Risk‐averse inflorescence departure in hummingbirds and bumble bees: could plants benefit from variable nectar volumes?

Abstract

Most hermaphroditic, many‐flowered plants should suffer reduced fitness from within‐plant selfing (geitonogamy). Large inflorescences are most attractive to pollinators, but also promote many flower visits during a single plant visit, which may increase selfing and decrease pollen export. A plant might avoid the negative consequences of attractiveness through modification of the floral display to promote fewer flower visits, while retaining attractiveness. This report shows that increasing only the variance in nectar volume per flower results in fewer flower visits per inflorescence by wild hummingbirds (Selasphorus rufus) and captive bumble bees (Bombus flavifrons) foraging on artificial inflorescences. Inflorescences were either constant (all flowers contained the same nectar volume) or variable (half the flowers were empty, the other half contained twice as much nectar as in the constant flowers). Both types of inflorescence were simultaneously available to foragers. Risk‐averse foraging behaviour was expressed as a patch departure preference: birds and bees visited fewer flowers on variable inflorescences, and this preference was expressed when resource variability could be determined only by concurrent sampling. When variance treatments were clearly labelled with colour and offered to hummingbirds, the departure effect was maintained; however, when preference was measured by inflorescence choice, birds did not consistently prefer to visit constant inflorescences. The reduced visitation lengths on variable inflorescences by both birds and bees documented in this study imply that variance in nectar production rates within inflorescences may represent an adaptive trait to avoid the costs of geitonogamy.

Feeding behavior of hummingbirds and perching birds on Erythrina speciosa Andrews (Fabaceae) flowers in an urban area, Londrina, Paraná, Brazil

“…The way in which foragers respond to variability in amount of reward has been addressed by theories on risk-sensitive foraging. Most experimental studies holding mean reward constant have shown that foragers prefer less variable rewards (SHAFIR 2000, BATESON 2002, BIERNASKIE et al 2002. Our data do not allow us to test this idea, but in E. speciosa, early morning visitors are likely to be more profitable in foraging (and encouraged to forage) due to the greater nectar availability and lower variability among flowers.…”

Influência de abelhas africanizadas na concentração de açúcares no néctar de soja (Glycine max L. Merrill) var. Coodetec 207

“…O néctar é considerado a principal recompensa para o polinizador (DELAPLANE;MAYER, 2000) e sua Acta Scientiarum. Animal Sciences Maringá, v. 32, n. 2, p. [189][190][191][192][193][194][195]2010 concentração de açúcares está associada a diferentes tipos de polinizadores, enquanto que a frequência e duração de visitas dependem da taxa de produção de néctar (BIERNASKIE et al, 2002;SHAFIR et al, 2003;NICOLSON;NEPI, 2005). Pierre et al (1999), estudando Brassica napus e Toledo et al (2005), estudando siratro (Macroptilium atropurpureum), observaram que a concentração de açúcares totais apresenta variação ao longo do dia, o que pode estar relacionado ao número de insetos visitantes, especialmente, abelhas que podem coletar pólen e néctar.…”

Hummingbird responses to gender-biased nectar production: are nectar biases maintained by natural or sexual selection?

“…Even so, the adaptive benefits of differential rewards, including those associated with floral gender, have rarely been tested or demonstrated (Biernaskie et al 2002). Carlson & Harms (2006) outlined two non-mutually exclusive mechanistic explanations for gender-biased nectar production: the sexual selection and geitonogamy avoidance hypotheses.…”