Behavior Non-putative Offspring

457

Symposium: Non-putative Offspring in Monogamy Conveners: Jiro Kikkawa and Terry A. Burke

Maintenance of Social Monogamy Despite Complete Cuckoldry in Fairy-Wrens Andrew Cockburn, Peter O. Dunn and Raoul A. Mulder Evol. Ecol. Group, Div. Bot. and Zool., Australian National Univ., Canberra, ACT 2601, Australia Extra-pair paternity is one of the most important sources of variation in avian mating systems. The highest known incidence of extra-pair paternity (78 %) occurs in the Superb Fairy-wren, Malurus cyaneus (1). Females in this species usually live with more than one male, and all those males cooperate in the rearing of offspring. However, most fertilizations are gained by males from outside the group which cooperate to rear young. We argue that this system has evolved and remained stable because of three sequential events. First, male exploitation of the presence of helpers to increase their courtship of extra-group females. Dominant males punish helpers who do not contribute care (2), and reduce their own rate of provisioning when they have helpers. Second, female exploitation of the alternative source of paternal care provided by helpers, which allows females unrestricted mate choice. Females without helpers allow their partners some paternity, presumably to persuade him to provide more care for her offspring, but routinely cuckold him when they have helpers (1). The ability to choose among extragroup males leads to the production of sons that are themselves successful in obtaining extragroup fertilizations. Third, because females exhibit consistent and near-unanimous preference for particular male genotypes (1), they impose such strong sexual selection that only males that devote most of their time to extra-group courtship are likely to leave many descendants. This need to display to extra-group females and female control of fertilization means that mate guarding and repeated copulation are unlikely to be effective deterrents to cuckoldry. We argue that one of the most important axis affecting mating systems is the trade-off between a female's need to choose male genotypes and her need for parental care. Current empirical and theoretical effort is devoted to establishing factors (e. g. parasites) which favor mate choice. We propose an akernative approach which suggests that the benefits of mate choice to females may be universal, but the need for parental care may be variable This allows a number of mating systems usually treated separately (display-site defense promiscuity, polyandry, monogamy) to be viewed in a common framework. For example, display site defense promiscuity arises in species which eat superabundant but low quality food (3) or which have extremely low growth rates (4).

Literature: 1. MULDeR, R. A. et al.: Proc. R. Soc. Lond. B: (in press). 2. MULDEt~,R. A. & LANGMORE,N. E.: Anita. Behav. 40: 830--833.3. BE~LEa, B. & PRUETT-JoNES,S. G.: Behav. Ecol. Sociobiol. 13: 229--238.4. LILL,A.: Aust. J. Zool. 34: 351-371.

458

Journal fiir Ornithologie 135, 1994

Parental Care by Males and Paternity Certainty Among Eastern Bluebirds Patricia Adair Gowaty Inst. Ecol., Univ. of Georgia, Athens, Georgia, 30602-2602, U. S. A. Among Eastern Bluebirds Sialia sialis breeding in South Carolina, USA, males contribute half of the feedings that nestlings receive. However, variance in provisioning rates by males usually is greater than by females, so it is not surprising that male parental care is not essential to female reproductive success in this species (1, 2). Extra-pair paternity also varies, with about 20 % of offspring from extra-pair matings (3), many of which may be solicited by females (4) paired on territories with younger males (4). Based on inferences about parentage from protein polymorphisms and observations of provisioning to nestlings, males adjust their feeding rates as a function of their probability of paternity (Fig. 1). Along with these data additional data has been gathered based on parentage inferences from single-locus DNA polymorphisms, which partially corroborate earlier findings. These findings beg the question, what cues do males use to adjust their provisioning rates? A final study has been carried out that demonstrates that males do not discriminate between their own versus others' nestlings during provisioning, suggesting that males may use cues based on variation in the behavior of females when females are fertil~ Literature: 1. GOWATY,P. A. (1983): Am. Nat. 121: 149--157. 2. GOWATY,P. A., ROBERTSON, R., G. BALL, & A. DUFTY. unpublished. 3. GOWATY,E A. & W. C. Bt{iI3oEs (1991): Anita. Behav. 41: 661--675.4. GOWAT%P. A. & W. C. BRIDGES(1991): Behav. Ecol. 2: 339--350.

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Male Feeding Rates in Relation to Mean Fig. 1. The distribution of males' provisioning trips to nests as a function of the number of nondirectly descendant nestlings in a nest. Fathers that feed the most have the lowest likelihood of having young from extra,pair paternity in their nests; fathers that feed the least have the highest probability of having young from extra-pair paternity in their nests.

Behavior Non-putative Offspring

459

How Do They Keep It? -- Monogamy in Zosterops lateralis cMorocephala Bruce C. Robertson and Jiro Kikkawa Dept. Zool., the Univ. of Queensland, Brisbane, Australia 4072. Silver-eyes are restricted to the coral cays of the Capricorn-Bunker Group in the southern Great Barrier Reef, where they breed asynchronously in high density during a protracted breeding season (6 months), producing up to five clutches (1). Despite the population possessing the potential for non-putative offspring, no such offspring or extrapair copulations (EPC) have been observed during 25 years of breeding season observations. Furthermore, DNA fingerprinting using two minisatellite probes (19.6 & 18.15) has revealed no illegitimate offspring among 139 nestlings from 63 clutches and 52 families. We argue that the lack of illegitimate offspring is a consequence of a strong lifelong pair bond. This pair bond is manifested in several ways. First, pair association in movement both in summer and in winter. This association was not a result of mate guarding by the male as male Silver-eyes initiated slightly more site changes than females (Table 1). Male Silver-eyes were not actively guarding their mates even during the female's fertile period. No sexual difference in site change initiation was observed between breeding and non-breeding pairs (Table 1), further indicating that this association is not guarding against EPC. Second, following separation females initiated pair reunion as often as males, irrespective of season (Table 1); hence both sexes are actively maintaining pair contact. Third, Silver-eyes spend considerable time throughout the year on allopreening and huddling (1) which may serve to strengthen the pair bond. We suggest this strong pair bond has evolved along with the interdependency of the pair for maintenance of a breeding territory and successful reproduction. On Heron Island the breeding density of Capricorn Silver-eyes is as high as 6.4 to 11 pairs per ha and dispersion of territories and reproductive success are positively correlated with the abundance of fig trees in the nesting territory (2, 3). Approximately 85 % of pairs occupied territories during the breeding season (2); hence territory defense should be of utmost importance to successful breeding. Both sexes contribute to territory defense and remain on or near the breeding territory over winter. The Silver-eye population on Heron Island provides an example of a "perfect" monogamy, permitting analysis of its causes and consequences. Literature: 1. KIKKAWA,J.: In: ITO, B~OWN& KIK~AWA:Animal Societies, 253--266. 2. CATTSRALL,C. P. et al.: Ibis 124: 405-421.3. KIKKAWA,J. & WILSON,J. M.: Emu 83: 181--198. Table 1. Initiation of site changes and pair reunion (after separation): mean of 30 minute observations for 8 breeding and 12 postbreeding pairs. Site change initiated by

Male

Female

Summer Winter Reunion initiated by

1.94 1.31 1.68 O.59

1.69 NS 1.26 NS 1.33 NS O.65 NS