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The influence of social cues on the reproductive endocrinology of male brown-headed cowbirds: Field and laboratory studies
HORMONES AND BEHAVIOR
20, 222-234 (1986)
The Influence of Social Cues on the Reproductive Endocrinology of Male Brown-headed Cowbirds: Field and Laboratory Studies ALFRED The Rockefeller
M. DUFTY, JR. ,’ AND JOHN C. WINGFIELD
University
Field Research Center, Millbrook,
New York 12545
Captive male brown-headed cowbirds exposed to long days exhibit gonadal growth and have elevated plasma testosterone (T) levels. This photoperiodic response is enhanced if males are housed with female cowbirds: Photostimulated males with females increase plasma testosterone levels sooner than do individually housed photostimulated males. Peak plasma T levels are similar in both groups, although peak levels are maintained longer in males housed with females. The gonadal cycle is similarly affected; males in the presence of females have earlier gonadal recrudescence and maintain mature gonads longer than do photostimulated males without females. Plasma corticosterone levels increase in the unpaired males, suggesting that removal of social cues is stressful for these birds. Freeliving paired males have significantly higher plasma testosterone levels than do unpaired/unknown males early in the season, when social relationships are being established; the levels are similar thereafter. There is no difference between the two groups in testicular maturation rates; nor do they differ in plasma corticosterone levels at any time of the season. These results suggest that social stimuli are important in modulating the secretion of testosterone in males early in the season when pairing occurs, and possibly late in the season as well, probably to prevent termination of breeding prior to that of females. o 1986 Academic press, IW.
Most species of birds that breed in temperate areas respond to the vernal increase in daylength with changes in endocrine secretion, culminating in gonadal growth and physiological preparation for the coming reproductive season (Murton and Westwood, 1977; Wingfield and Farner, 1980; Wingfield, 1983). Yet, despite the “switching-on” of reproductive development by long days, final maturation of the reproductive system is achieved only after additional cues relating to territory, weather, and nutrition are perceived from the environment (Wingfield, 1983). One additional source of environmental information is social stimulation such as that received from the behavior of the mate. Numerous studies have demonstrated that males, through vocalizations and courtship displays, ’ To whom all correspondence should be addressed. 222 0018-506x/86 $1.50 Copyright 8 1986 by Academic Press, Inc. AU tights of reproduction in any form reserved.
SOCIAL
MODULATION
OF HORMONES
223
provide cues which can stimulate hormonal and behavioral responses in their mates (e.g., Lehrman, 1965; Kroodsma, 1976). Likewise, recent investigations have also demonstrated the role of social cues in stimulating endocrine responses in males of avian species. In the laboratory, male ring doves (Streptopeliu risoria) exposed to females exhibit higher androgen levels than do males in the absence of females (Feder, Storey, Goodwin, Reboulleau, and Silver, 1977). Intact females produce a stronger response than do ovariectomized females, suggesting that the gonadal state of the female is important (O’Connell, Silver, Feder, and Reboulleau, 1981). Furthermore, auditory cues also appear to play a role in stimulating courtship behavior in males (O’Connell et al., 1981). Additionally, paired male domestic pigeons (Columba liviu) have higher levels of plasma luteinizing hormone (LH) and larger testes than do unpaired males (Haase, Paulke, and Sharp, 1976). These responses are not restricted to laboratory populations: In feral white-crowned sparrows (Zonotrichiu leucophrys) paired males have higher plasma levels of testosterone (T) than do unpaired males (Wingfield and Famer, 1980). Male white-crowned sparrows and song sparrows (Melospiza melodia) paired with females implanted with estradiol (which facilitates sexual responsiveness) have significantly higher androgen levels than control males paired with unimplanted females (Moore, 1982; Runfeldt and Wingfield, 1985). We examine here the effect of social cues on males of a species with an unusual breeding system, the brown-headed cowbird (Molothrus ater). Cowbirds are obligate brood parasites whose breeding cycle lacks all aspects of parental care (Friedmann, 1929). Males outnumber females in the adult population by approximately 3:2 (Darley, 1971; Fankhauser, 1971),and males compete for rank in a dominance hierarchy and, ultimately, for access to females (Darley, 1978; West, King, and Eastzer, 1981). Successful males guard their mates from other courting males, resulting in a significant number of unpaired individuals in the population (Darley, 1982; Dufty, 1982a). Mate guarding continues for the entire 8- to loweek breeding season in which females lay eggs (Payne, 1965; Scott and Ankney, 1980). The plasma androgen [T and dihydrotestosterone (DHT)] profiles that occur in plasma of male cowbirds during the breeding season have been presented by Dufty and Wingfield (1986). Androgens peak during hierarchy formation and mate acquisition, and then gradually decline through the end of the reproductive period. However, the mating status of the males was not determined in this earlier study, so it was not possible to compare plasma androgen titers from paired and unpaired individuals. In this communication we present data from field and laboratory studies in which the T levels of paired males are compared with those of other males.
224
DUFTY
AND WINGFIELD
MATERIALS
AND METHODS
Laboratory investigation. Male (n = 36) and female (n = 18) cowbirds were captured in August and September. Males were housed individually on an 8L:16D photoperiod, and females were housed together in an outdoor aviary. On 8 and 9 February blood samples were taken (see below) from all males and their breeding condition was noted by unilateral laparotomy. Each male was assigned at random to one of four treatment groups: (1) short days (8L: 16D), female absent; (2) short days, female present; (3) long days (16L: 8D), female absent; or (4) long days, female present. Each treatment group was housed in a separate chamber in visual and acoustic isolation from all other groups. Within groups, males were visually but not acoustically separated from each other. One week later, days were lengthened and adult females, that had likewise been kept on short days, were added to the appropriate groups. The behavior of the females was not assessed. A second blood sample was taken on the following day and again at l-, 2-, or 3-week intervals. Laparotomies were performed four additional times to estimate gonadal growth by comparison with preserved (70% ethyl alcohol) testes of known weight. The final laparotomies occurred on 10 and 11 May, when the study ended. Three of the females housed with photostimulated males were pecked by their cage mates after varying amounts of time spent on long days. The females were removed and the males were excluded from additional consideration in the experiment. One male injured a leg and had to be removed, and one male died of unknown causes. Field investigation. The male brown-headed cowbirds studied in the wild are part of a color-marked population in the environs of The Rockefeller University Field Research Center in Millbrook, NY (42”N). Details regarding the capture and handling of these birds are presented elsewhere (Dufty and Wingfield, 1986). Briefly, birds were captured in mist nets or Potter traps baited with grain. A blood sample (750-1000 ~1) was taken from the alar vein in heparinized Caraway blood-collecting microtubes, following the technique of Wingfield and Farner (1976). The blood from each sample was transferred to disposable centrifuge tubes and was centrifuged at 2000 rpm for 5 min. Plasma was removed using Pasteur pipettes and stored at -20°C until assayed. Behavioral observations were conducted throughout the spring and summer. Some birds were followed on their breeding grounds, but the majority were observed as they fed on the lawns surrounding the Field Research Center. Males can be considered to be paired if they are routinely seen with the same female and guard her from other males (Darley, 1982; Dufty, 1982a). Since unpaired males will also court and guard females (Darley, 1982; Dufty, 1982b), males are considered to be paired only if they were seen with a particular female on five or more different occasions.
