J . Zool., Lond. (1996) 239, 155-166

Reproductive seasonality in Kafue lechwe antelope

R O R Y J . C . NEFDT

Large Animal Research Group, Department of Zoology, University of Cambridge, Downing Street, Cambridge, CB2 3EJ, UK

(Accepted 6 March 1995)

(With 5 figures in the text)

Kafue lechwe antelope (Kobus leche kufuensis) inhabit a wetland area of the Kafue Flats in southern Zambia and have seasonal peaks in mating and calving. The construction of a hydroelectric scheme has recently altered the time of year when flooding starts, and there has been a corresponding change in peak mating and calving dates. In this study, Kafue lechwe mated mainly during the dry part of the year when grass quality and quantity was at its lowest and when water levels were increasing and thus inundating any potential food. As rising water levels corresponded with increases in numbers of oestrous females on leks (both before and after dam impoundment), and with increased mating rates in two other subspecies of lechwe (the black and red lechwe), floods appear to act as a proximate cue for the initiation of mating. Mating during rises in water levels results in most calving occurring seven months later (the gestation period) when the floodwaters are receding and exposing optimal forage, irrespective of the time of year, and this increases the survival of the lactating mothers and their calves.

Introduction

Kafue lechwe antelope (Kobus Zeche kafuensis: Bovidae) are endemic to the Kafue Flats, a wetland area in southern Zambia. The area inhabited by lechwe consists of extensive plains which are flooded annually after the single rainy season from November to March (Sheppe & Osborne, 1971; Schuster, 1976; Rees, 1978~; Howard, 1985; Ellenbroek, 1987; Nefdt, 1992). Previous studies show that lechwe mate mainly on leks during the rainy season and calve during the dry season when the flood waters are receding and exposing floodplain vegetation (Robinette & Child, 1964; Schuster, 1976, 1977, 1978). However, no work has attempted to investigate the proximate cues and ultimate reasons for reproductive seasonality in Kafue lechwe.

It has been suggested that ungulates such as lechwe should produce calves when forage quantity and quality is at its optimum levels (Baker, 1938). Such reproductive seasonality would be expected to be adaptive as calf and mother survival may depend on ideal foraging conditions (Baker, 1938; Delany & Happold, 1979; Clutton-Brock, Guinness & Albon, 1982; Leader- Williams, 1988; Clutton-Brock & Albon, 1989). The ultimate cause for reproductive seasonality here is that calving at the best time of the year would increase the fitness of both mothers and their calves and calving at times when forage is poor could result in high mortality rates. For animals such as lechwe to synchronize calving times with optimal foraging conditions, they require proximate cues to determine when they should mate and conceive (Baker, 1938).

Present address: Department of Medical Parasitology, London School of Hygiene and Tropical Medicine, Keppel Street, London WClE 7HT, U.K.

155 8 1996 The Zoological Society of London

156 R . J . C . NEFDT

For Kafue lechwe the proximate cue has not been clearly established. One possibility is that rising water levels may act as a cue for lechwe to initiate mating. If mating does coincide with rising water levels, then calving would be expected always to occur seven months later when water levels are dropping and the receding floodline is exposing optimal floodplain vegetation. Thus natural selection may have caused lechwe to mate during rising water levels, because calving would then take place at the most favourable time for survival of both mothers and their calves.

The Kafue Flats provide an interesting opportunity to tease apart which environmental factors act as proximate cues to synchronize mating in Kafue lechwe. This is because, during the last decade. the annual changes of water levels on the Kafue Flats have been altered by the construction of two hydroelectric dams: one upstream (Iteshi-Iteshi dam) and another down- stream (Kafue Gorge dam) of the area utilized by the lechwe (Rees, 1978d,e; Schuster, 1978, 1980; Howard, 1985: Ellenbroek. 1987). These dams have altered the flooding regime and in some years the onset of floods is delayed for up to five months (Nefdt, 1992). It is possible, therefore, that the seasonal changes in flooding and exposure of green forage may have affected the behaviour and reproductive seasonality of Kafue lechwe (Nefdt, 1992).

In this paper. I present data showing the peak times of the year for mating on leks and for calving. I then relate these peaks to times of flooding and to the seasonal variation in the quality and quantity of forage after dam impoundment. Possible effects of changes in flood dates on reproductive seasonality of Kafue lechwe resulting from dam impoundment are then assessed.

