Behavioral and ecological aspects of inbreeding in natural animal populations: Inferences from Umbonia treehoppers (Homoptera: Membracidae)

For a full century, evolutionary biologists have recognized the harmful effects of close inbreeding, that is, reproduction between related individuals. Substantial empirical evidence, principally from studies of livestock and captive populations, suggests that "inbreeding depression" may explain the general dearth of inbreeding in animals. Nevertheless, isolated cases of close and routine inbreeding in natural populations are known, and their existence begs explanation. Such cases also offer opportunities to explore potential selective factors in mating system evolution.;Field studies showed that the maximum percentage of copulations occurring between siblings of U. crassicornis is 12%. Outbreeding is promoted by (1) asynchrony in the sexual maturation of siblings, (2) dispersal of siblings from their natal site, and (3) the availability of non-kin mates in the comparatively high density population. In contrast, U. ataliba reproduces, perhaps exclusively, by sibling-mating. This is promoted by (1) synchronous maturation of siblings, (2) delayed dispersal of siblings, (3) positive kin assortative mating, and (4) few non-kin mating opportunities in the low density population. Experimental matings revealed that sibling-mated U. crassicornis females produced 32% fewer adult offspring, while U. ataliba females suffered no detectable inbreeding depression. This is consistent with predictions from population genetics that a history of inbreeding and selection purges deleterious alleles, and, hence, reduces inbreeding depression. A literature study of the biology of regular inbreeders revealed that close inbreeding is associated with high costs of outbreeding, resulting from a low density of mates or lost breeding opportunities.;This dissertation presents field studies, conducted in Monteverde and San Luis, Costa Rica, of two related insects, Umbonia crassicornis and U. ataliba (Homoptera: Membracidae). These species share similar life histories but differ in their tendency for sibling-mating. This work (1) documents mating system variation; (2) examines behavioral mechanisms promoting inbreeding and outbreeding; (3) investigates fitness consequences of inbreeding; and (4) identifies ecological selective factors that promote mating system variation in these insects.