- Email: [email protected]
Does sugar content matter? Blood plasma glucose levels in an occasional and a specialist avian nectarivore
Comparative Biochemistry and Physiology, Part A 167 (2014) 40–44
Contents lists available at ScienceDirect
Comparative Biochemistry and Physiology, Part A journal homepage: www.elsevier.com/locate/cbpa
Does sugar content matter? Blood plasma glucose levels in an occasional and a specialist avian nectarivore Minke Witteveen, Mark Brown, Colleen T. Downs ⁎ School of Life Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville 3209, South Africa
a r t i c l e
i n f o
Article history: Received 22 July 2013 Received in revised form 26 September 2013 Accepted 26 September 2013 Available online 2 October 2013 Keywords: Nectar Sugar Blood glucose Sucrose Hexose Nectarivore Homeostasis
a b s t r a c t Nectar composition within a plant pollinator group can be variable, and bird pollinated plants can be segregated into two groups based on their adaptations to either a specialist or an occasional bird pollination system. Specialist nectarivores rely primarily on nectar for their energy requirements, while occasional nectarivores meet their energy requirements from nectar as well as from seeds, fruit and insects. Avian blood plasma glucose concentration (PGlu) is generally high compared with mammals. It is also affected by a range of factors including species, gender, age, ambient temperature, feeding pattern, reproductive status, circadian rhythm and moult status, among others. We examined whether sugar content affected PGlu of two avian nectarivores, a specialist nectarivore the Amethyst Sunbird Chalcomitra amethystina, and an occasional nectarivore the Cape White-eye Zosterops virens, when fed sucrose–hexose sugar solution diets of varying concentrations (5%–35%). Both species regulated PGlu within a range which was affected by sampling time (fed or fasted) and not dietary sugar concentration. The range in mean PGlu was broader in Amethyst Sunbirds (11.52–16.51 mmol/L) compared with Cape White-eyes (14.33–15.85 mmol/L). This suggests that these birds are not constrained by dietary sugar concentration with regard to PGlu regulation, and consequently selective pressure on plants for their nectar characteristics is due to reasons other than glucose regulation. © 2013 Elsevier Inc. All rights reserved.
1. Introduction Nectar is one of the most common floral rewards offered by plants, and it is used by a wide range of pollinators (Simpson and Neff, 1981). Although originally thought to be a simple sugar solution, nectar has since been shown to be able to contain a variety of free amino acids, vitamins and lipids dissolved or suspended in an aqueous sugar solution made up of up to three different sugars: monosaccharides glucose and fructose and the disaccharide sucrose (Simpson and Neff, 1981; Brandenburg et al., 2009). Nectar can be highly variable in its composition due to environmental variability, and is pollinator specific (Brandenburg et al., 2009). Nectar composition within a plant pollinator guild can be variable, as can be seen in nectar produced by plants pollinated by birds. Bird pollinated plants can be segregated into two groups based on their adaptations to a bird pollination system (Johnson and Nicolson, 2008). Plants adapted to specialist avian nectarivores produce a small volume (~ 10–30 μL) of concentrated (~ 15–25% w/w) sucrose dominant (~40–60%) nectar while plants adapted to occasional avian nectarivores produce a large volume (~40–100 μL) of dilute (~8–12% w/w) hexose dominant (~0–5% sucrose) nectar (Johnson and Nicolson, 2008). Specialist nectarivores (e.g. hummingbirds and sunbirds) are birds which rely primarily on nectar for their energy requirements, while ⁎ Corresponding author. E-mail address: [email protected] (C.T. Downs). 1095-6433/$ – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.cbpa.2013.09.017
occasional nectarivores (e.g. bulbuls and white-eyes) meet their energy requirements from nectar as well as from seeds, fruit and insects (Johnson and Nicolson, 2008; Brown et al., 2010b). The nectar composition of bird pollinated plants has long been thought to be due to the selective pressure of avian pollinator preferences (Lotz and Schondube, 2006; Chalcoff et al., 2008; Medina-Tapia et al., 2012). Avian preferences are often due to physiological processes, mainly digestive, but sensorial physiological processes such as taste have also been seen to potentially influence preferences (Lotz and Schondube, 2006; Chalcoff et al., 2008; Medina-Tapia et al., 2012). The presence or absence of the enzyme sucrase has been an important factor that has influenced avian preferences and shows the importance of nectar composition (Brandenburg et al., 2009; Brown et al., 2012). Sucrase is the enzyme that hydrolyses sucrose into glucose and fructose which are then absorbed in the gastrointestinal tract (Martinez del Rio, 1990; Braun and Sweazea, 2008). The presence and activity of sucrase have been documented in Cape White-eyes (Bizaaré et al., 2012) while Amethyst Sunbirds have a high assimilation efficiency of sucrose indicating the presence and high activity of sucrase (Downs, 1997). Blood plasma glucose concentration (PGlu) in birds is regulated at a level averaged at twice that maintained in mammals of similar body mass (Braun and Sweazea, 2008; Polakof et al., 2011). If such high PGlu were maintained in mammals, a hyperglycaemic-induced increase in reactive oxygen species would cause oxidative stress leading to impaired cell function and tissue damage (King and Loeken, 2004). Birds, on the other hand, are able to maintain such concentrations and
M. Witteveen et al. / Comparative Biochemistry and Physiology, Part A 167 (2014) 40–44
mitigate the physiological consequences (Smith et al., 2011). There are two hypotheses as to how they are able to do this: 1) due to the high turnover rate of red blood cells in birds (21days vs. 120days in humans) they are able to avoid glycation, a symptom of glucose toxicity (Hargrove, 2005) and 2) endogenous and dietary antioxidant pathways are able to minimise oxidative stress (Smith et al., 2011). However, the physiological significance of these comparatively high PGlu is currently undetermined (Scanes and Braun, 2013). PGlu in birds can vary according to different factors including: species (Braun and Sweazea, 2008; Polakof et al., 2011); gender (Kern et al., 2005); age (Villegas et al., 2002; Prinzinger and Misovic, 2010); ambient temperature (Downs et al., 2010); feeding pattern (Jenni and JenniEiermann, 1996); reproductive status (Gayathri et al., 2004); circadian rhythm (Frelin, 1974; Jenni and Jenni-Eiermann, 1996) and moult status (Driver, 1981), among others (Lill, 2011). Despite this variation in PGlu, in some species it is under strict homeostatic control and is regulated around a set point whereas in other species the range around the set point is broad (Scanes and Braun, 2013). In birds, the antagonistic relationship of the glucagon–insulin pair does not function to regulate PGlu as it does in mammals (Polakof et al., 2011). As a result, high PGlu could be attributed to a low insulin:glucagon ratio also causing avian metabolism to be controlled by glucagon levels, but the ultimate cause of high PGlu is uncertain (Polakof et al., 2011; Scanes and Braun, 2013). The challenge of PGlu regulation is expected to be compounded in nectarivores, with a diet of sugar-rich nectar (Beuchat and Chong, 1998). This is evidenced by the high fasting and fed PGlu in hummingbirds (Beuchat and Chong, 1998). However, as specialist nectarivores, hummingbirds feed primarily on concentrated nectar (Johnson and Nicolson, 2008). The question is would they be able to maintain high PGlu on a varied diet? Occasional nectarivores naturally ingest a range of sugar concentrations through nectar as well as fruit (Lotz and Schondube, 2006). How does this affect their ability to regulate PGlu? Consequently, we examined if sugar content affected the PGlu of two nectarivores, a specialist the Amethyst Sunbird Chalcomitra amethystina, and an occasional nectarivore the Cape White-eye Zosterops virens, when fed sucrose–hexose sugar solution diets of varying concentrations (5%–35%). We hypothesised that PGlu in these nectarivores was affected by increased dietary sugar concentration. 