SOCIAL
MODULATION
OF HORMONES
225
All other males are categorized as unpaired/unknown. This category undoubtedly contains some paired individuals, but difficulty in accurately determining the mating status of (1) paired and unpaired males that were rarely observed, or (2) birds that were captured and bled but never subsequently observed made this the most conservative set of categories. Assays. Two steroid hormones, T and corticosterone (B), were measured by radioimmunoassay (RIA) using the technique of Wingfield and Farner (1975). Steroids were extracted with 5 ml dichloromethane, dried, and resuspended in 10% ethyl acetate in iso-octane. The extracts were added to columns containing a mixture of diatomaceous earth:propylene glycol:ethylene glycol (6: 1.5: 1.5, w : v : v) with a diatomaceous earth:water (3 : 1, w: v) “glycol trap.” T was eluted in 20% ethyl acetate in iso-octane and B was eluted in 50% ethyl acetate in iso-octane. Samples were then assayed and the results were corrected for hormone loss during purification. Plasma hormone levels in the laboratory samples were analyzed in four assays; interassay variation was 5.1% for T and 13.7% for B. Field samples were analyzed in three assays; interassay variation was 5.2% for T and 22.0% for B. Nonparametric statistical tests (two-tailed) were used to analyze the data. RESULTS
Corticosterone Corticosterone levels exhibited no consistent change in either long day or short day males housed with females (Table 1; short day, female: H = 9.63, P > 0.20; long day, female: H = 7.84, P > 0.30). However, in males housed alone, plasma B levels tended to change over time, both on short days (Table 1; Kruskal-Wallis-H = 17.62, P < 0.02) and long days (H = 17.97, P < 0.02). Levels of B increased by day +22 in short day males housed alone (Day - 7 vs Day + 22: Wilcoxon matched-pairs signed-rank test--T = 6, n = 9, P < 0.05); they declined significantly at Day +50 (Day +22 vs Day +50: T = 3, n = 9, P < 0.02), only to increase again at Day +64 (Day +50 vs Day +64: T = 0, n = 9, P < 0.01) to levels that were sustained for the remainder of the experiment. In long day males housed alone, B levels increased significantly at Day +8 (Day - 7 vs Day +8: T = 6, n = 9, P < 0.05) and again at Day + 85 (Day +64 vs Day + 85: T = 6, n = 9, P < 0.05). In the field, there was no difference in plasma B levels between paired and unpaired/unknown male cowbirds in any part of the breeding season (Fig. 1). Gonads Gonadal growth was stimulated in both groups of long day males (Table 2; long day, no female: H = 33.43, P < 0.001; long days, female, P < 0.001). A significant increase in testis size occurred by Day +22 (long
3.1 (n 1.9 (n 3.3 (n 3.2 (n
2 = -’ = 2 = + =
0.8” 9) 0.3 9) 0.5 9) 0.7 8)
2.7 (n 3.4 (n 3.3 (n 3.7 (n
2 0.4 = 9) -r- 0.7 = 9) f 0.8 = 9) f 0.5 = 8)
+1 2.7 (n 3.1 (n 2.0 (n 4.8 (n f = 5 = 2 = 2 =
+8 0.4 9) 0.7’ 9) 0.5 9) 1.1 8)
4 9 f 0 4b,’ ‘(n = 5, 3.8 2 0.8 (n = 9) 2.8 f 0.8 (n = 9) 4.2 + 0.6 (n = 8)
+22 3.5 (n 4.4 (n 2.2 (n 4.9 (n
f = ” = zt = t =
+36 0.6 9) 0.8 9) 0.6 9) 0.8 8)
2.9 ” 0.7c*d (n = 9) 4.3 l?r 0.7 (n = 9) 3.1 f 0.7 (n = 9) 2.6 e 0.6 (n = 5)
+50 4.4 (n 6.1 (n 2.8 (n 4.6 (n
k = r = + = k =
0.6* 9) 1.2’ 9) 0.3 9) 0.8 5)
+64
a Relative to the onset of photostimulation and the addition of females. ‘-’ For each group, values with the same superscript are diierent at P < 0.05 (Wilcoxon matched-pairs signed-ranks test).
Long day, females
Short day, females
Long day
Short day
-7
Days”
TABLE 1 Changes (rig/ml) in Plasma Corticosterone in Captive Paired and Unpaired Male Brown-headed Cowbirds
6.3 (n 7.2 (n 3.7 (n 4.0 (n
+85 + 1.2 = 9) f 1.6’ = 9) f 0.6 = 9) + 0.8 = 3)
3 $ u s 2 0 gj E;
s
SOCIAL MODULATION
2-10 APR
I l-23 APR
24 APR 3YAY
7-16 NAY
227
OF HORMONES
17-30 MAY
3, MAY 13 JUN
14-30 JUN
I-20 JUL
FIG. 1. Mean plasma corticosterone levels in paired (open bars) and unpaired/unknown (hatched bars) male brown-headed cowbirds during the breeding season. Vertical lines are standard errors.
day, no female: T = 0, n = 6, P C 0.05; long day, female: T = 0, n = 8, P < O.Ol), and gonads remained larger in long day than in short day birds for the duration of the investigation. Testes of long day males with females remained large through Day +85, whereas those of long day males without females began to regress by Day +85 (Day +64 vs Day +85: T = 5, n = 9, P < 0.05). Maximum gonad size in paired or unpaired males was only one-half to two-thirds that attained by freeliving males (cf. Dufty and Wingfield, 1986). The presence of females did not have an effect on gonadal development in short day males. TABLE 2 Changes in Testis Weight (rng) in Captive Paired and Unpaired Male Brown-headed Cowbirds Days”
Short day Long day Short day, females Long day, females
-7
+22
+36
+64
+85
All< 10 (?I = 9) A11
All<10 (n = 9) 19 f 3b (n = 9) All<10 (n = 7) 34 2 4d (n = 8)
All<10 (n = 9) 56 -c 3 (n = 9) All< 10 (n = 9) 59 f 5 (n = 7)
All<15 (n = 9) 58 f 5 (n = 9) All<15 (n = 9) 76 2 7 (n = 5)
All<15 (n = 9) 43 -c 8’ (n = 9) All<15 (n = 8) 77 2 12 (n = 3)
0 Relative to the onset of photostimulation and the addition of females. b-d For each group, values with the same superscript are different at P < 0.05 (Wilcoxon matched-pairs signed-ranks test).
