Composición química

Rutting took place during the long rains and was highly synchronised: more than 90% o f births took place within a 1 % month period around the short rainy season. Does this suggest higher degrees o f breeding synchrony than in non-lekking topi? Data from a non-lekking population in Serengeti NP showed a considerable degree o f synchrony as well: more than 70% o f the calves were bom within two months (Sinclair et al., 2000). The less pronounced peak in the Serengeti data could be explained by the fact that the Serengeti study covered far more temporal and spatial variation than the present study: the data set spanned 31 years and were collected throughout Serengeti NP (14,763 km^).

In other lekking ungulates no clear difference in breeding synchrony can be demonstrated by the available data either. In fallow deer, mtting has been reported to last 10-21 days in non-lekking populations (Alvarez et al., 1990; McElligott et al., 2001; Moore et al., 1995) and 12-18 days in lekking populations (Apollonio et al., 1989); however one study on a non-lekking population mentions a gradual decrease in mating activity after a three week rut, which could reflect a lesser degree o f

C hapter 3________________ D em oaraphv & S easonality o f the R u t_______________________ 50

synchronisation in this population (Braza et al., 1986). Data from lechwe may suggest slightly more synchronised breeding in lekking populations (peaks generally lasting 2-3 months) (Nefdt, 1996; Schuster, 1976; Schuster, 1980) than in non-lekking populations (peaks lasting 4 months) (Nefdt, 1996; Williamson, 1991).

W hile the overall picture demonstrate a tendency for higher breeding synchrony in lekking populations, differences in methodology and precision o f the various studies prevent any conclusion at present as to whether lekking enhances breeding synchrony. Rigorous measurements o f the duration o f the rut by consistent m ethodology in lekking as w ell as non-lekking populations are needed for proper testing the hypothesis.

The phenology o f calving in Serengeti topi has been explained by maximisation o f food availability during the peak nutritional demand around parturition (Sinclair et al., 2000). This could be the ultimate cause o f a similar phenology in the Burrungat Study Area as well: parturition in November-December coincides with the onset o f the short rainy season. The proximate cue for rutting may be provided by a surge in rainfall, through improved feeding conditions, as the oestrous cycle is condition- dependent in many mammals (Bronson, 1989). Thus, in 1999 and 2000 rutting began 20 and 19 days following the first heavy showers o f the long rains. It is noteworthy that the rut was estimated to begin unusually early during the el Nino year 1998 when rainfall was extraordinarily high from November o f the preceding year and throughout the first half o f the year; an extreme surge in early January preceded the estimated onset o f the rut by about a month.

The topi sub-population within the Burrungat Study Area was relatively stable averaging 1,060 individuals. That the area roughly encompassed a sub-population is supported by the general correspondence between the demographic descriptions o f the B SA topi and larger sub-populations (Table 3.3).

The inclusion o f yearlings in the adult categories explains the rise in numbers o f adult males in February 2000 (Figure 3.2). In the case o f females and calves, however, there was a moderate influx starting just before the rut eventually resulting in the total number reaching 23% above the yearly average in late April. Conversely, in June o f both 1999 and 2000 the total number fell to 31% and 18% below average respectively. The 2000 observations showed that again it was predominantly adult females and calves who left the area, as w ell as some young males. These results suggest that the 'catchment area' o f the Burrungat leks was in fact slightly bigger than the Burrungat Study Area.

C hapter 3 D em oaraphv & S easonality o f the R u t 51 Table 3.3 Population compositions of topi from different studies.

A dults (% ) Y earlings (% ) C alves (% ) Adult s e x ratio (f:m) S u b ­ p op u la­ tion s iz e S o u rce

Ishasha plains, 65 16 19 1.69 3,796 Jewell (1972)

Queen Elizabeth N P

Ishasha plains, 82 18 1.74 3,513= Yoaciel and van

Queen Eiizabeth NP^ Orsdol (1981)

Central plains. 67 13 20 1.82 3,850 Monfort-Braham,

Akagera N P (1974,1975)

Wooded savannah. 61 15 25 - ? Monfort-Braham

Akagera N P (1975)

Burrungat plains. 63 18 19 1.86 1,060 This study

M aasai M ara N R

^Mean of 8 counts, yearlings counted with adults. ^SE=477

The number o f calves in February 1999 and the number o f yearlings at the beginning o f the year 2000 refer to the same cohort, thus there is a discrepancy in the fact that the number o f juveniles is higher. This can be explained either by a net influx o f juveniles into the area or a bias towards undercounting young calves who can be difficult to see when lying down. If there was a tendency to undercount very young calves in the beginning o f the year the calf mortality is underestimated in the figures presented.

Both lek males and resource defenders had higher territorial attendance during the rut than at other times, however, the difference was most pronounced in lek males. The higher attendance could be due to mating benefits or, as the rut coincides with peak rainfall, it could be due to a reduction in the cost o f low food availability on the lek. Whether topi leks are indeed sites o f low resource availability w ill be investigated in the next chapter where I address the hotspot m odels o f lek evolution. In fact, according to one hypothesis, the nutrient hotspot model, leks form at sites o f extraordinarily high-quality forage.

52

Chapter 4

HOTSPOTS -

PATTERNS IN RANGING,