Calving time and foetus growth among wild reindeer in Norway

Mean calving dates among 7 wild reindeer (Rangifer t. tarandus) herds in southern Norway varied between the 6th and the 27th of May. Ln transformed foetus weights relate linearly to ln age in days. Regression analyzes based upon 225 days pregnancy predicts birth wet weights between 4600 and 7500 g in the different areas and years. At 130 days, foetus weights varied between 547 and 746 g (x=681, standard deviation s=63) in all areas and years, indicating that foetus growth up to this size is independent of mothers body weight or condition. During the remaining 95 days of pregnancy, very poor body condition, reflected in dressed weights, resulted in slower foetus growth and smaller regression estimated birth weights. The foetus weight variation recorded at similar dates within areas and sampling years indicates a dispersed breeding time mostly within two ovulations. A small sample of foetuses from pregnant yearlings and calves indicate that these cohorts conceive later than 2 yr + females. Assuming similar foetus growth pattern among all female age cohorts within areas, yearlings conceive around 1 week later and calves (in Ottadalen) more than 3 weeks later than 2 + yr olds.


Introduction
Variation in calving time and birth weight may play an important part in the demographic processes.They relate both to neonatal mortality and to growth later in life (Lenvik & Aune, 1988;Eloranta & Nieminen, 1986).Among wild reindeer (Rangifer t. tarandus) in southern Norway, mean calving time (the time when 50% of the females have given birth) relates to female body weights in autumn (Reimers, 1997) and possibly to genetic differences (Flydal & Reimers,in press Wildl. Biol. 8 (2)).Heavy weight females, as in North Ottadalen, gave birth 6 of May, a month before snow melt, while low weight females, as in Hardangervidda, calved three weeks later (i.e. a week before snow melt) (Table 1).The nutritional aspects relating to this difference is discussed in Rangifer, 22 (1), 2002 Reimers et al. (1983).Jacobsen et al. (1981), Rognmo et al. (1983), Skogland (1984), Eloranta & Nieminen (1986) and Lenvik & Aune (1988) found that calves from undernourished reindeer were born smaller and suffered higher neonatal mortality than those from mothers in prime condition.The effect of calf birth weight may extend beyond the neonatal stage.Low weights at birth may (Espmark, 1980) or may not (Rognmo et al., 1983) be sustained through summer.Determinants include milk production, as influenced by maternal nutrition (White & Luick, 1984;White, 1991), and the degree of compensatory growth that is possible on a given summer range.This paper: (1) compares differences in foetus growth rates in relation to mother carcass weights in 7 wild reindeer areas in southern Norway, (2) assesses the variation of  b From Holthe (1975).c From Skogland (1984).d From Reimers (1983a; 1997).
foetus weights in relation to sampling dates and (3) estimates the female age effect on conception dates.

Material and methods
The data include female reindeer killed under various sampling programs or in snow avalanches in 7 wild reindeer areas in southern Norway (Table 1).Three populations are reindeer from domestic origin released as wild in 1964-65 in North and South Ottadalen (Reimers, 1972) or had their status changed to wild in 1956 in Forelhogna (J.J. Meli, pers.comm.; Røed, 1986).The data from Forelhogna and Knutsh0 in 1984 and Hardangervidda 1983 were obtained from a joint sampling program (Skogland, 1984).Sampling of pregnant females occurred from February through May.Female dressed weights (Langvatn et al., 1966) were measured to the nearest 100 g and foetus total weights to the nearest g when smaller than 1500 g, and to the nearest 25 g when heavier.Age of females was determined on basis of tooth develop¬ ment or tooth wear (animals sampled in 1952-59) or annuli in the incisor teeth cementum (Reimers & Nordby, 1968).The gestation period was set at 225    2. days (7 1/2 months) (Skjenneberg & Slagsvold, 1968) and conception dates were estimated form mean calv¬ ing dates.Number of days pregnant was calculated on basis of sampling dates and conception dates in the various areas.Foetus weights and number of days pregnant were ln-transformed to give a linear rela¬ tionship.Foetus weights and birth weights were esti¬ mated from regression equations on basis of these transformations and a gestation period of 225 days.2.
Differences in regression slopes were evaluated from regression coefficients ±95% confidence limits and from analysis of variation (Nissen, 1994).

Results
Pregnant female dressed weights differed in the vari¬ ous areas as did the mean calving dates (Table 1).Foetus weights plotted against number of days preg- The foetus weight at 130 days and the birth weights are estimated from the regression lines and based upon a pregnancy period of 225 days.b From Skogland (1984).
Estimated foetus weight at age 130 days varied between 547 and 746 g in the different areas and years and the weights were apparently unrelated to female carcass weights (Table 2, Fig. 2).Among females with mean carcass weights over 30 kg the foetus growth rate between age 85 and 225 days apparently was unrelated to the weights of the moth¬ ers (Table 2, Fig. 3).The foetus growth rate in a pooled sample excluding Hardangervidda 1983 was significantly higher (F=16.1;P=0.02) than the rate estimated among foetuses from Hardangervidda in 1983 (Figs. 3, 4).Assuming a gestation period of 225 days, the estimated wet birth weight was 4596 g in Hardangervidda in 1983 (95% Conf.Int. between 4106 and 5144 g) and 6999 g in a pooled sample from the other areas (95% Conf.Int. between 6476 and 7564 g) (Table 2).Birth weights recorded in field were somewhat lower than those estimated from the regression lines (Table 2).The maximum and mini¬ mum foetus weights recorded within areas and years during February through May (Table 3) indicate con¬ siderable variation in conception dates.A small sam¬ ple of pregnant yearlings and calves indicate that these age groups conceive later than the older females (Table 4).The estimated later conception dates range from 1-31 days (average=8, s=8) in yearlings and 22¬ 51 days (average=34, s=12) in calves.

