Reproductive Performance in Wild and Game Farm Mallards

Reproductive Performance in Wild and Game Farm Mallards KIMBERLY M. CHENG,1 ROBERT N. SHOFFNER, and RICHARD E. PHILLIPS Department of Animal Science, University of Minnesota, St. Paul, Minnesota 55108 FORREST B. LEE Northern Prairie Wildlife Research Center, US Fish and Wildlife Service, Jamestown, North Dakota 58401 (Received for publication November 14, 1979) ABSTRACT Reproductive performance was studied in five strains of mallards with different degrees of wildness. Game farm mallards had a longer breeding season, higher egg production, larger clutch size, and higher male fertility than wild mallards. Incubation time and maximum emergence time were also longer for eggs laid by game farm dams than for eggs laid by wild dams. There was no significant difference between strains for embryonic mortality and hatchability. Hybrid strains were intermediate between the game farm and the wild strain in traits where significant differences were found. Although game farm and wild mallards differed substantially in several of the reproductive traits examined, and these differences may have a direct influence on the reproductive success of game farm strains in the wild, none seems to present any postcopulatory isolating mechanisms between these two strains of mallards. (Key words: domestication, ducks, reproduction, genetics) 1980 Poultry Science 59:1970-1976 INTRODUCTION

Large scale and repeated releases of handreared mallards have been proposed as a means to replenish wild stocks that have been depleted by destruction of breeding habitats (Gottschalk and Studholm, 1970; Pospahala et ah, 1974). Hand-reared birds of game farm origin are genetically different from those in the wild (Greenwood, 1975; Lee and Nelson, 1972; Prince et ah, 1970). Concern is expressed because genes carried by these released birds that have low "fitness value" in the wild may "contaminate" the wild gene pool (Banks, 1972;Shoffner, 1972). The measurement of gene flow between large populations of animals in the field is difficult and has rarely been achieved in population biology (Cooke et ah, 1975). As an alternative, experiments designed to study potential isolating mechanisms of wild and game farm strains may give indications of the amount of gene exchange between the two populations. If isolating mechanisms exist, gene flow may be slowed down or completely blocked.

Low fertility in eggs from wild X game farm reciprocal crosses has been reported by previous workers (Hunt et ah, 1958; Prince et ah, 1970; Greenwood, 1975). Hunt et al. (1958) attributed poor fertility to the fact that their wild mallards, recently trapped, had not adjusted to the confinement and were, therefore, incapable of normal breeding. Prince et al. (1970) simply commented that the two lines did not cross readily. Poor fertility or hatchability of eggs from crosses of two different parental types may be attributable to many factors, such as behavioral incompatibility or mate preference (Cheng et ah, 1978, 1979), physiological incompatibility of gametes (Shaklee and Knox, 1953), differences in karyotypes (Shoffner, 1974a), or chromosome rearrangements (Shoffner, 1974b) in one of the parental lines. The purpose of this experiment was to study 1) the difference in reproductive performance between wild, game farm, and wild X game farm hybrid mallards, and 2) to determine if postcopulatory isolating mechanisms exist between the two strains. MATERIALS AND METHODS

'Present address: J. F. Bell Museum of Natural History, University of Minnesota, 10 Church St. SE, Minneapolis, MN 55455.

The game farm mallards used in this experiment originated from captive wild birds, two females and one male, in 1949 at the Frost

1970

REPRODUCTION AND MALLARDS Game Farm in Wisconsin (Frost, 1972). Occasional outcrossing to free flying wild drakes was practiced in the early generations to minimize inbreeding. During the following 16 generations of breeding in captivity, selection was made for morphological resemblance to wild mallards and increased egg production. In 1967, 10 females and 4 males were acquired by Northern Prairie Wildlife Research Center (NPWRC) in North Dakota for research purposes. The population was expanded and random mated for another 6 generations before the start of the experiment. The wild mallards used were hatched from eggs collected from nests of wild mallards in central North Dakota. Reproductive Procedures. At the onset of the experiment, 10 game farm (G) males and 10 wild (W) females were pair-mated to produce Fj hybrid ducklings (50% wild) at NPWRC. The ducklings were banded and raised for breeders in the following season when pedigreed matings were made according to the breeding scheme for 1974 in Table 1. Ducklings hatched from these matings were randomly chosen within strains to provide parental stock for the next breeding season, and matings were made as shown in the breeding scheme for 1975 (Table 1). Each year, breeding birds were randomly assigned to breeding pens in mid-March. Full-sib matings were excluded. Each breeding pair occupied an individual 3.7m X 2.4m outdoor "A"-pen furnished with wooden nest boxes (Lee, 1974). A pelleted ration (18% protein) was fed ad lib with a supplement of cereal

