Natural Food Abundance Research Articles

Sex Investment in a Social Insect: The Proximate Role of Food

In eusocial Hymenoptera, relatedness asymmetries lead to conflict between parents and offspring over sex investment; workers strive for a 3:1 female—biased ratio of sex allocation, and queens strive for a 1:1 ratio. Many studies support this genetic relatedness hypothesis, but the variation in allocation ratios in natural populations is great and remains mostly unexplained. In this paper, we examined whether food supply determines sex investment of the ant Formica podzolica. We compare reproductive parameters of colonies and populations across habitats, and compare sex allocation of fed and unfed colonies. Nest density, worker size, sex ratio, and sex allocation were all greater along forest edges than in meadows, and these patterns were associated with natural food abundance. Furthermore, there was a strong tendency for individual colonies to produce either all—male or all—female sexuals, and worker size was greatest in colonies producing all females and smallest in those producing all males. Most important, sex investment was greatly affected by a supplemented diet, as the population investment ratio, R (i.e., ratio of males to males plus females), was female biased (0.36) for fed colonies and male biased (0.62) for unfed, control colonies. Finally, investment ratios were more male biased in polygynous than in monogynous colonies as predicted by a genetic relatedness hypothesis. These results demonstrate that food supply has an important proximate influence on sex investment, and may explain much of the natural variation in sex investment in populations of eusocial Hymenoptera.

The Influence of Food Abundance on Territory Size in the Iguanid Lizard Sceloporus jarrovi

This study was designed to determine how food abundance affects territory size for the insectivorous lizard Sceloporus jarrovi. Observations and experiments were performed in the field in southeastern Arizona during 1971 and 1972 summers. Territories were defined as defended areas, and the convex polygon method was used to determine territorial boundaries. Preliminary studies showed that all adult and most juvenile lizards defended territories against lizards of the same species, size, and sex. Sex and body size influenced territory size. Within size classes, territories of ♂ ♂ averaged more than twice territory size of ♀ ♀. Lizards in the larger size class had territories more than twice those of smaller animals. Natural food abundance, determined with adhesive—coated Masonite® boards, was inversely correlated with territory size. Summer territories compressed significantly when food was added even through natural food supplies seemed adequate. Terrtories returned to initial sizes with the removal of additional food. Experiments involving the locations of concentrated food sources within the territory provided some evidence for the hypothesis that ease of food obtainment may be an important factor in determining summer territorial size and location.