2. Material and methods 2.1. Study species Cape White-eyes (Z. virens) are small (~13.5 g) occasional nectarivores that feed on a variety of fruit, insects and nectar (Franke et al., 1998; Hockey et al., 2005). These birds are known to be important pollinators of a number of genera including Aloe (Johnson et al., 2006; Botes et al., 2008; Symes et al., 2008); Erythrina (M. Brown, unpublished data); Kniphofia (Brown et al., 2011); Salvia (Wester and ClaβenBockhoff, 2006) and Schotia spp. (M. Brown, unpublished data). Amethyst Sunbirds (C. amethystina) are small (~15 g) specialist nectarivores which feed primarily on floral nectar but have been seen to feed occasionally on insects (Hockey et al., 2005). They, too, are known to be important pollinators of a number of genera including Aloe (Botes et al., 2008); Erythrina (Bruneau, 1997); Kniphofia (Brown et al., 2011); Protea (Hockey et al., 2005) and Strelitzia spp. (Frost and Frost, 1981). 2.2. Bird capture and maintenance Capture was done under the authority of permits from Ezemvelo KZN Wildlife and trials with ethical clearance from the University of KwaZulu-Natal. Seven adult non-breeding Cape White-eyes (11.27 ± 0.24 g) were captured during March 2011 on the Pietermaritzburg campus of the University of KwaZulu-Natal (UKZN) (29°38′S, 30°24′E). Four non-breeding adult and three juvenile Amethyst
41
Sunbirds (13.55 ± 0.19 g) were caught at Riley Crescent, Howick (29°28′S, 30°13′E), Sakabula Estate, Howick (29°52′S, 30°23′E) and Shiraz Villas, Pietermaritzburg (29°37′S, 30°26′E) during May 2012 using Ecotone® (Ecotone, Gdynia, Poland) mist nets. Birds were kept in separate cages (45 × 45 × 40 cm) in a constant environment room (25 °C; 12L: 12D photocycle) at the Animal House, School of Life Sciences, UKZN, Pietermaritzburg campus. Cape White-eyes were fed a maintenance diet of fresh fruit (apples, pears, bananas, pawpaws and oranges) supplemented with Aviplus Softbill Mynah Pellets (AviProducts, Durban, South Africa), as well as fruit flies and water ad libitum. Amethyst Sunbirds were fed a maintenance diet of ~ 20% sucrose, fructose, and glucose (2:1:1) sugar solution with Ensure nutrient supplement (Abbott Laboratories, Hoofddorp, The Netherlands) as well as fruit flies ad libitum. 2.3. Blood glucose trials To determine the extent to which Amethyst Sunbirds and Cape White-eyes regulate their PGlu they were given sucrose–glucose–fructose (2:1:1) solutions of varying concentrations (5%, 10%, 15%, 20%, 25% and 35%) and PGlu was measured at each of these concentrations. Food was removed overnight prior to a trial day and the trial began when the lights came on at 06:00 h. For the extent of the trial day, 06:00 h to 18:00 h, birds were fed one of the six sugar solutions, offered ad libitum from 50 mL nectar feeders. After 9h of feeding, i.e. at 15:00h, a drop of blood was collected on an Accu-Chek® Performa Blood Glucose Test Strip (Roche Diagnostics, Randburg, South Africa) from the brachial vein after venipuncture with a 29 gauge syringe needle. The strip was immediately inserted into the Accu-Chek® Performa Blood Glucose Meter (Roche Diagnostics, Randburg, South Africa) and PGlu measured. At the end of the day, i.e. at 18:00 h, the nectar feeders were removed and birds were starved overnight until 07:30 h the following morning when PGlu was again measured taking a drop of blood from the other wing. Trials were run once a week to allow birds six days of maintenance diet to recover between trials. As a response to stress, glucocorticoids such as corticosterone are released, which can cause an increase in PGlu (Warne et al., 2009). However, a study done by Fokidis et al. (2011) showed that handling stress and stress of being in captivity did not significantly increase PGlu. Regardless, handling was kept to a minimum with birds being caught, placed in a cloth bag and processed within 30 min. 2.4. Statistical analyses Intraspecific PGlu were analysed against sugar solution concentration for the two sampling times using a Generalised Linear Model Repeated Measures Analysis of Variance (GLM RMANOVA), followed by a posthoc Tukey HSD test. All data were analysed using STATISTICA 7 (StatSoft, Tulsa, OK, USA). All results reported are mean ± standard error. 3. Results 3.1. Body mass Body mass of Cape White-eyes did not differ significantly in the morning (06:00 h) before feeding between the respective trials (RMANOVA: F5, 30 = 0.563, P = 0.727; Fig. 1; mean ± SE range 11.06 ± 0.25–11.30 ± 0.37 g), nor at 15:00 h (RMANOVA: F5, 30 = 2.034, P = 0.102; Fig. 1; mean ± SE range 11.24 ± 0.41–11.76 ± 0.46 g), nor the following morning at 07:30 h after the trials (RMANOVA: F5, 30 = 1.164, P = 0.350; Fig. 1; mean ± SE range 10.57 ± 0.25–10.91 ± 0.41 g). In contrast body mass of Amethyst Sunbirds did differ significantly in the morning (06:00 h) before feeding between the respective trials
42
M. Witteveen et al. / Comparative Biochemistry and Physiology, Part A 167 (2014) 40–44
A)
Vertical bars denote +/- standard errors 12.6
06h00 15h00 07h30 after
12.4 12.2
Body mass (g)
12.0 11.8 11.6 11.4 11.2 11.0 10.8 10.6 10.4 10.2 10.0
5%
10%
15%
20%
25%
35%
Hexose/Sucrose concentration
B)
Vertical bars denote +/- standard errors 17.5
06h00 15h00 07h30 after
17.0 16.5
Body mass (g)
16.0 15.5 15.0 14.5 14.0 13.5 13.0 12.5 5%
10%
15%
20%
25%
35%
Hexose/Sucrose concentration Fig. 1. Body mass (g) of (A) Cape White-eyes (Zosterops virens) and (B) Amethyst Sunbirds (Chalcomitra amethystina) fed sucrose–hexose sugar solutions of various concentrations, before the trial (06:00 h), after 9 h of feeding (15:00 h) and after an overnight fast (07:30 h after). Values represent means ± standard error.
(RMANOVA: F5, 30 = 5.253, P = 0.001; Fig. 1; mean ± SE range 14.21 ± 0.57–15.46±0.64g), and at 15:00h (RMANOVA: F5, 30 =4.66, P=0.003; Fig. 1; mean ± SE range 14.24 ± 0.52–16.03 ± 0.75 g), and the following morning at 07:30h after the trials (RMANOVA: F5, 30 = 5.556, P =0.001; Fig. 1; mean ± SE range 13.54 ± 0.58–14.94 ± 0.68 g). 3.2. Blood plasma glucose concentration (PGlu) For Cape White-eyes, both the sucrose–hexose sugar solution concentration and time of sampling significantly affected PGlu (RMANOVA: F5, 25 = 3.54, P = 0.015; F1, 25 = 28.14, P = 0.003 respectively, Fig. 2A). However, the combined effect of concentration and time did not significantly affect PGlu (F5, 25 = 0.74, P = 0.599). A post-hoc Tukey HSD test revealed that PGlu differed significantly because of time only on a 15% sucrose–hexose diet, where PGlu was higher at 15:00h (after 9h of feeding) than at 07:30h after fasting overnight (Fig. 2A). Furthermore, it was shown that within each sampling time (15:00 h and 07:30 h after) the PGlu did not differ significantly over any diet concentrations (Fig. 2A). PGlu averaged over all diet concentrations during the trials (15:00 h) was 15.85 ± 0.20 mmol/L, and after the trials (07:30 h after) was 14.33 ± 0.25 mmol/L. The highest mean PGlu was measured at 15:00 h during a 5% sucrose–hexose diet trial (16.8 ± 0.7 mmol/L), and the
lowest mean PGlu after overnight fasting at 07:30 h after a 20% sucrose–hexose diet trial (13.5 ± 0.3 mmol/L). The highest recorded PGlu (20.3 mmol/L) was at 15:00 h, after 9 h of feeding, during a 5% diet trial, and the lowest recorded PGlu (11.7 mmol/L) was at 07:30 h after fasting overnight, after a 5% diet trial. Similarly for Amethyst Sunbirds, both the sucrose–hexose sugar solution concentration and time of sampling significantly affected PGlu (RMANOVA: F5, 60 =5.48, P=0.001; F2, 60 =90.78, P=0.000 respectively, Fig. 2B). However, the combined effect of concentration and time did not significantly affect PGlu (F10, 60 = 1.93, P = 0.058). A post-hoc Tukey HSD test revealed that PGlu were significantly higher after 9 h of feeding at 15:00 h at all diet concentrations than after overnight fasting at 07:30 h (Fig. 2B). However, PGlu did not differ significantly within each sampling time (Fig. 2B). PGlu averaged over all diet concentrations during the trials (15:00 h) was 16.51 ± 0.33 mmol/L, and after the trials (07:30 h after) was 11.52 ± 0.19 mmol/L. The highest mean PGlu was measured at 15:00 h during a 35% sucrose–hexose diet trial (18.0 ± 0.8 mmol/L), and the lowest mean PGlu at 07:30 h after a 5% sucrose– hexose diet trial (11.1 ± 0.4 mmol/L). The highest recorded PGlu (22.1 mmol/L) was at 15:00 h, after 9 h of feeding, during a 15% diet trial, and the lowest recorded PGlu (8.3 mmol/L) was at 07:30 h after fasting overnight, after a 20% diet trial.