Methods

This study was carried out mainly in Lochinvar National Park (226 km'), located on the South Bank of the Kafue River. southern Zambia (Fig. I ) . The lechwe also ranged over another 223 km' area outside the Park. in the surrounding Kafue Flats Game Management Area. At the time of this study, approximately 25.000 Kafue lechwe inhabited the South Bank and constituted about half the total world population (Jeffery. Malambo & Nefdt. 1989; Jeffery ef a/., 1990). The remainder of the world population, numbering around 20,000, were located on the North Bank (which included Blue Lagoon National Park) about 30 km north of Lochinvar. The results from a number of aerial surveys have suggested that some mixing occurs between the North Bank and South Bank populations (Howard. Malambo & JeKery, 1988).

Kafue lechwe leks are traditional areas of approximately lO0m x lOOm in size, where males defend tiny territories which sexually receptive females visit for the sole purpose of mating (Schuster, 1976, 1977, 1978, 1980; Deutsch & Nefdt. 1992: Nefdt. 1992. 1995: Clutton-Brock, Deutsch & Nefdt, 1993). Thus counts of females o n leks provide an unusual opportunity to estimate the proportion of sexually receptive females in the entire population throughout the year. Counts of lechwe on leks at different times are likely to reflect the incidences of oestrous females and the numbers of matings at different times for the entire population for 2 main reasons. First. i t has been shown in ungulate populations that have leks, the majority of matings take place on these leks (Gosling, Petrie & Rainey, 1987; Gosling & Petrie, 1990; Balmford, 1990; Deutsch, 1992, 1994: Fiefdt. 1992. 1995; Clutton-Brock el d.. 1993). Secondly. most oestrous females in lekking populations are generally confined to lek sites and are seldom found away from them (Clutton-Brock et ul., 1988; Clutton-Brock & Albon, 1989; Balmford, 1990, 1992; Clutton-Brock, Price & MacColl, 1992; Nefdt, 1992, 1995: Balmford ef (11.. 1993).

During 1990-91. 5 leks were located within the range of Kafue lechwe on the Kafue Flats (Fig. I) . No additional leks could be found in spite of 3 years of intense surveys and detailed knowledge of the area inhabited by the entire South Bank lechwe population. Such detailed surveys were possible because a research camp was set up in the centre of the area inhabited by lechwe (Fig. I ) . Four permanent researchers were based at the research camp for a total period of 16 months. The surveys were conducted on foot, on

REPRODUCTIVE SEASONALITY IN LECHWE 157

FIG, 1. Map of Lochinvar National Park illustrating the nine transect lines used during censuses between August 1990 and July 1991, The location of the leks are given: 1 = Kalonga lek 1; 2 = Kalonga lek 2; 3 = Kalonga lek 3; 4 = Mainde lek; 5 =Channel lek.

motorcycle and in a 4-wheel-drive vehicle. During the rainy season when many swampy areas were inaccessible by land, a boat was used to survey islands and other inaccessible areas inhabited by lechwe. Despite 2 ground visits to the North Bank in 1990 and 2 aerial surveys of the entire population (Jeffrey et al., 1989, 1990), no additional leks could be located. From 9 August 1990 to 9 July 1991, all leks were censused nearly every day, recording the numbers of females and territorial males on each lek. Most counts were conducted between 16:OO h and 18:30 h. Thus the daily total of animals on all leks was recorded. From these results, mean monthly numbers of males and females on each lek were calculated.

For the 1990-91 period, weekly mean numbers of lechwe on leks were compared to mean percentage green grass measured in quadrats along transect lines and to total monthly rainfall. These factors were then also compared to the seasonality of calving which was calculated as monthly mean calf: female ratios from data collected from 15 transect censuses. Water levels were recorded by placing a measuring pole in Chunga Lagoon, 300 m NW of the research camp (Fig. 1). Rainfall was measured using a rain gauge placed 200 m from the research camp.

The counts of calves and estimation of mean percentage green grass were extracted from a data base

1 5 X R. J . C. N E F D T

compiled during stratified sample counts along fixed-width transects from August 1990 to May 1991. Nine transects covering a total distance of 26 km were used in the censuses (Fig. 1) . Each transect was divided into 150m sections. marked by a flagged and numbered pole and the composition of all herds 500m on either side was counted. Thus each section covered an area of 250 x IOOOm or 0.25 km2. There was a total of 152 of these squares and a total sampled area of38 km'. Perpendicular distances from the transect lines to the herds were estimated by eye and sometimes confirmed by measuring the distance with a motorcycle or vehicle odometer. Herds were defined as including all animals within 50 m of each other. A total of 15 censuses was undertaken over a 10-month period. and the shortest period between 2 different censuses was always greater than 2 weeks.

