The Evolution of Mating Systems in Grouse

Condor, 80:126-137 0 The Cooper Ornithological Society 1978

THE EVOLUTION OF MATING SYSTEMS IN GROUSE

JAMES F. WITTENBERGER

The evolution of polygyny in birds has been miscuous birds do not rely upon resources clarified by recent theory based on female located on male territories while nesting or choice of mates (Verner 1964, Orians 1969), rearing young, the Orians-Verner model can but the evolution of promiscuity has remained suggest that promiscuity evolved only as a re- difficult to explain ( Selander 1972). Wiley sult of variation in male genetic quality. This (1974) recently suggested that promiscuity hypothesis is plausible only if ecological dif- evolved in grouse as a consequence of delayed ferences between species make male genetic maturation of males. This hypothesis reverses quality a larger component of female fitness the usually accepted causal relationship be- in some species than in others. tween delayed breeding and polygamy. It The literature on mating systems is still con- also emphasizes the optimization of male fused by conflicting definitions of terms. In rather than female reproductive tactics. There- the following discussion, “polygyny” refers to fore, the intent of this paper is to examine the regular occurrence of males which are Wileys’ (1974) hypothesis and present an pair bonded to more than one female at a time alternative based on the assumption that fe- (after Lack 1968). “Promiscuity” is charac- male choice determines the evolution of grouse terized by the absence of prolonged pair bonds mating systems. and the insemination of multiple females by Polygamy must normally be advantageous at least some males in the population. This to both sexes before it can evolve. The only definition does not imply that mating occurs known exceptions occur when females cannot at random or that females necessarily copulate determine the true mated status of potential with more than one male per season. “Polyg- mates ( Wittenberger 1976). According to sex- amy” is used here as a general term encom- ual selection theory, polygamy should nearly passing all nonmonogamous types of mating always be advantageous to males because systems. males *can usually increase their fitness by copulating with as many females as possible CRITIQUE OF THE SEXUAL (Bateman 1948). In contrast, females usually BIMATURISM HYPOTHESIS gain little advantage by copulating with more than one male per season because their repro- Wiley ( 1974) proposed that promiscuity ductive output is limited by energetic con- evolved in grouse because natural selection straints rather than ability to get eggs fertil- favored sexual bimaturism (i.e., a sexual dif- ized. In addition, females risk losing a much ference in the age when breeding is first at- larger investment of time and energy than tempted). According to Wiley, sexual bi- males when they accept a mate ( Orians 1969). maturism could skew the ratio of breeding As a result, males generally compete among males to breeding females toward an excess themselves for mates, while females generally of females. This, in turn, would force at least select their mates from an array of competing some females to accept already mated males males. According to this principle of female as mates. As a result, males would spend a choice, factors affecting female success should greater proportion of their time seeking mates, normally determine the type of mating system and the species would become promiscuous. that evolves ( Orians 1969). In the following discussion, Wileys’ hypothesis The Orians-Verner model follows directly will be referred to as the sexual bimaturism from the female choice principle. It predicts hypothesis. that polygyny should evolve only when some An alternative hypothesis is that sexual bimaturism evolves as a response to competition females can be more successful as second among males for females after the species has mates of already mated males than as first become polygamous (Lack 1954, 1968; Selan- mates of any remaining unmated males (Ver- der 1965, 1972; Orians 1969). According to ner 1964, Orians 1969). This can occur when- the sexual selection hypothesis, breeding is de- ever habitat differences between male terri- layed in males because young males cannot tories or genetic differences between males compete effectively for mates with older, more are large enough to offset any costs incurred experienced males. If the sexual selection hy- by females as a result of their choice to be- pothesis is correct, the sexual bimaturism hy- come second mates. Because females of pro- pothesis cannot explain promiscuity in grouse. [I261 GROUSE MATING SYSTEMS 127