M. Witteveen et al. / Comparative Biochemistry and Physiology, Part A 167 (2014) 40–44
43
Plasma glucose concentration (m mol L-1)
A) Vertical bars denote +/- standard errors
20
15h00 07h30 after
19 18 17 16 15 14 13 12 11 10 9 8 5%
10%
15%
20%
25%
35%
Hexose/Sucrose concentration
Plasma glucose concentration (m mol L-1)
B)
Vertical bars denote +/- standard errors
20
15h00 07h30 after
19 18 17 16 15 14 13 12 11 10 9 8 5%
10%
15%
20%
25%
35%
Hexose/Sucrose concentration Fig. 2. Blood plasma glucose concentration (mmol/L) of (A) Cape White-eyes (Zosterops virens) and (B) Amethyst Sunbirds (Chalcomitra amethystina) fed sucrose–hexose sugar solutions of various concentrations, after 9 h of feeding (15:00 h) and after an overnight fast (07:30 h after). Values represent means ± standard error.
4. Discussion We found that within each sampling time, both Cape White-eyes and Amethyst Sunbirds PGlu were not affected by dietary sugar concentration. However, it appears that both species have a range that PGlu is regulated within with a lower value after fasting compared with a higher value after feeding on sugar solutions irrespective of concentration; i.e. PGlu was affected more by sampling time and whether fasted or not. Similarly Napier et al. (2008) found that diet treatment (sucrose concentration) did not significantly affect the mean steady-state concentration of radiolabelled L-glucose in blood plasma. This suggests, for both occasional and specialist avian nectarivores, that birds' PGlu is not affected by the sugar content of floral nectar or fruits, but rather whether they have fed or not. In most vertebrate homeostatic mechanisms, the parameter is usually regulated around a set point i.e. the body tries to keep the parameter at a particular value (Scanes and Braun, 2013). However, in the nectarivorous birds in this study it appears that PGlu is regulated over a range. The range over which Cape White-eyes regulated PGlu was narrower and less affected by sampling time (feeding 15:00 h or fasting 07:30 h after), and showed a simplified glucose–time curve. Lobban et al. (2010) sampled PGlu of the Cape White-eyes at four sampling times at 6 h intervals, and found at 06:00 h (after a night of fasting) and 18:00 h (after 12 h of feeding) that PGlu were not significantly
different. However, this excludes significant changes in PGlu including a peak at 12:00 h (18.1 ± 0.7 mmol/L) and a dip at 24:00 h (13.2 ± 0.5 mmol/L) (Lobban et al., 2010). These values are similar to those found in the current study, with the highest mean PGlu measuring 16.8 ± 0.7 mmol/L, and the lowest mean PGlu measuring 13.5 ± 0.3 mmol/L. In Amethyst Sunbirds, PGlu differed significantly from when birds were fasted compared with when fed; however they were able to regulate these levels within a range regardless of the dietary sugar concentration. Values were similar to those obtained for Malachite Sunbirds (Nectarinia famosa) at 04:00 h (13.6 ± 0.7 mmol/L) and 10:00 h (16.0 ± 0.8 mmol/L) (Downs et al., 2010). Again it suggests that PGlu in avian nectarivores is more affected by circadian or diel variation than diet. Meal size and frequency, and energy expenditure can cause PGlu to fluctuate, especially in birds which are migrating or do not have a territory and are unable to feed regularly enough to avoid an empty crop (Beuchat and Chong, 1998). In birds which are able to feed regularly PGlu is expected to remain at the upper end of the regulation range and relatively steady throughout the day (Beuchat and Chong, 1998). Avian nectarivores generally regulate energy intake by adjusting volumetric intake (Leseigneur and Nicolson, 2009; Brown et al., 2010a, b,c), including Amethyst Sunbirds (Köhler et al., 2010) and Cape White-eyes (Wellmann and Downs, 2009). Consequently the diel or
44
M. Witteveen et al. / Comparative Biochemistry and Physiology, Part A 167 (2014) 40–44
circadian variation in PGlu as seen in studies by Downs et al. (2010) and Lobban et al. (2010), questions why this variation? With so many factors uncontrolled in natural conditions, the importance of using measures of PGlu other than mean values for more meaningful interspecific comparisons, as promoted by Lobban et al. (2010), is reiterated. The nectar-feeding bat Glossophaga soricina is only able to regulate blood glucose levels, especially on concentrated sugar solutions, by spending 60–75% of their time flying, an energetically costly activity (Kelm et al., 2011). The activity of the birds on each sugar concentration in the current study was not observed, and it would be interesting in further studies to observe and contrast bird activity on low and high sugar concentrations, perhaps high levels of activity enable them to regulate blood glucose levels, along with physiological mechanisms. However, as birds regulate energy intake by regulating volumetric intake as mentioned, this would rescind the necessity of PGlu regulation through increased activity. Our tentative comparison between an occasional and a specialist nectarivore shows that dietary sugar concentration does not affect either species' ability to regulate PGlu. Both species regulate PGlu over a range rather than around one set point. However, Amethyst Sunbirds had a broader range of PGlu than Cape White-eyes. Consequently it appears that avian nectarivores are not constrained by dietary sugar concentration with regard to PGlu regulation, so it would seem that selective pressure on plants for their nectar characteristics is due to reasons other than glucose regulation. Physiological processes, mainly digestive and centred on sucrase activity, are most likely the cause of selective pressure on nectar characteristics (Lotz and Schondube, 2006; Napier et al., 2013). A wider range of occasional and specialist avian nectarivore species should be investigated for their tolerance to variations in dietary content as related to their ability to regulate PGlu concentration for a full phylogenetic comparison to be done. Furthermore, PGlu should be measured more regularly throughout the scoto- and photophase to determine fluctuations and so elucidate a time or level (maximum or minimum) for interspecific comparisons.
Acknowledgements We are grateful to T. Mjwara for assistance in animal maintenance. L. Thompson and S. McPherson are thanked for assistance in catching birds and during trial days. Two anonymous reviewers are thanked for valuable comments on a previous draft of this manuscript.
References Beuchat, C.A., Chong, C.R., 1998. Hyperglycemia in hummingbirds and its consequences for hemoglobin glycation. Comp. Biochem. Physiol. A 120, 409–416. Bizaaré, L., Coetzer, T.H.T., Downs, C.T., 2012. Disaccharidase presence and activities in a range of southern African frugivores. Ostrich 83, 165–168. Botes, C., Johnson, S.D., Cowling, R.M., 2008. Coexistence of succulent tree aloes: partitioning of bird pollinators by floral traits and flowering phenology. Oikos 117, 875–882. Brandenburg, A., Dell'Olivo, A., Bshary, R., Kuhlemeier, C., 2009. The sweetest thing: advances in nectar research. Curr. Opin. Plant Biol. 12, 486–490. Braun, E., Sweazea, K.L., 2008. Glucose regulation in birds. Comp. Biochem. Physiol. B 151, 1–9. Brown, M., Downs, C.T., Johnson, S.D., 2010a. Concentration-dependent sugar preferences of the Malachite Sunbird (Nectarina famosa). Auk 127, 151–155. Brown, M., Downs, C.T., Johnson, S.D., 2010b. Sugar preferences and digestive efficiency in an opportunistic avian nectarivore, the Dark-capped Bulbul Pycnonotus tricolor. J. Ornithol. 151, 637–643. Brown, M., Downs, C.T., Johnson, S.D., 2010c. Sugar preference of a generalist nonpasserine flower visitor, the African Speckled Mousebird (Colius striatus). Auk 127, 781–786. Brown, M., Downs, C.T., Johnson, S.D., 2011. Covariation of flower traits and bird pollinator assemblages among populations of Kniphofia linearifolia (Asphodelaceae). Plant Syst. Evol. 294, 199–206. Brown, M., Downs, C.T., Johnson, S.D., 2012. African Red-winged Starlings prefer hexose sugar solutions, but do not like them too sweet. J. Ornithol. 153, 265–272. Bruneau, A., 1997. Evolution and homology of bird pollination syndromes in Erythrina (Leguminosae). Am. J. Bot. 84, 54–71.
