Local differences in parasitism and competition shape defensive investment in a polymorphic eusocial bee

Abstract

Many colonial animals rely for their defense on a soldier caste. Adaptive colony demography theory predicts that colonies should flexibly adjust the investment in different worker castes depending on the colony needs. For example, colonies should invest more in defensive workers (e.g., soldiers) in dangerous environments. However, evidence for this prediction has been mixed. We combined descriptive and experimental approaches to examine whether defensive investment and worker size are adjusted to local ecology in the only known bee with polymorphic workers, Tetragonisca angustula. Colonies of this species are defended by a morphologically specialized soldier caste. Our study included three populations that differed in the density of food competition and the occurrence of a parasitic robber bee. We found that colonies coexisting with robber bees had on average 43% more soldiers defending the nest entrance, while colonies facing stronger foraging competition had soldiers that were -6-7% smaller. We then experimentally relocated colonies to areas with different levels of competition. When released from intense food competition, body sizes of guards and foragers increased. After introducing chemical robber bee cues at nest entrances, we found both a short-term and a long-term up-regulation of the number of soldiers defending the colony. Active soldier numbers remained high after the experiment for a duration equivalent to 2-3 worker life spans. How information about past parasite threat is stored in the colony is currently unknown. In summary, T. angustula adjusts both the number and the body size of active soldiers to local ecological conditions. Competitor density also affects forager (or minor) size, an important colony trait with potential community ecological consequences. Our study supports adaptive colony demography theory in a eusocial bee and highlights the importance of colony threats and competition as selective forces shaping colony phenotype.

Repeated evolution of soldier sub-castes suggests parasitism drives social complexity in stingless bees

“…Lestrimelitta bees pose a threat to many Neotropical species as robbers of resources 29 and are suspected to be the driving force behind the evolution of soldiers in T. angustula 9,16 . For example, T. angustula soldiers aggressively attack objects that chemically or visually resemble robber bees 35 and colonies have more soldiers in areas where Lestrimelitta attacks are more common 16 . Ten of the 28 studied species are known victims of Lestrimelitta, whose attacks frequently destroy colonies 29 .…”

“…In stingless bees, the emergence of dangerous robbers and an environment that favored the storage of valuable resources might represent two important incentives for having larger and, therefore, stronger entrance guards. Previous studies have shown that a more dangerous environment can lead to increased investment in colony defence in species that already have soldiers 16,40 . Our study extends this theme and identifies a specific threat ( Lestrimelitta robber bees) as a possible selective force for the evolution of new sub-castes and increased social complexity in Neotropical stingless bees.…”

“…All colonies foraged on natural food sources. We estimated relative colony size by measuring colony traffic at the entrance 16 . We counted bees entering a colony for 1–2 min on a day with good foraging conditions, once in the morning and once in the afternoon (between 10 a.m.–12 noon and 1 p.m.–3 p.m.).…”

“…However, the recent discovery of soldiers in the Neotropical stingless bee Tetragonisca angustula 9 suggests that more complex caste systems might exist in this relatively understudied tribe. In T. angustula , colonies are defended by a small 16 but dedicated group 17 of entrance guards that are both larger (~ 30%) and of different shape than their nestmates 9 . Having larger soldiers is beneficial for colonies because body size is directly linked to the fighting ability of T. angustula guards 9 .…”

Mecanismos e estratégias de defesa em Tetragonisca angustula (Hymenoptera: Apidae)

“…Uma reserva de soldados poderia levar a um sistema social resistente, que deveria evoluir quando o ambiente biótico contém ameaças potencialmente catastróficas ). Segers et al (2016), demonstraram que colônias que não estão expostas a ameaças podem apresentar menor quantidade de guardas em sua entrada. Característica que corrobora com a teoria de eficiência ergonômica (Oster & Wilson, 1978).…”