Delayed breeding is advantageous for males when their lifetime fitness is increased by post- poning reproduction until an older age. This can occur when delayed breeding increases b,_, ba expected longevity sufficiently to compensate for the reproductive output lost by not breeding immediately. Sexual bimaturism evolves when this effect favors delayed breeding in males but not in females. It is possible to calculate the threshold conditions determining whether a young male should attempt breeding at age x = a - 1 or defer breeding for one additional year (Witten- berger, in press). This derivation is based on the Euler-Lotka life table equation and as- sumes age-independent survival and reproductive rates following maturity. An approximation of the threshold for delayed breeding is given by FIGURE 1. The conditions determining age at onset of breeding. Delayed breeding is advantageous to ha-l As males when the value of &-l/b, lies below the line 7<- specified by so for a given value of As..,. Symbols are l-sa ’ a defined in Table 1. where a = age at maturity, ha_1= male reproductive ’ rate at age x = a - 1, b, = male reproductive rate at ages x 3 a, As = reduction in fluencing the age males first attempt breeding survival rate (i.e., risk) resulting from at- can be deduced from equation 1. These are tempts to breed at age x = a - 1, and sa = an- summarized in Table 1. Not all of these fac- nual male survival rate at ages r > a. In tors can be evaluated on the basis of present words, equation 1 states that males one year evidence. short of maturity should delay breeding an- The risk of breeding for subadult males is other year only if the ratio of their expected difficult to measure directly because it entails reproductive success to that of adult males is comparison of survival rates for breeding and less than the risk of attempting to breed nonbreeding subadults within the same pop- divided by adult male mortality rate. The ulation. However, the relative magnitude of accuracy of this approximation depends on ’ risk may be indicated by predation rates af- how closely the intrinsic rate of increase for fecting adults. If predation on breeding adult the population approaches zero. Threshold males is high, the risk for subadult males is values are shown in Fig. 1 for several values also likely to be high. Similarly, if predation of adult male survival rate. on breeding adults is low, the risk for sub- This derivation differs from Wileys’ (1974) adults is probably also low. original model in that it includes the factors Although mortality from all causes is low of risk and adult male survival. Wileys’ deriva- in monogamous grouse during most of the tion assumed that earlier breeding by sub- breeding season (Choate 1963, Watson 1965, adults reduces their survival rate by a con- Bergerud 1970, Watson and Moss 1971), pre- stant amount that affects every year of life dation mortality is often high in early spring rather than just the year they begin breeding. when pair formation commences (Watson This assumption is inappropriate because ear- 1965, Dementev’ and Gladkov 1967, Bergerud lier breeding should not affect survival rate in 1970). In promiscuous grouse, predation on subsequent years. breeding adult males is heavy in Ruffed According to Wiley ( 1974)) males typically Grouse (Bump et al. 1947, Eng and Gullion do not breed as yearlings in promiscuous 1962, Gullion and Marshall 1968, Rusch and grouse but typically do breed as yearlings in Keith 1971a) and Blue Grouse (Zwickel, pers. monogamous grouse. This is not always true, comm.), but it is rare in Sage Grouse (Wiley as will be pointed out later. In contrast, fe- 1973), Sharp-tailed Grouse ( Ammann 1957), males of all grouse species normally begin and Greater Prairie Chickens (Berger et al. breeding as yearlings. The basic question here 1963, Hamerstrom et al. 1965) (see Appendix is why delayed breeding by males is more like- for scientific names). Therefore, assuming ly to evolve in promiscuous grouse than in that the magnitude of risk for subadults is monogamous grouse. Factors potentially in- correlated with predation rates on breeding 128 JAMES F. WITTENBERGER

TABLE 1. Some factors potentially influencing the evolution of delayed breeding in male grouse. 2.0 . (b,_,/b, = ratio of subadult to adult male reproductive rate; As = risk of attempting to breed as a i subadult; so = adult male survival rate.) badbe 1. Male experience in competing for better breeding habitats. 2. Male experience in competing for more fit females. 3. Male experience in enhancing survival of offspring. 4. Male experience in competing for more females once polygamy has evolved.

As 1. Frequency or intensity of courtship displays. 2. Male experience in evading predators during displays. 3. Male experience in maximizing foraging efficiency. 4. Habitat structure.

