Zoo Biology 1:119-126 (1982)

Demographic Trends in a Captive Population of Red Pandas (Ailurus fulgens) Miles Roberts

Department of Zoological Research, National Zoological Park, Smithsonian Institution, Washington, DC

Birth and survivorship data based upon 39 litters born over a ten-year period are examined and discussed. Reproduction in red pandas is characterized by seasonal breeding, small mean litter size, and relatively high survivorship of young, and in general follows a typical “K-selected” life-history strategy.

The mean litter size was two, and the sex ratio at birth was approximately 1 male to 1.11 females. Higher male mortality in the early age classes resulted in a sex ratio in favor of females. Mortality was not correlated with age or experience of the mother but was found to be highly positively correlated with an increase in the in- breeding coefficient of the young. There was no significant difference in mortality among the different litter-size classes, and in general mortality decreased with age.

Recommendations concerning future management for the study population are given.

Key words: red pandas, demographics, reproduction, captivity

INTRODUCTION

In their demographic analysis of the captive world population of red pandas, Veeke and Glatston [1980] concluded that the population was destined for extinction. An age distribution that was not stable and reduced natality were the primary factors implicated as being responsible for the ultimate demise of the population. That conclu- sion is refuted, to some degree, by the overall performance of the National Zoological Park’s (NZP) red panda breeding program in which 79 young have been born over the last ten years. The National Zoo stock has grown from 2 animals in 1972 to over 50 animals in 1982 and includes 12 breeding pairs, most of which were themselves born in captivity.

Sufficient data have accumulated over the last ten years for a preliminary analysis of the parameters of growth of the National Zoo subpopulation. The data presented here should be viewed within the context of the management plans developed for the National Zoological Park [Roberts, 1975; Bush and Roberts, 1977; Roberts and Kessler,

Received for publication May 21, 1982; accepted June 8, 1982.

Address reprint requests to Miles Roberts, Department of Zoological Research, National Zoological Park, Smithsonian Institution, Washington, DC 20008.

0733-3188/82/0102-0119$02.50 0 1982 Alan R. Liss, Inc.

120 Roberts

1979; Roberts, 1981al. Other populations of red pandas may respond differently to dif- ferent management approaches.

MATERIALS AND METHODS

Data used in this analysis were collected from the species record files at the Na- tional Park. The sample size for each analysis is given in the text along with the animal identification numbers, where appropriate. A variety of statistical tests were utilized [Sokal and Rohlf, 19731.

RESULTS Patterns of Reproduction

The red panda is a seasonal breeder. In the Northern Hemisphere mating occurs between early January and mid-March, and births occur from early June to early August [Roberts and Kessler, 19791. Pairs are compatible, and trios consisting of one male and two females have been successfully established for matings; but separation of females is necessary prior to parturition as females tend to steal one another’s young [Roberts, 1981aI. There is one litter per year and females do not have a postpartum es- trus following the los? of neonates. Females and males are sexually mature at approxi- mately IS months of age, generally mating in the second mating season following birth [Roberts, 1981bl. The reproductive life span is potentially ten years for females and possibly longer for males.

In general, the reproductive strategy of the red panda reflects a low rate of rcpro- duction, long reproductive life span, small litter size, and prolonged adolescence, all characteristic of a “K-selected” life-history strategy [Pianka, 19701. Red pandas ap- pear to be solitary but exhibit a marked degree of social tolerance. There is little pater- nal involvement in the rearing of’ the young.

Size and Sex Ratio of Litters Litter sizes range from one to four [Dittoe, 1944; Glatston, 1980; Mottershead,

1958; Munro, 1969; Roberts and Kessler, 1979; Wall, 1908; Zuckerman, 19531. The dis- tribution and litter sizes of 39 litters born to the NZP population are shown in Table 1. These data may be biased in that some females may have killed and consumed infants before the litter was discovered, but because red pandas never consumed dead infants after the litter was discovered, 1 suggest that this source of bias was negligible.

The data shown in Table 1 indicate that 82010 of all litters were composed of one or two young with the litter size of two being predominant. The highest proportion of litters occurred in June and all triplet and quadruplet litters occurred in that month.