Chalcoff, V.R., Aizen, M.A., Galetto, L., 2008. Sugar preferences of the Green-backed Firecrown hummingbird (Sephanoides sephaniodes): a field experiment. Auk 125, 60–66. Downs, C.T., 1997. Sugar digestion efficiencies of Gurney's Sugarbirds, Malachite Sunbirds and Black Sunbirds. Physiol. Zool. 70, 93–99. Downs, C.T., Wellmann, A.E., Brown, M., 2010. Diel variations in plasma glucose concentrations of Malachite Sunbirds Nectarinia famosa. J. Ornithol. 151, 235–239. Driver, E.A., 1981. Hematological and blood chemical values of mallards, Anas p. platyrhynchos, drakes before, during and after remige moult. J. Wildl. Dis. 17, 413–421. Fokidis, H.B., Hurley, L., Rogowski, C., Sweazea, K., Deviche, P., 2011. Effects of captivity and body condition on plasma corticosterone, locomotor behavior, and plasma metabolites in Curve-billed Thrashers. Physiol. Biochem. Zool. 84, 595–606. Franke, E., Jackson, S., Nicolson, S., 1998. Nectar sugar preference and absorption in a generalist African frugivore, the Cape White-eye Zosterops pallidus. Ibis 140, 501–506. Frelin, C., 1974. Daily variations of serum glucose levels in moulting Redpolls Acanthis flammea and in migrating robins Erithacus rubecula. Ornis Scand. 5, 83–86. Frost, S.K., Frost, P.G.H., 1981. Sunbird pollination of Strelitzia nicolai. Oecologia 49, 379–384. Gayathri, K.L., Shenoy, K.B., Hegde, S.N., 2004. Blood profile of pigeons (Columba livia) during growth and breeding. Comp. Biochem. Physiol. A 138, 187–192. Hargrove, J.L., 2005. Adipose energy stores, physical work, and the metabolic syndrome: lessons from hummingbirds. Nutr. J. 4, 36. Hockey, P.A.R., Dean, W.R.J., Ryan, P.G. (Eds.), 2005. Roberts — Birds of Southern Africa. The Trustees of the John Voelcker Bird Book Fund, Cape Town. Jenni, L., Jenni-Eiermann, S., 1996. Metabolic responses to diurnal feeding patterns during the postbreeding, moulting and migratory periods in passerine birds. Funct. Ecol. 10, 73–80. Johnson, S.D., Nicolson, S.W., 2008. Evolutionary associations between nectar properties and specificity in bird pollination systems. Biol. Lett. 4, 49–52. Johnson, S.D., Hargreaves, A.L., Brown, M., 2006. Dark, bitter-tasting nectar functions as a filter of flower visitors in a bird-pollinated plant. Ecology 87, 2709–2716. Kelm, D.H., Simon, R., Kuhlow, D., Voigt, C.C., Ristow, M., 2011. High activity enables life on a high-sugar diet: blood glucose regulation in nectar-feeding bats. Proc. R. Soc. B Biol. Sci. 278, 3490–3496. Kern, M., Bacon, W., Long, D., Cowie, R.J., 2005. Blood metabolite and corticosterone levels in breeding adult pied flycatchers. Condor 107, 665–677. King, G.L., Loeken, M.R., 2004. Hyperglycemia-induced oxidative stress in diabetic complications. Histochem. Cell Biol. 122, 333–338. Köhler, A., Verburgt, L., McWhorter, T.J., Nicolson, S.W., 2010. Energy management on a nectar diet: can sunbirds meet the challenges of low temperature and dilute food? Funct. Ecol. 24, 1241–1251. Leseigneur, C.D.C., Nicolson, S.W., 2009. Nectar concentration preferences and sugar intake in the white-bellied sunbird, Cinnyris talatala (Nectariniidae). J. Comp. Physiol. B. 179, 673–679. Lill, A., 2011. Sources of variation in blood glucose concentrations of free-living birds. Avian Biol. Res. 4, 78–86. Lobban, K., Downs, C.T., Brown, M., 2010. Diel variations in plasma glucose concentration in some South African avian frugivores. Emu 110, 66–70. Lotz, C.N., Schondube, J.E., 2006. Sugar preferences in nectar- and fruit-eating birds: behavioural patterns and physiological causes. Biotropica 38, 3–15. Martinez del Rio, C., 1990. Dietary, phylogenetic, and ecological correlates of intestinal sucrase and maltase activity in birds. Physiol. Zool. 63, 987–1011. Medina-Tapia, N., Ayala-Berdon, J., Morales-Pérez, L., Melo, L.M., Schondube, J.E., 2012. Do hummingbirds have a sweet-tooth? Gustatory sugar thresholds and sugar selection in the broad-billed hummingbird Cynanthus latirostris. Comp. Biochem. Physiol. A 161, 307–314. Napier, K.R., McWhorter, T.J., Nicolson, S.W., Fleming, P.A., 2013. Sugar preferences of avian nectarivores are correlated with intestinal sucrase activity. Physiol. Biochem. Zool. 86, 499–512. Napier, K., McWorter, Fleming, P.A., 2008. Mechanism and rate of glucose absorption differ between an Australian honeyeater (Meliphagidae) and a lorikeet (Loriidae). J. Exp. Biol 211, 3544–3553. Polakof, S., Mommsen, T.P., Soengas, J.L., 2011. Glucosensing and glucose homeostasis: from fish to mammals. Comp. Biochem. Physiol. B 160, 123–149. Prinzinger, R., Misovic, A., 2010. Age-correlation of blood values in the Rock Pigeon (Columba livia). Comp. Biochem. Physiol. A 156, 351–356. Scanes, C.G., Braun, E., 2013. Avian metabolism: its control and evolution. Front. Biol. 8, 134–159. Simpson, B.B., Neff, J.L., 1981. Floral rewards: alternatives to pollen and nectar. Ann. Mo. Bot. Gard. 68, 301–322. Smith, C.L., Toomey, M., Walker, B.R., Braun, E.J., Wolf, B.O., McGraw, K., Sweazea, K.L., 2011. Naturally high plasma glucose levels in mourning doves (Zenaida macroura) do not lead to high levels of reactive oxygen species in the vasculature. Zoology 114, 171–176. Symes, C.T., Nicolson, S.W., McKechnie, A.E., 2008. Response of avian nectarivores to the flowering of Aloe marlothii: a nectar oasis during dry South African winters. J. Ornith. 149, 13–22. Villegas, A., Sánchez, J.M., Costillo, E., Corbacho, C., 2002. Blood chemistry and haematocrit of the black vulture (Aegypius monachus). Comp. Biochem. Physiol. A 132, 489–497. Warne, J.P., Akana, S.F., Ginsberg, A.B., Horneman, H.F., Pecoraro, N.C., Dallman, M.F., 2009. Disengaging insulin from corticosterone: roles of each on energy intake and disposition. Am. J. Physiol. Regul. Integr. Comp. Physiol. 296, R1366–R1375. Wellmann, A.E., Downs, C.T., 2009. Sugar preferences and digestion by Cape White-eyes, Zosterops virens, fed artificial fruit diets. Afr. Zool. 44, 106–116. Wester, P., Claβen-Bockhoff, R., 2006. Bird pollination in South African Salvia species. Flora 201, 396–406.
