Foragers have a high extrinsic mortality risk compared to hive bees, and the life of a forager is therefore short (typically 1-2 weeks: Neukirch, 1982) relative to the typical duration of the hive bees phase (normally 3-8 weeks: Free and Spencer-Booth, 1959) and the diutinus stage (up to 280 days: reviewed by Omholt and Amdam, 2004). Consequently, the transitions among those three distinct life history states are key determinants of the lifespan of any individual worker (reviewed by Amdam and Page, 2005). Within each temporal caste, however, longevity is further conditional on physiological factors (Maurizio, 1950) (see below).
Metabolic Rate (MR) MR is one physiological factor that may underlie the temporal, caste-associated mortality rates of honeybee workers. Forager MR is significantly higher than the MR of hive bees (for a discussion see Suarez et al., 1996), and the MR of a foraging bee constitutes one of the highest known mass-specific aerobe MRs among animals (approximately 3-fold higher than hummingbird flight muscle). Such intense activity is a major source of mechanical senescence in insects and causes mortality to increase as a function of age (reviewed by Finch, 1990). In accordance with their longevity, diutinus bees periodically exhibit low MRs compared to hive bees—and thus also relative to foragers (Crailsheim, 1986; Nerum and Buelens, 1997). The MR of diutinus workers can fall to half the MR of hive bees. Elevated MR causes an increase in the release of free radicals that can induce oxidative impairment (reviewed by Jazwinski, 1996), which has led to the suggestion that oxidative stress is a mediator of stage-dependent physiological frailty in honeybee workers (Amdam and Omholt, 2002).
Endocrine Status Juvenile hormone (JH) and ecdysteroids are systemic hormones that are implicated in the regulation of invertebrate lifespan by orchestrating several suites of physiological processes (Tatar et al., 2003).
The JHs form a family of related sesquiterpenoid compounds secreted by the corpora allata (CA) complex, and although insects (e.g., Lepidoptera) may secrete a blend of JHs, JH III is the only naturally occurring JH in the honeybee (Tobe, 1985). The JH titer of worker bees increases at the onset of foraging, and a comparable rise in the JH level is associated with accelerated rates of senescence in invertebrate models like Drosophila spp. (Tatar and Yin, 2001). Diutinus workers and hive bees, however, have very low JH titres (Fluri et al., 1982). In many insect species, this hormonal signature is associated with diapause, and removal of the CA complex often results in diapause-associated characteristics, e.g., low MR, reduced activity levels, regression of ovary development, and enlargement of the fat body—a storage organ analogous to the mammalian liver and white adipose tissue. Yet, removal of the CA does not trigger a diapause-like state in worker bees: workers without the CA progress through a period of hive tasks and initiate foraging (Sullivan et al., 2000). Moreover, MR increases in allatectomized foragers that do not have detectable JH titers (Sullivan et al., 2003). Therefore, the role of JH
in the regulation of honeybee longevity may not be directly comparable with its role in solitary insect species (see Regulation of Aging through Social Control of Behavioral Plasticity for further information).
The dominant ecdysteroid in the honeybee is Maki-sterone A (Robinson et al., 1991). Steroids generally have multiple and complex regulatory effects that may affect longevity (Tatar et al., 2003). Robinson et al. (1991), however, demonstrated that the ecdysteroid titer of adult worker bees is low (independent of age), and it is therefore unlikely that Makisterone A plays an important role in the regulation of adult honeybee physiology and lifespan. Yet, it has not been fully determined if ecdysteroids are involved in the control of worker reproductive maturation and oogenesis, a state that can be triggered if the queen is lost from a colony (Robinson et al., 1991; Hartfelder et al., 2002).
Nutritional State The protein content of worker bees is low at emergence, but it increases sharply during the first 10-12 days of adult life through pollen feeding. In the broodless diutinus workers, protein may subsequently accumulate to extreme concentrations because nutritional resources are retained in the adult bees rather than being transferred to young larvae (Fluri et al., 1977; Fluri et al., 1982). This state is associated with an overall increase in longevity (Maurizio, 1950). Buildup of stored protein reserves (storage protein) is commonly linked to reduced mortality in insects. A comparable pattern to that observed in the bee is found in the ant Camponotus festinatus, where the presence of brood also has an inhibitory effect on the accumulation of storage proteins in workers (Martinez and Wheeler, 1994).
At the onset of foraging, protein levels in the hemo-lymph (blood), fat body and hypopharyngeal glands (brood food producing head glands) are significantly reduced in the bee, and a similar pattern is seen for stored lipids (Toth and Robinson, 2005). This loss of stored nutrients from individuals that perform risky exterior hive activities has parallel examples from other social Hyme-nopterans (Porter and Jorgensen, 1981; O'Donnell and Jeanne, 1995) and has led to the proposition that foragers are stripped of nutritional resources as part of a colony-level resource-saving strategy (O'Donnell and Jeanne, 1995; Amdam and Omholt, 2002). Accordingly, foraging bees have fewer resources that can be allocated to individual somatic maintenance—a state that appears to translate into a higher level of physiological frailty.
The insect immune cells, or hemocytes, are involved in defense mechanisms such as phagocytosis, encapsulation and nodule formation (reviewed by Rolff and Siva-Jothy, 2002). The immune cells also produce antibacterial peptides and contain most of the recruitable enzyme prophenoloxidase, which catalyses crucial steps in the melanization immune response. The number of functional circulating hemocytes at any give time, therefore, reflects the organism's capacity to cope with immunogenic challenges.
Hive bees and diutinus workers have high numbers of functional hemocytes (5,000-12,000 immune cells per ^l hemolymph: Fluri et al., 1977; Amdam et al., 2004a). JH, however, affects the number of normal hemocytes in honeybee worker hemolymph, and causes the cellular immune system of foragers to become suppressed (Rutz et al., 1976). Specifically, under the influence of high JH titers, hemocytes change into inactive pycnotic forms. One effect of this loss of immunity is a complete deterioration of the nodulation response (Bedick et al., 2001).
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