1971

grains. Crushed oyster shells were also available. Eggs were collected daily, identified, disinfected, and stored in a cooler at 50 F (10 C) and 65% relative humidity. Each egg collected from the nest was replaced by a dummy egg (a mallard egg from our surplus stocks) with a maximum of 5 dummy eggs per nest. The 6th and following eggs collected from the nests were not replaced. When a female stopped laying eggs for 3 consecutive days, the dummy eggs were removed and the nest entrance was blocked to induce the female to start a new nest. The same egg collecting procedure was repeated for each clutch. Clutch size was measured by the number of eggs laid in the first clutch. When a female stopped laying eggs for 3 consecutive days, the last egg laid was counted as the last egg in the clutch. Incubation and Hatching. Pedigreed eggs were incubated every 4 days in a Petersime Model #1 incubator (37.5 C and 85% wet bulb reading). Each hatch consisted of approximately 120 eggs with all five strains represented. The eggs were candled on the 7th, 14th, and 21st days of incubation to determine viability. Viable embryos were transferred to a Petersime Model H145 hatcher on the 21st day of incubation. Each egg was placed in an individual hatching basket for proper identification of the hatched ducklings. Starting on the 24th day of incubation, eggs were checked every 4 hr for emergence time. After the first candling, any apparently infertile eggs were broken out to determine macroscopically (Kosin, 1944)

TABLE 1. Breeding scheme Types of mating Year

Male

1974

game farm game farm 50% wild 50% wild 50% wild wild wild

1975

game farm 25% wild game farm 50% wild wild 75% wild wild

Female

No. of matings

Progeny genotype

X X X X X X X

game farm 50% wild game farm 50% wild wild 50% wild wild

10 6 6 10 6 6 11

game farm 25% wild 25% wild 50% wild 75% wild 75% wild wild

X X X X X X X

game farm 25% wild wild 50% wild game farm 75% wild wild

10 10 2 10 2 10 12

game farm 25% wild 50% wild 50% wild 50% wild 75% wild wild

CHENG ET AL.

1972

whether they were early embryonic deaths or truly infertile. Traits Analyzed. Traits related to egg production including date of first egg, date of last egg, total egg number, and clutch size were analyzed. Since all mallards used in the matings were first year birds, these traits reflect the females' onset of sexual maturity, their response to photoperiod, as well as their consistency in egg laying. Other reproductive traits such as percent fertility, percent embryonic mortality, percent pips, percent hatchability, percent hatching from the small end of the egg, and traits associated with progeny fitness, including incubation time and emergence time, were also analyzed. All analyses were conducted according to the least-square method outlined by Harvey (1975). Constants were fitted for strain of dam, strain of sire, and mating types. A sire strain X dam strain interaction was also included. All effects were considered fixed. RESULTS

Egg Production Traits. There were significant differences (P<.01) between females of the different strains for date of first egg, date of last egg, total egg number, and clutch size. There was also a distinct linear effect (P<.01) on these measurements associated with the degree of wildness in the genotype of the females.

The laying season of the wild females was confined to the interval between May 6 and June 11 (Table 2). In contrast, game farm females started laying 4 weeks ahead of their wild conspecifics and they continued laying for 3 weeks after the wild females had stopped. Between these extremes were the 25% wild, 50% wild, and 75% wild females. The wilder the females, the shorter the laying season (significant linear effect). Total egg number is defined as the total number of eggs laid by a female between the period of March 24 to July 15 each year. Because of their longer laying period, game farm females averaged 36 more eggs per female per year than the wild females (Table 2). The clutch size of game farm females was also significantly larger than those of wild females. In the 2 years of this study, 1 game farm female was killed by an unknown predator, and 2 wild females failed to lay any eggs. These females were excluded from the analysis. Game farm females had a mean clutch size of 41 while wild females averaged only 17 eggs per clutch (Table 2). The means for the hybrid strains were between those for the wild and the game farm strains. In addition, game farm females laid more clutches than wild females. The mean number of clutches laid by game farm, 25% wild, 50% wild, 75% wild, and wild females were 3.0, 3.5, 2.5, 2.2, and 1.5, respectively.

