Behavior Genetics, Vol. 6, No. 2, 1976

Assortative Mating in Lesser Snow Geese

(Anser

caerulescens) F. C o o k e , ~ G. H . Finney, ~ and R. F. RockwelP, ~ Received 9 Dec. 1974--Final 9 Apr. 1975

Assortative mating occurs in the dimorphic lesser snow geese in the wild, Mixed matings between the blue and white phases are much less frequent than would be expected by chance. Evidence from marked birds in field conditions indicated that mate choice was correlated with familial color. Birds from white families usually chose white mates, birds from blue families usually chose blue mates, and birds from mixed families chose mates of either color. Similar results were obtained under captive conditions when offspring from foster families with particular parental and offspring color combinations were allowed to choose mates. Both parental color and sibling color appeared to influence mate choice. The bird's own color did not appear to be important in mate choice in either f e l d or experimental conditions, and in those cases where male and female parents differed in color neither parental color was more influential than the other in determining offspring mate choice. The results provide the first evidence, to our knowledge, that mate selection ,based on familial appearance operates intraspecifcally in the wild. KEY W O R D S : lesser snow goose; assortative mating; polymorphisrn; sympatric speciation; associative learning.

INTRODUCTION Many studies in behavioral genetics concentrate on identifying the genetic basis of behavioral characteristics. The work described in this report invesThese studies formed part of an investigation being carried out under contract with the Canadian Wildlife Service. Financial assistance was also provided by the National Research Council, the Manitoba Department of Mines, Resources, and Environmental Management, the Canadian National Sportsmen's Show, and the City University of New York. 1 Department of Biology, Queen's University, Kingston, Ontario, Canada. 2 Present address: Department of Biology, The City College, New York, New York. 127 (~) I976 Plenum Publishing Corporation, 227 West 17th Street, New York, N.Y. 10011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written permission of the publisher.

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tigated the role of behavior on the genetic structure of the population. Cooke and Cooch (1968) hypothesized that the assortative mating found in the dimorphic lesser snow goose could be explained on the basis of a bird's choosing a mate with a plumage color similar to that of one or both of its parents and that the choice was determined by the bird's early learning experiences. There are several examples which show that under captive conditions a bird raised with a particular morph will, in a free choice situation, pair with a bird of that morph (Warriner et al., 1963; Immelmann, 1969), but to our knowledge there is no conclusive evidence that this might operate in the wild. Kalmus and Maynard Smith (1966) and Seiger (1967) have produced theoretical models which explore the evolutionary consequences of the assortative mating which would result from this type of early learning experience. They showed that, if the mating preference for a particular phenotype were absolute, eventually gene flow between the morphs would cease, thus bringing about sympatric speciation. If the mating preference were less than absolute, a mechanism would be provided for the maintenance of the polym0rphism in the population. The aim of this report is to provide evidence that assortative mating based on prepairing learning experience does occur in the wild. The lesser snow goose is a suitable species for investigating this question: (1) It has a plumage color dimorphism, blue (the "blue" goose) and white (the "snow" goose), both phases present in the Hudson Bay populations. At the La PErouse Bay colony, where we work, approximately 25% of the birds are blue phase. (2) The genetic basis for the polymorphism is known (Cooke and Mirsky, 1972). Blue is incompletely dominant to white. Heterozygotes often have white plumage on the bellies. (3) Large breeding colonies occur where many pairs can be studied. (4) Pair bonds are usually lifelong and family groups stay together for almost a year (Cooch, 1958; Prevett, 1972). (5) They can be kept in captivity and will form pair bonds under captive conditions. (6) Assortative mating occurs, as first described by Cooch and Beardmore (1959). Two approaches were used, each essential to the resolution of the problem. They are the field approach and the experimental approach.

FIELD APPROACH Methods

An extensive banding and color-banding program has been carried out from 1969 to the present using the birds at the La P~rouse Bay snow goose colony in Northern Manitoba (Latitude 58~ longitude 94~

Assortative Mating in Lesser Snow Geese (Anser caerulescens)

129

This colony consists of around 3000 pairs. The color banding involved marking goslings of known parentage with a color code which indicated the color phase of the parent, waiting at least 2 years until these birds reached sexual maturity, and observing the phase of the mates which they chose. To do this, the nests were visited while the goslings were hatching, parental color phases were ascertained, and a small metal webtag was attached to each gosling. This tag was coded to indicate parental color. Each year 3000-4000 goslings were so tagged. The goslings were located 4-5 weeks later, when the web-tagged goslings were large enough to have plastic color-coded leg bands applied. Of the web-tagged goslings, 500-800 were color-banded each year, such that we could identify them in the field both as to the nest from which they hatched and as to the color of their parents. Two or more years later, some of these color-banded goslings returned to breed in their natal colony and at that time they were observed and the color of their mate was ascertained. Up to 20% of the female goslings banded at the La P6rouse Bay colony subsequently return to breed at their natal colony, but very few males ever return (Cooke et al., 1975). Results

Pairs of geese are nonrandom in terms of color phase at La P6rouse Bay, as had been previously reported for other colonies (Cooch and Beardmore, 1959). Table I shows data from La P6rouse Bay from 1968 to 1974. As can be seen, mixed pairs are less frequent than expected on the basis of random pairing of morphs. In addition, there is a reciprocal difference in the frequency of mixed pairs, white female • blue male crosses being more frequent than blue female • white male. This difference, first noted by Cooch and Beardmore (1959), has not been satisfactorily explained. Table II shows the results of birds of known parentage sighted in 1973 and 1974, respectively. Results from the 2 years could not be combined since the possibility exists that observations may occasionally be of the same pair seen in successive years. All except four of the birds whose choices are given in the tables were females. One can see in both tables that birds with white parents tend to choose white mates and birds with blue parents to choose blue mates. Offspring of mixed pairs choose either blue or white. In both years, the null hypothesis that parental color was unrelated to mate choice color was rejected. This established an association but not necessarily a cause-and-effect relationship. First, birds with white parents are themselves usually white because of the genetic basis of color inheritance, and, second, they also usually have white siblings. Mate selection may reflect not parental color but self or sibling color. Looking at the mate choices of offspring of mixed matings

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Cooke, Finney, and Rockwell

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Assortative Mating in Lesser Snow Geese

Table II.

Offspring choice

(Anser caerulescens)

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Mate Selection of Geese of Known Parentage, Field Data, La P6rouse Bay, 1973 and 19744 Parental colors

White pair

Mixed pair

Blue pair

Total

1973

White Blue Total

23 4 27

5 1 6

2 4 6

30 9 39

1974

White Blue Total

29 2 31

8 6 14

2 11 13

39 19 58

, Contingency x2~ for 1973 7.60, p < 0.025; contingency x22 for 1974 26.26, p

Assortative mating in lesser snow geese (Anser caerulescens).

Assortative mating occurs in the dimorphic lesser snow geese in the wild. Mixed matings between the blue and white phases are much less frequent than ...
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