Reply to Nylin, Wickman & Wiklund regarding sex ratios of California overwintering monarch butterflies

In an earlier work (Frey & Leong 1993), we (1) determined that roosting height of central coastal California overwintering monarch butterflies, Danaus plexippus (L), was independent of sex, (2) determined that males and females were equally ‘catchable’, thereby eliminating sampling bias as the cause of male-biased sex ratios, and (3) eliminated, on logical grounds, three other widely held hypotheses regarding male-biased sex ratios often reported in Lepidoptera populations (Brussard & Ehrlich 1970). We suggested that male-dominated overwintering populations of western North American monarchs may be attributed to a disproportionate mortality factor between the sexes prior to and during fall migration. Nylin et al. (1995), drawing from evolutionary stable strategy models on adaptive variation in timing of life-history stages (e.g. Bulmer 1983; Iwasa et al. 1983; Parker & Courtney 1983) and their own work on Pierid butterflies (Wiklund et al. 1992), present an alternative explanation for male-biased ratios among overwintering California monarch butterfly populations. They clearly state that their hypothesis represents a different level of analysis and is therefore not necessarily mutually exclusive of ours (see Holekamp & Sherman 1989 regarding levels of analysis). Our reply here is to (1) clarify several points regarding monarch demographics and clustering phenology and (2) comment on their alternate hypothesis. Sakai (1991) did an extensive monarch mark– release–recapture census and found male-biased ratios throughout the range of California overwintering sites. His study involved 47 overwintering sites between San Diego and Marin County and 65 585 individuals. During the 1991–1992, 1992–1993 and 1993–1994 seasons at the North Beach campground site, Pismo Beach, California, we recorded male-biased mark–release–recapture sex ratios throughout each overwintering season, suggesting that male-biased ratios were both spatially and temporally pervasive (Leong & Frey 1992; unpublished data). Nylin et al. (1995) propose that in late summer and early autumn monarch populations, males should have ‘a greater propensity to enter diapause development, and accordingly be overrepresented among overwintering individuals’. Their evolutionary stable strategy-based model further implies that males ought to initiate migration to overwintering sites earlier than females. We found neither a pattern of greater male bias during the early phase (autumn) of colony increase nor a decline in male relative abundance at peak population levels, which normally occur in late December (Table I). James (1984) reported similar data for two overwintering seasons at a site in Camden, New South Wales, Australia. Selective advantage of early autumn arrival by males (relative to females) at overwintering sites is questionable because the critical aspects of evolutionary stable strategy life-history theory is focused on the timing of diapause emergence prior to and/or during the reproductive phase (February to early March in the case of D. plexippus (L) at central coastal California overwintering sites). The model of Nylin et al. (1995) also assumes that the earlier that males enter diapause, the sooner they will become sexually mature in the spring to maximize the number of matings. Their model does not, however, account for the state of reproductive diapause nor, once diapause is broken, the rate at which reproductive development occurs between the sexes. We have dissected field-collected monarchs (N=20 males and 20 females) twice monthly, between 18 October 1990 and 22 March 1991 (K. L. H. Leong, unpublished data). The microscopic examination of female reproductive tracts for egg development revealed that they remained undeveloped until the second week of January, when 20% of the females showed signs of ovarian development (i.e. increase in egg