ELSEVIER Applied Animal Behaviour Science 55 (1997) I-10

APPLIED ANIMAL BEHAVIOUR

SCIENCE

Agonistic behaviour associated with orphan bison ( Bison bison L.! calves released into a mixed

resident population

B.R. Coppedge a, * , T.S. Carter b, J.H. Shaw b, R.G. Hamilton ’ ’ Oklahoma Cooperative Fish and Wildlife Research Unit, Oklahoma State University, Stillwater,

OK 74078, USA b Department of Zoology, Oklahoma State University, Stillwater, OK 74078, USA

’ The Nature Consewancy, Tallgruss Prairie Preserve, Pawhuska, OK 74056, USA

Accepted 3 February 1997

Abstract

Aggressive interactions of 43 bison (Bison bison L.) calves (10 months old) released motherless into a 28%member resident bison population of mixed composition were observed. Based on expected observations of aggressive interactions from x2 analyses, these ‘orphan’ calves were recipients of significantly more aggression than any resident age class, including similarly aged resident calves. Most aggression directed towards orphans was initiated by resident yearlings. We did not observe orphan calves initiate any aggression towards other animals during the study, even towards other orphans. These findings support previously suggested hypotheses that: (1) dominance rank in bison is linear and established by older cohorts over each subsequent year’s cohort; and (2) maternal presence is socially beneficial to young bison by reducing aggression directed towards the offspring from other herd members and facilitating their social integration. Surprisingly, no differences were found in growth rates between orphan calves and resident calves with mothers, despite initial weight differences that were significant. This finding suggests that the higher levels of hostility experienced by the orphan calves had no significant physical effects. 0 1997 Elsevier Science B.V.

Keywords: Bison; Calves; Orphans; Aggression

* Corresponding author.

0168-1591/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO168-1591(97)0003S-X

2 B.R. Coppedge et al./Applied Animal Behauiour Science 55 (1997) l-10

1. Introduction

Although orphaned mammals are sometimes fostered by conspecifics or close kin, adoption is still relatively rare (Reidman, 1982). Death from starvation or predation probably claims most orphans, resulting in a scarcity of information concerning the socioecology of orphaned mammals in many wild species. Lent (1974) presented several examples of young ungulates that were socially maladjusted due to maternal and social deprivation during their development, and recommended more study into the social behaviour of orphaned offspring in both domestic and wild ungulate species. However, since Lent’s recommendation, there has been little information on orphan ungulate behaviour gained apart from anecdotal observations. It has been reported that orphan red deer (Ceruus elaphus) and mountain goats (Oreamnos americana) suffer more aggres- sive displacements by other herd members than young animals near their mothers (Clutton-Brock et al., 1982; Masteller and Bailey, 1988). These studies also reported that the displacement of orphans reduced their access to food and forced them to occupy positions on the herd periphery. In addition to the displacement from feeding sites, orphans would suffer additional nutritional stress from the lack of suckling, likely resulting in decreased growth rates as compared to mothered animals.

This study presents an investigation of the social interactions of motherless bison (Bison bison L.) calves introduced into a free-ranging bison population of mixed age and sex composition. These calves were weaned at 7 months of age. Because weaning usually varies in this species from 8 to 20 months (Green, 19871, usually occurs gradually instead of abruptly and is initiated by the cow (Green, 1992), we predicted that these calves would attempt to integrate with the resident animals. Based on observations of orphans in other ungulate species (Clutton-Brock et al., 1982; Masteller and Bailey, 1988), we predicted that the lack of maternal presence and intervention and integration efforts would lead to these orphan calves receiving significantly higher levels of aggression than their resident calf peers. We also predicted that orphan calves would exhibit different spatial behaviour and be found more often on herd peripheries as compared to mothered resident calves. Finally, as a result of aggressive displacement leading to reduced feeding and the lack of suckling, we predicted that the orphans would have lower growth rates during the course of the study than resident calves.