Contents lists available at ScienceDirect
Comparative Biochemistry and Physiology, Part A journal homepage: www.elsevier.com/locate/cbpa
Does sugar content matter? Blood plasma glucose levels in an occasional and a specialist avian nectarivore Minke Witteveen, Mark Brown, Colleen T. Downs ⁎ School of Life Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville 3209, South Africa
a r t i c l e
i n f o
Article history: Received 22 July 2013 Received in revised form 26 September 2013 Accepted 26 September 2013 Available online 2 October 2013 Keywords: Nectar Sugar Blood glucose Sucrose Hexose Nectarivore Homeostasis
a b s t r a c t Nectar composition within a plant pollinator group can be variable, and bird pollinated plants can be segregated into two groups based on their adaptations to either a specialist or an occasional bird pollination system. Specialist nectarivores rely primarily on nectar for their energy requirements, while occasional nectarivores meet their energy requirements from nectar as well as from seeds, fruit and insects. Avian blood plasma glucose concentration (PGlu) is generally high compared with mammals. It is also affected by a range of factors including species, gender, age, ambient temperature, feeding pattern, reproductive status, circadian rhythm and moult status, among others. We examined whether sugar content affected PGlu of two avian nectarivores, a specialist nectarivore the Amethyst Sunbird Chalcomitra amethystina, and an occasional nectarivore the Cape White-eye Zosterops virens, when fed sucrose–hexose sugar solution diets of varying concentrations (5%–35%). Both species regulated PGlu within a range which was affected by sampling time (fed or fasted) and not dietary sugar concentration. The range in mean PGlu was broader in Amethyst Sunbirds (11.52–16.51 mmol/L) compared with Cape White-eyes (14.33–15.85 mmol/L). This suggests that these birds are not constrained by dietary sugar concentration with regard to PGlu regulation, and consequently selective pressure on plants for their nectar characteristics is due to reasons other than glucose regulation. © 2013 Elsevier Inc. All rights reserved.
1. Introduction Nectar is one of the most common floral rewards offered by plants, and it is used by a wide range of pollinators (Simpson and Neff, 1981). Although originally thought to be a simple sugar solution, nectar has since been shown to be able to contain a variety of free amino acids, vitamins and lipids dissolved or suspended in an aqueous sugar solution made up of up to three different sugars: monosaccharides glucose and fructose and the disaccharide sucrose (Simpson and Neff, 1981; Brandenburg et al., 2009). Nectar can be highly variable in its composition due to environmental variability, and is pollinator specific (Brandenburg et al., 2009). Nectar composition within a plant pollinator guild can be variable, as can be seen in nectar produced by plants pollinated by birds. Bird pollinated plants can be segregated into two groups based on their adaptations to a bird pollination system (Johnson and Nicolson, 2008). Plants adapted to specialist avian nectarivores produce a small volume (~ 10–30 μL) of concentrated (~ 15–25% w/w) sucrose dominant (~40–60%) nectar while plants adapted to occasional avian nectarivores produce a large volume (~40–100 μL) of dilute (~8–12% w/w) hexose dominant (~0–5% sucrose) nectar (Johnson and Nicolson, 2008). Specialist nectarivores (e.g. hummingbirds and sunbirds) are birds which rely primarily on nectar for their energy requirements, while ⁎ Corresponding author. E-mail address: [email protected] (C.T. Downs). 1095-6433/$ – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.cbpa.2013.09.017
occasional nectarivores (e.g. bulbuls and white-eyes) meet their energy requirements from nectar as well as from seeds, fruit and insects (Johnson and Nicolson, 2008; Brown et al., 2010b). The nectar composition of bird pollinated plants has long been thought to be due to the selective pressure of avian pollinator preferences (Lotz and Schondube, 2006; Chalcoff et al., 2008; Medina-Tapia et al., 2012). Avian preferences are often due to physiological processes, mainly digestive, but sensorial physiological processes such as taste have also been seen to potentially influence preferences (Lotz and Schondube, 2006; Chalcoff et al., 2008; Medina-Tapia et al., 2012). The presence or absence of the enzyme sucrase has been an important factor that has influenced avian preferences and shows the importance of nectar composition (Brandenburg et al., 2009; Brown et al., 2012). Sucrase is the enzyme that hydrolyses sucrose into glucose and fructose which are then absorbed in the gastrointestinal tract (Martinez del Rio, 1990; Braun and Sweazea, 2008). The presence and activity of sucrase have been documented in Cape White-eyes (Bizaaré et al., 2012) while Amethyst Sunbirds have a high assimilation efficiency of sucrose indicating the presence and high activity of sucrase (Downs, 1997). Blood plasma glucose concentration (PGlu) in birds is regulated at a level averaged at twice that maintained in mammals of similar body mass (Braun and Sweazea, 2008; Polakof et al., 2011). If such high PGlu were maintained in mammals, a hyperglycaemic-induced increase in reactive oxygen species would cause oxidative stress leading to impaired cell function and tissue damage (King and Loeken, 2004). Birds, on the other hand, are able to maintain such concentrations and
M. Witteveen et al. / Comparative Biochemistry and Physiology, Part A 167 (2014) 40–44
mitigate the physiological consequences (Smith et al., 2011). There are two hypotheses as to how they are able to do this: 1) due to the high turnover rate of red blood cells in birds (21days vs. 120days in humans) they are able to avoid glycation, a symptom of glucose toxicity (Hargrove, 2005) and 2) endogenous and dietary antioxidant pathways are able to minimise oxidative stress (Smith et al., 2011). However, the physiological significance of these comparatively high PGlu is currently undetermined (Scanes and Braun, 2013). PGlu in birds can vary according to different factors including: species (Braun and Sweazea, 2008; Polakof et al., 2011); gender (Kern et al., 2005); age (Villegas et al., 2002; Prinzinger and Misovic, 2010); ambient temperature (Downs et al., 2010); feeding pattern (Jenni and JenniEiermann, 1996); reproductive status (Gayathri et al., 2004); circadian rhythm (Frelin, 1974; Jenni and Jenni-Eiermann, 1996) and moult status (Driver, 1981), among others (Lill, 2011). Despite this variation in PGlu, in some species it is under strict homeostatic control and is regulated around a set point whereas in other species the range around the set point is broad (Scanes and Braun, 2013). In birds, the antagonistic relationship of the glucagon–insulin pair does not function to regulate PGlu as it does in mammals (Polakof et al., 2011). As a result, high PGlu could be attributed to a low insulin:glucagon ratio also causing avian metabolism to be controlled by glucagon levels, but the ultimate cause of high PGlu is uncertain (Polakof et al., 2011; Scanes and Braun, 2013). The challenge of PGlu regulation is expected to be compounded in nectarivores, with a diet of sugar-rich nectar (Beuchat and Chong, 1998). This is evidenced by the high fasting and fed PGlu in hummingbirds (Beuchat and Chong, 1998). However, as specialist nectarivores, hummingbirds feed primarily on concentrated nectar (Johnson and Nicolson, 2008). The question is would they be able to maintain high PGlu on a varied diet? Occasional nectarivores naturally ingest a range of sugar concentrations through nectar as well as fruit (Lotz and Schondube, 2006). How does this affect their ability to regulate PGlu? Consequently, we examined if sugar content affected the PGlu of two nectarivores, a specialist the Amethyst Sunbird Chalcomitra amethystina, and an occasional nectarivore the Cape White-eye Zosterops virens, when fed sucrose–hexose sugar solution diets of varying concentrations (5%–35%). We hypothesised that PGlu in these nectarivores was affected by increased dietary sugar concentration. 2. Material and methods 2.1. Study species Cape White-eyes (Z. virens) are small (~13.5 g) occasional nectarivores that feed on a variety of fruit, insects and nectar (Franke et al., 1998; Hockey et al., 2005). These birds are known to be important pollinators of a number of genera including Aloe (Johnson et al., 2006; Botes et al., 2008; Symes et al., 2008); Erythrina (M. Brown, unpublished data); Kniphofia (Brown et al., 2011); Salvia (Wester and ClaβenBockhoff, 2006) and Schotia spp. (M. Brown, unpublished data). Amethyst Sunbirds (C. amethystina) are small (~15 g) specialist nectarivores which feed primarily on floral nectar but have been seen to feed occasionally on insects (Hockey et al., 2005). They, too, are known to be important pollinators of a number of genera including Aloe (Botes et al., 2008); Erythrina (Bruneau, 1997); Kniphofia (Brown et al., 2011); Protea (Hockey et al., 2005) and Strelitzia spp. (Frost and Frost, 1981). 2.2. Bird capture and maintenance Capture was done under the authority of permits from Ezemvelo KZN Wildlife and trials with ethical clearance from the University of KwaZulu-Natal. Seven adult non-breeding Cape White-eyes (11.27 ± 0.24 g) were captured during March 2011 on the Pietermaritzburg campus of the University of KwaZulu-Natal (UKZN) (29°38′S, 30°24′E). Four non-breeding adult and three juvenile Amethyst