228
DUFTY AND WINGFIELD
Testicular maturation rates were similar in paired and unpaired/unknown birds during April and early May in the field (Table 3). Additional laparotomies of paired males were infrequent and occurred at irregular intervals, thus precluding further comparisons. Testosterone Laboratory investigation. Testosterone secretion in both groups of short day males did not change over the course of the experiment, although a slight rise in short day males with females bordered on significance (Table 4; short day, no female: H = 11.14, P > 0.1; short day, female: H = 14.05, P > 0.05). Long days stimulated T secretion in both groups of long day males (Table 4; long day, no female: H = 30.74, P < 0.001; long day, female: H = 17.28, P < 0.02). One male housed with a female did not show any response to long days. Since 17/18 males did exhibit the well-known photoperiodic response, this male was considered an anomaly and was dropped from the analysis. An increase in T secretion from that of Day -7 in long day males paired with females was evident by Day + 8 (T = 1, n = 8, P < 0.02), while in long day males without females a significant rise did not occur until Day +22 (T = 0, it = 9, P < 0.01). Plasma T levels in the paired males remained elevated until the completion of the experiment; those of long day males without females declined by Day +85 (Day + 50 vs Day +85: T = 0, n = 9, P < 0.01). Field investigation. A total of 21 males were considered to be paired. Sixty-eight blood samples were taken from them between 2 April and 14 July (range = 1-7; median = 3). Unpaired/unknown males totaled 109 in the population, and 157 blood samples were obtained from them between 2 April and 20 July (range = 1-7; median = 1). Figure 2 illustrates the difference in plasma T values in the two groups of male cowbirds. Paired males exhibited higher T levels than unpaired/unknown males early in the season, and this difference was significant from mid-April through the beginning of May. Thereafter, plasma TABLE 3 Changes in Testis Weight (mg) in Free-Living Paired and Unpaired/Unknown Brown-headed Cowbirds during the Early Stages of Recrudescence
Male
Date
Paired Unpaired/unknown
4/2-4/ 10
4/11-4123
4/24-S/3
33 f 9 (n = 7) 35 2 5 (n = 14)
54 + 9 (n = 5) 47 s 5
(n = 6)
(n = 22)
(n
88 2 15 80 2 5 = 22)
160 * (n = 136 ‘(n = 132 2 (n = 149 2 (n =
11 8) 13’ 9) 13 9) 39d 8)
98 f 7 (n = 9) 131 % 42 (n = 9) 114 t 10 (n = 9) 189 ? 28 (n = 8)
+1 33 9) 406 9) 11 9) 47d
(n = 8)
137 ‘(n = 761 2 (n = 108 ? (n = 450 2
+8
1460 k 498 (n = 8)
(n = 9)
121 -c 22 (n = 9) 860 + 359’ (n = 9) 102 f 7
+22
102 * (n = 1242 t (n =
7 9) 414 8)
(n = 9)
212 ” 60 (n = 9) 1653 + 506
+36
822 e 479 (n = 5)
(n = 9)
149 2 39 (n = 9) 1082 f 282 (n = 9) 110 -e 12
+50
1088 2 282 (n = 5)
(n = 9)
164 ” 38 (n = 9) 872 + 288 (n = 9) 320 ?z 115
+64
a Relative to the onset of photostimulation and the addition of females. b-d For each group, values with the same superscript are different at P < 0.05 (Wilcoxon matched-pairs signed-ranks test).
Short day, females Long day, females
Long day
Short day
-7
Days”
TABLE 4 Changes (pg/ml) in Plasma Testosterone in Captive Paired and Unpaired Male Brown-headed Cowbirds
209 f (n = 397 k (n = 180 2 (n = 1523 t (n =
+85 64 9) 158’ 9) 34 9) 600 3)
g
8
%
5 g F =I g
z
w
230
DUFTY AND WINGFIELD
2600
600
Z-10 APR
II-23
APR
I
24 APR 3MAY
7-16 MAY
17-30 MAY
31 MAY 13 JUN
14-M
JUN
I-20 JUL
FIG. 2. Mean plasma testosterone levels in paired (open bars) and unpaired/unknown (hatched bars) male brown-headed cowbirds during the breeding season. Vertical lines are standard errors. *Means are different at the 0.05 level (Mann-Whitney U test).
levels of T were indistinguishable in the two groups, with values for both groups declining by late May and throughout June. DISCUSSION Corticosterone Plasma B levels were unchanged in males housed with females but were elevated in males housed alone, regardless of the photoperiod. This suggests that the removal of social stimuli was stressful for these cowbirds. In free-living male cowbirds, plasma B was generally highest during the first half of the breeding season and declined during the second half, as had been noted earlier for this cowbird population (cf. Dufty and Wingfield, 1986) and for other avian species (e.g., Wingtield and Farner, 1978a,b; Wingfield, 1984). Gonads Gonadal maintenance in captive male cowbirds was enhanced by the presence of females, a result that has been noted by others (Haase et al., 1976; Moore, 1983). Maximum testicular weights never approached those of free-living conspecifics, a result that is consistent with earlier findings (Scott and Middleton, 1968). No differences in gonadal size were found between free-living paired and unpaired/unknown male cowbirds early in the season, perhaps because other environmental cues, such as intrasexual interactions or nutrition, may act in concert with stimuli from females and thus obscure any effect on testicular growth.
SOCIAL
MODULATION
OF HORMONES
231
Testosterone Laboratory investigation. The results suggest that the increase in T of male cowbirds exposed to long days can be modulated, at least at certain times, by social cues. This social effect is apparent at the beginning and at the end of the breeding cycle in captive males. The presence of a female will elicit the response, although it is unknown whether the effect can be produced by other birds, as well. Plasma T levels in males housed with females increased earlier than those of solitary males; T levels in the former were still elevated at Day +85, while levels in the latter had declined. These results are similar to those reported for other species (Feder et al., 1977; O’Connell et al., 1981; Moore, 1982, 1983, 1984; Runfeldt and Wingfield, 1985). Nonphotostimulated male cowbirds showed no endocrine response to females. Despite differences at the beginning and end of the cycle, plasma T levels were similar in paired and unpaired captive males for most of the investigation. T secretion in captive male cowbirds appears to be maximally stimulated by long days. Any influence of females (or other birds) is masked by the photoperiodic response, except at the beginning and end of the cycle. The presence of females has similarly minor effects on adult male Japanese quail (Coturnix coturnixjuponica) (Delville, Sulon, Hendrick, and Balthazart, 1984). The peak T values of captive males are comparable to those of freeliving male cowbirds during much of the season. This is unlike the situation in monogamous nesting birds, where captive unpaired males and males paired with untreated females have plasma T levels that are 12-14 times lower than those reached by free-living males (Wingfield and Farner, 1980). This suggests that the amplitude of plasma T secretion in cowbirds is less affected by social and other external stimuli than in nesting species, and that all or most male cowbirds eventually secrete maximal levels of T at some point during the breeding season. Thus all male cowbirds may be capable of competing for a mate and breeding, regardless of their immediate pairing status. Indeed, in the wild, unpaired males frequently initiate aggressive song-and-display bouts with paired individuals and court unguarded females; paired males that disappear are quickly replaced (Darley, 1982; Dufty, 1982a,b), attesting to the availability of unpaired males that are in breeding condition. Field investigation. Free-living paired male cowbirds possesssignificantly higher T levels than do unpaired/unknown males during the early part of the breeding season, when males are courting females and aggressively displaying to other males. Once egg laying begins (in late April/early May), T levels in the two groups are indistinguishable but are still well above basal, and T declines gradually thereafter through the end of the season (see also Dufty and Wingfield, 1986).