2. Methods

2.1. Study area description and animals

Observations were carried out from 3 February to 20 May 1995 at the Tallgrass Prairie Preserve (TPP) located approximately 25 km northwest of Pawhuska in northcen- tral Oklahoma, USA. TPP is a 15 342 ha nature preserve operated by the Nature Conservancy. The area was predominantly tallgrass prairie composed of big bluestem ( Andropogon gerardii), little bluestem (Schizachyrium scoparium), switchgrass (Pani- cum uirgatum) and indiangrass (Sorghastrum nutans) with small areas of oak (Quercus spp.) and riparian woodland. A bison herd ranged on and was familiar with a 1973 ha

B.R. Coppedge et al./Appiied Animal BehaGour Science 55 (1997) l-10 3

portion of the preserve. Herd management was limited to roundups for inoculations, weighing, and culling of excess animals. In late October 1994, TPP received a donation of 43 (14 male, 29 female) calves (approximately 7 months old) from the bison population at the Wichita Mountains Wildlife Refuge (WMWR) in southwestern Okla- homa. These calves had been weaned at the WMWR in early October 1994 at 7 months of age. The calves were quarantined from the TPP herd until a January 1995 roundup, where they (and the resident animals) were weighed and fitted with colored ear tags to permit field identification. Both groups of animals were released together into the ranging area on 27 January 1995, and observations of aggressive interactions began 1 week after mixing. Thus, the study group of 331 animals consisted of 17 resident bulls 2-12 years of age, 169 resident cows ages 2- 12, 54 resident yearlings (12-24 months of age), 48 resident calves (O-12 months of age), and the 43 orphan calves. Animals were categorized into these five age/sex classes for this study.

2.2. Social behaviour

The TPP bison population forms large herds only during the summer rut (July- August), which is common in bison (McHugh, 1958). For the remainder of the year, they usually remain in two types of small, dynamic herds; mixed herds which contain cows, yearlings, calves, and usually young bulls, and bull herds containing mature and occasionally young bulls (McHugh, 1958). As part of a larger study of bison grazing habits on the TPP, we conducted daily surveys of the bison ranging area 4- 12 times per month. When herds were encountered, herd size, composition and the presence of any orphans were noted and herd locations mapped on 1:24000 topographic maps.

2.2.1. Aggressive interactions For this study, mixed herds located during daily surveys were observed ad libitum

(Altmann, 1974) during l-2 h observation periods (128 h total). In small herds (fewer than 20) all members were observed for behavioural interactions with other herd members. In larger herds, a small focal subgroup (about 20) of animals was observed. All identifiable agonistic interactions between herd members were recorded. These included buts, lunges, threats, avoidances, and displacements (Lott, 1979; Rutberg, 1983; Rothstein and Griswold, 1991). Data concerning each interaction included time of occurrence and the age or sex class of both the initiator and receiver of the aggression. In subsequent observations involving the same individual, only interactions more than 5 min apart were recorded to avoid bias due to lack of independence.

2.2.2. Spatial behaviour The relative position of orphan and resident calves within focal herds was noted

every 30min during observation periods. Positions were adopted from Green et al. (1989) and were defined as: central, with other herd members on all four sides; peripheral, with herd members on 2-3 sides; and outside, with herd members on only one side and sometimes an obvious distance from the herd. Data resulting from these calf observations were used to facilitate a comparison between orphan and resident calf spatial behaviour.

4 B.R. Coppedge et al./Applied Animal Behauiour Science 55 (1997) I-IO

2.3. Statistical analysis

Data were grouped into two time periods for analyses; the pre-calving period from 3 February to 20 March 1995 (the date of the first calf born in 1995); and the calving period from 21 March to 20 May 1995. Statistical tests used include one- and two-way (including interactions terms) ANOVA and x2 analysis.

There were no interactions involving mature bulls observed since only mixed herds were found to contain orphan calves and, thus, were observed for this study. We also did not observe any aggressive interactions involving young bulls despite their presence in some of the mixed herds we observed. We, therefore, dropped the bull age/sex class from further consideration in this study.

