Interplay among nocturnal activity, melatonin, corticosterone and performance in the invasive cane toad (Rhinella marinus)
Introduction
Animals typically exhibit daily sleep–wake cycles with segregated periods of active (e.g. foraging and social interactions) and inactive (e.g. sleep, rest) behavior. The conserved evolution of discreet behavioral partitioning within 24 h activity cycles, suggests animals respond to strong temporal selection from daily variation in environmental and ecological processes to maximise fitness (Daan, 1981). However, despite the generality of discreet daily patterns of activity, animal may also exhibit phase shifts of varying durations in daily activity schedules in response to temporal variation in resources, to accommodate life-history requirements, or because environmental or ecological constraints limit daily activity. The capacity for animals to mediate flexibility (i.e. plasticity) in the timing of daily activity patterns is likely to confer higher fitness in temporally variable or rapidly changing environments (Daan, 1981, Cotton and Parker, 2000, Kronfeld-Schor and Dayan, 2003, Webb et al., 2014).
The endocrine regulation of sleep wake cycles in diurnal vertebrates (particularly under captive conditions or using laboratory model species) is well understood (Underwood, 1990, Reiter, 1991). Here two hormones, melatonin (peak levels at night) and corticosterone (peak levels pre awakening or during the day) often exhibit opposing daily profiles that regulates the interplay between daily cycles of behavior (e.g. activity), physiology (e.g. metabolism/digestion) and entrainment with environmental timing cues such daily as photo- and thermal-cycles (Firth et al., 1989, Underwood, 1990, Reiter, 1991). In some diurnal species, melatonin sets the phase of the daily activity cycle, but also reduces arousal state that causes inactivity or sleepiness (Underwood, 1990, Hyde and Underwood, 2000, Zhdanova et al., 2001, Azpeleta et al., 2010) and can even inhibit activity (Chiba et al., 1985, Phol, 2000). Diel variation in basal corticosterone levels typically peak before or during the active phase of an animal daily sleep–awake cycle (Breuner et al., 1999, Jessop et al., 2002). The peak in corticosterone up regulates multiple physiological and behavioral processes consistent with increased arousal, activity and metabolism associated with the active period of the sleep–wake cycle (Dallman et al., 1993, Breuner et al., 1999).
How then does temporal flexibility in daily activity schedules of animals influence these physiological processes? Some nocturnal species (e.g. rodents) maintain high melatonin and corticosterone levels during periods of peak nocturnal activity (Mendelson et al., 1980, Tobler et al., 1994), whilst others, including nocturnal birds exhibit limited increase in night time plasma melatonin levels (Taniguchi et al., 1993, Wikelski et al., 2006). By contrast, diurnal species that exploit nocturnal activity for seasonal life-history events (e.g. vernal migration in diurnal birds: Gwinner et al., 1993, Gwinner, 1996, Fusani and Gwinner, 2001, nocturnal nesting in green sea turtles: Jessop et al., 2002) facultatively reduce the nocturnal peak in melatonin, nor exhibit any commensurate phase shift in the hormone’s profile. These examples intuitively suggest some capacity for physiological plasticity in regulation of the daily melatonin cycle outside the typical nocturnal cycle exhibited by diurnal species. Because phase shifts in these activities are often highly predictable (e.g. due to seasonal environmental cues) and important, it suggests that selection may act on individuals to resynchronize their endocrine cycles with phase shifts in daily activity to maximise organismal performance (Gwinner et al., 1993, Fusani and Gwinner, 2004, Fusani and Gwinner, 2005). Otherwise, such animals might, as suggested by human shift work research, be exposed to the broad scale negative health effects that arise from uncoupling between daily behavioral and physiological cycles (Knutsson, 2003, Schernhammer et al., 2003).
However, some animals display even greater flexibility, and adjust daily activity in response to short term fluctuations in environmental (e.g. rainfall, temperature) or ecological processes (e.g. food pulses, breeding activities or predation risk). Both diurnal and nocturnal species may vary the onset of daily activities to accommodate local conditions. For example, under field conditions nocturnal animals such as many anurans, bats, and rodents exhibit intermittent periods of nocturnal activity interspersed by nightly bouts of inactivity (Zug and Zug, 1979, Halle and Stenseth, 2000). How these animals regulate their daily melatonin and corticosterone profiles to accommodate highly flexible daily activity cycles is not well understood. Presumably, such animals also require compensation among activity cycles, hormones and performance (Kronfeld-Schor and Dayan, 2003, Fusani et al., 2011).