232
DUFTY AND WINGFIELD
The difference in plasma T levels between paired and unpaired/unknown male cowbirds probably reflects, in part, the stimulatory effect of females on T secretion. Aggressive interactions with other males may also contribute to the endocrine response. The sensory modality (or modalities) responsible for the facilitating effect, of females or males is presently ‘unknown. However, recent evidence suggests that individual differences in androgen titer occur primarily during the establishment of social relationships or when established systems are challenged (Ramenofsky, 1984; Wingfield and Ramenofsky, 1985). Once the social system stabilizes, T levels in both dominant and subordinate individuals are similar and the social positions are maintained by social inertia (Ramenofsky, 1984). Thus the high T levels of paired male cowbirds early in the season may also be a function of intrasexual interactions over access to females. The subsequent decline in plasma T of paired males to levels found in unpaired/unknown males may represent the relative stabilization of the social hierarchy. Plasma T levels in nonparasitic species are also high during the egglaying period, but these decline markedly with the onset of incubation and parental activities (Wingfield and Famer, 1978a,b; Wingfield, 1984). A reduction in T levels at the onset of parental duties has adaptive value in males that normally provision their young, for if T levels are artificially maintained at high levels males will not feed nestlings, resulting in reduced reproductive success (Silverin, 1980). Since cowbirds are emancipated from parental care they are not subject to this deleterious effect of sustained high plasma T levels. On the contrary: Throughout the breeding season (1) females lay eggs almost daily (Payne, 1965; Scott and Ankney, 1980), (2) paired males regularly engage in display bouts with intruder males (Darley, 1982; Dufty, 1982a),and (3) solitary males quickly approach and court unguarded females (Dufty, 1982b). These behaviors tend to keep the social system unsettled; and as a consequence, androgen levels remain elevated in all male cowbirds. Thus, the cowbird’s unusual breeding system has resulted in the evolution of a maximal T response, which is achieved even in the absence of females and in captivity. Comparable patterns of plasma T secretion may be found in males of other species subject to similar ecological pressures as the cowbird. For example, in western populations of red-winged blackbirds (Ageluius phoeniceus) males have harems of asynchronously breeding females, and the males rarely feed young (Orians, 1980). Territorial male redwings are similar to male cowbirds in that they are frequently challenged by nonbreeding males and have regular access to a fertile female: In the case of the redwing it is several females that are receptive intermittently, while for cowbirds it is one primary female that is sexually receptive almost continuously. The plasma T profiles for males of the two species are also similar, for peak levels of T in redwings are likewise found early
SOCIAL MODULATION
OF HORMONES
233
in the breeding season and gradually decline thereafter (W. A. Searcy, unpublished). We suggest that the temporal patterns of hormone secretion in the brood parasitic brown-headed cowbird and nesting species, though generally dissimilar in appearance, have evolved in response to similar evolutionary pressures. The details of these endocrine secretion patterns are influenced both by social stimuli and by the breeding phenology of the specific mating system under consideration. ACKNOWLEDGMENTS This research was supported by funds from Biomedical Research Grant PHS RR07065 15 to The Rockefeller University, NIMH Postdoctoral Fellowship Grant HD06464 to A.M.D., and a Charles H. Revson Foundation Fellowship in Biomedical Research and Grant PCM8118522 from the National Science Foundation to J.C.W. We also thank C. Levesque for preparing the figures.
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Dufty, A. M., Jr. (1982a). Movements and activities of radio tracked brown-headed cowbirds. Auk 99, 316-327. Dufty, A. M., Jr. (1982b). Response of brown-headed cowbirds to simulated conspecific intruders. Anim. Behav. 30, 1043-1052. Dufty, A. M., Jr., and Wingheld, J. C. (1986). Temporal patterns of circulating LH and steroid hormones in a brood parasite, the brown-headed cowbird, Molothrus ater. J. Zool. (London), 208, 191-203. Fankhauser, D. P. (1971). Annual survival rates of blackbirds and starlings. Bird-Banding 42, 36-42. Feder, H. H., Storey, A., Goodwin, D., Reboulleau, C., and Silver, R. (1977). Testosterone and “5a-dihydrotestosterone” levels in peripheral plasma of male and female ring doves (Streptopelia risoria) during the reproductive cycle. Biol. Reprod. 16, 666-677. Friedmann, H. (1929). The Cowbirds: A Study in the Biology of Social Purusirism. C C Thomas, Springfield, Ill. Haase, E., Paulke, E., and Sharp, P. J. (1976). Effects of seasonal and social factors on testicular activity and hormone levels in domestic pigeons. J. Exp. Zool. 197, 81-88. Harvey, S., Merry, J. B., and Phillips, J. G. (1980). Influence of stress on the secretion of corticosterone in the duck (Anus plutyrhynchos). J. Endocrinol. 87, 161-171. Kroodsma, D. E. (1976). Reproductive development in a female songbird: differential stimulation by quality of male song. Science 192, 574-575. Lehrman, D. S. (1%5). Interaction between internal and external environments in the regulation of the reproductive cycle of the ring dove. In F. A. Beach (Ed.), Sex and Behavior, pp. 355-380. Wiley, New York. Moore, M. C. (1982). Hormonal response of free-living male white-crowned sparrows to experimental manipulation of female sexual behavior. Horm. Behav. 16, 323-329.