41
Sunbirds (13.55 ± 0.19 g) were caught at Riley Crescent, Howick (29°28′S, 30°13′E), Sakabula Estate, Howick (29°52′S, 30°23′E) and Shiraz Villas, Pietermaritzburg (29°37′S, 30°26′E) during May 2012 using Ecotone® (Ecotone, Gdynia, Poland) mist nets. Birds were kept in separate cages (45 × 45 × 40 cm) in a constant environment room (25 °C; 12L: 12D photocycle) at the Animal House, School of Life Sciences, UKZN, Pietermaritzburg campus. Cape White-eyes were fed a maintenance diet of fresh fruit (apples, pears, bananas, pawpaws and oranges) supplemented with Aviplus Softbill Mynah Pellets (AviProducts, Durban, South Africa), as well as fruit flies and water ad libitum. Amethyst Sunbirds were fed a maintenance diet of ~ 20% sucrose, fructose, and glucose (2:1:1) sugar solution with Ensure nutrient supplement (Abbott Laboratories, Hoofddorp, The Netherlands) as well as fruit flies ad libitum. 2.3. Blood glucose trials To determine the extent to which Amethyst Sunbirds and Cape White-eyes regulate their PGlu they were given sucrose–glucose–fructose (2:1:1) solutions of varying concentrations (5%, 10%, 15%, 20%, 25% and 35%) and PGlu was measured at each of these concentrations. Food was removed overnight prior to a trial day and the trial began when the lights came on at 06:00 h. For the extent of the trial day, 06:00 h to 18:00 h, birds were fed one of the six sugar solutions, offered ad libitum from 50 mL nectar feeders. After 9h of feeding, i.e. at 15:00h, a drop of blood was collected on an Accu-Chek® Performa Blood Glucose Test Strip (Roche Diagnostics, Randburg, South Africa) from the brachial vein after venipuncture with a 29 gauge syringe needle. The strip was immediately inserted into the Accu-Chek® Performa Blood Glucose Meter (Roche Diagnostics, Randburg, South Africa) and PGlu measured. At the end of the day, i.e. at 18:00 h, the nectar feeders were removed and birds were starved overnight until 07:30 h the following morning when PGlu was again measured taking a drop of blood from the other wing. Trials were run once a week to allow birds six days of maintenance diet to recover between trials. As a response to stress, glucocorticoids such as corticosterone are released, which can cause an increase in PGlu (Warne et al., 2009). However, a study done by Fokidis et al. (2011) showed that handling stress and stress of being in captivity did not significantly increase PGlu. Regardless, handling was kept to a minimum with birds being caught, placed in a cloth bag and processed within 30 min. 2.4. Statistical analyses Intraspecific PGlu were analysed against sugar solution concentration for the two sampling times using a Generalised Linear Model Repeated Measures Analysis of Variance (GLM RMANOVA), followed by a posthoc Tukey HSD test. All data were analysed using STATISTICA 7 (StatSoft, Tulsa, OK, USA). All results reported are mean ± standard error. 3. Results 3.1. Body mass Body mass of Cape White-eyes did not differ significantly in the morning (06:00 h) before feeding between the respective trials (RMANOVA: F5, 30 = 0.563, P = 0.727; Fig. 1; mean ± SE range 11.06 ± 0.25–11.30 ± 0.37 g), nor at 15:00 h (RMANOVA: F5, 30 = 2.034, P = 0.102; Fig. 1; mean ± SE range 11.24 ± 0.41–11.76 ± 0.46 g), nor the following morning at 07:30 h after the trials (RMANOVA: F5, 30 = 1.164, P = 0.350; Fig. 1; mean ± SE range 10.57 ± 0.25–10.91 ± 0.41 g). In contrast body mass of Amethyst Sunbirds did differ significantly in the morning (06:00 h) before feeding between the respective trials
42
M. Witteveen et al. / Comparative Biochemistry and Physiology, Part A 167 (2014) 40–44
A)
Vertical bars denote +/- standard errors 12.6
06h00 15h00 07h30 after
12.4 12.2
Body mass (g)
12.0 11.8 11.6 11.4 11.2 11.0 10.8 10.6 10.4 10.2 10.0
5%
10%
15%
20%
25%
35%
Hexose/Sucrose concentration
B)
Vertical bars denote +/- standard errors 17.5
06h00 15h00 07h30 after
17.0 16.5
Body mass (g)
16.0 15.5 15.0 14.5 14.0 13.5 13.0 12.5 5%
10%
15%
20%
25%
35%
Hexose/Sucrose concentration Fig. 1. Body mass (g) of (A) Cape White-eyes (Zosterops virens) and (B) Amethyst Sunbirds (Chalcomitra amethystina) fed sucrose–hexose sugar solutions of various concentrations, before the trial (06:00 h), after 9 h of feeding (15:00 h) and after an overnight fast (07:30 h after). Values represent means ± standard error.
(RMANOVA: F5, 30 = 5.253, P = 0.001; Fig. 1; mean ± SE range 14.21 ± 0.57–15.46±0.64g), and at 15:00h (RMANOVA: F5, 30 =4.66, P=0.003; Fig. 1; mean ± SE range 14.24 ± 0.52–16.03 ± 0.75 g), and the following morning at 07:30h after the trials (RMANOVA: F5, 30 = 5.556, P =0.001; Fig. 1; mean ± SE range 13.54 ± 0.58–14.94 ± 0.68 g). 3.2. Blood plasma glucose concentration (PGlu) For Cape White-eyes, both the sucrose–hexose sugar solution concentration and time of sampling significantly affected PGlu (RMANOVA: F5, 25 = 3.54, P = 0.015; F1, 25 = 28.14, P = 0.003 respectively, Fig. 2A). However, the combined effect of concentration and time did not significantly affect PGlu (F5, 25 = 0.74, P = 0.599). A post-hoc Tukey HSD test revealed that PGlu differed significantly because of time only on a 15% sucrose–hexose diet, where PGlu was higher at 15:00h (after 9h of feeding) than at 07:30h after fasting overnight (Fig. 2A). Furthermore, it was shown that within each sampling time (15:00 h and 07:30 h after) the PGlu did not differ significantly over any diet concentrations (Fig. 2A). PGlu averaged over all diet concentrations during the trials (15:00 h) was 15.85 ± 0.20 mmol/L, and after the trials (07:30 h after) was 14.33 ± 0.25 mmol/L. The highest mean PGlu was measured at 15:00 h during a 5% sucrose–hexose diet trial (16.8 ± 0.7 mmol/L), and the
lowest mean PGlu after overnight fasting at 07:30 h after a 20% sucrose–hexose diet trial (13.5 ± 0.3 mmol/L). The highest recorded PGlu (20.3 mmol/L) was at 15:00 h, after 9 h of feeding, during a 5% diet trial, and the lowest recorded PGlu (11.7 mmol/L) was at 07:30 h after fasting overnight, after a 5% diet trial. Similarly for Amethyst Sunbirds, both the sucrose–hexose sugar solution concentration and time of sampling significantly affected PGlu (RMANOVA: F5, 60 =5.48, P=0.001; F2, 60 =90.78, P=0.000 respectively, Fig. 2B). However, the combined effect of concentration and time did not significantly affect PGlu (F10, 60 = 1.93, P = 0.058). A post-hoc Tukey HSD test revealed that PGlu were significantly higher after 9 h of feeding at 15:00 h at all diet concentrations than after overnight fasting at 07:30 h (Fig. 2B). However, PGlu did not differ significantly within each sampling time (Fig. 2B). PGlu averaged over all diet concentrations during the trials (15:00 h) was 16.51 ± 0.33 mmol/L, and after the trials (07:30 h after) was 11.52 ± 0.19 mmol/L. The highest mean PGlu was measured at 15:00 h during a 35% sucrose–hexose diet trial (18.0 ± 0.8 mmol/L), and the lowest mean PGlu at 07:30 h after a 5% sucrose– hexose diet trial (11.1 ± 0.4 mmol/L). The highest recorded PGlu (22.1 mmol/L) was at 15:00 h, after 9 h of feeding, during a 15% diet trial, and the lowest recorded PGlu (8.3 mmol/L) was at 07:30 h after fasting overnight, after a 20% diet trial.
