Photoperiodism, pineal clock and seasonal reproduction in the Indian Weaver Bird (Ploceus...

ORIGINAL ARTICLE

Photoperiodism, pineal clock and seasonal reproductionin the Indian Weaver Bird (Ploceus philippinus)

Sangeeta Rani Æ Sudhi Singh Æ Vinod Kumar

Received: 24 April 2007 / Revised: 24 September 2007 / Accepted: 25 September 2007 / Published online: 24 October 2007

� Dt. Ornithologen-Gesellschaft e.V. 2007

Abstract Previous studies show that, in birds, pineal

melatonin is important for circadian rhythmicity, and cir-

cadian rhythms mediate photoperiodic effects. The effect

of pinealectomy or melatonin administration in photo-

periodic induction of testicular growth is not found in many

bird species. This is inconsistent with the fact that avian

pineal is a self-sustained circadian biological clock and

decodes both the daily and annual photoperiodic informa-

tion. Does this mean that the pineal clock in birds regulates

circadian rhythms, but not the one that is involved in the

photoperiodic induction of the seasonal response? We have

examined this in experiments on the subtropical Indian

Weaver Bird (Ploceus philippinus). We investigated the

effects of the absence of the pineal gland or exogenous

melatonin administration on circadian rhythmicity in

activity, and on photoperiodic induction of testicular

growth, androgen-dependent beak pigmentation and

luteinizing hormone-specific plumage coloration. Weaver

birds were subjected for several weeks to short day length

(8 h light: 16 h darkness, 8L:16D), long day length

(16L:8D) and to light–dark cycles that tested the involve-

ment of circadian rhythmicity in photoperiodic time

measurement. The results show that circadian pineal clock

may regulate the circadian rhythm in activity, but is not

essential for the expression of photoperiodism associated

with the photoperiodic induction of testicular growth. This

suggests that the circadian activity and photosensitivity

rhythms are outputs of different circadian oscillators or of

the same set of circadian oscillators, but that they are not

closely coupled.

Keywords Circadian rhythm � Melatonin � Photoperiod �Pineal � Ploceus philippinus

Introduction

Day length regulates seasonal cycle of gonadal growth and

regression in many birds (Gwinner and Hau 2000; Dawson

et al. 2001). An endogenous clock system sensitive to light

enables the bird to synchronize its physiological activities

at the appropriate time of the day and time of the year

(Kumar and Follett 1993a; Kumar et al. 1996a). In a typical

photoperiodic species that uses long days to initiate its

gonadal growth and development, endogenous circadian

rhythm sensitive to light becomes photoinducible about

12 h after dawn each day. Hence, spring and summer day

lengths with light period ‡12 h per day are read as a ‘‘long

day’’ and result in induction of gonadal growth and

development. Conversely, autumn and winter day lengths

with light period \12 h per day are read as ‘‘short day’’

and, consequently, there is no photoperiodic induction of

gonadal growth (Kumar and Follett 1993a).

The clock system in birds appears to have multiple

independent pacemakers sited in a minimum of three pla-

ces—the hypothalamus (avian SCN), the pineal gland, and

the retina of eyes (Kumar et al. 2004; Kumar and Singh

2006). Unlike in mammals, where the input, pacemaker

and output are localized in different structures, all the three

avian clocks have independent input–pacemaker–output

systems. Hence, the avian pineal is an autonomous clock

with its own input–oscillator–output systems (summary in

Communicated by F. Bairlein.

S. Rani (&) � S. Singh � V. Kumar

DST-IRHPA Unit on Biological Rhythm Research,

Department of Zoology, University of Lucknow,

Lucknow 226 007, India

e-mail: [email protected]

123

J Ornithol (2007) 148 (Suppl 2):S601–S610

DOI 10.1007/s10336-007-0236-z

Gwinner et al. 1997; Kumar et al. 2004; Kumar and Singh

2006). The melatonin produced at night is the best known

output of the pineal gland. The secretion of melatonin

tracks the night length (= day length; Kumar and Follett

1993b). It is also shown that the avian pineal decodes daily

and annual photoperiodic information both in the duration

and amplitude of its melatonin output (Kumar and Follett

1993b; Brandstatter et al. 2000a, b; Kumar 2001). There-

fore, melatonin could play a key role in photoperiodic

control of the daily and seasonal rhythms in vertebrates. In

fact, both winter and summer breeding mammals use

melatonin in photoperiodic regulation of their reproductive

cycle (for references, see review in Lincoln et al. 2003).

In birds, however, the effect of pinealectomy or mela-

tonin administration in photoperiodic induction of testic-

ular growth is not significant, although a few studies do

show the effects of pinealectomy and exogenous mela-

tonin on gonadal growth and regression (see review in

Kumar 2001). It will be interesting to consider whether

the absence of the effects of pinealectomy or exogenous

melatonin on photoperiodic induction of gonadal devel-

opment would mean the absence of the role of pineal

melatonin in regulation of reproduction in birds. This

might not be the case since, in a recent study, Ubuka

et al. (2005) have clearly shown the effects of pinealec-

tomy on the expression of gonadotropin-inhibitory

hormone (GnIH) in the Japanese Quail (Coturnix coturnix

japonica). GnIH is a recently identified neuropeptide

whose expression is photoperiodically controlled. The

expression of GnIH is high under short days when the

duration of nocturnal melatonin is increased, and low

under long days when the duration of nocturnal melatonin

is decreased (Ubuka et al. 2005). Ubuka et al. (2005)

further found that pinealectomy combined with orbital

enucleation (in Japanese Quail, the eyes produce a sig-

nificant amount of melatonin, Underwood et al. 1984)

decreased the expression of GnIH precursor mRNA and

content of peptide in the brain, and melatonin adminis-

tered to pinealectomized and enucleated quails restored

both of them in a dose-dependent manner.

