dsopt para la planificación de tareas

Observations on breeding behaviour of wild and captive elephants mark the onset of sexual maturity between seven and fifteen years of age for African

elephants (Perry 1953; Buss and Smith, 1966; Hanks and Short, 1972; Smith and Buss, 1973) and six to seven years for Asian elephants (Eisenberg, McKay and Jainudeen, 1971; Eisenberg, 1980; Dittrich, 1967).

1.2.2 Pregnancy, Parturition and Lactation

The reproductive physiology associated with pregnancy, parturition and lactation in both species of elephants is limited. Much of what has been described in the literature is either anecdotal or factual accounts of breeding and births (Mayberry, 1972; Lang, 1963; Krishne Gowda, 1969; Dittrich, 1967; Leuthold and Leuthold, 1975; Slade, 1903).

Once puberty has been attained, wild elephants have a conception rate of between 75-80% (Moss, 1983) and thus do not display the regular ovarian cycles seen in captive animals who have been shown to have a conception rate of less than 20% (Niemuller, Shaw and Hodges, in press).

The duration of pregnancy has been quoted to range from 18-22 months in Asian elephants, with most births occurring around 21 months (eg. Ratnasooriya et al., 1992). In the African elephant the figures obtained from observation of wild elephants would indicate a slightly longer gestation period ranging between 20-24 months or 634 to 683 ± 5 days (Moss 1983). Generally, one offspring is bom, although in both species there appears to be an approximate 1 % probability of twins occurring (Dittrich, 1967; Smuts, 1975; Laws, 1969). After parturition the calf is solely dependent on it s mothers’ milk for the first six months of it s life, after which the baby starts experimenting with solid food. A lactational anoestrus

of two years is typical in a nursing wild elephant, although in captivity this may be shortened by up to a year. If a calf should die at birth, the removal of the

suckling stimulus returns a cow (in good health) to oestrus in approximately three months (Lee and Moss, 1986).

Smith and Buss (1975) found that follicular development was repressed during gestation but that lactation had little influence on follicular development. Based on their data, they surmised that one ovum was ovulated at each oestrus, whereupon the ovulated follicle became luteinized irrespective as to whether or not conception occurred. At the same time, other large follicles may or may not become luteinized. These CL (termed ovulatory and accessory CL) persist

through to the next oestrus period whereby the process is repeated until conception occurs.

However, the African elephant’s intercalf interval appears to be quite

variable even within the same study area with factors such as high density, drought and food availability playing critical roles. Perry (1953) reported an intercalf interval of approximately four years which increased to eight years by 1964 in the same area (Smith and Buss, 1973). The calving interval in wild African elephants has been reported to range between three to five years (Smuts, 1975; Moss, 1983; Williamson, 1976; Hanks, 1972; Kerr, 1978; Sherry, 1975), although in harassed or stressed populations this interval increases (Laws 1969). The data on captive Asian elephants indicates an interbirth interval of approximately three years which also matches a range of two and a half to five years observed in wild herds (Gee,

1964; Kurt, 1974; Eisenberg, 1980). 1.2.3 Seasonality

Elephants are capable of breeding all year as has been observed in wild African elephants (Perry, 1953; Buechner, Buss and Longhurst, 1963; Buss and Smith, 1966; Smith and Buss, 1973), and in captive Asian and African elephants (Hess, Schmidt and Schmidt, 1983; Plotka, Seal, Zarembka, Simmons, Teare, Phillips, Hinshaw and Wood, 1988; Brannian, Griffith, Papkoff and Terranova,

1988). None-the-less, there is strong evidence of the seasonality of reproduction for those animals living in areas of extremes between dry and wet seasons (Laws,

Buss, 1973; Sherry, 1975; Kerr, 1978; Williamson, 1976). Most of these studies demonstrated that peak conception rates occurred during peak rainfall although Hanks (1972) noted peak conception occurring in the dry season during one year of his study. Laws (1974) suggested that suppression of female fertility during the rest of the year may occur due to limited food availability. Certainly in more temperate climates, areas with more than one rainy season or in captivity, seasonality of reproduction is not seen.

1.2.4 Senescence

Perry (1953) was unable to demonstrate the cessation of reproduction with age in the African elephant specimens he examined, although he surmised that the interbirth interval increased and felt that there was the potential for some females to lose the capacity to breed. Smuts (1975) found no age specific pattern in reproductive activity between African elephant cows of between 16-50 years. In the 50-60 year olds, he found 81% to be lactating, 29% to be pregnant and only 12% not reproductively active. In contrast. Hanks (1972) found peak fertility in 18-19 year olds with a significant decline in fertility after 40 years of age. Laws, Parker and Johnstone (1970) found a peak conception rate in 31-35 year olds, as did Kerr (1978), which declined with age, so that conception rate in 51-55 year olds was 8.3%, while 56-60 years olds were reproductively inactive. The differences in reproductive senescence noted in these studies are most probably related to population density dependent factors and physiological, nutritional and social stresses, although this is an area which has received very little attention.

In both wild and captive Asian elephants Sukumar (1989) noted

reproductively active cows over 50 years of age. However, population dynamics indicate that since only a small proportion of cows represent this older population, their overall contribution to the elephant population growth is negligible.

1.3 REPRODUCTIVE ANATOMY