Reproductive Interdependence of Pinon Jays and Pinon Pines

Abstract

Pinon Jays (Gymnorhinus cyanoephalus) and pinon pine trees (Pinus edulis) interact in a mutualistic fashion, in that the jays provide a primary means of seed dissemination for pinon trees in the Southwest, while at irregular intervals the trees provide the jays with an abundant and highly nutritious source of food. Availability of pinon seeds permits both late winter (February) and late summer (August) breeding by the jays. Seeds and photoperiod interact synergistically to accelerate gonadal development in late winter. When seeds are abundant, considerable testis growth takes place in some Pinon Jay ♂ ♂ even before the winter solstice. This relationship was investigated experimentally. Use of a specific food (pinon seeds) as a proximate timer for breeding, the ability of individual jays to breed in spring, molt, and breed again in August without an intervening period of gonadal refractoriness, and testicular development in early December, when day length approaches its minimum, are extraordinary characteristics among north—temperate—zone passerine birds. The synchrony of seed production by pinon pines over large geographic areas is interpreted as an evolved mechanism that (1) overwhelms invertebrate seed and cone predators, and (2) increases the numbers of seeds per tree that will be cached by Pinon Jays and other vertebrates. Caching sites of Pinon Jays are often at locations especially conducive to germination and growth of pinon pine seedlings.