Foraging in the Eastern Grey Kangaroo and the Wallaroo

Foraging in the Eastern Grey Kangaroo and the WallarooAuthor(s): Robert J. TaylorSource: Journal of Animal Ecology, Vol. 53, No. 1 (Feb., 1984), pp. 65-74Published by: British Ecological SocietyStable URL: http://www.jstor.org/stable/4342 .

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Journal of Animal Ecology (1984), 53, 65-74

FORAGING IN THE EASTERN GREY KANGAROO AND THE WALLAROO

BY ROBERT J. TAYLOR*

Department of Ecosystem Management, University of New England, A rmidale, N.S. W. Australia 2351

SUMMARY

(1) Foraging by two species of kangaroo, the eastern grey kangaroo and the wallaroo, in different habitats was quantified for two sites which differed in pasture quality. On the basis of optimal foraging theory it was hypothesized that use of an area should be related to the abundance of high quality food.

(2) The amount of foraging undertaken in a habitat was positively correlated with the abundance of the most preferred food category, high-protein grass leaf. Other less preferred components of the pasture, although sometimes important in the diet, had no significant influence on the distribution of either species.

(3) Within the category high-protein grass leaf, the distribution of kangaroos was influenced by the species of grass as much as by their quality. The two species of kangaroo differed in their use of some high quality grasses.

(4) For a given biomass of high-protein grass leaf, the density of both species was significantly greater on the study area with the higher quality pasture.

INTRODUCTION

Optimal allocation of time by predators to different patches within an environment has received much recent attention (Tullock 1971; Krebs, Ryan & Charnov 1974; Cook & Hubbard 1977; Cook & Cockrell 1978; Waage 1979). Examination of optimal foraging in mammalian herbivores, however, has mainly been concerned with predicting the content of the diet (Westoby 1974, 1978; Belovsky 1978; Owen-Smith & Novellie 1982). This study on foraging by two species of large herbivorous marsupials, the eastern grey kangaroo (Macropus giganteus Shaw) and the wallaroo (M. r. robustus Gould), examines the use made of areas differing in pasture quality and composition. Both species occur sympatrically in dry sclerophyll forests and open woodland over large areas of eastern Australia. Wallaroos, however, are present in an area only when suitable shelter in the form of rocky hills or steep slope is available. Both species are grazers. They feed mainly from late afternoon through to early morning and retire to suitable sheltering habitat during the day (Frith & Calaby 1969).

Theoretical considerations suggest that a greater amount of time should be spent foraging in good patches than in poor ones (Charnov 1976; McNair 1982). It was thus hypothesized that, all else being equal and assuming the quantity of food available was not limiting, the abundance of high quality food should influence the degree of usage of an area for foraging. Since grass is the major component of the diet of both the grey kangaroo and the wallaroo (Kirkpatrick 1965; Griffiths & Barker 1966; Taylor 1983), it was expected

* Present address: Department of Zoology, University of Tasmania, Box 252C, G.P.O. Hobart, Tasmania, Australia 7001.

65



Foraging by two species of kangaroo

that the abundance of high quality grass would be the most important component of the pasture influencing use of an area.

STUDY AREAS

The study was conducted on two areas (Lana and Newholme) in the New England Tablelands of New South Wales, Australia. The study areas are within 40 km of each other and have similar temperature and rainfall regimes. However, there are marked differences between the two areas in the intensity of pasture management. On Newholme no pasture improvement has occurred and large areas with a dense tree canopy remain. Newholme is dominated topographically by Mt Duval (rising 300 m above adjacent flat country) and adjoining smaller rocky hills. Areas of woodland and grassland occur around these hills. Coarse tussock grasses (Poa) dominate the pasture in most areas. On Lana the pastures have been fertilized regularly for the past 20 years and high-protein grasses now predominate. Clover (Trifolium) seed has been spread to build up soil nitrogen content and some paddocks have been harrowed and sown with the introduced grasses Phalaris and Festuca and lucerne (Medicago sativa L.). A chain of rocky hills runs across the centre of the study area on Lana. These areas are well wooded but other areas have been selectively cleared and now support open woodland.