To determine the relationship between forage quality and quantity to the times of mating and calving, lcchwe densities along the transects were compared to the mean percentage green grass in each 250m section. Mean percentage green grass was quantified by randomly throwing five 0.5 m' quadrats within a radius of IOm from each flagged pole. The proportion of grass which was green in each 0.5m2 quadrat was then assessed by eye. The mean values of all 5 quadrats were then calculated for the area surrounding each pole. This method produced satisfactory results over an entire year because seasonal changes in the percentage of green grass bvithin a number of individual transect sections varied from 100% during the rains 10 O'b at the end o f the 7-month dry season.

Results

Swsontil Ihcrriges irr Irk attciidancr

The number of male and female Kafue lechwe recorded on five different lek-sites is shown in Fig. 2. In contrast to previous reports that lechwe only attended leks during the rains from November to February (Schuster. 1976. 1977, 1978, 1980), this study showed that inales and females were seen on leks during every month of the year. Only from 7-22 March 1991 were all lek sites completely abandoned. There was a pronounced peak in lek attendance during the cool dry months of April to July 1991. As the number of females on the lek is likely to reflect the number of sexuaIIy receptive females in the population and the incidence ofmatings (Nefdt, 1992), these results suggest that most matings take place during a four-month period after the rainy season.

Despite pronounced variation in lek sizes, the changes in the number of males and females was closely synchronized across all five leks (Fig. 2). Increases of lechwe numbers on one lek were nearly always associated with equal increases on the other leks, unless the lek was totally inundated or when the water-line receded more than 300 m. This synchrony across leks was unlikely to be the result purely of overall variations in densities of lechwe across the entire region because some of the leks were more than 10 km apart (Fig. l) , and population changes at these different sites were independent of one another (Nefdt. 1992; for Uganda kob see Balmford, 1990: Deutsch, 1992. 1994).

Figure 3 compares monthly female attendance on leks (a) with (c) total rainfall (bars) and water levels (line) and clearly shows that rises in water levels are followed one month later by rises i n female attendance on leks; a remarkably close association. Female attendance was most closely and positively correlated with increases in water levels during the previous month (Fig. 4: Spearman rank correlation: r, = 0.91. 17 = 11. P = 0.004). From August to March, there was a decline i n water levels of nearly one metre, and this was closely followed by a reduction of female attendance from an average of 70 females in August to less than 30 in March (rs = 0.76, n = 8, P < 0.05). From March to May. there was a two metre rise in water level as a result of the opening of the gates of the dam-wall upstream. Numbers of females on leks began to rise rapidly

REPRODUCTIVE SEASONALITY I N LECHWE

K 0

s - 8 200 c

a, m -

c E

159

-

Y a, C 0

-

- 0 : 100 n 5

$ 0

C C (0

A S O N D J F M A M J J

(b)

A S O N D J F M A M J J Month

FIG. 2 Mean monthly numbers of (a) female dnd (b) male lechwe on f ive different leks in the entire populdtion of Kdfue Channel lek lechwe (August 1990 to July 1991). 0 KalOngd lek I ; Kalonga lek 2, 0 Kalonga lek 3 , B4 Mainde lek,

in April, one month after the first rises in water levels, and increased from less than a mean of 30 females to around 130 females in June and July. This suggests a one month lag in female attendance on leks after the start of rising flood-waters.

The lowest numbers of females on leks were recorded during the rainy season from November to March. Number of females on leks was negatively correlated with monthly rainfall during the previous month rs = -0.58, n = 10, P = 0.023), but did not correlate with total rainfall two months before (r, = -0.10, n = 10, P > 0.OOl). Rainfall predicted mean monthly percentage green grass (r, = 0.95, n = 10, P < O.OOl), with a mean of 73.0% green grass in quadrats for the rainy season (November to March), compared to a mean of 7.1 % green grass in quadrats during the remaining dry period. Female numbers, however, were not different between these wet and

160 R . J . C. NEFDT

0.2

a a - - f I g 0.1 In a, > m 0 -

0.0 A S O N D J

5.5

h

E

- a 4.5

v - a > L

c a, z 3.5 ---h

F

-

0

L

F Month

A S O N D J

M A M J J

1 150

L - J O M A M J J

Fic,. 3 . The reproductive seasonality of Kafue lechwe in relation to monthly water levels and rainfall in Lochinvar National Park from August 1990 to July 1991. (a) Mean number of females on all leks; (b) mean calves per female; and (c) water levels (line) and total monthly rainfall (histogram) at Chunga Lagoon, Lochinvar National Park.