SO L 1 I 1. Body size. 1.2 1.4 1.6 13 210 212 2. Resource availability. MALE:FEMALE WEIGHT RATIO 3. Habitat structure. FIGURE 2. The relationship between increased sex- Population density. ual size dimorphism and autumn male:female sex 2: Densities of predator and buffer prey populations. ratios. The regression line is significant at P < 0.01. 6. Weather. Weight ratio data are from Johnsgaard (1973) and Wiley ( 1974). Sex ratio data are from Baker ( 1953), Bergerud ( 1970), Bezdek ( 1944), Boag ( 1966), adults, differences in risk cannot explain why Campbell ( 1972), Choate ( 1963)) Dorney ( 1963), sexual bimaturism is more likely to evolve in Girard ( 1937 ). Tenkins et al. ( 1967 ). Patterson ( 1949)) Pulliainen and Loisa ( 1972), Raiala ( 1974), promiscuous grouse. Robe1 et al. (1972), Rusch and Keith (1971a), Viht Wiley (1974) proposed that sexual bimat- ( 1974)) Watson ( 1965), and Zwickel ( 1972). urism evolved because male survival rates were higher than female survival rates in grouse populations ancestral to present-day females in Spruce Grouse, 15% larger in Les- promiscuous species. He attributed this dif- ser Prairie Chickens, 16% larger in Ruffed ference to sexual size dimorphism in ancestral Grouse, and 17% larger in Sharp-tailed forms. Large-sized males would, according to Grouse, as compared to lo-15% larger in Wiley, survive longer than smaller-sized fe- monogamous species of ptarmigan (data from males because their energy balance would be Johnsgaard 1973, Wiley 1974). more favorable and their vulnerability to pre- Wileys’ (1974) hypothesis rests on the as- dation would be less. Larger body size could sumption that larger size increases male lon- reduce heat loss, lower metabolic require- gevity in sexually dimorphic species. This as- ments, and increase resistance to temperature sumption can be tested directly, since the fluctuations ( Rensch 1960, Kendeigh 1972). survival rate of males relative to females de- Large size could also reduce the number of termines the adult (i.e., nonjuvenile) sex ratio. predators capable of attacking adult males. A positive correlation between sexual size di- According to Wiley, females remained small morphism and sex ratio would therefore sup- in these species because larger size would have port Wileys’ hypothesis. A negative correlation conflicted energetically with egg production. would support the alternate hypothesis that Thus, Wiley ( 1974) suggested that sexual size increased size dimorphism is a reproductive dimorphism led to sexual bimaturism, which cost paid by males of polygamous species in in turn forced females to become polygamous. order to compete more effectively for females. According to this hypothesis, monogamy A plot of data from the literature shows a sig- should evolve only when ecological constraints nificant negative correlation (Fig. 2) support- prevent males from becoming larger than fe- ing the sexual selection hypothesis that size males. dimorphism is a consequence rather than a One problem with Wileys’ hypothesis is that cause of polygamy in grouse. some promiscuous species of grouse are no It is possible that this negative correlation more dimorphic in size than monogamous is an artifact of biased data. Some data used species. Thus, males are about 8% larger than in the analysis were based on the composition GROUSE MATING SYSTEMS 129 of hunter kills, which are frequently biased Grouse (Bendell and Elliott 1967, Bendell et in favor of excess males. Kill data were used al. 1972, Zwickel 1972)) Ruffed Grouse (Boag only if there was evidence that hunter kills and Sumanik 1969, Rusch and Keith 1971a, were not biased toward either sex or if the Fischer and Keith 1974), and Sharp-tailed data were the best available and the expected Grouse (Rippin and Boag 1974). In every bias ran counter to the sexual selection hy- study, yearling males occupied the removal pothesis. Thus, Campbell (1972) stated on area, established territories, advertised for the basis of eariler studies that vulnerability mates, and presumably copulated with one or to hunting was not a function of either age or more females. These results are corroborated sex in Lesser Prairie Chickens. Kill data for by observations of subadult males attempting Capercaillie (Pulliainen and Loisa 1972) and to breed under natural conditions. Yearling Sage Grouse (Girard 1937, Patterson 1949) Ruffed Grouse sometimes display on vacated were the best available, and the expected bias territories following natural mortality of adult toward excess males would weaken the evi- males (Marshall 1965). Yearling Sharp-tailed dence for a negative correlation. Kill data for Grouse were observed occupying the center of Ruffed Grouse should not be biased because one lek and successfully copulating despite the males are only slightly larger than females. presence of adult males on the lek (Hjorth This assumption is supported by evidence that 1970). Yearling Greater Prairie Chickens per- sex ratios based on hunter kills (Bezdek 1944, formed 18% of all observed copulations by Dorney 1963, Rusch and Keith 1971a) are known-aged males during a 20-year study similar to the sex ratio obtained by eliminat- (Hamerstrom 1972). Thus, the evidence ing an entire population (Bump et al. 1947). shows that yearling males of promiscuous A second source of bias stems from the ef- grouse defer breeding only when they cannot fects of long-term selective hunting on extant compete effectively for mates. There is no population sex ratios. If hunters kill more than evidence that yearling males who have access the “surplus” males for many years in succes- to receptive females ever defer breeding be- sion, there is likely to be an abnormally low cause of an associated high risk, and present proportion of males in the population. This evidence contradicts the sexual bimaturism bias may have had a particularly pronounced hypothesis proposed by Wiley ( 1974). effect on the most dimorphic species. Rajala (1974) pointed out that this bias probably in- A FEMALE CHOICE MODEL fluenced sex ratios in Capercaillie, but he ar- gued that the observed preponderance of fe- As explained in the introduction, females usu- males cannot be entirely explained by selective ally select mates while males usually compete for mates. According to sexual selection hunting pressure. He noted that females also predominate in unhunted populations and that theory, females should generally select mates juvenile sex ratios before the hunting season on the basis of phenotypic traits which reflect are the same as adult sex ratios during and male genetic quality (Fisher 1958). When fe- after the hunting season. In relatively non- males nest on male territories, they should also dimorphic species, hunting should have little select mates on the basis of territory quality effect on sex ratios unless the sexes are un- (Orians 1969). By the same reasoning, fe- equally distributed in habitats receiving dif- males should, in general, select the best breed- ferent intensities of hunting pressure. Palmer ing situations available, where “breeding situa- ( 1956), for example, found no difference in tion” refers to the breeding context a female fall-to-spring mortality between hunted and accepts by mating with a particular male (after Emlen 1957). unhunted populations of Ruffed Grouse. Nevertheless, additional studies of unhunted Two behavioral components define avian populations would be desirable to evaluate mating systems: the duration of pair associa- this potential source of bias. tion and the extent that individuals of either The sexual bimaturism hypothesis implies sex obtain more than one mate at a time. Both that males delay breeding because the risks are components can be influenced by female mat- too high, not because they are unable to obtain ing preferences. Females can preferentially mates. Therefore, subadult males of promis- select males with a propensity toward pro- cuous grouse should not attempt breeding longed pair bonding by withholding copula- even when they can gain access to receptive tions until an extensive courtship period has females. This can be tested by removing all been completed. They can minimize the adult males from an area and observing the duration of pair bonding by copulating as behavior of colonizing subadults. Removal ex- quickly as possible after testing the pheno- periments have been performed for Blue typic suitability of the male. Likewise, fe- 130 JAMES F. WITTENBERGER

