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per individual (Kroodsma 1974). The wrentit has a relatively simple song, which shows little withinindividual variability (Boarman 1985). The energy spectra of the two species' songs do partially overlap, however; the modal frequency of the wrentit's is considerably lower than that of the Bewick's wren (Boatman 1985). Hence, some acoustical interference may occur when the two songs overlap, but the extent of this is limited (but see Bremond 1977). The wrentit, having a single stereotyped song, may be affected more by this interference. The Bewick's wren may avoid interference with greater variability in its song; fewer portions of its song overlap spectrally with the wrentit's song than vice versa. Results from further analyses of an experimental nature would be desirable (i.e. playbacks, removals, etc.). We thank S. I. Rothstein for permission to use Hughell's data and J. P. Myers for suggesting cross-correlation. S. I. Rothstein, R. Payne, L. Baptista, E. Morton and an anonymous referee commented on previous drafts.

dispersal in the Bewick's wren. Z. Tierpsychol., 35, 352-380. Marler, P. 1960. Bird songs and mate selection. In: Animal Sounds and Communication (Ed. by W. E. Lanyon & W. N. Tavolga), pp. 348-367. Washington, D.C.: American Institute of BiologicalScience. Miller, E. H. 1982. Character and variance shift in acousticsignalsof birds. In: Acoustic"Communication in Birds (Ed. by D. E. Kroodsma & E. H. Miller), pp. 253 295. New York: Academic Press. Wiley, R. H. & Richards, D. G. 1982. Adaptations for acoustic communication in birds: sound transmission and signal detection. In: Acoustic Communication in Birds (Ed. by D. E. Kroodsma & E. H. Miller), pp. 131-181. New York: Academic Press.

(Received 29 March 1984; revised 12 October 1984; MS. number As-278)

Dispersal and Inbreeding Avoidance

In a recent highly selective review, Moore & Ali (1984) argue that dispersal patterns in vertebrates have evolved independently of the consequences of ROBERT C. FLEISCHER* inbreeding avoidance, but they virtually ignore a WILLIAMI. BOARMAN~'w considerable body of evidence to the contrary. MARTIN L. CODY~ They selectively cite my paper on olive baboons *Department of Biological Sciences, (Papio anubis) to give the impression that I consiUniversity o f California, dered inbreeding avoidance to be the only cause of Santa Barbara, CA 93106, U.S.A. dispersal, when in fact I made detailed comparisons t Department of Biological Sciences, of natal dispersal and breeding dispersal by males, San Francisco State University, and I emphasized that considerable dispersal San Francisco, CA 94132, U.S.A. occurred that could not have been the result of ~Department o f Biology, inbreeding avoidance (Packer 1979, pages 10-13). University o f California, For example, a large proportion of males transfer Los Angeles, CA 90024, U.S.A. several times during their lives. Secondary movements by males do not coincide with their w address: Department of Biological daughters reaching sexual maturity, but instead Sciences, Rutgers University, Piscataway, NJ consist of migration into troops with greater 08854, U.S.A. numbers of oestrous females. However, I specifically tested the possibility that male natal dispersal was a mechanism for avoiding close inbreeding, and my results consistently supReferences Boarman, W. I. 1985. Habitat acoustics and the vocal ported that hypothesis. Moore & Ali seriously structure of a Califorinia songbird community. M.A. misrepresent my work and cursorily dismiss the work of others who have made similar findings. thesis, San Francisco State University. Bremond, J. C. 1977. Acoustic competition between the Since space does not allow a comprehensive reply song of the wren (Troglodytes troglodytes) and the (instead, see Pusey & Packer, in press), I will only songs of other species. Behaviour, 65, 89-98. address the following six points concerning nay own Cody, M. L. & Brown, J. H. 1969. Song asynchrony in work. neighbouring bird species. Nature, Lond., 222, First, the only quantitative data presented in 778-780. their paper concern the infant mortality in A troop Ficken, R. W., Ficken, M. S. & Hailman, J. P. 1974. Temporal pattern shifts to avoid acoustic interference prior to the migration of a male to a branch of his natal troop (see Packer 1979 for details). They in singingbirds. Science, N.Y., 183, 76~763. Gochfeld, M. 1978. Intraspecific social stimulation and suggest that infant mortality in that troop was temporal displacement of songs of the lesser skylark, already high prior to the male's achieving high Alauda gulgula. Z. Tierpsyehol., 48, 337-344. mating success, and thus that the mortality that I Kroodsma, D. E. 1974. Song learning, dialects and had attributed to inbreeding depression may have

