 Reference Base  Mortality factors and breeding performance of translocate... |
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Brett, R.A., 1998. Mortality factors and breeding performance of translocated black rhinos in Kenya: 1984-1995. Pachyderm 26: 69-82, figs. 1-8, tables 1-2
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| Location: |
Africa - Eastern Africa - Kenya |
| Subject: |
Translocation - Methods |
| Species: |
Black Rhino |
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Analysis of 121 black rhino, Kenya, 1984-1995. Breeding performance of females
A total of 44 calves were born to translocated females, of which 36 survived to the end of 1995. Of the 59 females translocated (30 adult, 29 immatures, 33 had sufficient time in residence in recipient reserves to produce at least one calf (ie. more than one gestation period of 16 months, and assuming that on average young females produce their first calves at the age of seven), a total of 227 rhino years of potential breeding. Four females had calved out of the 12 immature rhinos which became adults after translocation. Seven of the latter had sufficient time in residence to have calved.
A total of 25 females had calved by the end of 1995 (76% of potential mothers), of which 21 were translocated as adults, and four as immatures. The mean time spent by females as adults after introduction to new reserves was 6.9 ? 3.2 years. Eight calves were conceived in donor reserves (ie. before translocation), and their mothers were pregnant when released.
Adult females (including animals pregnant at translocation) took an average of 3.4 ? 2.2 years to produce their first calves in the new reserve (n=21), with nonpregnant animals calving 4.6 ? 2.1 years on average after translocation (n= 13). At the end of 1995, eight females had spent an average of 3.6 ?1.6 years as adults in residence, and had still not calved. Of these, two appear to be infertile (no calf after 6 years). Only a small number of females were of known age at translocation, and of those which had calved, the mean age at first calving was 8.7 ?1.0 years (n=6, range 7-10).
Adult, not pregnant females who were translocated had similar calving rates in recipient reserves whether they were captured as stragglers (0.18 calves/year, n=7) or surplus rhinos moved from other sanctuaries (0.19 calves/ year, n=6), with a comparable proportion of females breeding (80 vs. 67%) in each case. However, straggler took significantly longer than ex-sanctuary females to have their first calf (5.4 vs. 3.6 years).
There was poor survivorshipof calves whose mothers were translocated while pregnant (ie. calves conceived in previous locations). Of all four calves dying in the first week after birth, three were born to mothers translocated while pregnant. Half of the eight calves conceived before translocation of their mothers did not survive their first year.
Overall there was no bias in the sex ratio of calves born to translocated rhinos (22 males, 21 females, 1 unsexed). However, sex ratios of calves born in particular sanctuaries did show a marked, but not consistent bias. For example, male calves exceeded female calves by approximately 2:1 in the Ngulia and Lake Nakuru sanctuaries (6:3 and 9:5 respectively) but the reverse was found for calves born at Lewa Downs (4 males: 8 females).
Population performance and breeding output in females.
The translocation of black rhinos into sanctuaries over the last 12 years has resulted in a small overall population increasse, and has also fulfilled the guidelines adopted for genetic and demographic management of the Kenya black rhino metapopulation. In itself, the removal of 21 stragglers from highly vulnerable situations, and of 87 surplus rhinos from overstocked sanctuaries was essential for the immediate survival of those stragglers (and any future breeding on their part) and the future health of key breeding populations (eg. Solio and Nairobi NP).
However, the reproductive performance of this translocated sample was below expectation. Compared with a modest prediction of each adult female having a mean calving interval of three years, the conception and calving output in recipient reserves was less than half (45%) of an expected minimum total (78 calves). The overall annual growth rate of translocated populations (mean of 2.9%) was well below that of the two source populations, Solio (4-10 % annual growth) and Nairobi (10% annual growth). Reduced performance of translocated rhinos has also been noted with similar sizes of translocated samples, populations and reserves in southern Africa.
Responsibility for at least some of the relatively poor initial performance of these translocated poulations lies with the low output of some adult females, with around a quarter not having calved, and two animals with nothing to show after six years in residence. Even those that did calve after translocation (when not in calf) took around 40 months to conceive. In addition, young known-age females were slow to conceive for the first fiine, doing so well after the earliest possible time (4-5 years old). It is possible that stress and lowered nuttritional status after introduction to a new reserve, and disruption of established relationships of adult females with breeding males in previous reserves have some effect in delaying conceptions after translocation. The relative delay in production of first calves in a new reserve for straggler females, compared with ex-sanctuary animals, is also understandable, particularly considering the long periods of isolation that some straggler females have endured as the last renmants of formerly large populations which had been severely reduced through poaching (eg. Tsavo NP). However, lack of breeding (or any) contact with other rhinos, does not necessarily result in reduced subsequent fertility. The last al captured in the southern area of Tsavo West NP in 1993, a particularly aggressive female which had probably been isolated for at least six years, took only a year to conceive after introduction to Ngulia sanctuary, and calved in 1995.
Although the sample was small, the indication that there was reduced survival of calves whose mothers had been translocated while pregnant gives cause for some concern about the wisdom of routinely translocating pregnant females. This was evident in poor calf survival rather than early abortion, perhaps due to lowered nutritional status and reduced milk production of lactating cows. Visual diagnosis of pregnancy is problematic in black rhinos, with only the presence of a swollen udder just prior to parturition giving a reliable (but not early) warning. If additional data to these confirm that the translocation of pregnant females constitutes a significant risk to unborn calves, a reliable field-based test of diagnosis of pregnancy using urine or faeces could assist selection of females for translocation and thus improve overall calf survival.
The marked bias in sex ratio of calves born in some sanctuaries demonstrates the potential for early demographic upset in small breeding groups of rhinos. Extreme sex ratio bias in the Lewa Downs population prior to 1992, and the loss of single adult breeding males resulted in complete cessation of breeding for several years. The results of recent simulations of Kenya rhino populations found that demographic instability is more of an initial problem for small populations than loss of genetic variation or inbreeding in causing early extinction.
Using this data set, an additional analysis of potential effects on the probability of conception and calving in individual females found that none of 30 explanatory variables tested had any effect except calendar year. The sex/age composition of founder stock for new reserves may be an important influence on breeding output in the first five years after introduction. Obviously a bias in adult sex ratio of founder stock towards females should result in an early high yield of calves. The founder stock of the Solio reserve was strongly female-biased (8 males: 14 females) and early and rapid population growth resulted. Recent modelling of southern African black rhino populations has suggested that translocating adults is also more favourable to total population growth due to the immediate enhancement of fecundity in a new lower density reserve; these authors recommended the translocation of as many adults from the source population as it can tolerate. Clearly the selection of the age and sex composition of translocated founder groups of rhinos should be adapted to the sex/ age composition of donor populadons and should avoid any negative effect on the future breeding potential or viability of the latter.
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