Regulation of life history determines lifespan of worker honey bees (Apis mellifera L.)
Introduction
The evolutionary theory of aging predicts that a high extrinsic mortality rate in age-structured populations promotes rapid organismal aging and thus increases the intrinsic mortality. This argument is based on the notion that the force of natural selection opposing aging and senescence is directly related to the survivorship to any given age (Rose, 1991, Kirkwood and Austad, 2000). Important empirical support comes from social insects (Keller and Genoud, 1997, Chapuisat and Keller, 2002). Their social group living habits and the construction of sophisticated nests have reduced the external mortality rate for their reproductives and favored exceptionally long lives (Keller and Genoud, 1997). In general, the reproductives live an order of magnitude longer than their sterile helpers, never leave the safety of the nest center, and act simultaneously as stem cells and gonads of their colony “super-organism”.
The honey bee (Apis mellifera L.) is an emerging model for aging research for multiple reasons (Omholt and Amdam, 2004, Rueppell et al., 2004a). Honey bees have one queen per colony as the sole reproductive. In the queen’s presence the thousands of female workers are functionally sterile and perform all non-reproductive tasks in the colony. Worker/queen caste determination is based on nutrition. Queens receive high-quality food, develop in 16 days, and can live several years, producing up to 2000 eggs per day (Page and Peng, 2001). In contrast, the life expectancy of workers varies seasonally from only 3 to 4 weeks in the summer to over 6 months in the winter (Omholt and Amdam, 2004). Thus, aging in female honey bees is highly variable between and within castes but the proximate factors underlying this plasticity are not clear (Finch, 1990, Corona et al., 2005). This study addresses the proximate causes of worker mortality from a demographic perspective as a first step to understanding the proximate causes for the queen/worker aging plasticity.
The aging rate varies considerably within the honey bee worker caste and this variation is central for understanding the proximate causes of worker mortality (Omholt and Amdam, 2004). Honey bee workers display an unusual type-2 survivorship pattern (Sakagami and Fukuda, 1968) with low mortality rates as young hive bees and high mortality at older ages. The pronounced age-dependent mortality increase is believed to be intimately linked to the workers’ behavioral changes throughout life (Page and Peng, 2001). Honey bee workers display a largely age-based division of labor (Winston, 1987, Beshers and Fewell, 2001): Young bees perform in-hive tasks, such as nest maintenance, food processing, and brood care before a relatively sharp transition to foraging outside the hive. Until their death, foragers collect mainly nectar, pollen, water and plant resins, specializing to varying degrees on these resources (Winston, 1987). The significant increase in age-specific mortality has been regarded as evidence for a high mortality caused by foraging activity (Sakagami and Fukuda, 1968, Page and Peng, 2001) and one early report has estimated that up to 98% of workers die outside the hive (Lundie, 1925). However, these reports present data that confound the mortality effects of chronological aging, behavioral and physiological profile, and extrinsic mortality.
Several hypotheses to explain the peculiar worker mortality patterns have been suggested. Down-regulation of the workers’ free protein reserves at the onset of foraging is central for the elevated mortality in foragers (Amdam and Omholt, 2002, Omholt and Amdam, 2004). This hypothesis that foragers deplete themselves of protein in face of their high external mortality rate to preserve colony resources (Amdam et al., 2005) focuses on vitellogenin, a major hemolymph protein with immune (Amdam et al., 2005) and anti-oxidant functions (Seehuus et al., 2006). In addition, several other mechanistic hypotheses have been suggested to account for the high mortality rate of foraging workers. First, the external mortality hazards of foraging may be sufficient to explain the high mortality rate of foragers (Sakagami and Fukuda, 1968, Visscher and Dukas, 1997). Possible causes of death include predation, accidents, dehydration, or disorientation. Another influential hypothesis proposes that the limited glycogen reserves in foragers cannot be replenished at older ages, leading to death by exhaustion (Neukirch, 1982). Simple wear-and-tear can lead to the inability to fly and ultimately death (Page and Peng, 2001), or oxidative tissue damage may limit lifespan (Corona et al., 2005). While experimental data show that some of these processes take place, but with the exception of vitellogenin titers (Nelson et al., 2007, Amdam et al., 2007) they have not been linked quantitatively to mortality rates. It is difficult to distinguish between these alternative hypotheses because it is difficult to determine the cause of death for individual worker honey bees.