‘TABLE 1. Annual distribution of births and litter size for 3Y red oanda litters ~

Litter Size May June July August September ‘Total

1 0 5 3 0 0 8 2 0 17 5 2 0 24 3 0 6 0 0 0 6 4 0 1 0 0 0 1 Total 0 29 8 2 0 39

~- ~ ~ ~ _ _

Red Panda Demographics 121

Singleton litters accounted for 10.3% of all young born, twins for 61.5%, triplets for 23.1 Yo, and quadruplets for 5.1 (70.

Age-specific survivorship data were calculated for red pandas up to 3 years of age (Table 2). By combining the data in Tables 1 and 2, a matrix of the relative contribution of animals to the breeding pool (here, animals reaching 3 years of age) by litter-size class is developed (Table 3). This shows that the litter-size class of two contributed sig- nificantly more to the pool of potentially reproducing animals than any other litter-size class (x2 = 60.871; P < 0.001; df = 3). There was no significant difference between survivorship of each litter ske class and the overall survivorship rate of 58.9% (x2 = 0.301; P > 0.90; df = 3). This was in large part due to the small sample sizes registered by litter-size classes 1 and 4.

The mean litter size was 2.00. In general, the reproductive life span of females having consistently more than the mean was lower than that for females having consis- tently less, but the sample sizes for both classes of females was too small to allow ade- quate statistical testing.

The sex ratio at birth for 39 liters was 35 males, 39 females, and 4 of undeter- mined sex. If the four of unknown sex were assigned to the male class the sex ratio was equality, but even when assigned to the female class the sex ratio did not vary signifi- cantly from a 1: 1 ratio (x' = 0.82; df = 1 ; P > 0.1). In 31 litters of more than one cub, 16 litters (51.6%) contained both sexes.

The specific mortality rates of cubs of both sexes were examined for differential mortality between sexes up to the age of 3 years. There were 74 young born in the sam- ple (ie, the total sample minus the four young of unidentified sex). The sex ratio was 35 males to 39 females and the total mortality rate up to 3 years was 37.2%. Table 2 dem- onstrates the mortality distribution up to the age of 3 years.

The data in Table 2 indicate that overall male and female mortality up to 3 years of age did differ significantly (x' = 3.86; P < 0.05; df = 1) and that this difference was

TABLE 2. Mortality schedule for the red panda population*

1-30 days 31-90 days 91-120 days 121 days-3 years Total - .

Males 8 (0.229) 4 (0.114) 2 (0.057) 5 (0.143) 19

Total 12 6 4 7 29

*The table shows the number of captive born animals dying in each age class. Figures in parentheses indi- cate the proportion each number represents of the total number of males and females.

Females 4 (0.103) 2 (0.051) 2 (0.05 1) 2 (0.051) 10

TABLE 3. Litter class-specific survivorship rates*

Survivorship Total no. Contribution to Litter-size class to 3 years of young born rcproducing animals

1 0.50 8 4 2 0.625 48 30 3 0.55 18 10 4 0.so 4 2

*The table indicates the specific survivorship rate of each litter-size class, the total number of young born to each, and the total contribution of each class to the breeding-age population.

~ ~ .

122 Roberts

due in large part to the highly significant number of male deaths in the 0-30-day age class (x2 = 6.14; P = 0.0128; df = 1). There were no significant differences in mortali- ty for any of the other age classes. This finding suggests that more females than males will survive to 3 years if the current trend continues. Male survivorship was approxi- mately 45.7Vo while that of females was approximately 74%. Thus, the expected ratio of males to females at 3 years of age under these population trends would be slightly more than one male to two females.

Survivorship in Litters of Different Sizes To determine if there was any relationship between the survivorship of young and

the size of the litter a regression of mortality rates against litter size was performed for the 39 litters.

The regression of mortality against litter size indicated no significant decrease in survivorship with increased litter size (y = 0.4375 + 0.0075X; r = + 0.1639; P > 0.9). These data suggest that survivorship to 3 years of age does not decrease with an increase in litter size and that survivorship EO 3 years is roughly equivalent across all litter size classes.