We analyzed the distribution of aggressive interactions by bison age classes in a two-way contingency matrix to determine how aggression was allotted during each study period. The matrix is based on which age class ‘initiated’ and which age class ‘received’ the aggressive interactions we observed. Statistical analysis was conducted using x2 goodness-of-fit tests. Expected values for within-class interactions were based on the proportion of the total population made up of the respective class. Expected values for between-class interactions were based on the average proportion of the population made up of the two classes. Since dominance was assumed to be linear, based on age (Rutberg, 1983), expected values were 0 when younger age classes were the initiator and an older age class the recipient. Row and column totals were used to assess the aggression initiated and received by each class as an overall group with separate x2 tests. However, we present the results of all tests for each study period in a single table for convenience. When x2 analysis showed significance, we calculated 95% simultaneous Bonferroni confidence intervals (Byers et al., 1984) to determine if the frequency of observed aggressive interactions were significantly higher or lower than expected.

3. Results

3.1. Aggressive interactions

3.1.1. Pre-calving period Aggressive interactions were not distributed as expected during the pre-calving period

( x2 = 60.34, df = 9, P < 0.001). Cows initiated significantly less aggression towards other cows than was anticipated, while resident yearlings exhibited high levels of aggression towards the orphan calves (Table 1). We did not observe orphan calves initiating any aggression towards any other animals. We point this out because the lack of these observations prevented proper statistical tests, but the fact that orphan calves did not initiate any aggression is a significant result nonetheless.

Overall, the amount of aggressive interactions initiated by each age class during the pre-calving period was different than expected ( x2 = 7.99, df = 3, P < 0.051, although no individual class initiated more or less aggression than expected (Table 1; right column). Overall, the amount of aggression received differed greatly ( x2 = 28.43,

B.R. Coppedge et a/./Applied Animal Behaciour Science 55 (1997) l-10 5

Table I Bison age class aggression matrix for the pre-calving period. A positive symbol ( +) indicates the observed frequency of aggressive interactions in the category was significantly higher than expected, whereas negative symbols (-) indicate aggressive interactions were observed significantly less than expected based on simultaneous Bonferroni confidence intervals. Categories marked by NS were observed frequencies not significantly different than expected, while those marked with 0 were categories with no interactions observed and thus not testable statistically.

Receivers

Initiators cows Yearlings Resident calves Orphans Overall (initiated)

cows _ NS 0 NS NS Yearlings 0 0 0 + NS Resident Calves 0 0 NS NS NS Orphans 0 0 0 0 0 Overall (received) - NS - +

df = 3, P < 0.001). Both cows and resident calves received aggression significantly less than expected, while orphaned calves received aggression significantly more (Table 1; bottom row).

3.1.2. Calving period We conducted an additional x2 test to determine whether the allotment of aggression

differed between study periods or whether data should be pooled. The observed distribution of aggressive interactions differed significantly between study periods (x2=20.17,df= 9, P < 0.025). Thus, data were not pooled across study periods.

The distribution of aggressive interactions during the calving period also differed significantly from expectations ( x2 = 25.72, df = 9, P < 0.005). Only one class of interactions varied significantly from what was expected. Cows again initiated less aggression towards other cows than expected (Table 2). Orphaned calves again failed to initiate any aggression towards any other animals, which again prevented the calculation of statistical tests.

The overall amount of aggression initiated was different from that expected ( x2 = 15.14, df = 3, P < 0.005). Yearlings initiated more aggression than expected as a group (Table 2; right column). The distribution of received aggression was also different

Table 2 Bison age class aggression matrix for the calving period. See Table 1 for explanation of symbols.

Receivers

Initiators cows Yearlings Resident Calves Orphans Overall (initiated)

cows _ Yearlings 0 Resident Calves 0 Orphans 0 Overall (received) -

NS 0 NS NS NS NS NS + 0 0 NS NS 0 0 0 0 NS NS NS

6 B.R. Coppedge et al/Applied Animal Behauiour Science 55 (1997) I-10

( x2 = 9.47, df = 3, P < 0.025). As earlier, cows in the calving period received signifi- cantly less aggression than expected (Table 2; bottom row).