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Moore, M. C. (1983). Effect of female sexual displays on the endocrine physiology and behaviour of male white-crowned sparrows, Zonotrichiu leucophyrs. J. Zool. (London) 199, 137-148. Moore, M. C. (1984). Changes in territorial defence produced by changes in circulating levels of testosterone: A possible hormonal basis for mate-guarding behavior in whitecrowned sparrows. Behaviour 88, 215-226. Mutton, R. K., and Westwood, N. J. (1977). Avian Breeding Cycles. Oxford Univ. Press (Clarendon), London/New York. O’Connell, M. E., Silver, R., Feder, H. H., and Reboulleau, C. (1981). Social interactions and androgen levels in birds. II. Social factors associated with a decline in plasma androgen levels in male ring doves (Streptopeliu risoria). Gen. Comp. Endocrinol. 44, 464-469. Orians, G. H. (1980). Some Adaptations of Marsh-nesting Blackbirds. Princeton Univ. Press, Princeton, N.J. Payne, R. B. (1%5). clutch size and numbers of eggs laid by brown-headed cowbirds. Condor 67, 44-60. Ramenofsky, M. (1984). Agonistic behaviour and endogenous plasma hormones in male Japanese quail. Anim. Behav. 32, 698-708. Runfeldt, S., and Wingfield, J. C. (1985). Experimentally prolonged sexual activity in female sparrows delays termination of reproductive activity in their untreated mates. Anim. Behav. 33, 403-410. Scott, D. M., and Ankney, C. D. (1980). Fecundity of the brown-headed cowbird in southern Ontario. Auk 97, 677-683. Scott, D. M., and Middleton, A. L. A. (1968). The annual testicular cycle of the brownheaded cowbird (Molothrus ater). Cunud. J. Zool. 46, 77-87. Silverin, B. (1980). Effects of long-acting testosterone treatment on free-living pied flycatchers, Ficedula hypoleucu, during the breeding season. Anim. Behuv. 28, 906-912. West, M. J., King, A. P., and Eastzer, D. H. (1981). Validating the female bioassay of cowbird song: Relating differences in song potency to mating success. Anim. Behuv. 29, 490-501. Wingfield, J. C. (1983). Environmental and endocrine control of avian reproduction: An ecological approach. In S. Mikami, K. Homma, and M. Wada (Eds.), Avian Endocrinology: Environmental and Ecological Perspectives, pp. 265-288. Springer-Verlag, New York. Wingfleld, J. C. (1984). Environmental and endocrine control of reproduction in the song sparrow, Melospizu melodiu. I. Temporal organization of the breeding cycle. Gen. Camp. Endocrinol. 56, 406-416. Wingfield, J. C., and Famer, D. S. (1975). The determination of five steroids in avian plasma by radioimmunoassay and competitive protein binding. Steroids 26, 31l-327. Wingfield, J. C., and Famer, D. S. (1976). Avian endocrinology-Field investigations and methods. Condor 78, 570-572. Wingfteld, J. C., and Famer, D. S. (1978a). The endocrinology of a natural breeding population of the white-crowned sparrow. Physiol. Zool. 51, 188-205. Wingfield, J. C., and Famer, D. S. (1978b). The annual cycle of plasma irLH and steroid hormones in feral populations of the white-crowned sparrow, Zonotrichiu leucophrys gumbelii. Biol. Reprod. 19, 1046-1056. Wingtield, J. C., and Famer, D. S. (1980). Control of seasonal reproduction in temperatezone birds. In P. 0. Hubinont, (Ed.), Progress in Reproductive Biology, Vol. 5, pp. 62-101. Karger, Basel. Wingtield, J. C., and Ramenofsky, M. (1985). Testosterone and aggressive behaviour during the reproductive cycle of male birds. In R. Gilles and J. Balthazart, (Eds.), Comparative Neurobiology, pp. 92-104. Springer-Verlag, Berlin.
20, 222-234 (1986)
The Influence of Social Cues on the Reproductive Endocrinology of Male Brown-headed Cowbirds: Field and Laboratory Studies ALFRED The Rockefeller
M. DUFTY, JR. ,’ AND JOHN C. WINGFIELD
University
Field Research Center, Millbrook,
New York 12545
Captive male brown-headed cowbirds exposed to long days exhibit gonadal growth and have elevated plasma testosterone (T) levels. This photoperiodic response is enhanced if males are housed with female cowbirds: Photostimulated males with females increase plasma testosterone levels sooner than do individually housed photostimulated males. Peak plasma T levels are similar in both groups, although peak levels are maintained longer in males housed with females. The gonadal cycle is similarly affected; males in the presence of females have earlier gonadal recrudescence and maintain mature gonads longer than do photostimulated males without females. Plasma corticosterone levels increase in the unpaired males, suggesting that removal of social cues is stressful for these birds. Freeliving paired males have significantly higher plasma testosterone levels than do unpaired/unknown males early in the season, when social relationships are being established; the levels are similar thereafter. There is no difference between the two groups in testicular maturation rates; nor do they differ in plasma corticosterone levels at any time of the season. These results suggest that social stimuli are important in modulating the secretion of testosterone in males early in the season when pairing occurs, and possibly late in the season as well, probably to prevent termination of breeding prior to that of females. o 1986 Academic press, IW.
Most species of birds that breed in temperate areas respond to the vernal increase in daylength with changes in endocrine secretion, culminating in gonadal growth and physiological preparation for the coming reproductive season (Murton and Westwood, 1977; Wingfield and Farner, 1980; Wingfield, 1983). Yet, despite the “switching-on” of reproductive development by long days, final maturation of the reproductive system is achieved only after additional cues relating to territory, weather, and nutrition are perceived from the environment (Wingfield, 1983). One additional source of environmental information is social stimulation such as that received from the behavior of the mate. Numerous studies have demonstrated that males, through vocalizations and courtship displays, ’ To whom all correspondence should be addressed. 222 0018-506x/86 $1.50 Copyright 8 1986 by Academic Press, Inc. AU tights of reproduction in any form reserved.
SOCIAL
MODULATION
OF HORMONES
223
provide cues which can stimulate hormonal and behavioral responses in their mates (e.g., Lehrman, 1965; Kroodsma, 1976). Likewise, recent investigations have also demonstrated the role of social cues in stimulating endocrine responses in males of avian species. In the laboratory, male ring doves (Streptopeliu risoria) exposed to females exhibit higher androgen levels than do males in the absence of females (Feder, Storey, Goodwin, Reboulleau, and Silver, 1977). Intact females produce a stronger response than do ovariectomized females, suggesting that the gonadal state of the female is important (O’Connell, Silver, Feder, and Reboulleau, 1981). Furthermore, auditory cues also appear to play a role in stimulating courtship behavior in males (O’Connell et al., 1981). Additionally, paired male domestic pigeons (Columba liviu) have higher levels of plasma luteinizing hormone (LH) and larger testes than do unpaired males (Haase, Paulke, and Sharp, 1976). These responses are not restricted to laboratory populations: In feral white-crowned sparrows (Zonotrichiu leucophrys) paired males have higher plasma levels of testosterone (T) than do unpaired males (Wingfield and Famer, 1980). Male white-crowned sparrows and song sparrows (Melospiza melodia) paired with females implanted with estradiol (which facilitates sexual responsiveness) have significantly higher androgen levels than control males paired with unimplanted females (Moore, 1982; Runfeldt and Wingfield, 1985). We examine here the effect of social cues on males of a species with an unusual breeding system, the brown-headed cowbird (Molothrus ater). Cowbirds are obligate brood parasites whose breeding cycle lacks all aspects of parental care (Friedmann, 1929). Males outnumber females in the adult population by approximately 3:2 (Darley, 1971; Fankhauser, 1971),and males compete for rank in a dominance hierarchy and, ultimately, for access to females (Darley, 1978; West, King, and Eastzer, 1981). Successful males guard their mates from other courting males, resulting in a significant number of unpaired individuals in the population (Darley, 1982; Dufty, 1982a). Mate guarding continues for the entire 8- to loweek breeding season in which females lay eggs (Payne, 1965; Scott and Ankney, 1980). The plasma androgen [T and dihydrotestosterone (DHT)] profiles that occur in plasma of male cowbirds during the breeding season have been presented by Dufty and Wingfield (1986). Androgens peak during hierarchy formation and mate acquisition, and then gradually decline through the end of the reproductive period. However, the mating status of the males was not determined in this earlier study, so it was not possible to compare plasma androgen titers from paired and unpaired individuals. In this communication we present data from field and laboratory studies in which the T levels of paired males are compared with those of other males.
