in relation to sun angle

Transcription

1 Orientation of wildebeest in relation to sun angle and wind direction H.H. Berry. Etosha Ecological Institute P.U. Okaukuejo YWO South Wcs~ Africa/Naibia W.K. Siegfried and T.M. Crowe FitzPairick lns~itute Un~versity ol' Cape Town Kondebosuh 77 l<epubltc of South hl'rica Subiued: 6 Deceber 1982 Accepted: 13 June 1983 CONTENTS 1 Introduction Methods ,F Results... L W 4 Discussion... LYH 5 Conclusioi~s h Acknowledgeents...?U I 7 Kef'erences....1U I The wildebeest Connochacrcs taurinus in thc Elosha National Park, northern South West AfricaINaibia. is exposed to a severe cliale, characterised by daily tcperalure and relative huidity changes 1' up lo 28'C and 85%) (Berry, 198). In addition, desiccating winds and high incoing and rellecled solar radiation can be expected to exert considerable influence on the behavioural responses of a large ungulate, such as wildebeesl. In order to tesl whether a significant relationship exists betwccn sun angle. wind direction and the body orientation of' wildebeest, our data were sub. jccted to statistical analysis by application 1' a copuler progra tered DIRECT which is designed for the inkrprctation of two-diensional directional data (Min~ack ui al., 198). 2 METHODS ABSTRACT Response of wildebeest to direct solar radiation and wind was rnea~urcd at hourly ~niervals. fro sunrise to sunsel, during cold and hot seasons In the Etosha National Park. The two-dienslonal data were slatistically analysed by a coputer prograe package DIRECT). The findings indicate that w~tdebces~ orientate \~gn~licantly In relation to tlie sun when its allitudc IS low (< 459 and that during the idday sector wlnd intlucnces body position. 2.1 In a iotal OF 32 separate, hourly saples ot" lhe orientation OF wildebeest, the I'oliawing data were recordcd during October 1977, May and Deceber I Y Angle OF the long axis ol' the anial's body in relalion to O (agnetic North), by allocating one 1' eighi vectors, naely O " - 9, through 36q Le each anial Wind direction, using the sae vectors, and wind speed as recorded by autoatic aneoeter at a peranent weather stalion situated 18 k fro where held observaions were ade Unscreened teperature, using a black bulb lheroeter to sirnulake the dark pelage ot' wildebeest Direct sunlight or sun obscured by cloud Wildebeest stationary or oving. 2.2 Data processing and siatistical procedures were in accordance with Miack ci al. ( 198) using ulti-saple tesks to deterine whether the dispersion paraeters were equal. The tesl to copare odal vcclars was perfored only if the null hypothesis was accepted.

2 3 RESULTS [or Deceber 1978 ("hot, dry season"), showing the preferred orientation of wildebeest in relation to sun The results and their analyses for a typical behavioural angel and wind direction (Figs, 1 to 4). The sun aziuth response during October 197'1, the so-called "hot, dry and its angle above the horizon are particularly relevant season" (Berry, 198) are given in a coplete for in LO our findings For tlie Deceber suer solstice and Tables I to 3. We then present two further data sets, they are given in Table 4. The easureent of sun naely one for May 1938 ("cold, dry season"') and one aziuth is shown in Fig. 5. TABLE 1: General statistics for saples copered when recording orientation of wildckst in the Etosha National Park to edvironental stiuli during the hot, dry season (5 October 1977). Saple No. 1 ' 2 I B No. in saple Coponent 43,288-2,335 4,346 4,48 3 1, ,85 1,45 1 Coponent 9 23,375 5,993 7,497 1,497-3,62-3 1,746-15,445-7,291 Resultant (length) 49,196 6,439 8,666 4, , ,76 3,35 R/N,623.23R,377,26,843,473, Angle of R (dcgs) ,29 316, ,158 28, ,54 Confidence intervals For angles of odal vector 95% (degs) 99% (dcgrecs) 15, ,6 26, ,9 329,5 336, ,s ,O ,2 36,1 342,s TABLE 2: Multi-saple test for orientation data of wildebeest in the Etosha National Park to deterine whether the dispersion paraeters are the sac. The null hypothcsis IS ihat H, = k, = k, =...,... ks Observed value of Z = *The test to copare odal vectors was not perfored due to the rejection of the null hypothesis. Critical value Significant level Conclusion* 27,24 Reject H, 38,144.5 Reject H, Rejcct H, 32,852,25 Rcject H, 38,582 Reject H, TABLE 3: Difference between the angle of the sun, the direction ofthe wind, and the preferred angle of orientation of wildebeest in the Etosha National Park during thc hot, dry season (5 October 1977). Resultan1 Difference between I 1 Difference lxtween Saple No. in Wind direction sun and resultant wind and resullant No. saple (degrees) (degrees) angles (degrees) angles (degrees) TABLE 4: Sun aziuth and sun altitude in the Okaukuejo area of 4 DISCUSSION the Etosha National Park (latitude 19's: longitude 16'E) Deceber Tie S.A.S.T il loo I l R 19 2 Aziuth ' logl 1' 15O35' 14a5' 14'33' 1 1 2O26' 224O37' 251'4' 255'55' 255'37' 253'45' 2553' 247W3' Altitude on l9-4'5' (Below horizon) YIS' 2293' 369' 49O54' 63O4 1' 7T1 5' 8S9' 73-21' 59O39' 45O52' 3 2W9' 1 8 O3 7' S28' -7 41' (Below horizon) 4.1 Cold, dry season: 26 May I978 (Figs. 1 and 2) Under the prevailing cloudless conditions our hypothesis is rhal wildebeest will present thc long axis ol their bodies to the sun when teperatures are low to facilitate heat uptake. Conversely, when teperatures increase, we predicted that wildebeest would orientate their long axis parallel to the incoing radiation, exposing the sallest surface to direct sun rays and thereby copensating for excessive heat load. In Fig. I it is evident that at a teperature of 1 C there was a tendency to orientate laterally. As the teperature rose to 22OC the wildebeest Faced increasingly into the sun. However, 3,s hours after sunrise they oved fro the grazing area to drink water and this resulted in a ore rando orientation in relation to sun angle. Hav-