3.2. Spatial behaviour

The spatial distribution of orphan and resident calves within resident herds was compared to determine if orphans exhibited different spatial behaviour while in resident herds. During the pre-calving period, orphan spatial patterns were not different from those of resident calves ( x2 = 1.40, df = 2, P > 0.25). All calves were found with approximately equal frequency at all locations relative to the main herd body (Fig. 1). However, calf distributions were not the same during the calving period ( x2 = 32.70, df = 2, P < 0.001). Orphan calves were much more common in both the center and outside of herds, while resident calves were found mostly on herd peripheries (Fig. 1).

3.3. Size and growth rates

All animals were weighed at the January 1995 roundup and again in November 1995. These pre- and post-study weights were compared for orphan and resident calves to assess whether stress due to increased aggression affected orphan growth. Growth rates were calculated as a percentage of the starting weight. The average (+ 1 SE) pre-study weight for orphan calves (117 f 6 kg) was significantly (F = 8.10, df = 1, P = 0.006) less than the average resident calf weight (141 f 6 kg). There was no difference in weight by sex (F = 2.13, df = 1, P > 0.1) at this time. For post-study weights, resident calves (by this time yearlings) remained significantly (F = 20.81; df = 1, P < 0.0001) larger (X = 275 + 7 kg) than the orphan calves (X = 232 + 7 kg). However, differences between the sexes were also significant (F = 6.09, df = 1, P < 0.02) at this time, males being larger than females (males, 265 + 8 kg; females, 242 f 6 kg). Although resident calves remained larger than orphan calves during 1995, there was no difference in growth rates between resident and orphan animals (F = 1.06, df = 1, P > 0.3) or between the sexes (F = 0.04, df = 1, P > 0.8).

k% CENTRAL i 1 PERIPHERAL k? OUTSIDE

Residents

Orphans

Residents

Orphans

I I I I I 0 20 40 GO 80 100

% of Calf Observations

Fig. 1. Positioning of orphan and resident calves relative to herd body by study period. See text for definition of positions.

B.R. Coppedge et al. /Applied Animal BehaL’ionr Science 55 (1997) l-10

4. Discussion

The results in Table 1 support our prediction that orphan calves would receive proportionally higher levels of aggression than resident calves. Although there is evidence that maternal presence influences dominance in bison calves (Giovengo and Waring, 1991) the immediate advantages of dominance in bison of pre-weaning age is not known. However, the long-term benefits of early social dominance in bison has been investigated. Green and Rothstein (1993) reported that earlier-born calves are socially dominant to calves born later, and have higher long-term growth rates and fecundity. Studies of adult bison have shown that higher social rank is positively correlated with increased feeding opportunities and higher fecundity in cows (Rutberg, 1986; Lott and Galland, 1987) and higher breeding rates in bulls (Lott, 1979). These results also support the suggestion by Rutberg (1983) that dominance in bison is established at young ages when adjacent cohorts are of different sizes. Resident yearlings appeared to establish dominance over the smaller orphans calves with frequent aggressive interactions, initiating significantly more aggression towards orphans than expected (Table 1). Surprisingly, though, resident yearlings did not initiate aggression against resident calves with the same frequency. It may be that the presence of cows shielded the resident calves against aggression (as found by Green et al., 1989) from resident yearlings, who then directed all of it towards the undefended orphans. It apparently did not take long to establish dominance over the orphan calves, as they did not receive the overt levels of aggression during the second study period as compared to the first.