REPRODUCTIVE SEASONALITY IN LECHWE 161

I I I

3 4 5 6

Water level (m)

FIG. 4. Association between water levels at Chunga Lagoon and the total number of female lechwe on leks one month later.

dry seasons (Mann-Whitney U test: n1 = 7, n2 = 3, U = 21, P = 0.016), suggesting that the quality of food has no effect on the numbers of females visiting leks.

Seasonal changes in calving

There was a pronounced seasonality for calving. Calves were born throughout the year but with a distinct peak during January and February, reflected by the higher calf: female ratios from that time onwards (Fig. 3b). Most antelope that are similar in size to lechwe have a gestation period of between seven and eight months. Thus the number of conceptions in the study population of Kafue lechwe was likely to have peaked during July 1990. (Unfortunately, no data were collected during May-June 1990.) In support of this, a positive correlation was found between the calf: female ratio and the expected number of females on leks eight months previously if the 1989/90 female numbers on leks followed the same changes recorded during the 1990/91 year (r, = 0.94, n = 10, P = 0.005).

Unlike the numbers of females on leks, peak calving dates (Fig. 3b) coincided with months of high rainfall and low water levels (Fig. 3c). Figure 5a illustrates the positive correlation between total monthly rainfall and calving (calf: female ratios) two months later (rs = 0.95, n = 10, P < 0.001). The first counts along transects resulting in high calf: female ratios were in January and February. This reflected the time of highest numbers of births, since the rise in numbers of calves would be cumulative. Over 100 mm of rain fell during December, followed by over 80 mm during both January and February (Fig. 3c). Mean percentage green grass coincided with these rains and was positively correlated with total monthly rainfall (r, = 0.95, n = 10, P < 0.001), clearly showing that the amount of green grass was dependent on rainfall. The number of calves per female was also positively correlated with mean percentage green grass in the preceding two months (Fig. 5b: r, = 0.56, n = 10, P < 0.001), confirming that calves are born at times when green grass is most abundant.

Most calves were born during low water levels, after the floods had receded and exposed

162 R . J. C . NEFDT

0.2 r (a) L

c a, m - I

L a,

cn a, > - Q l 0.0

0 20 40 60 80 100 120

Total rainfall (mm)

I P

- E a, 0.1 - m

L a, Q 111 a, > -

0 . 0 .

0

0 20 40 60 80

Mean percentage green grass (YO)

Fib. 5 Correlation of ( a ) total rnonthl) rainfall and (b) mean percentage green grass on calves per female lechwe two month\ la trr.

swards of floodplain grass (Fig. 3b, c). Calves were still with their mothers, however, well into the period of floods from May to July. As a result, water levels did not correlate with ca1f:female ratios (r, = 0.05. I I = 10, P = 0.88), because calves were present both during low and high flood. Those calves present during high flood were mostly older than three months and were thus more likely to survive any deterioration in quality of food. These data show that Kafue lechwe calves are born both when water levels are at their lowest and also during the rainy season, the time when there is an abundant food supply.

Discussion

Alrhough Kafue lechwe mate on leks and calve throughout the year, they calve mainly during times of best forage availability. Calving also occurs at a time when water levels are decreasing and exposing highly nutritious green vegetation (Rees. 1978a, h, c; Ellenbroek, 1987). This

REPRODUCTIVE SEASONALITY IN LECHWE 163

exposure of wetland vegetation may in some years (as in this study) also coincide with the rainy season, from November to March. Thus Kafue lechwe produce calves at times of optimal forage availability. This supports Baker’s (1 938) inference that selection often determines the breeding time of individual genotypes, and that the ultimate cause of reproductive seasonality in Kafue lechwe is increased survival of calves and their mothers at times of the year when food conditions are optimal.

Results from this study also show that large numbers of females visit leks during the dry season when flood waters are rising. Since rising flood waters inundate grazing areas, it is unlikely that this would be the time of best forage availability (Sayer & van Lavieren, 1975).

Matings therefore occur at times of poor forage conditions. What proximate cues might Kafue lechwe use to synchronize mating seven months (the gestation period) before the time of optimal food supply? In temperate regions day-length has been shown to be a proximate cue which initiates mating in most ungulates (Yeates, 1949; Leader-Williams, 1988; Jewel, 1989), but closer to the equator, day-length becomes more constant and is less likely to be of use as a mating cue. The results in this project show that mating in Kafue lechwe occurs during rising water levels on the extensive Kafue Flats floodplains. The results also suggest that rainfall is not the immediate cue that stimulates lechwe to mate, as mating coincided with rising water levels some time after the rains had ended, and not during the rainy season itself.