TABLE 2. Least-square means for reproductive traits with significant dam effect Dam genotype Traits

Game farm

25% Wild

50% Wild

75% Wild

Wild

Date of first egg N

April 10 a 28

April l l a 10

April 22b 32

April 28b<: 10

May 6 C 29

Date of last egg N

June 30 a 27

July 7 a 10

June 2 5 a 32

June 2 1 a b 10

June l i b 29

Total egg no. N Clutch size N Incubation time N Max emergence time N

59 a 27 41a

27 629.4 a 27 59.5 a 27

58 a 10 25bc 10 628.7 a 10 52.6 a b 10

45b 32

40b 10

b

b

27 32

625.6 b 30 55.4 a 30

23D

31

23 c 10

17C 29

618.4 d 10

620.3 C 27

54.5ab 10

48.7 b 23

' ' ' Across each row, subclass means that aro followed by different letter superscripts are significantly different (P<.05) by Duncan's multiple range test (Kramer, 1957).

REPRODUCTION AND MALLARDS

Out of a total of 27 game farm females in the breeding pens, 7 laid continuously (i.e., without stopping for more than 2 days) from early April to late June or early July. No female from the other strains did so (Table 4). Since the 6th and subsequent eggs collected from each nest were not replaced, the estimates for total egg number and clutch size are higher than if the females were left to finish laying their clutches undisturbed (Prince et ah, 1970). Although sire strain effect was not significant in any of the egg production traits examined, there are indications that the male of the breeding pair did have some effects on the laying period of the female. There was a significant sire strain X dam strain interaction (P <.05) in date of last egg. The nature of the interaction, however, was not apparent. Furthermore, the 2 wild females that failed to lay eggs were mated to males of a different strain. Within strains, 50% and 100% wild females mated to game farm males tended to start laying eggs sooner than the females of the same strains mated to wild males. The effect, however, was not significant. The game farm males may have started their courting and copulating behavior earlier than the wild males, therefore providing some stimulation to their females promoting earlier egg laying (Desforges, 1972). Fertility and Hatchability Traits. Totals of 1,805 and 1,958 eggs were incubated in 1974 and 1975, respectively. Of the six fertility and hatchability traits analyzed, no significant difference was found between sire strains, dam strains, and mating types in the percent of early embryonic deaths (embryos that died before the 7th day of incubation), the percent of late embryonic deaths (embryos that died between the 7th day of incubation and pipping), percent

1973

hatchability (percent of fertile eggs that hatched), and the percent of ducklings hatching from the small end of the egg. The population means for percent early and late embryonic deaths were 7.1% and 9.3%, respectively. The average hatchability for the whole population was 71% and of those ducklings that hatched, 1.5% hatched from the small end of the egg. The strain of the sire was a significant factor in the percent of fertile eggs (P<.05) and in percent pips (embryos that pipped but died before they got out of the shell ) (P<.01), but the strain of the dam was not significant and there was no significant sire strain X dam strain interaction in either trait. The mean fertility over all strains was 98%, and there was no significant difference in fertility of eggs sired by game farm, 25% wild, 50% wild, and 75% wild males, but eggs sired by wild males had a significandy lower fertility of 82% (Table 3). These results support those reported by Batt and Prince (1978). The male fertility duration profile of the different strains is given in Fig. 1. Poor fertility of wild males in captivity has been reported previously (Greenwood, 1975; Prince et al., 1970). The poor fertility perhaps was attributable to their short duration of fertility, or the inhibitory effect of being in captivity (Phillips, 1964). The difference in percent pips of eggs sired by males from different strains was quadratic (P<.01). That is, of the eggs that were fertile, a higher percent of ducklings sired by hybrid (25% wild, 50% wild, and 75% wild) males than of ducklings sired by pure strain (wild and game farm) males pipped but died (Table 3). No logical explanation for this phenomenon is available after close examination of the data. The difference in percent pips, however, was