M. Witteveen et al. / Comparative Biochemistry and Physiology, Part A 167 (2014) 40–44
43
Plasma glucose concentration (m mol L-1)
A) Vertical bars denote +/- standard errors
20
15h00 07h30 after
19 18 17 16 15 14 13 12 11 10 9 8 5%
10%
15%
20%
25%
35%
Hexose/Sucrose concentration
Plasma glucose concentration (m mol L-1)
B)
Vertical bars denote +/- standard errors
20
15h00 07h30 after
19 18 17 16 15 14 13 12 11 10 9 8 5%
10%
15%
20%
25%
35%
Hexose/Sucrose concentration Fig. 2. Blood plasma glucose concentration (mmol/L) of (A) Cape White-eyes (Zosterops virens) and (B) Amethyst Sunbirds (Chalcomitra amethystina) fed sucrose–hexose sugar solutions of various concentrations, after 9 h of feeding (15:00 h) and after an overnight fast (07:30 h after). Values represent means ± standard error.
4. Discussion We found that within each sampling time, both Cape White-eyes and Amethyst Sunbirds PGlu were not affected by dietary sugar concentration. However, it appears that both species have a range that PGlu is regulated within with a lower value after fasting compared with a higher value after feeding on sugar solutions irrespective of concentration; i.e. PGlu was affected more by sampling time and whether fasted or not. Similarly Napier et al. (2008) found that diet treatment (sucrose concentration) did not significantly affect the mean steady-state concentration of radiolabelled L-glucose in blood plasma. This suggests, for both occasional and specialist avian nectarivores, that birds' PGlu is not affected by the sugar content of floral nectar or fruits, but rather whether they have fed or not. In most vertebrate homeostatic mechanisms, the parameter is usually regulated around a set point i.e. the body tries to keep the parameter at a particular value (Scanes and Braun, 2013). However, in the nectarivorous birds in this study it appears that PGlu is regulated over a range. The range over which Cape White-eyes regulated PGlu was narrower and less affected by sampling time (feeding 15:00 h or fasting 07:30 h after), and showed a simplified glucose–time curve. Lobban et al. (2010) sampled PGlu of the Cape White-eyes at four sampling times at 6 h intervals, and found at 06:00 h (after a night of fasting) and 18:00 h (after 12 h of feeding) that PGlu were not significantly
different. However, this excludes significant changes in PGlu including a peak at 12:00 h (18.1 ± 0.7 mmol/L) and a dip at 24:00 h (13.2 ± 0.5 mmol/L) (Lobban et al., 2010). These values are similar to those found in the current study, with the highest mean PGlu measuring 16.8 ± 0.7 mmol/L, and the lowest mean PGlu measuring 13.5 ± 0.3 mmol/L. In Amethyst Sunbirds, PGlu differed significantly from when birds were fasted compared with when fed; however they were able to regulate these levels within a range regardless of the dietary sugar concentration. Values were similar to those obtained for Malachite Sunbirds (Nectarinia famosa) at 04:00 h (13.6 ± 0.7 mmol/L) and 10:00 h (16.0 ± 0.8 mmol/L) (Downs et al., 2010). Again it suggests that PGlu in avian nectarivores is more affected by circadian or diel variation than diet. Meal size and frequency, and energy expenditure can cause PGlu to fluctuate, especially in birds which are migrating or do not have a territory and are unable to feed regularly enough to avoid an empty crop (Beuchat and Chong, 1998). In birds which are able to feed regularly PGlu is expected to remain at the upper end of the regulation range and relatively steady throughout the day (Beuchat and Chong, 1998). Avian nectarivores generally regulate energy intake by adjusting volumetric intake (Leseigneur and Nicolson, 2009; Brown et al., 2010a, b,c), including Amethyst Sunbirds (Köhler et al., 2010) and Cape White-eyes (Wellmann and Downs, 2009). Consequently the diel or
44
M. Witteveen et al. / Comparative Biochemistry and Physiology, Part A 167 (2014) 40–44
circadian variation in PGlu as seen in studies by Downs et al. (2010) and Lobban et al. (2010), questions why this variation? With so many factors uncontrolled in natural conditions, the importance of using measures of PGlu other than mean values for more meaningful interspecific comparisons, as promoted by Lobban et al. (2010), is reiterated. The nectar-feeding bat Glossophaga soricina is only able to regulate blood glucose levels, especially on concentrated sugar solutions, by spending 60–75% of their time flying, an energetically costly activity (Kelm et al., 2011). The activity of the birds on each sugar concentration in the current study was not observed, and it would be interesting in further studies to observe and contrast bird activity on low and high sugar concentrations, perhaps high levels of activity enable them to regulate blood glucose levels, along with physiological mechanisms. However, as birds regulate energy intake by regulating volumetric intake as mentioned, this would rescind the necessity of PGlu regulation through increased activity. Our tentative comparison between an occasional and a specialist nectarivore shows that dietary sugar concentration does not affect either species' ability to regulate PGlu. Both species regulate PGlu over a range rather than around one set point. However, Amethyst Sunbirds had a broader range of PGlu than Cape White-eyes. Consequently it appears that avian nectarivores are not constrained by dietary sugar concentration with regard to PGlu regulation, so it would seem that selective pressure on plants for their nectar characteristics is due to reasons other than glucose regulation. Physiological processes, mainly digestive and centred on sucrase activity, are most likely the cause of selective pressure on nectar characteristics (Lotz and Schondube, 2006; Napier et al., 2013). A wider range of occasional and specialist avian nectarivore species should be investigated for their tolerance to variations in dietary content as related to their ability to regulate PGlu concentration for a full phylogenetic comparison to be done. Furthermore, PGlu should be measured more regularly throughout the scoto- and photophase to determine fluctuations and so elucidate a time or level (maximum or minimum) for interspecific comparisons.
Acknowledgements We are grateful to T. Mjwara for assistance in animal maintenance. L. Thompson and S. McPherson are thanked for assistance in catching birds and during trial days. Two anonymous reviewers are thanked for valuable comments on a previous draft of this manuscript.
References Beuchat, C.A., Chong, C.R., 1998. Hyperglycemia in hummingbirds and its consequences for hemoglobin glycation. Comp. Biochem. Physiol. A 120, 409–416. Bizaaré, L., Coetzer, T.H.T., Downs, C.T., 2012. Disaccharidase presence and activities in a range of southern African frugivores. Ostrich 83, 165–168. Botes, C., Johnson, S.D., Cowling, R.M., 2008. Coexistence of succulent tree aloes: partitioning of bird pollinators by floral traits and flowering phenology. Oikos 117, 875–882. Brandenburg, A., Dell'Olivo, A., Bshary, R., Kuhlemeier, C., 2009. The sweetest thing: advances in nectar research. Curr. Opin. Plant Biol. 12, 486–490. Braun, E., Sweazea, K.L., 2008. Glucose regulation in birds. Comp. Biochem. Physiol. B 151, 1–9. Brown, M., Downs, C.T., Johnson, S.D., 2010a. Concentration-dependent sugar preferences of the Malachite Sunbird (Nectarina famosa). Auk 127, 151–155. Brown, M., Downs, C.T., Johnson, S.D., 2010b. Sugar preferences and digestive efficiency in an opportunistic avian nectarivore, the Dark-capped Bulbul Pycnonotus tricolor. J. Ornithol. 151, 637–643. Brown, M., Downs, C.T., Johnson, S.D., 2010c. Sugar preference of a generalist nonpasserine flower visitor, the African Speckled Mousebird (Colius striatus). Auk 127, 781–786. Brown, M., Downs, C.T., Johnson, S.D., 2011. Covariation of flower traits and bird pollinator assemblages among populations of Kniphofia linearifolia (Asphodelaceae). Plant Syst. Evol. 294, 199–206. Brown, M., Downs, C.T., Johnson, S.D., 2012. African Red-winged Starlings prefer hexose sugar solutions, but do not like them too sweet. J. Ornithol. 153, 265–272. Bruneau, A., 1997. Evolution and homology of bird pollination syndromes in Erythrina (Leguminosae). Am. J. Bot. 84, 54–71.