Variations in the flooding patterns in different years is the result of a number of factors, but depends mainly on the time of the year when the flood-gates of the Iteshi-iteshi dam upstream are opened to release water. Gate-opening in turn, is dependent on the extent of filling of the dam during and after the rainy season, and particularly the amount of rainfall in the catchment area in north-western Zambia (Nefdt, 1992). Hence, the delay of five months of the floods recorded in 1990/91 may not necessarily be the same in other years.

Such changes in the annual timing of flooding would therefore be expected to influence the time of the year when lechwe mate and calve. Comparisons of results of reproductive seasonality of Kafue lechwe studied 20 years ago support this prediction. Robinette & Child (1964) and nine years later, Sayer & van Lavieren (1975) estimated that most calves were born during the cool dry season (June-September) (calculated by use of foetuses’ weights from culled females). After the completion of dam impoundment, the calving season of Kafue lechwe shifted five months later in parallel with a five-month shift in the time of flooding.

Similarly, a comparison of the times of the year when most mating takes place suggest that changes in the time of the year when water rises have changed the time when most females visit leks and mate. The close synchrony between rising water levels and peak numbers of females on leks suggests that rising floods on the Kafue Flats stimulate females to visit leks. Before the timing in the start of the floods was altered by the construction of the second dam upstream, which had the greatest effect on the flooding regime, lechwe mainly occupied leks during the rises in flood-waters, which at that time was from November to March (Schuster, 1976, 1977, 1978, 1980). Robinette & Child (1964) also reported that rutting took place during rises in flood-waters.

Studies of two other subspecies of lechwe also show that mating takes place during rises in flood- waters (Table I). For red lechwe, K. 1. Zeche, Williamson (1991) found that calving was restricted to the dry, cool season (June-October) and mating peaked during the rains, again when flood-waters began to rise. It seems therefore that in the Linyanti swamps, where the red lechwe were studied, best forage conditions occur during the cool, dry months after the rains (Williamson, 1990) and that, also in this subspecies, calving takes place at the optimal time of the year. It also showed that rising flood-waters are associated with increases in sexual activity.

1 64

Date

Red lechue 1963 1976 Black lechue 1975 1990 Kafue lechi4e I963

-

1971

1974 1975 1976 1991

Mating season

Calving season

Dec. -Apr. Dee.- Apr.

Dec.-Mar

Dec.- Mar.

No\. --Dee. No\.-Feb. Dec.- Feb. All year Peak in Aug. ~ Jul.

Jan.-Mar. Jun.-Oct.

Jun.-Oct. Jun.-Oct.

Jun.--Sept

Jill.-Oct

Jan.-Mar

Peak condition ~.

Jun.- Oct.

Jun.--0ct.

Feb.-Apr.

Refs

Lent, 1969 Williamson, 1991

Grimsdell & Bell, 1975 Thirgood rf nl., 1992

Robinette & Child, 1964 Sayer & van Lavieran. I975 Schuster, 1980 Schuster, 1980 Schuster, 1980 This study

A third subspecies. the black lechwe, K . 1. si?iitheniani, also mate mainly during rises and peaks in water levels on the Bangweulu swamps (Grimsdell & Bell, 1975; Thirgood et al., 1992) and calve seven months later when water levels drop (Table I). Calving also coincides with peak condition of females, whereas mating does not (Grimsdell & Bell, 1975), suggesting that the ultimate reason for calving during receding flood-water, was the improved supply of forage for the calves and their mothers.

The results from this study suggest that increased survival of calves and their mothers has led to selection favouring calving at times of best forage availability, and that the modification of the calving season since dam construction is most probably because rising water levels provide the proximate cue for mating in Kafue lechwe. Mating activity, either on leks, as in Kafue lechwe, or on single resource based territories, as in both red (Williamson, 1979) and black lechwe (Thirgood et al., 1992), usually coincides with rising water levels. Since i t takes several months for the water levels to recede after initial rises, calves will always be born when floodplain grass has been exposed seven months later.

I would like to thank T. H. Clutton-Brock for advice, and the Zambian National Parks and Wildlife Services for permission to work in Lochinvar National Park. For field assistance I thank Katongo Chanda, Chungu Katebe. Lembe Lungu. and Lionel Nefdt. Nicky Price and an anonymous referee kindly commented on the manuscript. I thank Chris Curtis for getting the proofs to me while I was in Zambia. During this study, 1 was supported by a Beit Trust Research Fellowship.

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