Discussion
The estimated wet birth weights and the weights of the newborn (Table 2) were between 4.5 and 7.5 kg.This is mostly within the expected range between 3.7 and 7.1 kg (see review by Reimers, 1997).In Hardangervidda, Skogland (1984) found a mean birth weight of 2.98 kg among 7 dead calves and 3.72 kg among 11 live-captured calves.He pooled the two samples and arrives at an average newborn weight of 3.43 kg, 1.17 kg less than predicted from the regres¬ sion model (  2).Foetus growth up to this size and during this period was independent of the mothers body condition.During the remaining 95 days of pregnancy, poor body con¬ dition reflected in dressed weights apparently has a major influence on the foetus growth as found by Skogland (1984).Factors explaining foetus weight variations may include area, mother's age (Table 4), dressed weight (Skogland, 1984; Table 1), the ovula¬ tion pattern (Ropstad et al., 1995) and the conception date.The ovulation pattern, i.e. the number of days between two ovulations, is 21 days among reindeer (Ropstad et al., 1995).Without conception, the female continues to ovulate every 21 day at least up to 6 times (Ropstad et al., 1995).Judging from the calv¬ ing season, which within areas are rather concentrat¬ ed, the majority of females give birth within 10 days, the females breed synchronized and most of them apparently during two ovulations (Table 3).Alternatively, as discussed by Krog et al. (1980), breeding is spread out in time and foetus growth is later synchronized by delayed implantation to give a concentrated calving time.The foetus weight varia¬ tion (Table 3) does not support a synchronization of foetus growth.Rather, it indicates a spread out breed¬ ing or differences in the intrauterine environment.If breeding involved more than two ovulations, the maximum difference between foetus weights meas¬ ured at the same time would have been more than recorded in Table 3, as would the calculated difference in number of days pregnant.At similar dates, calves and yearlings bore smaller foetuses than adults (Table 4).Krog et al. (1980) report corresponding results for yearlings.Yearling females had foetuses weighing 38% less than foetuses of 2 1/2+ year old females when measured at the same date in February.This foe¬ tus weight difference could either imply a later con¬ ception and thus calving date or a smaller birth weight.Assuming similar birth weights, the foetus weight differences represents a birth date delay of 5-6 days among yearlings and more than 3 weeks among calves compared with the 2 1/2+ years old.It is well documented that birth weight relates to the mothers weight (Eloranta & Nieminen, 1986).In feeding experiments, Rognmo et al. (1983) and Eloranta & Nieminen (1986) found that newborn weights decreased with decreasing weights of their mothers.
Female calves and yearlings have smaller carcass weights than the older females (Reimers, 1983a).Hence, some of the birth date delay indicated in Table 4 may be compensated by smaller birth weights.However, it is possible that the apparent age effect in reality is a body weight or body composition effect (Reimers, 1983b;Ropstad et al., 1991).Being in a growth phase, it is conceivable that yearlings, and in particular calves meet the weight or fat:lean ratio needed for ovulation (Ropstad et al., 1991) later than the older females.

Fig. 1 .
Fig. 1.Foetus weights from wild reindeer sampled in different areas and years and plotted against estimated number of days preg¬ nant.Fig. 2. Female carcass weights (fromTable 1) +95% confidence intervals (CI) and regression esti¬ mated ln foetus weights ±95% CI at age 130 days.Foetus weights are calculated on basis of regression equations in Table 2.

Table 1 .
Dressed weights from pregnant females 1 yr + sampled from Nov through May and calving time and estimated conception date among wild reindeer herds in Norway.s = standard deviation.
Calving date, the date when 50% of the pregnant females have given birth, is registered in field.Conception date is estimated on basis of a pregnancy period of 225 days including conception and calving dates. a

Table 2
what to account for the water loss occurring during drying up.Foetus weights at age 130 days were rea¬ sonably similar in all areas and years (Table
b Difference in calving date is corrected for the number of days between measuring dates: (Xmax -Xmin).

Table 4 .
Ln transformed regressions of foetus weights (Y) in g from pregnant females 2 1/2 yr + at concep¬ tion in relation to number of days pregnant (X) in Feb, Mar, Apr and May (211>X>85 days).ConclusionsFoetus growth rate in wild reindeer up to around age 130 days and weight 550-750 g appears not to be influenced by the mothers body weights.Later in pregnancy females in very poor condition (mean car¬ cass weights around 25 kg) support a foetus growth rate which is significantly lower than among females with mean carcass weights of 29 kg or above.Foetus weight variations within areas and years indicate that conception in two year or older females mostly occur within two eustrous cycles.Yearling females conceive within a week later than older females while calves apparently conceive 3-4 weeks later.