Chalcoff, V.R., Aizen, M.A., Galetto, L., 2008. Sugar preferences of the Green-backed Firecrown hummingbird (Sephanoides sephaniodes): a field experiment. Auk 125, 60–66. Downs, C.T., 1997. Sugar digestion efficiencies of Gurney's Sugarbirds, Malachite Sunbirds and Black Sunbirds. Physiol. Zool. 70, 93–99. Downs, C.T., Wellmann, A.E., Brown, M., 2010. Diel variations in plasma glucose concentrations of Malachite Sunbirds Nectarinia famosa. J. Ornithol. 151, 235–239. Driver, E.A., 1981. Hematological and blood chemical values of mallards, Anas p. platyrhynchos, drakes before, during and after remige moult. J. Wildl. Dis. 17, 413–421. Fokidis, H.B., Hurley, L., Rogowski, C., Sweazea, K., Deviche, P., 2011. Effects of captivity and body condition on plasma corticosterone, locomotor behavior, and plasma metabolites in Curve-billed Thrashers. Physiol. Biochem. Zool. 84, 595–606. Franke, E., Jackson, S., Nicolson, S., 1998. Nectar sugar preference and absorption in a generalist African frugivore, the Cape White-eye Zosterops pallidus. Ibis 140, 501–506. Frelin, C., 1974. Daily variations of serum glucose levels in moulting Redpolls Acanthis flammea and in migrating robins Erithacus rubecula. Ornis Scand. 5, 83–86. Frost, S.K., Frost, P.G.H., 1981. Sunbird pollination of Strelitzia nicolai. Oecologia 49, 379–384. Gayathri, K.L., Shenoy, K.B., Hegde, S.N., 2004. Blood profile of pigeons (Columba livia) during growth and breeding. Comp. Biochem. Physiol. A 138, 187–192. Hargrove, J.L., 2005. Adipose energy stores, physical work, and the metabolic syndrome: lessons from hummingbirds. Nutr. J. 4, 36. Hockey, P.A.R., Dean, W.R.J., Ryan, P.G. (Eds.), 2005. Roberts — Birds of Southern Africa. The Trustees of the John Voelcker Bird Book Fund, Cape Town. Jenni, L., Jenni-Eiermann, S., 1996. Metabolic responses to diurnal feeding patterns during the postbreeding, moulting and migratory periods in passerine birds. Funct. Ecol. 10, 73–80. Johnson, S.D., Nicolson, S.W., 2008. Evolutionary associations between nectar properties and specificity in bird pollination systems. Biol. Lett. 4, 49–52. Johnson, S.D., Hargreaves, A.L., Brown, M., 2006. Dark, bitter-tasting nectar functions as a filter of flower visitors in a bird-pollinated plant. Ecology 87, 2709–2716. Kelm, D.H., Simon, R., Kuhlow, D., Voigt, C.C., Ristow, M., 2011. High activity enables life on a high-sugar diet: blood glucose regulation in nectar-feeding bats. Proc. R. Soc. B Biol. Sci. 278, 3490–3496. Kern, M., Bacon, W., Long, D., Cowie, R.J., 2005. Blood metabolite and corticosterone levels in breeding adult pied flycatchers. Condor 107, 665–677. King, G.L., Loeken, M.R., 2004. Hyperglycemia-induced oxidative stress in diabetic complications. Histochem. Cell Biol. 122, 333–338. Köhler, A., Verburgt, L., McWhorter, T.J., Nicolson, S.W., 2010. Energy management on a nectar diet: can sunbirds meet the challenges of low temperature and dilute food? Funct. Ecol. 24, 1241–1251. Leseigneur, C.D.C., Nicolson, S.W., 2009. Nectar concentration preferences and sugar intake in the white-bellied sunbird, Cinnyris talatala (Nectariniidae). J. Comp. Physiol. B. 179, 673–679. Lill, A., 2011. Sources of variation in blood glucose concentrations of free-living birds. Avian Biol. Res. 4, 78–86. Lobban, K., Downs, C.T., Brown, M., 2010. Diel variations in plasma glucose concentration in some South African avian frugivores. Emu 110, 66–70. Lotz, C.N., Schondube, J.E., 2006. Sugar preferences in nectar- and fruit-eating birds: behavioural patterns and physiological causes. Biotropica 38, 3–15. Martinez del Rio, C., 1990. Dietary, phylogenetic, and ecological correlates of intestinal sucrase and maltase activity in birds. Physiol. Zool. 63, 987–1011. Medina-Tapia, N., Ayala-Berdon, J., Morales-Pérez, L., Melo, L.M., Schondube, J.E., 2012. Do hummingbirds have a sweet-tooth? Gustatory sugar thresholds and sugar selection in the broad-billed hummingbird Cynanthus latirostris. Comp. Biochem. Physiol. A 161, 307–314. Napier, K.R., McWhorter, T.J., Nicolson, S.W., Fleming, P.A., 2013. Sugar preferences of avian nectarivores are correlated with intestinal sucrase activity. Physiol. Biochem. Zool. 86, 499–512. Napier, K., McWorter, Fleming, P.A., 2008. Mechanism and rate of glucose absorption differ between an Australian honeyeater (Meliphagidae) and a lorikeet (Loriidae). J. Exp. Biol 211, 3544–3553. Polakof, S., Mommsen, T.P., Soengas, J.L., 2011. Glucosensing and glucose homeostasis: from fish to mammals. Comp. Biochem. Physiol. B 160, 123–149. Prinzinger, R., Misovic, A., 2010. Age-correlation of blood values in the Rock Pigeon (Columba livia). Comp. Biochem. Physiol. A 156, 351–356. Scanes, C.G., Braun, E., 2013. Avian metabolism: its control and evolution. Front. Biol. 8, 134–159. Simpson, B.B., Neff, J.L., 1981. Floral rewards: alternatives to pollen and nectar. Ann. Mo. Bot. Gard. 68, 301–322. Smith, C.L., Toomey, M., Walker, B.R., Braun, E.J., Wolf, B.O., McGraw, K., Sweazea, K.L., 2011. Naturally high plasma glucose levels in mourning doves (Zenaida macroura) do not lead to high levels of reactive oxygen species in the vasculature. Zoology 114, 171–176. Symes, C.T., Nicolson, S.W., McKechnie, A.E., 2008. Response of avian nectarivores to the flowering of Aloe marlothii: a nectar oasis during dry South African winters. J. Ornith. 149, 13–22. Villegas, A., Sánchez, J.M., Costillo, E., Corbacho, C., 2002. Blood chemistry and haematocrit of the black vulture (Aegypius monachus). Comp. Biochem. Physiol. A 132, 489–497. Warne, J.P., Akana, S.F., Ginsberg, A.B., Horneman, H.F., Pecoraro, N.C., Dallman, M.F., 2009. Disengaging insulin from corticosterone: roles of each on energy intake and disposition. Am. J. Physiol. Regul. Integr. Comp. Physiol. 296, R1366–R1375. Wellmann, A.E., Downs, C.T., 2009. Sugar preferences and digestion by Cape White-eyes, Zosterops virens, fed artificial fruit diets. Afr. Zool. 44, 106–116. Wester, P., Claβen-Bockhoff, R., 2006. Bird pollination in South African Salvia species. Flora 201, 396–406.