Sheep and cattle are grazed on both study areas. Small areas of pasture intensively affected by the grazing and sheltering activities of sheep are present on both areas. The study area on Lana covers 995 ha and that on Newholme 970 ha. Within their occupied habitats the density of wallaroos was estimated to be seven times greater on Lana (54 km-2) than on Newholme (7 km-2) and the density of grey kangaroos on Lana (31 km-2) was estimated to be double that on Newholme (14 km-2) (Taylor 1981).

The two study areas were subdivided into habitats based on slope, tree cover, size and abundance of rocks, shrub density, and biomass and composition of the pasture. Structural features of the two areas were broadly similar. Thus, on both areas rocky hills are present with flat country surrounding the slopes. However, on Newholme increased variability in pasture composition within areas with the same structural features led to different names being applied to areas which were otherwise similar. On Lana, most of the rocky hills were classified according to structural features because pasture composition in these areas is relatively uniform. On Newholme, however, the slopes were separated depending mainly on the species of Poa present and its biomass in relation to other grass species. This was done so that the role of pasture biomass and composition in determining use of habitat could be investigated. The range of tree densities was also greater on Newholme than on Lana leading to a greater range of habitats on Newholme.

Habitat types recognized on Lana were: savanna woodland; dense woodland; steep slope with (a) large boulders, (b) scattered rocks, (c) abundant rocks, (d) dense shrubs; gentle slope and sheep-affected pasture. Habitats recognized on Newholme were: Eragrostis grassland; Poa-Pennisetum grassland; steep slope with (a) abundant rocks, (b) dense shrubs, (c) open woodland; steep slope with pasture dominated by (a) Poa sieberana, (b) Poa costiniana, (c) Poa labillardieri, (d) Cymbopogon, (e) other grasses; woodland with (a) pasture dominated by Poa, (b) Acacia understorey; forest with (a) pasture dominated by Poa, (b) Acacia understorey; and sheep-affected pasture. On Lana, three pasture types were included in the sheep-affected habitat, i.e. pasture dominated by Microlaena stipoides (Labill.) R. Br., pasture dominated by other grass species and pasture dominated by forbs. On Newholme all sheep-affected pasture was dominated by Microlaena.

66



Sheep-affected pasture could be further subdivided into areas occurring on slopes and areas occurring in woodland. A detailed description of each habitat is given by Taylor (1981).

METHODS

To quantify the amount of foraging in each habitat, transects were set out, four on Lana and five on Newholme, so as to sample all habitats. The total length of transects was 13*2 km on Lana and 14.1 km on Newholme. Individual transects ranged between 1.9 and 4.1 km. Density of kangaroos in each habitat was estimated using methods similar to that employed by Lamprey (1964). Visibility profiles of each transect were constructed. Distances to which it was estimated that all kangaroos present could be seen were measured along the length of the transects. These distances ranged from 15 to 300 m. Distances from the transect were measured (with a calibrated range finder) each time the visibility changed rather than at fixed intervals (cf. Lamprey 1964). There were no significant changes in vegetation density between seasons and hence the same profile distances could be used all year. When transects were walked in fog, any changes in visibility were noted and the area of each habitat seen changed accordingly. Each transect was sampled twice a month in October and November 1976 and January to September and Decemberl977. The transects sampled 23% of the study area on Lana and 18% on Newholme. The area of each habitat sampled on the transects was proportional to the area they occupied on the study area. The area of each habitat sampled on the transects as a proportion of the total area sampled varied between 2% for steep slope with abundant rock and 49% for savanna woodland.

The transects were walked during early morning (within 1.5 h of dawn) and late afternoon (within 1-5 h of dusk) when it was expected that kangaroos would be actively grazing. For the grey kangaroo, the distribution of individuals among habitats did not differ between early morning and late afternoon and so the results for these two periods were grouped for the analysis. For the wallaroo, however, the percentage of time spent grazing is greater during late afternoon than during early morning (Osazuwa 1978) and use of habitats during late afternoon best reflect this species' preferences for grazing areas (Taylor 1981). The density of kangaroos recorded for a habitat will be influenced both by the number of individuals using the area and the amount of time spent by each individual in that area. Thus, provided that the amount of time spent grazing in a habitat as a proportion of the total time spent in that habitat does not differ between habitats, the density of individuals can be used as an index of the proportion of time spent foraging by the population in each habitat.