Short Communications been too high. Moore & Ali analysed data over a period during which no information on possible paternity was available; and over the same period there were several natal males of breeding age in A troop. Therefore, the higher infant mortality in A troop from 1970 to 1973 may also have been due to inbreeding depression. Whether or not this is the case, my original analysis showed that there was no higher mortality in the presumed offspring of males that had immigrated into A troop from elsewhere, than in the offspring of immigrants in the other two troops over the period 1972 1976. There is no reason why infants of low viability should have been restricted to one male unless there was some deleterious factor associated with that male. Furthermore, Moore & All concede that the infant mortality in A troop increased after the immigration of the male in question. Even accepting that A troop had inherently higher mortality than any of the other troops, this increase in infant mortality would still be high enough to predict male emigration. According to Bengtsson's (1978) model, dispersal to avoid close inbreeding should occur when i> m/(l +f), where i is the proportion of offspring that fail to reproduce as a result of inbreeding depression, f is the mean degree of relatedness between the dispersing male and all the females in his troop (estimated to be about 0.10 in the Gombe baboons) and m is the proportion of males that die during dispersal (estimated to be less than 0' 17). My original estimate of i was 0-40, and from Moore & Ali's data i=0.25. In both cases i>0-17/(1 +0.1)=0.16, and my original conclusions remain unaffected. Second, they suggest that males are able to recognize close genetic relatives and that, in general, dispersal should not be necessary for close inbreeding to be avoided, since males could do so while remaining within their natal group. This suggestion is not tenable for two reasons. (1) Kin recognition is not universal (Holmes 1984), and the direct recognition of paternal kin has not yet been convincingly demonstrated in a complex social group of primates (see Berenstain et al. 1981; Stern & Smith 1984), nor has paternal kinship been shown to influence mating preferences (Smith 1982). (2) Baboons are highly polygynous, and a male's reproductive success is likely to depend on the number of receptive females with which he can mate. With perfect information of kinship, a male would be expected to mate only with those females to which he was sufficiently distantly related. However, a male's natal troop contains a much lower proportion of acceptable mates than does an adjacent troop. The natal troop not only contains a male's close matrilineal relatives, but also a cohort

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of patrilineal sisters (see Altman 1979; Packer 1979). By emigrating to an adjacent troop, a male would gain access to a much greater proportion of potential mates, and such dispersal would still be due to the avoidance of close inbreeding: Third, Moore & Ali cite a number of cases where a male remains in his natal troop but they lose sight of how exceptional this is in some areas. Of the 43 oldest males born in the Gombe troops whose fates could be followed, 42 are now known to have emigrated prior to breeding (including the male discussed in the first point above), and only one became a breeding male without having first transferred (Packer et al., in preparation). However, this one male did not gain breeding status until after his natal troop had split in two, and his close matrilineal relatives were in the other troop. The exceptional nature of this one case further emphasizes the overwhelming regularity of male emigration in the olive baboons at Gombe, a regularity that is remarkable in any biological system. Obviously there may be individual differences in the costs and benefits of dispersing, and so exceptional cases of males remaining in their natal troop are not surprising. In some cases the costs of dispersal to particular individuals may be especially high (e.g. a male that enjoys high dominance rank owing to the aid of his female relatives, Chapais 1983); in others the costs of breeding in the natal group may be low (e.g. after a troop has split along kin lines, as above). By focusing on such exceptions without reference to the vast majority of individuals that do disperse, however Moore & Ali fail to address adequately the question that prompted the investigations in the first place: why do virtually all members of one sex disperse in so many different species? Fourth, implicit in their arguments that dispersal is due to intra-sexual competition is the idea that a young male would receive considerable harassment if he remained in his natal group. Males are indeed forcibly evicted from their natal troop in some primate species and remain outside bisexual troops until they can successfully take over a new troop. However, in a number of species males transfer directly from their natal troop to another in spite of receiving considerably more aggression from members of the new troop than from members of their natal troop (olive baboons: Packer 1979; vervet monkeys, Cercopithecus aethiops: Henzi & Lucas 1980; Cheney 1983; toque macaques, Macaca sinica: Dittus 1977). The intra-sexual competition hypothesis would predict male retention rather than dispersal in these species. On a related point, an important feature of male migration in several species is that males almost never return to their natal troop even though they