TABLE 2. Behavioral comparison of monogamous and promiscuous grouse. General information is based on Hjorth (1970), Johnsgaard (1973), and Wiley (1974).” English names are in the Appendix.

Mating system Duration Female use of Male Winter and species of pair-bond! ’ male territoryc vigilamx+ dispersione Referencesi

Monogamous species Lagopus lagopus Y C,F,C F,N,Y UF 16,20,67 L. 1. scoticus Y C,F,N,Y F,N,Y P 36,66 L. leucurus EI-I G,F,(N) F,(N) MF,S 6,17,37,60 L. mutus EI-I G,F,N,(Y) F,N UF,P,S 20,50,65,67 Tetrastes bonasia MI-I G,F,N F,(N) PS 16,20,65,67 T. sewerzowi MI? G,F,N F,N? ? 20

Promiscuous species, males dispersed Bonasa umbellus C C None B,S,UF 6,13,23,30 Falcipennis falcipennis C None MF? 20,34 Canachites canadensis : C None MF,S 25,26,49 Dendragapus obscurus C CE None B,S? 4,5,46 Tetrao ,urogallus” C C None UF,S 20,34,42,48,63 T. parvirostris” C C None ? 16,20

Promiscuous species, males on leks Pedioecetes phasianellus C None MF 1,33,34 Tympanuchus cupido C E None MF,UF 2,6,32,34,47 T. pallidicinctus C None MF 6,15,35 Lyrurus tetrix C : None MF 42,43,45,63 L. mlokosiewiczi C C None MF,S 20 Centrocercus urophasianus C C None UF l&56,69

