Evidence for alloethism in stingless bees (Meliponinae)1

Many ant species and some social bees display size-related polyethism, often referred to as alloethism (Wilson, 1971). For example bumblebees (Bombus spp.) exhibit alloethism; there is great size variation within the worker caste, and large bees tend to be foragers, while smaller bees tend to perform within-nest tasks (reviewed in Goulson, 2003). In contrast, honeybees workers are approximately uniform in size, and they exhibit age-related polyethism; young workers remain in the nest, older workers forage. Here we assess whether three stingless bee species exhibit alloethism, something which to our knowledge has never been examined. The size of foragers and nest bees was compared for Melipona beecheii Bennett (4 nests), Scaptotrigona mexicana (Guérin-Méneville) (3 nests) and Tetragonisca angustula Illiger (2 nests). All nests were kept in private apiaries near Tapachula, Chiapas, Mexico. Approximately 30 bees were netted as they returned to the nest. Henceforth these bees were termed “foragers” for brevity, but some may have been returning from trips to dump waste products from the nest. Once a sample of foragers had been taken, the nest was opened and an approximately similar number of workers taken at random from within the nest using a aspirator (this sample is likely to include some foragers that happened to be within the nest at the time). Bees were stored in alcohol and thorax widths subsequently measured under a dissecting microscope with an eyepiece graticule. Data were analyzed separately for each species using twoway Anova, with nest and forager/nest bee as explanatory factors For all three bee species, average sizes differed greatly between nests (F3,209 = 21.5, P < 0.001 for M. beechii; F2,164 = 50.5, P < 0.001 for S. mexicana; F1,109 = 9.81, P = 0.002 for M. augustula). For M. beechii, the size of foragers and nest bees did not differ (Tab. I). However, for S. mexicana and T. angustula, foragers were significantly larger than nest bees (Tab. I). This suggests the possibility that these bee species exhibit alloethism. Alternatively, selective mortality of small foragers may result in the survivors being on average larger than nest bees. Another possibility is that these bee species exhibit agerelated polyethism, as some stingless bees are known to do (Sommeijer, 1982), and that food availability to the colonies had recently declined (assuming, as seems plausible, that worker size declines when food is scarce). This could result in older bees being larger. Unfortunately we have no means of determining age. Wing wear is often used as an indicator of age, but it accrues at higher rates in foragers than nest bees. Thus we would expect foragers to have more wing wear than nest bees regardless of their age. More detailed studies using individual marking of bees as soon as they eclose