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may transfer numerous times (Packer t979). No mate has yet returned to breed in his natat troop in either the Gombe baboons or in toque macques (Dittus 1979), and such returns are exceedingly rare both in Japanese macaques (Macaca fuscata, Sugiyama 1976) and rhesus macaques (M. mulatta, Drickamer & Vessey 1973). Intra-sexual competition alone cannot explain this pattern. Fifth, Moore & Ali state that lower reproductive activity in the natal troop could be due to age. I specifically dealt with this question, and my Fig. 8 shows that immigrant males of the same age as natal males engaged in consistently higher levels of consorting activity. Consorting activity is probably the most reliable measure of reproductive success in these animals since a male may thereby monopolize a female over her entire oestrous period (Packer 1979). Figure 8 actually overestimates the breeding success of natal males, since they preferred consorting with adolescent females, an age class that was infertile and rarely consorted by immigrant males (my Table XIII). Natal males appeared to prefer consorting with adolescent females because there was so little competition for access to them. Sixth, implicit in many of their arguments is the concept that, as Bengtsson (1978) and I both emphasized, dispersal is only expected to occur as an inbreeding avoidance mechanism when the costs of dispersal are low compared to the costs of close inbreeding. However, Moore & Ali dismiss the possibility that costs of close inbreeding could ever be high enough to lead to dispersal. This assertion is unfounded in the absence of any quantitative data other than my own on the relative costs of dispersal and close inbreeding; and, as shown above, my data suggest that costs of dispersal can indeed be sufficiently low. While it may be a mistake for authors to assume that this will always be the case, it is at least as mistaken to assume the opposite. CRAIG PACKER

Department of Ecology and Behavioral Biology, University o f Minnesota, Minneapolis, M N 55455, U.S.A.

References Altmann, J. 1979. Age cohorts as paternal sibships. Behav. Ecol. Sociobiol., 6, 161-164. Bengtsson, B. O. 1978. Avoidinginbreeding: at what cost? J. theor. Biol., 73, 439444. Berenstain, L., Rodman, P. S. & Smith, D. G. 1981. Social relations between fathers and offspring in a captive group of rhesus monkeys (Macaca mulatta). Anim. Behav., 29, 1057-1063. Chapais, B. 1983. Matriline membership and male rhesus reaching high ranks in natal troops. In: Primate Social

Relationships (Ed. by R. A. Hinde), pp. 171-175. Sunderland, Massachusetts: Sinauer. Cheney, D. L. 1983. Proximate and ultimate factors related to the distribution of mate migration. In: Primate Social Relationships (Ed. by R. A. Hinde), pp. 241-249. Sunderland, Massachusetts: Sinauer. Dittus, W. P. J. 1977. The social regulation of population density and age-sex distribution in the toque monkey. Behaviour, 63, 281-322. Dittus, W. P. J. 1979. The evolution of behaviors regulating density and age-specific sex ratios in a primate population. Behaviour, 69, 265-301. Drickamer, L. C. & Vessey, S. H. 1973. Group changing in free-living male rhesus monkeys. Primates, 14, 359-368. Henzi, S. P. & Lucas, J. W 1980. Observations on the intertroop movement of adult vervet monkeys (Cercopithecus aethiops). Folia primat., 33, 220-235. Holmes, W. G. 1984. Sibling recognition in thirteen-lined ground squirrels: effects of genetic relatedness, rearing association, and olfaction. Behav. Ecol. Sociobiol., 14, 225-233. Moore, J. & Ali, R. 1984. Are dispersal and inbreeding avoidance related? Anim. Behav., 32, 94-112. Packer, C. 1979. Inter-troop transfer and inbreeding avoidance in Papio anubis. Anim. Behav., 27, 1 36. Pusey, A. E. & Packer, C. In press. Dispersal and philopatry in primates. In: Primate Societies (Ed. by B. B. Smuts, D. L. Cheney, R. M. Seyfarth, T. T. Struhsaker & R. W. Wrangham). Chicago: University of Chicago Press. Smith, D. G. 1982. Inbreeding in three captive groups of rhesus monkeys. Am. J. phys. Anthropol., 58, 447-451. Stern, B. R. & Smith, D. G. 1984. Sexual behaviour and paternity in three captive groups of rhesus monkeys (Macaca mulatta). Anita. Behav., 32, 23-32. Sugiyama, Y. 1976. Life history of male Japanese monkeys. Adv. Study Behav., 7, 255-284. (Received 9 May 1984; revised 11 October 1984; MS. number: ,~s-283)

Fading Short-term Memory for Patch Quality in Sticklebacks An efficient predator, feeding in a patch, should continuously decide whether to stay or leave, the decision being largely affected by its present feeding success compared with that expected in alternative patches. Where food items are usually scattered the predator should monitor its success rate over a certain span of time that is long enough to make up for the scatter but short enough to avoid suffering from depletion. To solve this problem, two mechanisms have been proposed, i.e. the 'memory window', which produces a sliding average over some last units of time (Krebs & Cowie 1976), and the 'memory factor' by which former payoffs are decremented by a constant fraction of their remaining value at regular intervals (e.g. Ollason 1980; Harley 1981). Both mechanisms produce a decay with time in the weight given to payoffs in the