224
DUFTY
AND WINGFIELD
MATERIALS
AND METHODS
Laboratory investigation. Male (n = 36) and female (n = 18) cowbirds were captured in August and September. Males were housed individually on an 8L:16D photoperiod, and females were housed together in an outdoor aviary. On 8 and 9 February blood samples were taken (see below) from all males and their breeding condition was noted by unilateral laparotomy. Each male was assigned at random to one of four treatment groups: (1) short days (8L: 16D), female absent; (2) short days, female present; (3) long days (16L: 8D), female absent; or (4) long days, female present. Each treatment group was housed in a separate chamber in visual and acoustic isolation from all other groups. Within groups, males were visually but not acoustically separated from each other. One week later, days were lengthened and adult females, that had likewise been kept on short days, were added to the appropriate groups. The behavior of the females was not assessed. A second blood sample was taken on the following day and again at l-, 2-, or 3-week intervals. Laparotomies were performed four additional times to estimate gonadal growth by comparison with preserved (70% ethyl alcohol) testes of known weight. The final laparotomies occurred on 10 and 11 May, when the study ended. Three of the females housed with photostimulated males were pecked by their cage mates after varying amounts of time spent on long days. The females were removed and the males were excluded from additional consideration in the experiment. One male injured a leg and had to be removed, and one male died of unknown causes. Field investigation. The male brown-headed cowbirds studied in the wild are part of a color-marked population in the environs of The Rockefeller University Field Research Center in Millbrook, NY (42”N). Details regarding the capture and handling of these birds are presented elsewhere (Dufty and Wingfield, 1986). Briefly, birds were captured in mist nets or Potter traps baited with grain. A blood sample (750-1000 ~1) was taken from the alar vein in heparinized Caraway blood-collecting microtubes, following the technique of Wingfield and Farner (1976). The blood from each sample was transferred to disposable centrifuge tubes and was centrifuged at 2000 rpm for 5 min. Plasma was removed using Pasteur pipettes and stored at -20°C until assayed. Behavioral observations were conducted throughout the spring and summer. Some birds were followed on their breeding grounds, but the majority were observed as they fed on the lawns surrounding the Field Research Center. Males can be considered to be paired if they are routinely seen with the same female and guard her from other males (Darley, 1982; Dufty, 1982a). Since unpaired males will also court and guard females (Darley, 1982; Dufty, 1982b), males are considered to be paired only if they were seen with a particular female on five or more different occasions.
SOCIAL
MODULATION
OF HORMONES
225
All other males are categorized as unpaired/unknown. This category undoubtedly contains some paired individuals, but difficulty in accurately determining the mating status of (1) paired and unpaired males that were rarely observed, or (2) birds that were captured and bled but never subsequently observed made this the most conservative set of categories. Assays. Two steroid hormones, T and corticosterone (B), were measured by radioimmunoassay (RIA) using the technique of Wingfield and Farner (1975). Steroids were extracted with 5 ml dichloromethane, dried, and resuspended in 10% ethyl acetate in iso-octane. The extracts were added to columns containing a mixture of diatomaceous earth:propylene glycol:ethylene glycol (6: 1.5: 1.5, w : v : v) with a diatomaceous earth:water (3 : 1, w: v) “glycol trap.” T was eluted in 20% ethyl acetate in iso-octane and B was eluted in 50% ethyl acetate in iso-octane. Samples were then assayed and the results were corrected for hormone loss during purification. Plasma hormone levels in the laboratory samples were analyzed in four assays; interassay variation was 5.1% for T and 13.7% for B. Field samples were analyzed in three assays; interassay variation was 5.2% for T and 22.0% for B. Nonparametric statistical tests (two-tailed) were used to analyze the data. RESULTS
Corticosterone Corticosterone levels exhibited no consistent change in either long day or short day males housed with females (Table 1; short day, female: H = 9.63, P > 0.20; long day, female: H = 7.84, P > 0.30). However, in males housed alone, plasma B levels tended to change over time, both on short days (Table 1; Kruskal-Wallis-H = 17.62, P < 0.02) and long days (H = 17.97, P < 0.02). Levels of B increased by day +22 in short day males housed alone (Day - 7 vs Day + 22: Wilcoxon matched-pairs signed-rank test--T = 6, n = 9, P < 0.05); they declined significantly at Day +50 (Day +22 vs Day +50: T = 3, n = 9, P < 0.02), only to increase again at Day +64 (Day +50 vs Day +64: T = 0, n = 9, P < 0.01) to levels that were sustained for the remainder of the experiment. In long day males housed alone, B levels increased significantly at Day +8 (Day - 7 vs Day +8: T = 6, n = 9, P < 0.05) and again at Day + 85 (Day +64 vs Day + 85: T = 6, n = 9, P < 0.05). In the field, there was no difference in plasma B levels between paired and unpaired/unknown male cowbirds in any part of the breeding season (Fig. 1). Gonads Gonadal growth was stimulated in both groups of long day males (Table 2; long day, no female: H = 33.43, P < 0.001; long days, female, P < 0.001). A significant increase in testis size occurred by Day +22 (long
3.1 (n 1.9 (n 3.3 (n 3.2 (n
2 = -’ = 2 = + =
0.8” 9) 0.3 9) 0.5 9) 0.7 8)
2.7 (n 3.4 (n 3.3 (n 3.7 (n
2 0.4 = 9) -r- 0.7 = 9) f 0.8 = 9) f 0.5 = 8)
+1 2.7 (n 3.1 (n 2.0 (n 4.8 (n f = 5 = 2 = 2 =
+8 0.4 9) 0.7’ 9) 0.5 9) 1.1 8)
4 9 f 0 4b,’ ‘(n = 5, 3.8 2 0.8 (n = 9) 2.8 f 0.8 (n = 9) 4.2 + 0.6 (n = 8)
+22 3.5 (n 4.4 (n 2.2 (n 4.9 (n
f = ” = zt = t =
+36 0.6 9) 0.8 9) 0.6 9) 0.8 8)
2.9 ” 0.7c*d (n = 9) 4.3 l?r 0.7 (n = 9) 3.1 f 0.7 (n = 9) 2.6 e 0.6 (n = 5)
+50 4.4 (n 6.1 (n 2.8 (n 4.6 (n
k = r = + = k =
0.6* 9) 1.2’ 9) 0.3 9) 0.8 5)
+64
a Relative to the onset of photostimulation and the addition of females. ‘-’ For each group, values with the same superscript are diierent at P < 0.05 (Wilcoxon matched-pairs signed-ranks test).