a Parentheses indicate that the trait varies among populations. b EI = male leaves early in incubation; MI = male leaves in middle of incubation;, I = male leaves after young hatch; Y = male remains with female and young; C = female leaves after courtship and copulation. c C z copulation; F = foraging,; N = nesting; Y = rearing of young. d F = male guards pre-incubatmg female; N = male guards incubating female and nest; Y = male guards female and young. p UF = unisexual flocks; MF = mixed sex flocks; S = solitary; P = pairs; B = broods. f 1. Ammann 1957. 2. Baker 1953. 3. Beer 1943. 4. Bendell 1955. 5. Bendell and Elliott 1967. 6. Bent 1932. 7. Bergerud 1970. 8. Berner and Gysel 1969. 9. Blackford 1958. 10. Blackford 1963. 11. Boag 1966. 12. B#rset and Krafft 1973. 13. Brander 1967.’ 14. Brown 1946. 15. Campbell 1972. 16. Cheng 1964. 17. Choate 1963. 18. Dalke et al. 1963. 19. Darrow 1939. 20. Dementev’ and Gladkov 1967. 21. Doerr et al. 1974. 22. Eastman and Jenkins 1970. 23. Edminister 1947. 24. Ellison 1966. 25. Ellison 1971. 26. Ellison 1973. 27. Fowle 1960. 28. Girard 1937. 29. Gullion and‘ Mar- shall 1968. 30. Hale and Dorney 1963. 31. Hamerstrom 1939, 1963. 32. Hamerstrom and Hamerstrom 1955. 33. Hamerstrom and Hamerstrom 1951. 34. Hjorth 1970. 35. Hoffman 1963. 36. Jenkins et al. 1963, 1967. 37. Johnsgaard 1973. 38. Jones 1963. 39. Jones 1964. 40. Jones 1966. 41. Klebenow 1969. 42. Koskimies 1957. 43. Krnijt and Hogan 1967. 44. Kruijt et al. 1972. 45. Lack 1939. 46. Lance 1970. 47. Lehmann 1941. 48. Lumsden 1961. 49. MacDonald 1968. 50. MacDon- a.d 1970. 51. Marshall 1946. 52. Marshall and Jensen 1937. 53. Martin et al. 1951. 54. May 1970. 55. Moss 1969. 56. Patterson 1952. 57. Pendergast and Boag 1970. 58. Peters 1958. 59. Pynniinen 1954. 60. Quick 1947. 61. Robinson 1969. 62. Rusch and Keith 1971b. 63. Seiskari 1962. 64. Stewart 1944. 65. Watson 1965. 66. Watson and Jenkins 1964. 67. Weeden 1963, 1964. 68. Weeden 1969. 69. Wiley 1973. C In some populations females form prolonged pair bonds and nest on male territories (Blackford 1963). 1‘ Sometimes considered a lek species (e.g. Wiley 1974), but display arenas are larger than typical leks (see Lumsden 1961).