Pasture sampling Pasture biomass was assessed by weighing samples from quadrats of 0.25 m2. The

number of quadrats harvested on each study area during a sampling period ranged between 103 and 158. Quadrats were placed in habitats on a stratified random basis. On Lana, pasture was sampled in February, May, July and November and on Newholme in March, May, July and November.

Samples were dried in an oven at 85 ?C for 48 h and separated into species and plant part groups of similar quality. Samples were sorted into grasses, non-grass monocotyledons and forbs (i.e. dicotyledons and pteridophytes). Grasses were further separated into stem plus inforescence and leaf plus sheath components. The leaf and sheath were

R. J. TAYLOR 67




separated into high-protein species (e.g. Bothriochloa, Eragrostis, Sporobolus, Microlaena), low-protein species (e.g. Aristidia, Pennisetum, Imperata) and tussock species (e.g. Poa, Stipa). The stems of Aristida were also included in the low-protein category because of the growth form of this species. An additional category consisting of Lomandra longifolia Labill. and Pteridium esculentum (Forst. f.) Cockayne was included for Newholme. For all categories except tussock grasses only the live portions of the plants were included in the biomass estimates. A sample of seventy grey kangaroos and sixty- seven wallaroos was shot during grazing periods. Examination of plant material found in the mouths of these kangaroos showed that both live and dead leaves of tussock grasses were eaten whereas the leaves of other grasses were largely green.

Additional samples of green plants were collected and dried at 85 ?C for determination of forage quality. Both the grey kangaroo and wallaroo rely on foregut fermentation by micro-organisms for the majority of their energy requirements (Dellow 1979). The two measures of food quality most relevant to animals with foregut fermentation are the protein and digestible energy content of forage. Sorted samples were thus analysed for nitrogen content (nitrogen x 6.25 = protein) by the Kjeldahl method on a Technicon auto-analyser and acid-detergent fibre content (an index to digestible energy content) by the method of Van Soest (1963).

RESULTS

Pasture quality Since the fibre and protein content of plants were negatively correlated (Lana r =

-0 80, P < 0.001; Newholme r= -0-81, P < 0.001), only the results for protein content are given (Table 1). Grasses occurring in sheep-affected pasture and introduced grasses (i.e. Phalaris) contained the highest levels of protein. High-protein grasses were of a higher quality than tussock (P < 0.001) or low-protein grasses (P < 0.001). Grass leaf was of a higher quality than stem and inflorescence (P < 0.001). On Lana, the protein content of non-grass monocotyledons and forbs was not significantly different from that for high-protein grass leaf (not in sheep-affected pasture). On Newholme, forbs were higher (P < 0.05) and non-grass monocotyledons lower (P < 0.01) in protein content than high-protein grass leaf (not in sheep-affected pasture). For the tussock (P < 0.001) and high-protein (P < 0.01) grass categories, species on Lana were of a higher quality than

TABLE 1. Protein content (% dry matter) of plant categories on the two study areas. Figures are means + S.E. with sample sizes given in brackets

Plant category Lana Newholme

Grass Stem and inflorescence 6-9 + 0.6 (13) 6.9 + 0-5 (3) Leaf and sheath

Tussock 8.8 + 0.7 (12) 5-6 + 0.6 (20) Low-protein 5.6 + 0.6 (10) 6.3 + 0-5 (9) High-protein

Microlaena in sheep-affected pasture 22-5 + 1.9 (5) 22.4 + 1.3 (5) Other species in sheep-affected pasture 22.4 + 3.1 (3) -

Phalaris 21.9 + 1.8 (5) -

Range of species (not in sheep-affected pasture) 12.5 + 0.6 (30) 10.0 + 0-6 (31) Non-grass monocotyledons 10.0 + 2.5 (11) 7.5 + 0.6 (13) Forbs 14.4 + 1.4 (19) 13.1 + 1-3 (15)

68



those on Newholme. A comparison of the same species occurring on both study areas which had been sampled in the same season also showed that pasture plants on Lana were of a higher quality than those on Newholme (Wilcoxon matched-pairs signed-ranks test, P < 0.01).