Long day, females
Short day, females
Long day
Short day
-7
Days”
TABLE 1 Changes (rig/ml) in Plasma Corticosterone in Captive Paired and Unpaired Male Brown-headed Cowbirds
6.3 (n 7.2 (n 3.7 (n 4.0 (n
+85 + 1.2 = 9) f 1.6’ = 9) f 0.6 = 9) + 0.8 = 3)
3 $ u s 2 0 gj E;
s
SOCIAL MODULATION
2-10 APR
I l-23 APR
24 APR 3YAY
7-16 NAY
227
OF HORMONES
17-30 MAY
3, MAY 13 JUN
14-30 JUN
I-20 JUL
FIG. 1. Mean plasma corticosterone levels in paired (open bars) and unpaired/unknown (hatched bars) male brown-headed cowbirds during the breeding season. Vertical lines are standard errors.
day, no female: T = 0, n = 6, P C 0.05; long day, female: T = 0, n = 8, P < O.Ol), and gonads remained larger in long day than in short day birds for the duration of the investigation. Testes of long day males with females remained large through Day +85, whereas those of long day males without females began to regress by Day +85 (Day +64 vs Day +85: T = 5, n = 9, P < 0.05). Maximum gonad size in paired or unpaired males was only one-half to two-thirds that attained by freeliving males (cf. Dufty and Wingfield, 1986). The presence of females did not have an effect on gonadal development in short day males. TABLE 2 Changes in Testis Weight (rng) in Captive Paired and Unpaired Male Brown-headed Cowbirds Days”
Short day Long day Short day, females Long day, females
-7
+22
+36
+64
+85
All< 10 (?I = 9) A11
All<10 (n = 9) 19 f 3b (n = 9) All<10 (n = 7) 34 2 4d (n = 8)
All<10 (n = 9) 56 -c 3 (n = 9) All< 10 (n = 9) 59 f 5 (n = 7)
All<15 (n = 9) 58 f 5 (n = 9) All<15 (n = 9) 76 2 7 (n = 5)
All<15 (n = 9) 43 -c 8’ (n = 9) All<15 (n = 8) 77 2 12 (n = 3)
0 Relative to the onset of photostimulation and the addition of females. b-d For each group, values with the same superscript are different at P < 0.05 (Wilcoxon matched-pairs signed-ranks test).
228
DUFTY AND WINGFIELD
Testicular maturation rates were similar in paired and unpaired/unknown birds during April and early May in the field (Table 3). Additional laparotomies of paired males were infrequent and occurred at irregular intervals, thus precluding further comparisons. Testosterone Laboratory investigation. Testosterone secretion in both groups of short day males did not change over the course of the experiment, although a slight rise in short day males with females bordered on significance (Table 4; short day, no female: H = 11.14, P > 0.1; short day, female: H = 14.05, P > 0.05). Long days stimulated T secretion in both groups of long day males (Table 4; long day, no female: H = 30.74, P < 0.001; long day, female: H = 17.28, P < 0.02). One male housed with a female did not show any response to long days. Since 17/18 males did exhibit the well-known photoperiodic response, this male was considered an anomaly and was dropped from the analysis. An increase in T secretion from that of Day -7 in long day males paired with females was evident by Day + 8 (T = 1, n = 8, P < 0.02), while in long day males without females a significant rise did not occur until Day +22 (T = 0, it = 9, P < 0.01). Plasma T levels in the paired males remained elevated until the completion of the experiment; those of long day males without females declined by Day +85 (Day + 50 vs Day +85: T = 0, n = 9, P < 0.01). Field investigation. A total of 21 males were considered to be paired. Sixty-eight blood samples were taken from them between 2 April and 14 July (range = 1-7; median = 3). Unpaired/unknown males totaled 109 in the population, and 157 blood samples were obtained from them between 2 April and 20 July (range = 1-7; median = 1). Figure 2 illustrates the difference in plasma T values in the two groups of male cowbirds. Paired males exhibited higher T levels than unpaired/unknown males early in the season, and this difference was significant from mid-April through the beginning of May. Thereafter, plasma TABLE 3 Changes in Testis Weight (mg) in Free-Living Paired and Unpaired/Unknown Brown-headed Cowbirds during the Early Stages of Recrudescence
Male
Date
Paired Unpaired/unknown
4/2-4/ 10
4/11-4123
4/24-S/3
33 f 9 (n = 7) 35 2 5 (n = 14)
54 + 9 (n = 5) 47 s 5
(n = 6)
(n = 22)
(n
88 2 15 80 2 5 = 22)
160 * (n = 136 ‘(n = 132 2 (n = 149 2 (n =
11 8) 13’ 9) 13 9) 39d 8)
98 f 7 (n = 9) 131 % 42 (n = 9) 114 t 10 (n = 9) 189 ? 28 (n = 8)
+1 33 9) 406 9) 11 9) 47d
(n = 8)
137 ‘(n = 761 2 (n = 108 ? (n = 450 2
+8
1460 k 498 (n = 8)
(n = 9)
121 -c 22 (n = 9) 860 + 359’ (n = 9) 102 f 7
+22
102 * (n = 1242 t (n =
7 9) 414 8)
(n = 9)
212 ” 60 (n = 9) 1653 + 506
+36
822 e 479 (n = 5)
(n = 9)
149 2 39 (n = 9) 1082 f 282 (n = 9) 110 -e 12
+50
1088 2 282 (n = 5)
(n = 9)
164 ” 38 (n = 9) 872 + 288 (n = 9) 320 ?z 115
+64
a Relative to the onset of photostimulation and the addition of females. b-d For each group, values with the same superscript are different at P < 0.05 (Wilcoxon matched-pairs signed-ranks test).
Short day, females Long day, females
Long day
Short day
-7
Days”
TABLE 4 Changes (pg/ml) in Plasma Testosterone in Captive Paired and Unpaired Male Brown-headed Cowbirds
209 f (n = 397 k (n = 180 2 (n = 1523 t (n =
+85 64 9) 158’ 9) 34 9) 600 3)
g
8
%
5 g F =I g
z
w
230
DUFTY AND WINGFIELD
2600
600
Z-10 APR
II-23
APR
I
24 APR 3MAY
7-16 MAY
17-30 MAY
31 MAY 13 JUN
14-M
JUN
I-20 JUL
FIG. 2. Mean plasma testosterone levels in paired (open bars) and unpaired/unknown (hatched bars) male brown-headed cowbirds during the breeding season. Vertical lines are standard errors. *Means are different at the 0.05 level (Mann-Whitney U test).
levels of T were indistinguishable in the two groups, with values for both groups declining by late May and throughout June. DISCUSSION Corticosterone Plasma B levels were unchanged in males housed with females but were elevated in males housed alone, regardless of the photoperiod. This suggests that the removal of social stimuli was stressful for these cowbirds. In free-living male cowbirds, plasma B was generally highest during the first half of the breeding season and declined during the second half, as had been noted earlier for this cowbird population (cf. Dufty and Wingfield, 1986) and for other avian species (e.g., Wingtield and Farner, 1978a,b; Wingfield, 1984). Gonads Gonadal maintenance in captive male cowbirds was enhanced by the presence of females, a result that has been noted by others (Haase et al., 1976; Moore, 1983). Maximum testicular weights never approached those of free-living conspecifics, a result that is consistent with earlier findings (Scott and Middleton, 1968). No differences in gonadal size were found between free-living paired and unpaired/unknown male cowbirds early in the season, perhaps because other environmental cues, such as intrasexual interactions or nutrition, may act in concert with stimuli from females and thus obscure any effect on testicular growth.