TABLE 3. Least-square means for reproductive traits with significant sire effect Sire genotype Traits 1

Game farm

25% Wild

50% Wild

75% Wild

Wild

% Fertility N

99.55* 27

99.25* 10

98.50* 31

99.70* 10

81.60 b 31

% Pips N

2.2* 27

13.4 b 10

7.5 b 31

5.7* b 10

9.8 b 25

Arcsine transformation applied to all percentages during analyses. a' bAcross each row, subclass means that are followed by different letter superscripts are significantly different (P<.05).

1974

CHENG ET AL. TABLE 4. Frequency distribution of clutches within genotype N u m b e r of clutches per female

Genotype

0

1

2

3

4

5

c

G a m e farm 2 5 % wild 50% wild 7 5 % wild Wild

0 0 0 0 2

0 0 8 2 14

5 0 7 5 13

11 5 11 2 2

4 5 5 1 0

0 0 1 0 0

7 0 0 0 0

1

Total no females 27 10 32 10 31

C, females laying continuously for more than 60 days.

not large enough to affect the overall hatchability of the different strains. Progeny Fitness Traits. Eggs laid by females from different strains differed significantly (P<.005) in incubation time, which is the period from onset of incubation to the time the duckling is completely out of the shell. Eggs laid by game farm females took an average of 629.4 hr to hatch and incubation time decreased (linear effect: P<.005) with the increased degree of wildness in the females (Table 2). Sire strain effect and sire strain x dam strain interaction were not significant. Maximum emergence time is defined as the time interval between the hatching of the earliest and the last ducklings from the same female (not necessarily from the same clutch). Females from the different strains differed significantly (P<.05) in this trait (Table 2). Maximum emergence time was longer for eggs laid by game farm females than for eggs laid by wild females. There was also a significant sire strain X dam strain interaction, but the sire strain effect was not significant.

FIG. 1. Duration of male fertility among the five mallard strains.

A separate analysis on incubation time was conducted treating each duckling as a sample. Constants were fitted for years, progeny strain, hatch within year, and year X strain interaction. All three main effects were highly significant (P<.01), but the interaction was not. Game farm ducklings required the longest time to hatch (628.4 hr) and incubation time decreased with the increased wildness in the ducklings (621.6 hr for wild ducklings, linear effect: P<.01). Eggs set in 1974 required longer time to hatch (626.8 hr) than eggs set the following year (624.3 hr). Within each year, eggs in the earlier hatches hatched out in a shorter time than eggs from later hatches. The analysis was conducted on eggs laid between April 22 (day 30) and June 1 (day 70) of each year, a period when eggs from all five strains were available. DISCUSSION Although game farm and wild mallards differed substantially in the several reproductive aspects examined, and a few of those differences may have a direct influence on the reproductive success of game farm strains in the wild, none seems to present any potential barrier to the cross matings of these two strains under the present experimental conditions. Game farm mallards have a longer breeding season, higher egg production, and higher fertility than wild mallards. Under domestic conditions, these are probably the traits selected for, either intentionally or unintentionally, but some of the same characteristics may present problems in the wild. Selection for high egg production resulted in extending the breeding season and causing females to nest early. Increased body size (Greenwood, 1975) also makes them more resistant to cold temperature (Lee, unpublished data). These changes may be the reasons why many game farm