males can determine how many mates males sive ecological differences between monog- obtain by accepting or rejecting already mated amous and promiscuous species are appar- males as their mates. ently those affecting females during and Behavioral comparisons of promiscuous and immediately preceding egg laying. monogamous grouse should indicate the time There are two major differences in female of year when ecological differences are most behavior during the prelaying and laying likely to influence the evolution of mating sys- periods. First, females of all monogamous spe- tems. Table 2 shows that females of monog- cies forage on male territories, while females amous species remain paired with males from of promiscuous species do not to any signifi- the beginning of courtship until at least the on- cant extent (Table 2). Females of monog- set of incubation, while females of promiscuous amous species usually also nest on male species associate with males only briefly during territories, although female White-tailed Ptar- courtship and copulation (by definition). On- migan do not (Choate 1963). Females of pro- ly in Red Grouse and some populations miscuous species nest on male territories only of Willow Ptarmigan do males help in any way coincidentally, and they do not associate with with the rearing of offspring. Therefore, male males during the nesting period except to cop- parental care and ecological conditions during ulate. Secondly, females of monogamous spe- the rearing period apparently do not deter- cies apparently rely on male vigilance to mine which mating system evolves. Likewise, detect predators during egg laying and some- wintering behavior appears unrelated to the times during part or all of incubation, while evolution of grouse mating systems. The deci- females of promiscuous species do not. For GROUSE MATING SYSTEMS 131 instance, female Rock Ptarmigan forage con- scarcity should favor nesting on male terri- tinuously in spring and are vigilant only when tories and reliance on male vigilance. This is alerted by the male, who remains close to her possible only if females form prolonged pair until the onset of incubation (MacDonald bonds with males. Hence, competition for 1970). Male Hazel Grouse and Amur Grouse food resources should favor monogamy or remain in the treetops while females forage in polygyny, provided that these resources are the understory (Cheng 1964, Dementev’ and defensible. Gladkov 1967). Male White-tailed Ptarmigan When food is abundant, females need not and Willow Ptarmigan accompany females forage on male territories to obtain adequate continuously until the onset of incubation and food. Females gain little advantage by re- presumably also serve as sentinels (Choate ducing competition for food when their suc- 1963, Dementev’ and Gladkov 1967). Two cess is limited by digestive rate rather than ecological factors potentially influencing the food intake rate or nutritive quality. They evolution of these differences in female be- can avoid any increased conspicuousness havior are availability of food resources and caused by the presence of a male by minimiz- vulnerability to predation during the prelaying their association with males. If food is ing and laying periods. abundant, females can rely on their own vigil- Food availability during the prelaying and ance for detecting predators, or they can form laying periods can affect female reproductive flocks to obtain more effective vigilance than success in several ways. In Red Grouse, there a male can provide. Unless suitable nesting is evidence that early chick mortality is largely substrate is scarce, food abundance should determined before the eggs hatch, possibly re- therefore lead to promiscuity. flecting the condition of laying females (Jen- A comparison of the diets and habitats of kins et al. 1963, 1965). In Capercaillie and monogamous and promiscuous grouse suggests Black Grouse, viability of eggs and chicks is two trends (Table 3). First, monogamous spe- correlated with early spring weather condi- cies of ptarmigan occupy open arctic or alpine tions ( Siivonen 1957), which are presumably habitats, and the monogamous Hazel Grouse correlated with early spring foraging condi- and Amur Grouse occupy open temperate for- tions (Lack 1966). However, conflicting evi- ests. In contrast, promiscuous grouse occupy dence seems to minimize the importance of temperate forests, boreal forests, or grasslands, this effect in at least some species (Marcstrom all of which provide relatively dense cover. 1960, Helminen 1963, Zwickel and Bendell Second, all but one of the species predomi- 1967). nantly exploiting a single type of food during Food availability probably limits clutch size spring are promiscuous. Species with more in all species of grouse. Lack (1964, 1968) diversified spring diets can be either monog- suggested that clutch size in grouse and other amous or promiscuous. precocial birds should be determined by aver- It would be interesting to know whether age availability of food during laying, along monogamy is associated with more open hab- with the mean size of each egg. Johnsgaard itats because food availability is generally (1973) has questioned this hypothesis and lower, because predation vulnerability is gen- proposed instead that clutch size is limited by erally higher, or because of other factors. I the cumulative effects of vulnerability to nest can find no data to test these hypotheses. It predation through time. According to Johns- may be significant that a similar trend exists gaard, the longer a female spends laying eggs, in pheasants. Monogamous pheasants prevail the higher her risk of losing the entire clutch at high altitudes, polygynous pheasants occur to a predator. Food availability remains an within a broad range of intermediate altitudes, important determinant of clutch size in Johns- and promiscuous and harem polygynous pheas- gaards’ hypothesis because it should limit the ants are restricted to low altitudes (Beebe rate at which females can lay eggs. Food 1926, Delacour 1951, Smythies 1953, Ali 1962, availability during the laying period must Cheng 1964). surely be important, since female energy re- The sole exception to the correlation be- quirements increase by an estimated 2030% tween monotypic diet and promiscuity is the during that time (King 1973). Red Grouse, which exploits heather. Avail- Theoretically, females should be able to al- ability of food for this species is limited by leviate competition for food by nesting on nutritive quality of the heather and the length male territories (Brown 1964). They should of the growing season prior to the end of egg also be able to devote more time to foraging laying (Moss 1969, Watson and Moss 1972, and Iess time to surveillance by relying on Moss et al. 1975). In contrast, availability of males to detect predators. Therefore, food conifer needles taken by several species of 132 JAMES F. WITTENBERGER

TABLE 3. Comparison of habitats and diets of monogamous and promiscuous grouse. English names are in the Appendix.

Mating system Preferred Staple spring and species habitata diet of females~ Reference+ ’

Monogamous species Lagopus lagopus arctic and alpine tundra buds, berry seeds, foliage; 20,58,68 b erries L. 1. scoticus heather heather 22,55 L. leucurus rocky alpine tundra heath, moss, new shoots, 17,37 leaves, flowers L. mutus arctic and alpine tundra leaves, flowers, berries, 20,50,55,68 buds Tetrastes bonasia spruce-aspen-birch forest catkins, buds, leaves; 16,20,34 insects T. sewerzowi alpine aspen-birch-alder herbaceous flowers and 20,34 leaves

Promiscuous species, males dispersed Bonasa umbellus mixed deciduous’ forest buds, twigs, leaves; 14,19,23,29,51,53,62 herbaceous foliage Falcipennis falcipennis spruce-fir; spruce-larch conifer needles; shoots, 20 buds? Canachites canadensis spruce; pine conifer needles; leaves, 24,25,49,57,61 berries Dendragapus obscurus open conifer-deciduous wide variety of plant 3,5,9,10,27,51,53,64 materials Tetrao urogallus” spruce-fir forest conifer needles; new 12,16,20,34 shoots, buds T. parvirostris” spruce-fir forest herbaceous foliage; berries 20