The initial weight differences between orphan and resident calves may have been influenced by several factors prior to this study. Green and Rothstein (1993) found that earlier-born calves were significantly larger than calves born only a few weeks later. Thus, slight variation in age could have lead to the differences in initial weights between calf groups in this study. In addition, bison calves usually suckle longer than one year of age (Wolff, 1988). Since the orphan calves were weaned at approximately 7 months of age, the resident calves may have been larger because of a longer suckling period. However, Houpt and Hintz (1983) found that in domestic foals, orphans spent signifi- cantly more time grazing than foals with mothers. Thus, orphan calves could have compensated for the lack of suckling with more grazing, which may have also countered the reduction in feeding time lost to aggressive displacement. This would explain the similar growth rates found between the two calf groups. Unfortunately, we did not attempt to quantify the grazing efforts of orphans or the suckling time of resident calves to more fully investigate this hypothesis.

Another prediction was that orphan calves would exhibit spatial behaviour different from that of resident calves. However, as shown in Fig. 1, orphan calves and resident calves occupied similar spatial positions during the pre-calving period, despite higher levels of aggression towards orphans, while spatial behaviour was different between resident and orphan calves during the calving period. However, these patterns may have confounding influences in addition to integration efforts of the orphan calves. Bison cows show significant variation in spatial behaviour during parturition (McHugh, 1958: Lott and Galland, 1985) and average herd size almost doubled during the course of the study. Thus, the differences in spatial behaviour during this period may be due to a

8 B.R. Coppedge et &./Applied Animal Behaviour Science 55 (1997) l-10

confounding combination of the calving behaviour of the cows, which are then followed by their calves (Green, 19921, a concurrent increase in herd size, and integration efforts by the orphans.

The relative lack of aggression directed towards orphans during the calving period (Table 2) as compared to the pre-calving period (Table 1) supports the idea of their achieving social integration. Integration into social groups is important for ungulates because group members benefit from increased vigilance and feeding opportunities (Berger and Cunningham, 1988). These benefits would be especially important for orphaned animals who lack maternal protection and guidance on feeding choices. Integration may have also been promoted by the social composition of the TPP herd, being a more natural mixing of age and sex classes. More natural environments and group age/sex structures are known to ease the stress associated with mixing and subsequent integration in domestic pigs (Petersen et al., 1989) and sheep (Stolba et al., 1990). Lott and Minta (1983) found that social groupings in bison were essentially random aside from those between cows and their calves, which suggests that social integration and acceptance may be easier in bison of post-weaning age.

4.1. Long-term implications

Berger and Cunningham (1995) documented the effects of mixing discrete lineages in a closed population of North American plains bison. In their 5 year study, they documented the breeding activities, offspring survival, and growth rates after two lineages were mixed in 1984 in Badlands National Park, South Dakota, USA. Among their findings, was that males of one lineage failed to mate at all because of lower social standing. These results are surprising and seem contradictory to the findings of Lott (1979) of frequent reversals of dominant-subordinate status in bison bulls. It may be that in some closed bison populations, dominance hierarchies in bulls are established at early ages and not contested later, similar to hierarchies of bison cows (Rutberg, 1983). This is plausible considering that there are differences in social patterns in many species depending on the circumstances of the population (Lot& 1984).

This certainly implies that mixing bison lineages to enhance genetic diversity may be in vain if all animals are not contributing to the gene pool. Interestingly, Berger and Cunningham (1995) observed that females of both lineages were readily mated. Lott et al. (1987) have already suggested that trading female bison may be a good approach to mixing lineages and enhancing genetic diversity in the North American population. The apparent easy integration of the orphan calves into the TPP herd suggests that trading young animals may be yet another viable approach to managing closed bison popula- tions. Their smaller size would certainly make handling and transport easier. However, it remains to be seen how both sexes of the orphaned calves in our study will fare as adults. Their lower social ranking relative to their resident peers may decrease their later breeding success.

Acknowledgements

Financial and logistical support for this study was provided by the Nature Conser- vancy and the Oklahoma Cooperative Fish and Wildlife Research Unit (U.S. National

B.R. Coppedge et al. /Applied Animal Behaciour Science 55 (1997) l-10 9

Biological Service, Oklahoma Department of Wildlife Conservation, Oklahoma State University, and Wildlife Management Institute cooperating). We thank G.H. Waring for reviewing an earlier manuscript draft.

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