REPRODUCTION AND MALLARDS

mallards released experimentally not only fail to migrate at the proper time (Brakhage, 1972) but also lay eggs too early (Greenwood, personal communication). Ducklings may also hatch before spring weather conditions moderate sufficiently to permit good survival. Large clutch size and decreased broodiness may be the underlying reasons for strong tendencies of nest or brood desertion by game farm females (Greenwood, personal communication; Alexander, 1971). However, feral mallard populations seem to flourish around city parks, hydroelectric power plants, and game reserves where water is kept open in the winter and artificial feeding is practiced. In light of the increasing rate of urban development and destruction of natural habitat, some game farm mallard characteristics may in fact have a selective advantage for adaptation to these semidomestic situations. The feral pigeon is a good example of such successful adaptation. Eggs from game farm females required a longer incubation time than eggs from wild females (Table 2). Greenwood (1975) reported that game farm mallards laid larger eggs than wild mallards. It has long been known in chickens that egg weight is an important source of variation in the length of incubation period (Byerly, 1934). Heavier eggs generally require longer time for incubation than lighter ones. Maximum emergence time was also longer in game farm strain than in the wild strain. This supports the notion of Prince et al. (1970) that under domesticated conditions artificial incubation of eggs relaxed the selection for short emergence time and increased the variation in the length of incubation period. In the wild, the longer incubation time and emergence time, which may keep the female on the nest longer and thus making her and her brood more vulnerable to predation, would be a selective disadvantage. Previous studies reported poor fertility in reciprocal crosses of wild and game farm mallards (Hunt et al., 1958; Prince etal, 1970; Greenwood, 1975). However, breeding birds used in their crosses were either wild trapped (Hunt et al., 1958) or raised in separate pens by strain (Greenwood, 1975). In both cases, birds of one strain did not have an extended experience with breeding birds of the other strain, especially during the imprinting period, before they were put into the breeding pens. Cheng et al. (1978, 1979) showed that wild and game

1975

farm males were successful in pairing with females of either strain if the males were raised with females of the opposite strain. Males raised with females of their own strain only paired with females of their own strain during their first breeding season. In the present study, breeding birds of both strains were combined in a mixed group 30 hr after hatching. Fertility in the crosses were not lower than the fertility in the pure strain matings. It is likely, therefore, that the low fertility reported previously was because of behavioral incompatibility of the breeding pair rather than physiological incompatibility of the gametes from the two strains. Fisher's fundamental theorem of natural selection (Fisher, 1958) predicts that traits that are major components of fitness contain little additive genetic variation ( c * 2 ) when the population concerned is in relative equilibrium with its environment. Such traits will have been subjected to extensive prior selection that reduces Oj^2 but favors the accumulation of nonadditive genetic variation, a major component of heterosis (Crow, 1952) when two such populations are crossed. A comparison of the reproductive traits among our five strains showed that most of the strain differences in these traits were linear, indicating that considerable aj^2 remains in these traits. The mallard has holartic distribution, and environmental conditions for reproduction vary among different geographical populations. Batt and Prince (1978) reported differences in reproduction between females of two different geographical origins. Dzubin and Gallop (1972) also reported that in their study areas in Manitoba, and Saskatchewan, mallards breeding in the aspen parkland had low nest success and high brood survival while those breeding in the grassland had high nest success and low brood survival. Direction and intensity of selection pressure may also vary from generation to generation in both the wild and game farm populations. These variations, plus frequent gene migrations between various geographical populations, may be sufficient to maintain the observed Op^ in the reproductive traits studied, even though these traits are major components of fitness. The results of this study suggest that, with respect to the reproductive traits examined, game farm females would not be reproductively as fit as wild females under most natural conditions. There is no evidence, however, to show any potential postcopulatory isolating

CHENG ET AL.

1976

mechanisms against the crossbreeding of these t w o strains. ACKNOWLEDGMENTS We would like t o t h a n k F r a n k McKinney and H. H. Prince for reviewing t h e manuscript, W. J. Boylan, D e p a r t m e n t of Animal Science, and J. E. Colten, senior analyst programmer, Agricultural E x p e r i m e n t Station, University of Minnesota for assistance in statistical analyses. We would also like to t h a n k the staff of N o r t h ern Prairie Wildlife Research Center for technical assistance. This article is s u b m i t t e d by the first a u t h o r as part of a thesis for the partial fulfillment of the requirements for a Ph.D. degree at the University of Minnesota. T h e s t u d y was s u p p o r t e d b y a c o n t r a c t (No. 14— 1 6 - 0 0 0 8 - 8 1 1 ) from t h e Northern Prairie Wildlife Research Center, US Fish and Wildlife Service. Additional s u p p o r t was received from t h e Max McGraw Wildlife F o u n d a t i o n and the University of Minnesota C o m p u t e r Center. This is c o n t r i b u t i o n N o . 1 0 , 4 8 9 of the Scientific Journal Series of the Minnesota Agricultural E x p e r i m e n t Station.

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