Use of habitats forforaging

The results of a dietary study of the grey kangaroo and wallaroo on the two study areas are given in Taylor (1983). The diet of both species of kangaroo contained a large proportion of grass (77-98%) in all seasons. High-protein grass leaf was selected by both

species in all seasons. During winter the proportion of high-protein grass leaf in the diet was lowest. The proportion of tussock grasses eaten increased in winter with levels for

kangaroos on Newholme (grey kangaroo 42%; wallaroo 49%) being greater than for

kangaroos on Lana (grey kangaroo 13%; wallaroo 12%). In order to assess the influence of food abundance and quality on use of a habitat by

kangaroos, the biomass of high-protein grass leaf was regressed against kangaroo density. Other pasture components were then tested in multiple regression with high-protein grass leaf but they were not significant. The density values used were averages for all monthly samples and the pasture biomass was a yearly average from the four seasonal samples.

Grey kangaroo Grey kangaroos avoided areas with steep slope (Taylor 1981) so these were excluded

from the analysis. The density of grey kangaroos was significantly correlated with the biomass of high-protein grass leaf on both Lana (r = 0. 91, P < 0.05) and Newholme (r =

0.94, P < 0.01) (Fig. 1). An analysis of covariance showed no significant differences in the

slopes of the regression lines but a significant difference in the elevations for Lana and Newholme (F(i,i0) = 6.8, P < 0.05). Thus, for a given biomass of high-protein grass leaf, the density of grey kangaroos was higher on Lana than on Newholme. The increased density of grey kangaroos for a given biomass of high-protein grass leaf on Lana may be due to the higher quality of grasses on Lana compared with Newholme.

0-6 - 0.''

o / 0c0/

0-" -

/

c * / c 0.2- /

0/ /

o- 0"o ,

5 10 15 20 25 30 35

Biomass of high-protein gross leaf (g m 2)

FIG. 1. Relationships between the density of grey kangaroos and the biomass of high-protein grass leaf of habitats on Lana and Newholme. Habitats on Lana -- (OA); habitats on New-

holme ..... (OA); sheep-affected pasture (AA).

R. J. TAYLOR 69




There is also other evidence of the effects of grass quality influencing the density of grey kangaroos foraging in an area. For savanna woodland on Lana the biomass of high-protein grass leaf in sown pasture areas (17.6 g m-2) was not different from that found in other areas (18.2 g m-2). However, the density of grey kangaroos in sown pasture areas (79 km-2) was greater (P < 0.05) than in other areas (37 km-2). The introduced grass Phalaris (protein content 22%) which is abundant in sown pasture is of a higher quality (P < 0.01) than high-protein grasses in other areas of savanna woodland (protein content 12%). The increased density on sown pasture may thus be a result of the increased quality of grass in these areas. However, the quality of grasses in sheep-affected pasture is equivalent to that in sown pasture (Table 1) yet there is no increased use made of this habitat above that predicted on the basis of the biomass of high-protein grass leaf present for either Lana or Newholme (Fig. 1).

Wallaroo Areas isolated (>200 m) from rocky hills were not included in the analysis as these

areas are not utilized by wallaroos (Taylor 1981). Steep slope with abundant rock or large boulders was also excluded from the analysis. Very little high-protein grass occurs in these areas and few wallaroos were seen feeding here. These habitats are favoured resting areas (Taylor 1981) and the density values thus overestimate their importance as feeding habitats. The density of wallaroos per habitat on Lana was not significantly correlated with the biomass of high-protein grass leaf present (Fig. 2). If a line of best fit is drawn through the points in Fig. 2, gentle slope occurs above the line and savanna woodland below the line. This greater use of gentle slope and lower use of savanna woodland than expected on the basis of the biomass of high-protein grass leaf is probably related to the way in which wallaroos utilize these habitats. Wallaroos had a tendency to congregate in gentle slope, near where it joined savanna woodland, prior to entering or after leaving woodland areas. Any disturbance would cause individuals in savanna woodland to flee to gentle slope where they would sometimes wait before returning to savanna woodland. Thus, the assumption that density values can be used as an estimate of use of an area for foraging is probably not justified for the wallaroo. To properly investigate the relationship between food abundance and use for foraging for the wallaroo measurements on the

1--

E c 0-5-

0

5 10 15 20

Biomass of high-protein grass leaf (g m 2)

FIG. 2. Relationship between the density of wallaroos and the biomass of high-protein grass leaf for habitats on Lana. Savanna woodland (R); gentle slope (A); other habitats (0).