Using field and laboratory experiments we investigated: (1) if animals that utilize flexible activity cycles alter daily hormone profiles of melatonin and corticosterone; and (2) if melatonin affects physiological aspects of exercise and metabolic performance which under ecological conditions could infer fitness costs. We used the cane toad (Rhinella marina), to investigate the interplay among daily hormone profiles, variable activity cycles and performance. Toads are an interesting model for such studies as they exhibit ontogenetic shifts from diurnal to nocturnal activity (Pizzatto et al., 2008). Once nocturnally active, toads exhibit highly variable daily activity patterns, alternating between days with and without nocturnal activity (Zug and Zug, 1979). These activity shifts arise because of inter-daily variation in temperature and rainfall. However, even when conditions are favorable, toads can be inactive to reconcile time lags associated with digestion and gut clearance (Zug and Zug, 1979). Simply toads may alter between days of nocturnal activity and inactivity and such schedules are likely to be highly variably due to dynamic variation in biophysical constraints (e.g. rainfall) and individual foraging success. We predicted that if toads compensate hormone cycles due to daily variation in nocturnal activity, daily melatonin and corticosterone profiles should match periods of low and high activity, respectively. Further if daily melatonin profiles are indeed altered, does this suggest nocturnal performance compensation to prevent the potentially inhibitory effects of melatonin on activity? Here we tested the effects of exogenous melatonin on two measures or whole organism physiological performance. First we examined the effects of exogenous melatonin on post exercise plasma lactate (a metabolic waste product causing fatigue) and glucose recovery as markers of melatonin’s potential performance costs for toads conducting nocturnal activity. Similar to most vertebrates, toads need to utilize both anaerobic and aerobic metabolisms to facilitate different behaviors appropriate to specific ecological contexts. Typically high performance and intense physical activities associated with escape or social conflict involve anaerobic metabolism (Pough, 1989). Clearance and release of plasma lactate and glucose during such behaviors can have profound effects on animal performance and in turn have important fitness consequences (Pough, 1989). Melatonin has been reported in some studies to have an inhibitory effect on the regulation of intermediate metabolites such as lactate and glucose (Soengas et al., 1996, Prunet-Marcassus et al., 2003). Similarly there is evidence that melatonin can inhibit glucocorticoid and thyroid hormones that also regulate intermediate metabolites necessary for optimal physical performance (Appa-Rao et al., 2001, Saito et al., 2005, Azpeleta et al., 2010). Second, we investigated the effects of melatonin on resting metabolism in toads. Putative effects of melatonin suggest that it has a regulatory role related to energy metabolism through inhibition of thyroid hormones (John et al., 1990, Krotewicz and Lewinski, 1994) or glucocorticoid hormones (Azpeleta et al., 2010). These hormones can have an important stimulatory effect on resting metabolic rate in ecotherms (Chiu and Tong, 1979, John-Alder, 1983, John-Alder, 1990; Durant et al., 2008). Any reduction in metabolic rate through melatonin inhibiting these hormones could again have important performance implications for toads undertaking nocturnal activity.
Section snippets
Ethics statement
All animals were maintained and tested in accordance with the Institutional Animal Care and Use Committee of the University of Queensland (zoo/225/96/urg).
Study animals
For all experiments we captured adults cane toads (∼130 g), a large and now wide spread terrestrial anuran introduced intro Australia from South America (via Hawaii) in the 1930’s (Phillips et al., 2007). Animals used in our experiments were predominantly males (∼80%) reflecting the natural sex bias in Australian toad populations. Thus we did
Plasma hormone profiles of toads sampled under natural field conditions
Time of day (Wald χ2 = 7.91, P = 0.16) had no significant effect on plasma melatonin levels of toad sampled under field conditions (Fig. 1). In contrast, plasma corticosterone levels of toad significantly varied with time of day (Wald χ2 = 20.72, P < 0.001) being much higher in toads sampled during nocturnal activity compared to those sampled inactive during the day (Fig. 1).
Effects of nocturnal activity on plasma endocrine profiles of toads under captive conditions
Time of day (Wald χ2 = 43.82, P < 0.001), activity treatment (Wald χ2 = 30.41, P < 0.001) and their interaction (Wald χ2 = 37.11, P < 0.001)
Discussion
Our results suggested that cane toads (1) altered daily hormone profiles of melatonin and corticosterone in response to nocturnal activity and (2) that exogenous melatonin could influence anaerobic recovery post physical exercise. Nocturnal activity resulted in a suppression of melatonin and elevation of corticosterone levels in toads. Other nocturnally active animals can reduce plasma melatonin levels, but the mechanisms for this remain unknown. Potential pathways include increased sensitivity
Conclusions
Under field and captive conditions, nocturnally active cane toads exhibited conspicuous changes to daily hormone cycles of melatonin and corticosterone. Clearance of plasma lactate post exercise was reduced in melatonin treated toads compared to control toads. Hence, by reducing melatonin during nocturnal activity, cane toads may avoid performance costs similar to those purported for human shift workers. In particular, we suggest that a reduction in nocturnal melatonin during nocturnal activity
Acknowledgment
Funding for this study was provided by a Australian Post Graduate Award to Tim Jessop.
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