3 - 3 DIFFERENCE BETWEEN WIND AND RESULTANT 7 : C P DIFFERENCE BETWEEN SUN AND RESULTANT I I I 1 L I I I 1 -* 4 h) h) W Ln U1 BLACK BULB TEMPERATURE tcl $ ru N N UI Ln Cn rci ' BLACK BULB TEMPERATURE ('C)

4 DIFFERENCE BETWEEN WIND AND RESULTANT -' z c a SUN ALTITUDE AND DIFFERENCE BETWEEN SUN AND RESULTANT 4 N P Ol W l W. -Q- \l, 'I' d d 4 4!+ N P Q1 3 TI a g S it;' 2? S g. A N BLACK BULB THERMOMETER ('C) 3 BLACK BULB THERMOMETER ('C) r:

5 SUN AZIMUTH MO! TH AZIMUTH /' easured fr d North thugh East FIGURE 5: Measureent of sun aziuth at latitude 19OS; longitude lbe in the Etosha National Park. Read this figure in conjunction with Table 4. ing drunk. they rfsted and ruinated between h lo 1 llours aficr sunrise (Berry cl al., 1982). once ore displaying a prelerence to face towards the sun. During the lasl two hours of daylight sun leperature declined and thc wildebeest began oving back to the grazing area, again exposing the grealest body surface to the sun. Fig. 2 is designed to show the inllucnce of wind on wildcbeesl body orientation, taking into account the accopanying theral regie. As wind speed increased LO 1 2 k/h two hours after sunrise. the wildebeest changed horn a "tail-1-wind" posture 1 race into the wind. a position which they aintained whilst walking to water. During this period teperature rose fro 1DC to 25OC. Once he herd had drunk (4,5 hours aiter sunrise) they began thc rest-ruination period. during which tliey orientated broadside to the wind, now reaching a force of up to 28 kjh. 'This period coincided with the hottest tie of' the day. The two hours preceding sunsei were the period that wildebeest chose to leave the vicinity 1' the water-hole and ove ta the night grazing area (Berry cl al ). Their body posture in relation to wind direction becae rando during this period and this ay have also been related to the decreasing wind Force b krn/h. Thus it appears that the hottest tie oc day, when wind speed was also at ils highest. caused wildebeest to orientate in such a way that they obtained thc greatesl sounl OS airflow over their body surface. Their daily drinking habit partly asked this tcndcncy although it is worth noting that wildebeest preferred to approach water facing into the wind and thereby facilitating their awareness of possible predators which were lying in abush. 4.2 Hot, dry season: 19 Deceber 1978 (Figs. 3 and 4) The sapling was done on a hot. cloudless day with wind force gradually increasing to 2 k/h at idday. after which it siilarly declined to less than 5 k/h. Interpretation of these data ust be ade cautiously since the sun's altitude (Table 4) increases to 85' at this tie of year. Consequently, the sun will have little direc- tionality on the horizontal plane between four and eight hours aiter sunrise. Wildebeests' response to the sun's angle was only evident during the tirst hour of sunlight when they raced away fro the sun. and & to 13 hours after sunrise when a prd'erred head-to-sun orientation prevailed. The response to wind is also ore or less rando for the lirst tight hours of sun wilh a tendency. however. to present he body surlhce laterally to the wind. Fro eight hours arter sunrise onwards. wildebeest showed a strong preterence for a (ail-to-wind orientation which correlates closely with a preverred head-to-sun orientation at lhe sae period. 5 CONCLUSIONS Wildcbecst in Etasha responded predictably to the abient sun and wind regies, although two factors appeared to ipinge on Ihe basic patlern asking il to soe cxtcnt. These were first. the daily drinking habit OF wildebeest which altered their body orien~alion as they inovcd to a watering point and. secondly, he idday altitude ol' the sun, notably during the suer solstice in Ileceber, when body orientation had litlle bearing on t hc sun's overhead position. Sun direction is probably of greatest iporlance to body orientation in wildebeest towards either cnd or the day. whilst wind ay influence the body position during the idday heat. ACKNOWLEDGEMENTS WE appreciate the assistance or Mr. A. McKenzie. SAERING. University of Cape Town. in obtaining coputer printouts and copiling the tables and figures. The inforation on sun aziuth and altitude was supplied by Prof. L. P. Adas. Land Survey Departent. University of Cape Town. 7 REFERENCES BERRY H.H. 198: Behavioural and eco-physiological studies on blue wildebeest (Connochae~es iaurinus) at the Etosha National Park. Unpubl. Ph.D, thesis, University of Cape Town. BERRY H.H.; SIEGFRIED, W.R. and CROWE, T.M. 1982: Activity patterns in a population OF free-ranging wildckst Contnochaetes raurinus at Etosha National Park. Z. Ticrps~~hol., 59: MIMMACK, G.M.; MORANT, P.D.; CROWE, T.M. and McKENZIE, A. 198: Biological applications of a coputer prograe package (DIRECT) for statistical anaiys~s oc twodiensional directional data. S. Rfr. J. CVildl. Res., 1 (1): 29-33