SOCIAL
MODULATION
OF HORMONES
231
Testosterone Laboratory investigation. The results suggest that the increase in T of male cowbirds exposed to long days can be modulated, at least at certain times, by social cues. This social effect is apparent at the beginning and at the end of the breeding cycle in captive males. The presence of a female will elicit the response, although it is unknown whether the effect can be produced by other birds, as well. Plasma T levels in males housed with females increased earlier than those of solitary males; T levels in the former were still elevated at Day +85, while levels in the latter had declined. These results are similar to those reported for other species (Feder et al., 1977; O’Connell et al., 1981; Moore, 1982, 1983, 1984; Runfeldt and Wingfield, 1985). Nonphotostimulated male cowbirds showed no endocrine response to females. Despite differences at the beginning and end of the cycle, plasma T levels were similar in paired and unpaired captive males for most of the investigation. T secretion in captive male cowbirds appears to be maximally stimulated by long days. Any influence of females (or other birds) is masked by the photoperiodic response, except at the beginning and end of the cycle. The presence of females has similarly minor effects on adult male Japanese quail (Coturnix coturnixjuponica) (Delville, Sulon, Hendrick, and Balthazart, 1984). The peak T values of captive males are comparable to those of freeliving male cowbirds during much of the season. This is unlike the situation in monogamous nesting birds, where captive unpaired males and males paired with untreated females have plasma T levels that are 12-14 times lower than those reached by free-living males (Wingfield and Farner, 1980). This suggests that the amplitude of plasma T secretion in cowbirds is less affected by social and other external stimuli than in nesting species, and that all or most male cowbirds eventually secrete maximal levels of T at some point during the breeding season. Thus all male cowbirds may be capable of competing for a mate and breeding, regardless of their immediate pairing status. Indeed, in the wild, unpaired males frequently initiate aggressive song-and-display bouts with paired individuals and court unguarded females; paired males that disappear are quickly replaced (Darley, 1982; Dufty, 1982a,b), attesting to the availability of unpaired males that are in breeding condition. Field investigation. Free-living paired male cowbirds possesssignificantly higher T levels than do unpaired/unknown males during the early part of the breeding season, when males are courting females and aggressively displaying to other males. Once egg laying begins (in late April/early May), T levels in the two groups are indistinguishable but are still well above basal, and T declines gradually thereafter through the end of the season (see also Dufty and Wingfield, 1986).
232
DUFTY AND WINGFIELD
The difference in plasma T levels between paired and unpaired/unknown male cowbirds probably reflects, in part, the stimulatory effect of females on T secretion. Aggressive interactions with other males may also contribute to the endocrine response. The sensory modality (or modalities) responsible for the facilitating effect, of females or males is presently ‘unknown. However, recent evidence suggests that individual differences in androgen titer occur primarily during the establishment of social relationships or when established systems are challenged (Ramenofsky, 1984; Wingfield and Ramenofsky, 1985). Once the social system stabilizes, T levels in both dominant and subordinate individuals are similar and the social positions are maintained by social inertia (Ramenofsky, 1984). Thus the high T levels of paired male cowbirds early in the season may also be a function of intrasexual interactions over access to females. The subsequent decline in plasma T of paired males to levels found in unpaired/unknown males may represent the relative stabilization of the social hierarchy. Plasma T levels in nonparasitic species are also high during the egglaying period, but these decline markedly with the onset of incubation and parental activities (Wingfield and Famer, 1978a,b; Wingfield, 1984). A reduction in T levels at the onset of parental duties has adaptive value in males that normally provision their young, for if T levels are artificially maintained at high levels males will not feed nestlings, resulting in reduced reproductive success (Silverin, 1980). Since cowbirds are emancipated from parental care they are not subject to this deleterious effect of sustained high plasma T levels. On the contrary: Throughout the breeding season (1) females lay eggs almost daily (Payne, 1965; Scott and Ankney, 1980), (2) paired males regularly engage in display bouts with intruder males (Darley, 1982; Dufty, 1982a),and (3) solitary males quickly approach and court unguarded females (Dufty, 1982b). These behaviors tend to keep the social system unsettled; and as a consequence, androgen levels remain elevated in all male cowbirds. Thus, the cowbird’s unusual breeding system has resulted in the evolution of a maximal T response, which is achieved even in the absence of females and in captivity. Comparable patterns of plasma T secretion may be found in males of other species subject to similar ecological pressures as the cowbird. For example, in western populations of red-winged blackbirds (Ageluius phoeniceus) males have harems of asynchronously breeding females, and the males rarely feed young (Orians, 1980). Territorial male redwings are similar to male cowbirds in that they are frequently challenged by nonbreeding males and have regular access to a fertile female: In the case of the redwing it is several females that are receptive intermittently, while for cowbirds it is one primary female that is sexually receptive almost continuously. The plasma T profiles for males of the two species are also similar, for peak levels of T in redwings are likewise found early
SOCIAL MODULATION
OF HORMONES
233
in the breeding season and gradually decline thereafter (W. A. Searcy, unpublished). We suggest that the temporal patterns of hormone secretion in the brood parasitic brown-headed cowbird and nesting species, though generally dissimilar in appearance, have evolved in response to similar evolutionary pressures. The details of these endocrine secretion patterns are influenced both by social stimuli and by the breeding phenology of the specific mating system under consideration. ACKNOWLEDGMENTS This research was supported by funds from Biomedical Research Grant PHS RR07065 15 to The Rockefeller University, NIMH Postdoctoral Fellowship Grant HD06464 to A.M.D., and a Charles H. Revson Foundation Fellowship in Biomedical Research and Grant PCM8118522 from the National Science Foundation to J.C.W. We also thank C. Levesque for preparing the figures.
REFERENCES Darley, J. A. (1971). Sex ratio and mortality in the brown-headed cowbird. Auk 88, 560566. Darley, J. A. (1978). Pairing in captive brown-headed cowbirds (Molothrus arer). Canad. J. Zool. 56, 2249-2252. Darley, J. A. (1982). Territoriality and mating behavior of the male brown-headed cowbird. Condor 84, 15-21. Delville, Y., Sulon, J., Hendrick, J.-C., and Balthazart, J. (1984). Effect of the presence of females on the pituitary-testicular activity in male Japanese quail (Corurnix coturnix japonica).
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