70



proportion of time spent feeding when in each habitat would need to be obtained. Casual observation indicated that wallaroos do spend a higher proportion of time feeding when in savanna woodland compared with when in gentle slope.

As for grey kangaroos, in savanna woodland the density of wallaroos was greater in sown pasture (136 km-2) than in normal pasture (32 km-2). Hence, it is likely that the quality of grass is also influencing the number of wallaroos feeding in a habitat. To investigate the effects of differences in quality of high-protein grass leaf on density, sheep-affected pasture was separated into its subhabitats. Savanna woodland was excluded from the analysis because of probable differences in the proportion of time spent feeding when in this habitat compared to when in other habitats. However, areas of gentle slope with no steeper slope above were included. This was thought to provide a better measure of use of gentle slope for feeding because of less bias related to wallaroo use of gentle slope as a 'stepping stone' into woodland. It can be seen from Fig. 3 that sheep-affected areas with Microlaena pasture are greatly overused in relation to the biomass of high-protein grass leaf present in comparison with other habitats. When sheep-affected pasture with Microlaena dominant is excluded there is a significant correlation between the biomass of high-protein grass leaf and the density of wallaroos (r = 0.90, P < 0.01). Sheep-affected pasture with other grass species dominant also contains grasses of a high quality (Table 1). It therefore appears that these high values for Microlaena pasture also indicate a preference for Microlaena rather than just a preference for high quality grasses per se.

The relationship between the biomass of high-protein grass leaf and density for wallaroos on Newholme is shown in Fig. 4. The two are significantly correlated (r = 0.71, P < 0.05). As on Lana, the greatest density of wallaroos per unit of biomass of high-protein grass leaf occurs in sheep-affected pasture (dominated only by Microlaena on

Newholme). When the regression line for Newholme is compared with that for Lana (excluding

sheep-affected pasture with Microlaena dominant) there is no difference in the slope of the

2 8-

2-4 -

T 1 2-

, 1 . 6 -

-

3 08- A

0 4- - 0

0

5 0 15 2 25 30 .

Biomass of high-protein gross leaf (g m 2)

FIG. 3. Relationship between the density of wallaroos and the biomass of high-protein grass leaf for habitats on Lana when savanna woodland is excluded and sheep-affected pasture is divided into its subhabitats. Sheep-affected pasture areas: Microlaena pasture in woodland (A); Microlaena pasture on slope (V); other species pasture in woodland (E); other species pasture

on slope (A); forb-dominated pasture in woodland (0). Other habitats (0).

R. J. TAYLOR 71




*- 04- /

a 02 - ,

5 1 1 20 25

Biomass of high-protein grass leaf (g m 2)

FIG. 4. Relationship between the density of wallaroos and the biomass of high-protein grass leaf for habitats on Newholme. Sheep-affected pasture (A), other habitats (0).

lines but the elevations are significantly different (F( 1,1) =18-2, P < 0.01). Thus, for a given biomass of high-protein grass leaf there is a higher density of wallaroos on Lana than of Newholme. As for the grey kangaroo, this is probably associated with the higher quality of grasses on Lana compared with Newholme.

DISCUSSION

It was hypothesized in the introduction that the abundance of high-quality food should influence the degree of use of an area for feeding. The only component of the pasture which was found to be correlated with the dispersion of kangaroos was high-protein grass leaf. High-protein grass leaf was the major component of the diet of both species and was selected for in all seasons (Taylor 1983). However, other pasture components, notably tussock grasses, sometimes comprised up to half of the diet. Even though the distribution of tussock grasses and other components of the pasture were generally not correlated with the distribution of high-protein grasses, the addition of these components to high-protein grass leaf in multiple regression was not significant. This was still the case when data for winter only (when the intake of tussock grasses was greatest) were analysed separately. The degree of use of an area for foraging therefore appears to be related to the abundance of the most preferred food category. During winter, when the availability of high-protein grasses is reduced, both kangaroo species increase their intake of the more abundant lower quality items, but do not appear to adjust their distribution to any significant extent to respond to the distribution of these less preferred items. The abundance of these lower quality plants in most habitats (especially on Newholme) is probably such that the quantity of food available is not limiting. Thus, in winter the kangaroos can still respond to the distribution of the higher quality food because of this excess of low quality food. Correlations between the abundance of food and the distribution of individuals have also been found for other species of herbivorous mammals (e.g. Frith 1964; Holsworth 1967; Franklin, Mossman & Dole 1975; Henry 1981).