Survivorship of Different Age Classes To determine whether there was a relationship between mortality and offspring

age a regression between mortality rate and age class up to 1 year of age was performed for the 39 litters.

The regression indicated a significant decrease in mortality (1 - survivorship) with an increase in age (y = 14.00 - 3.2X; r = - 0.95; P < 0.05; df = 3) for all litter- size classes lumped into one sample. These data suggest that mortality is highest in the youngest age classes and decreases over the course of the first year. Adequate data were not available to determine a subsequent increase in mortality at ages of over 3 years. Such data will not be available until the population has matured.

Survivorship of Litters With Increased Age and Experience of Females The reproductive performance of ten females with more than two litters was ex-

amined to determine if there was a relationship between litter number (implying experi- ence) and survivorship. All ten females had at least 2 litters, seven had 3, four females had 4 litters, and only one female had more than 4 (7). Thus, the data for 5,6, and 7 lit- ters were lumped into the category 5 +.

The regression of litter number against survivorship indicated a decline in survi- vorship with increased litter number with no significant correlation (r = - 0.332; P > 0.05; df = 3) (Fig. 1).

Inbreeding and Survivors hip

Inbreeding coefficients were calculated for all 39 litters following the method of Lasley [ 19781.

A regression of inbreeding coefficient and survivorship revealed a significant negative correlation (y = 0.734 - 1.14X; r = - 0.961; P < 0.01; df = 3). These data suggest that survivorship decreased concomitantly with an increase in the inbreeding coefficient (Fig. 2).

A regression between the variance in the survivorship of litters and the inbreeding coefficient of each litter suggested that there was a significantly higher variance in sur-

Red Panda Demographics 123

Y = 0.682 - 0.007X r = - 0.332

1 2 3 4 54

Litter Number Fig. I . Regression analysis of mean survivorship and litter number: The decreasing function y = 0.682 - 0.007X with r = - 0.332 indicated no significant correlation between mean survivorship and litter number. The Spearman Rank Correlation Coefficient derived from this analysis was rs = - 0.20 (P > 0.1) and indi- cated that there was no significant correlation between the two variables. Thus, survivorship was not seen as increasing or decreasing with increasing litter number of females.

Y = 0.734 - 1.14X r = -0.961

.8

.6 Q .- f b

9 .4 .- L 3 m

.2

I I I I I

0 0.05 .1 .15 .2 2 5 .3 .35 .4

Y = 0.734 - 1.14X r = -0.961

-

1 I 1 I I I I I

0 0.05 .1 .15 .2 2 5 .3 .35 .4

Inbreeding Coefficient

Fig. 2. Regression analysis of inbreeding coefficient and mean survivorship: The function y = 0.734 - 1.14X with r = -0.961 was derived from data on 74 animals of both sexes born at NZP. This indicated a high degree of correlation between inbreeding coefficient and mean survivorship. The Spearman Rank Correlation Coefficient derived from comparing the variables was rs = 1 .OO (P < 0.05) indicating a signifi- cant correlation between inbreeding coefficient and mean survivorship and that survivorship declines as in- breeding coefficient increases.

124 Roberts

Y = -0.02 + 0.289X r = + 0.987

.1 .2 .3 .4 .5 .6 .7 .8 .9 1.0

Variance in Survivorship

Fig. 3. Regression analysis of the variance in survivorship with inbreeding coefficient: the increasing function y = - 0.02 + 0.289X with r = + hr 0.987 indicates a high degree of correlation between the vari- ance in survivorship and inbreeding coefficient. The Spearman Rank Correlation Coefficient derived from comparing the variables was rs = 1 .oO (P < 0.05) indicating a high degre2 of correlation betwen the vari- ables. These analyses suggest that populations of animals with high inbreeding coefficients will have a higher variance in survivorship than will populations with low overall inbreeding coefficients.

vivorship with an increase in the inbreeding coefficient (y = -0.02 + 0.289X; r = +0.987; P < 0.01; df = 3) (Fig. 3 ) .

