have reported a preference for heavy women with high waist-to-hip ratios (WHR) in a culturally isolated population in southeast Peru. Their findings are interesting because a preference for low WHR is widespread in westernized populations 2-5 . However, we disagree with their argument that cultural invariance is necessary for an adaptionist interpretation of WHR preference.WHR and waist circumference are positively correlated with testosterone and negatively associated with oestrogen 6 . Women with low WHR have better health and fertility than women with high WHR 5 . However, women in England and Texas with high scientific correspondence 214 NATURE | VOL 399 | 20 MAY 1999 | www.nature.com

While most spiders are solitary and opportunistically cannibalistic, a variety of social organisations has evolved in a minority of spider species. One form of social organisation is subsociality, in which siblings remain together with their parent for some period of time but disperse prior to independent reproduction. We review the literature on subsocial and maternal behaviour in spiders to highlight areas in which subsocial spiders have informed our understanding of social evolution and to identify promising areas of future research. We show that subsocial behaviour has evolved independently at least 18 times in spiders, across a wide phylogenetic distribution. Subsocial behaviour is diverse in terms of the form of care provided by the mother, the duration of care and sibling association, the degree of interaction and cooperation among siblings, and the use of vibratory and chemical communication. Subsocial spiders are useful model organisms to study various topics in ecology, such as kin recognition and the evolution of cheating and its impact on societies. Further, why social behaviour evolved in some lineages and not others is currently a topic of debate in behavioural ecology, and we argue that spiders offer an opportunity to untangle the ecological causes of parental care, which forms the basis of many other animal societies.

Colonial web-building spiders respond to trade-offs between selective forces relative to spatial position within colonies and thus provide support for the selfish herd theory. The size distribution of spiders within colonies of Metepeira incrassata, a colonial orb-weaver (Araneae: Araneidae) from tropical Mexico is nonrandom; larger (mature) spiders and females guarding eggsacs are more prevalent in the center, whereas more small (immature) spiders are found on the periphery. Experimental field studies with spiders of selected size classes show that larger spiders actively and aggressively seek protected positions in the center of the colony webbing, even though prey availability and capture rates are significantly higher on the periphery. Attacks by predatory wasps, other spiders, and hummingbirds are more frequent on the periphery than in the core of the colony. Reproductive females on the periphery are at greater risk because they are captured more often than smaller spiders, and if their egg sacs consequently remain unguarded, chances of cocoon parasitism are increased. As a result, spiders in the core of the colony have greater reproductive success, producing more egg sacs with greater hatching frequency. Colonial spiders thus appear to be making a trade-off between foraging and protection from predation and show a spatial organization predicted by the selfish herd theory. The influence of such trade-offs on individual fitness and the structure of colonies is discussed.

Populations of the highly social Gunnison's prairie dog (Cynomys gunnisoni) were studied at two sites in south central Colorado. The sites differed in availability of water, duration of the growing season, and the diversity and quantity of edible vegetation. At the lusher site, Quartz Creek, all age- and sex classes weighed significantly more than their counterparts at the more barren site, Blue Mesa. Growth rates and overwinter survival of Quartz Creek young were higher than at Blue Mesa. Two of 5 yearling females raised litters at Quartz Creek, whereas none of 15 yearling females weaned litters at Blue Mesa. Quartz Creek females produced larger litters [Formula: see text] than Blue Mesa females [Formula: see text]. At Quartz Creek, 6 of 17 yearlings dispersed from their natal harem, but none of the 31 yearlings at Blue Mesa dispersed. Thus, in a favorable habitat, Gunnison's prairie dogs were heavier, reached sexual maturity more rapidly, and dispersed at a younger age than in a less favorable habitat. These results provide intraspecific support for Armitage's hypothesis that delayed sexual maturity and dispersal in the large-bodied, social ground squirrels is associated with the age at which immatures attain adult weight.

No abstract