When the pasture was separated into different categories the distribution of kangaroos was, as predicted, correlated with the distribution on the highest quality food category, high-protein grass leaf. However, when the influence on distribution of differences in

72



quality among grasses in the high-protein grass category was examined, kangaroos appeared to respond to the species of grass as much as to their quality. Both the grey kangaroo and wallaroo increased their use of high quality pasture in sown pasture areas above that expected on the basis of the biomass of high-protein grasses present. However, grey kangaroos did not increase their use of the high quality pasture in sheep-affected pasture and in these areas wallaroos responded only if Microlaena was the dominant species. It is possible that the kangaroos were responding to something other than protein content (the measure of quality used in this study) when selecting among high-protein grasses. Thus, individuals may be choosing items so as to obtain the best nutritional balance within a fixed bulk of food as Westoby (1974) hypothesizes for large herbivores. However, as Owen-Smith & Novellie (1982) note, nutrients tend to covary in their availability within plant tissues and thus quality rankings should be able to be expressed on the basis of the availability of the most limiting nutrient. Energy appears to be limiting for both the grey kangaroo and wallaroo on the two study areas (Taylor 1981). The digestive physiology of the two species also appears to be relatively similar (Foley 1977; Dellow 1979). Thus, one might expect both species to be selecting plants on the basis of the same limiting nutrient. The differences between the species in their use of Microlaena pasture in sheep-affected areas indicates that this is probably not the case. These differences may be related to the results of interspecific competition leading to the evolution of a degree of dietary separation. The greater proportion of Microlaena selected by the wallaroo was the major difference between the diet of the two species (Taylor 1983). However, regardless of any differences between the species in their response to different plant nutrients, in terms of optimal allocation of time between habitats the differences in use of Microlaena pasture may simply be related to the differences in the abundance of preferred food items for the two species in this habitat.

The lack of a significant correlation between density and biomass of high-protein grass leaf for the wallaroo when all habitats were included was thought to be due to differences in the proportion of time spent feeding in different habitats. Quantification of foraging times may show use of a habitat for feeding to be related to the biomass of high-protein grasses. However, wallaroos rarely feed in areas far from rocky hills and when disturbed head quickly back up into the hills. Their use of pasture adjacent to rocky hills may thus be a cQmpromise between predator avoidance and maximization of foraging efficiency. Under these conditions areas out from the rocky hills would be utilized less than predicted on the basis of optimal foraging theory.

In the present study it was found that the abundance of high quality food was important in determining distribution. However, in periods such as drought, when the quantity of food available becomes limiting, a herbivore may achieve a higher rate of energy assimilation by concentrating on lower quality foods. Under these conditions distribution may become influenced by the quantity of food available rather than by food quality.

When distribution was examined in relation to broad-scale pasture categories, the kangaroos were found to allocate foraging times between habitats as predicted by optimal foraging theory. However, the influence on foraging of species within the high-protein grasses was not as predicted. An animal is defined as foraging optimally if its rate of energy assimilation is maximized (Charnov 1976). In the present study it is not known whether different nutrients co-occur or if such things as secondary compounds are affecting digestibility. It is thus uncertain as to whether food quality (and thus energy assimilated) can be equated with single measurements such as protein content. It may prove difficult to show that a species is, in fact, not foraging optimally because of these uncertainties regarding food quality for herbivores.

R. J. TAYLOR 73




ACKNOWLEDGMENTS

Staff of the University of New England Herbarium and the Herbarium of New South Wales helped with the identification of many plant species. Gary Martin helped with the sorting of some of the pasture samples. Lesley Jenkins carried out the fibre analysis of many of the pasture samples. This study was supported by an Australian Postgraduate Research Award. Drs Peter Jarman and Peter Dwyer and Mr Randy Rose commented on a previous draft of the manuscript.

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(Received I November 1982)

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Foraging in the Eastern Grey Kangaroo and the Wallaroo

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