These analyses suggest that inbreeding is a major factor in the mortality of young of all litter-size classes. Females giving birth to young sired by a closely related male can expect not only higher mortality in their young but can also expect the interlitter survi- vorship to have a higher degree of variance (ie, unpredictability) than females having young by unrelated males.

The rate of mortality of 54% at an inbreeding coefficient of 0.25 compared with a moderate increase in mortality at an inbreeding coefficient of 0.0625 indicates that the most successful management practice is to mate individuals no more closely related than second cousin.

DISCUSSION

The pattern of reproduction in the population of red pandas examined in this study can be characterized by seasonal mating, relatively small litter size, and high sur- vival of offspring. The sex ratio at birth favors females slightly but becomes more heavily skewed in favor of females in older age classes due primarily to the significantly higher mortality in males less than 30 days of age. Increased litter size and the age, and

Red Panda Demographics 125

apparently experience, of the female have no significant effect upon infant survivor- ship. Mortality was highest for both sexes in the early age classes. Inbreeding caused an increase in mortality across age classes and suggested that the red panda is sensitive to the effects of inbreeding.

These trends suggest a number of appropriate management adjustments for this population. Clearly, if we continue to maintain animals in pairs, the rate of reproduc- tion and the fecundity of the population will be a function of the less abundant sex, in this case males, and there will be an overabundance of the more common sex. I have shown previously [Roberts, 1981al that red pandas can be kept in trios of one male and two females for breeding purposes as long as one female is separated from the trio prior to parturition. Increasing the number of trios will alleviate the problem of reduced pop- ulation fecundity but will, in its turn, also tend to increase the genetic contribution of males. The detrimental effects of increasing the number of trios can be ameliorated to some degree by ensuring that males do not mate with the same females year after year. In doing so, the inevitable increase in the mean inbreeding coefficient will be slow and relatively smooth rather than abrupt as would be the case when closely related animals are mated out of necessity. Thus, the maximal mixing of bloodlines now will not stop inbreeding over the long run, but it will force inbreeding to increase at a slower rate. If the social system of the red panda is as labile as it appears to be from our captive studies [Roberts, 1981b], such group manipulation should be easily attained.

Overall, survivorship was highest in litters of two young, and this litter-size class contributed almost twice as many animals to the breeding pool as did all other litter-size classes combined. The poor survivorship of litters of three and four can be explained in terms of intralitter competition for milk and maternal stress during the most difficult time of the year for this temperate species (summer). The poor rate of survivorship for litters of one cub is more difficult to explain, but in general these young appear to be either less fit or succumb to overzealous attention on the part of the mother. Conceiv- ably, litters of one young may reflect poor maternal health during gestation and therefore may be less fit. Managers may be able to take measures that improve survi- vorship on litters of less than or greater than two cubs. If three or four are born, one or more young can be removed for handrearing or can be supplemented with milk while being left with the mother [Conway, 19811. Cross-fostering to females who have lost their young may also be successfully accomplished [Roberts, 1981bJ. If, in fact, fe- males with one young appear to be harming the infant because of overzealous attention rather than poor performance, fostering another infant of similar age to her may be at- tempted. In general, fostering of young is more desirable than handrearing, as hand reared young, even when reintroduced to siblings at an early age, tend to be behaviorally retarded and may be poor breeders. For this reason given a choice between “pulling” a male or female infant the female will be chosen because of the potentially greater value of males as breeders at this stage in the population history.

In conclusion, the analysis of the National Zoo’s population of red pandas sug- gests that several management adjustments could be made to enhance population growth and survivorship. Inbreeding must be reduced and bloodline contributions brought as close as possible to parity. Increased matings of animals in trios may im- prove the lowered population fecundity caused by the female-biased inequality in re- productive age-sex ratios. Finally, cub management strategies may have to be devised to reduce the mortality rate of cubs of certain litter sizes and ages.

126 Roberts

ACKNOWLEDGMENTS

I would like to thank Jon Ballou, Ron Evans, Devra Kleiman, and an anonymous reviewer for providing many suggestions for the improvement of the manuscript. Jon Ballou critically reviewed the statistical procedures used in the manuscript.

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