Demographic techniques cannot directly establish individual causes of death but actuarial analysis of the mortality patterns can test general mortality models and aid in the discrimination of competing hypotheses on major mortality factors (Carey, 2001). We performed three large-scale demographic experiments to analyze worker mortality patterns and better understand aging in honey bees. Specifically, we assessed (1) the importance of extrinsic risk on worker mortality, (2) how foraging is quantitatively related to mortality, (3) how variation in life history between two selected strains correlates with mortality, and (4) how chronological age affects mortality.
The first experiment was designed to eliminate most extrinsic mortality factors by training bees to forage within a flight cage. We predicted the cage-restricted workers to have a significantly lower mortality, particularly as foragers, if extrinsic mortality factors such as predation, disorientation, or other accidents (Visscher and Dukas, 1997) played a major role in limiting honey bee worker lifespan.
The second experiment addressed the quantitative aspect of foraging by varying the amount of foraging within flight cages by restricting access to food. We predicted significantly lower worker mortality, particularly as foragers, in the limited colony if worker mortality is quantitatively related to foraging effort (Neukirch, 1982).
In the third experiment, we compared worker mortality between two honey bee strains (high and low pollen-hoarding strains; Page and Fondrk, 1995) to assess how their life-history differences, including life expectancy (Amdam et al., 2007), relate to age-specific mortality during the in-hive and foraging state. Compared to the low pollen-hoarding strain, the high strain workers specialize more on pollen foraging (Page et al., 1995), initiate foraging earlier (Pankiw and Page, 2001, Rueppell et al., 2004b), and have larger and more active ovaries (Amdam et al., 2006). When young, the high pollen-hoarding bees have higher levels of vitellogenin. However, vitellogenin levels drop faster in adult high pollen-hoarding bees causing an earlier initiation of foraging (Amdam et al., 2007). Thus we predicted that the worker mortality is higher in the high pollen-hoarding strain then in the low strain specifically at intermediate ages when they exhibit lower vitellogenin titers and earlier foraging.
Section snippets
Experiments
We studied focal cohorts of honey bees (Apis mellifera L.) in colonies of a natural age composition. Honey bee queens in the source colonies were induced to lay eggs in empty combs. These combs were brought into a humidity- and temperature-controlled incubator (33 °C/60% Rel. Humid.) 1 day prior to emergence of the focal cohort bees. Within 12 h of emergence, worker bees were marked with individually numbered color-tags (BeeWorks, Canada) and introduced into an unrelated host colony. The host
Experiment 1
Experiment 1 compared the age-specific mortality between workers that foraged into protected flight cages with naturally foraging controls, while maintaining similar internal colony conditions. The initial survival (during the first 5 days) of the 960 marked and introduced workers ranged from 74% to 85% and left 816 and 736 focal individuals in the two freely-foraging colonies and 714 and 774 in the two caged colonies for the subsequent analyses.
The age of first foraging
The transition from pre-foraging hive activities to foraging behavior (age of first foraging: AFF) emerged as the key determinant of honey bee worker lifespan. In all three experiments the AFF showed a significant positive influence on overall lifespan. This pattern is consistent with previous reports (Sakagami and Fukuda, 1968, Neukirch, 1982, Guzmán-Novoa et al., 1994). The average AFF coincides well with the maximal increase of mortality across all 10 experimental cohorts. The majority of
Conclusion
Overall, our experiments confirmed the type-2 survivorship patterns of honey bee workers (Sakagami and Fukuda, 1968). The age-dependent mortality followed in most cases a logistic model (Pletcher, 1999). The main reason for these logistic mortality dynamics in honey bee workers is the age-dependent pattern of the onset of flight activity, which also follows a logistic pattern (Rueppell et al., 2004b). A logistic mortality pattern has been reported from a variety of organisms with sufficiently
Acknowledgments
We thank LoriAnn Rodriguez for great practical help with the third experiment. Two anonymous reviewers made helpful suggestions to improve the manuscript. This research was supported by the National Institute on Aging (PO1 AG22500).
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