Promiscuous species, males on leks Pedioecetes phasianellus bogs, short grass, brush; aspen forbs, grass, leaves; 1,31,34,40,53 insects Tympanuchus cupido prairie, meadows seeds, fruits, buds, leaves 1,31,38,47,53 T. pallidicinctus short-grass prairie insects, seeds, leaves 38,39,53 Lyrurustetrix bogs, open birch-aspen herbaceous foliage; heather 12,16,20,34 L. mlokosiewiczi alpine meadows, low herbaceous foliage; shoots, 20,34 scrub buds Centrocercus urophasianus sagebrush sagebrush; weeds, insects 28,41,56

u Predominant habitat or diet precedes semicolon. b See Table 2 for references. c Sometimes considered a lek species (see footnote h of Table 2). promiscuous grouse is not limited by nutritive species of ptarmigan by the regular, though quality (Zwickel and Bendell 1967, 1972, El- infrequent, occurrence of polygyny (Choate lison 1976). Th is correlation is significant be- 1963, Watson and Jenkins 1964, MacDonald cause monoculture food resources such as 1970, Weeden and Theberge 1972). Polygyny conifer needles and sagebrush seem to be should not occur unless some females cannot superabundant, thus supporting the prediction find unmated males in suitable habitats that promiscuity evolves when food is plenti- (Orians 1969). However, the importance of ful in spring. Further data are needed to con- food availability in determining habitat qual- firm this conclusion and to test whether food ity for these species has not been established. availability is higher for promiscuous than for The value of nesting on male territories ap- monogamous grouse with diversified diets. pears somewhat different for female Hazel Some evidence indicates that females of Grouse. In this species male territories are monogamous grouse nest on male territories centered around spruce groves which provide because food is scarce. Competition for food dense cover amid the open birch, aspen, and has been demonstrated in Red Grouse, where willow where females forage (Dementev’ and many individuals of both sexes are excluded Gladov 1967). This suggests that female Hazel from suitable habitats (Watson and Moss Grouse nest on male territories because it en- 1971, 1972). Habitat quality in this species is ables them to forage near dense cover. greatly influenced by the quality and quantity There is also evidence that females of pro- of heather (Moss et al. 1975). Competition miscuous grouse compete for habitats. Re- for suitable nesting habitat is indicated in all moval experiments show that adult female GROUSE MATING SYSTEMS 133

Blue Grouse exclude yearling females from where cover is poor but food is abundant, fe- suitable nesting habitat (Bendell and Elliott males might best avoid predation by forming 1967, Bendell et al. 1972, Zwickel 1972). Nest- flocks to enhance their ability to detect preda- ing female Spruce Grouse and Blue Grouse are tors. For example, female Sage Grouse breed overdispersed (Bendell and Elliott 1967, in relatively open sagebrush and remain in Lance 1970, Ellison 1971,1973), and evidence flocks throughout the breeding season (Pat- for Blue Grouse suggests that this may result terson 1952). This interpretation of how from conflicts between females (Stirling 1968, grouse mating systems evolved has by no Zwickel, pers. comm.) . Whether these spatial means been proven, but it is consistent with interactions are a response to competition for both the existing evidence and sexual selection food or to other factors has not been deter- theory. It also provides concrete hypotheses mined. that are amenable to future testing. It is not presently clear whether females of Two additional aspects of grouse mating monogamous grouse are more vulnerable to systems are of general interest. Both can be predation than females of promiscuous grouse explained in terms of the female choice prin- during early spring. m ciple. Mating typically occurs at traditional ptarmigan are relatively high (Watson 1965, display sites in all promiscuous grouse, and Dementev’ and Gladkov 1967, Bergerud 1970), these display sites are generally organized into but predation on female Ruffed Grouse is also leks in promiscuous species that occupy open high (Edminister 1947). Very high vulner- habitats (reviewed by Hjorth 1970). Blue ability to predation may even preclude reli- Grouse usually display at dispersed sites in ance on male vigilance. For example, Gard- their forest habitats, but they sometimes narsson (cited by Johnsgaard 1973) reported breed in open habitats following forest fires, that Willow Ptarmigan in Iceland are promis- where they have a tendency to perform com- cuous under conditions of heavy predation by munal displays (Hoffmann 1956, Blackford Gyrfalcons (F&o rusticoh). Males suffer 1958, 1963). This adds support to the causal much higher mortality from predation than relationship between habitat structure and females, suggesting that association with males lekking behavior. Capercaillies are sometimes may be sufficiently hazardous to preclude pro- considered lek species (Wiley 1974)) but their longed pair bonding. Therefore, differences display arenas are much larger than those of in predator pressure alone seem insufficient typical lek species (Lumsden 1961). Lek be- for explaining why some grouse are monog- havior probably evolved in open country amous while others are promiscuous grouse after they invaded open habitats To summarize, evidence is somewhat con- (Hjorth 1970, Johnsgaard 1973). fusing, and the relative importance of various Lack (1968) suggested that traditional dis- ecological factors may differ among species. play sites are used by males because they have ne possible interpretation is that female proven safe from predation. However, the lo- grouse in relatively open habitats compete cation and appearance of traditional sites can strongly for food and are often required to be quickly learned by predators, so vulner- orage in areas containing little cover during ability may actually be higher at such sites. the prelaying and laying periods. Under these For example, Gullion and Marshall (1968) conditions, females can alleviate competition found that male Ruffed Grouse using peren- for food by foraging on male territories, and nial drumming logs have a lower life expec- they can enhance foraging efficiency and im- tancy than males using transient logs. An al- prove their chances for survival by relying on ternate explanation is that females can more 1 vigilant males to detect predators. Conceal- safely approach and mate at familiar sites ment alone would be ineffective against pred- than at unfamiliar _ In this event, males ators because of the openness of their hab- would attract and copulate with more females itat and females could ill afford the time at traditional sites even though their expected necessary to maintain vigilance for themselves. longevity is reduced. The use of traditional In more closed habitats females are not re- sites should evolve whenever the propensity quired to forage in the open, and they might for females to mate at familiar sites offsets any \ best avoid predation by maximizing conceal- reduction in male life expectancy. ’ ’ ment. One way to enhance concealment is to Two factors probably select for lek behavior stay away from males and prevent close in grouse. Lack (1939) and later Hjorth proximity to other females. This should be (1970) suggested that the combined displays advantageous only if the rate of food con- of several males might increase the attractive- sumption is not limiting and if there is no scar- ness or conspicuousness of a display arena, \ ity of suitable nest sites. In open habitats thereby increasing the average number of fe- 134 JAMES F. WITTENBERGER

males obtained by each male on the arena. evidence for consistent behavioral differences The same hypothesis was advanced by Snow during other stages of the breeding cycle. (1963) to explain manakin leks. Hamerstrom The observed behavioral differences can be and Hamerstrom (1960) found that the num- explained by the hypothesis that females of ber of female Greater Prairie Chickens at- monogamous grouse are limited by the avail- tracted to a lek per territorial male increased ability of food in spring, while females of pro- as predicted with lek size, until 11-15 males miscuous grouse are limited by digestive rate. were present on the lek. Males on still larger In monogamous species females should be leks averaged fewer females than on leks of able to increase their rate of food intake by this size. Koivisto (1965), however, failed to foraging on male territories to reduce com- find any relationship between lek size and the petition and by relying on male vigilance for number of visiting female Black Grouse. Simi- predators to increase the amount of time avail- larly, Lill (1976) found no correlation be- able for finding food. Females of promis- tween lek size and number of visiting females cuous species would not be able to increase in Golden-headed Manakins (Pipra erythro- digestive rate by these behaviors, and they cephala) . A major problem with this hypothe- would be able to avoid increased conspicuous- sis is that it fails to explain why lek behavior ness by minimizing their association with has evolved only in open country grouse. A males. Hence, scarce food resources should second hypothesis resolves this problem. Lek favor monogamy, while abundant food re- behavior should reduce vulnerability of males sources should favor promiscuity. The avail- displaying in open habitats because several in- able evidence is consistent with this hypothe- dividuals can detect predators better than can sis but is insufficient to prove it. a single individual (Koivisto 1965, Lack 1968, Powell 1974). Leks may also be more attrac- ACKNOWLEDGMENTS tive to females than solitary males because females should be safer there for the same This paper is an outgrowth of discussions with R. L. Tilson concerning the evolution of monogamy. I am reason. The hypothesis that leks enhance pro- grateful to W. J. Hamilton III, R. L. Wilson, J. F. tection against predators is supported by the Bendell, A. T. Beraerud. F. N. Hamerstrom. Tr.. F. fact that leks are usually on elevated terrain in Hamerstrom, S. D. MacDonald, F. C. Zwickel, -G: H. the most open parts of the habitat available Orians, R. C. Roberts, and D. Taub for their many helpful comments and criticisms of the manuscript. (reviewed by Hjorth 1970). LITERATURE CITED SUMMARY ALI, S. 1962. 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