Regardless of its effect on photoperiodism, the pineal

clock is important for avian circadian rhythmicity. Nearly

four decades ago, Gaston and Menaker (1968) reported

arrhythmicity in locomotor activity pattern of pinealec-

tomized House Sparrow (Passer domesticus). In a later

study, the same group reported that after a pineal from

another sparrow was transplanted into the anterior chamber

of the eye, pinealectomized arrhythmic sparrows become

re-rhythmic with the phase of the donor (Zimmerman and

Menaker 1979). Disruption of circadian rhythmicity was

further confirmed in several other bird species when either

the pineal was removed or birds were implanted with

melatonin-filled silastic tubes which elevated the blood

melatonin levels such that the rhythmicity in melatonin

itself was abolished (Gwinner et al. 1997; Kumar 2001).

Therefore, it is important to consider whether in birds

the photoperiodic induction and activity-rest cycles, both

mediated by the circadian system, are outputs of the same

circadian oscillator or of the same set of circadian osci-

llators that are very closely coupled or of different

circadian oscillators. The present mini-review article will

focus on this, with the background of a brief summary of

the results on role of pineal in activity rhythms and pho-

toperiodic induction of primary and secondary sexual

characteristics in the Indian Weaver Bird (Ploceus philip-

pinus), a photoperiodic subtropical passerine finch (also

known as the Baya Weaver). Interestingly, a few previous

studies have recorded activity rhythms in support of the

evidence that circadian system is involved in the photo-

periodic time measurement (see, e.g., Hamner and Enright

1967; Elliott et al. 1972). This was done with the

assumption that circadian activity rhythm reflects the

characteristics of the circadian rhythm that underlies

photoperiodism.

Photoperiodism and circadian rhythms in the Indian

Weaver Bird

The Indian Weaver Bird (Ploceus philippinus) is a sea-

sonally breeding species with the breeding season

extending between May and September (Ali and Ripley

1974) (Fig. 1). The onset of gonadal growth is triggered by

increasing late spring/summer day lengths, and the breed-

ing season ends in early fall with the development of

photorefractoriness which is evidenced by the onset of

gonadal regression and molt (Saxena 1964; Nair and

Chandola-Saklani 1998; S. Rani et al., unpublished).

Annual gonadal cycle of the Indian Weaver Bird is

described in four phases: preparatory, progressive, breed-

ing, and regressive (Saxena 1964). Laboratory

investigations have also confirmed the role of environ-

mental photoperiod in regulation of the Weaver Bird’s

reproductive cycle. Day lengths ‡12 h per day initiate

testicular recrudescence, and post-stimulation Weaver

Birds become refractory, albeit briefly, to stimulatory

photoperiods equivalent to what they experience in the

wild (Thapliyal and Tewary 1964; Singh nee Pavgi and

Chandola 1981; Chakravorty and Chandola-Saklani 1985;

Nair and Chandola-Saklani 1998). Using standard lighting

protocols (see Kumar and Follett 1993a), it has been

demonstrated that the circadian rhythm sensitive to light

appears mediating photoperiodic induction of gonadal

recrudescence in Weaver Birds (Chandola et al. 1976;

Pavgi and Chandola 1983).

S602 J Ornithol (2007) 148 (Suppl 2):S601–S610

123

The Indian Weaver Bird is diurnal species being active

during the day. The circadian control of activity and its

synchronization has been readily documented in the fol-

lowing experiment. Birds were released into constant dim

light condition (LLdim) following exposure to a pro-

grammed light–dark (LD) cycle in order to examine the

phase and period of the circadian rhythm as assayed by a

behavioral (activity) rhythm. The details of the experiment

are as follows. Birds (n = 5) were subjected to a complete

photoperiod [12 h light (400 lux):12 h darkness

(= \1 lux); 12L:12D] for 24 days and then released into

constant dim light conditions (LLdim, \1 lux; nighttime

illumination). On day 13 of LLdim, a single 4-h light period

(400 lux) was introduced in the subjective night beginning

at zeitgeber time 12 (ZT 12; ZT 0 = light onset of previous

12L:12D) to examine the response of circadian system to a

light perturbation. Birds remained in LLdim for another 7

days and then returned to 12L:12D. After 17 days of

resynchronization to 12L:12D, the lighting protocol was

changed to the corresponding skeleton photoperiod

(1L:10D:1L:12D) by introducing a 10-h dark period

(\1 lux) beginning at ZT 2. After 9 days of skeleton

photoperiod, birds were again released into LLdim.

Activity-rest pattern was continuously measured as the

bird’s general activity within its cage, as described else-

where (Malik et al. 2004; Trivedi et al. 2006). Briefly, each

bird was housed in a specially designed activity cage fur-

nished with two perches and mounted with a Passive

Infrared Motion Sensor [C & K Systems (Intellisense

XJ-413T) Conrad Electronic, Germany, Haustier PIR-

Melder]. Each sensor detected the movement of the bird

within the cage, and this information was stored on the

computer and analyzed using a software program from

Stanford Software Systems, Stanford, USA.

Figure 2 presents the data. There was a clear bimodal

activity pattern under 12L:12D: the activity in the morning

(*3 h) and evening (*1 h) was significantly higher

(P \ 0.05; one-way RM ANOVA, Student Newman Keuls

post hoc test;) than in the rest of the light period (Fig. 2a).

Such activity pattern seen in several other species is con-

sistent with a two-oscillator, the morning (M) and evening

(E), circadian system governing and synchronizing circa-

dian rhythms of Weaver Birds with the environmental

photoperiod (Aschoff 1981; Pittendrigh 1981; Daan et al.

2001). That the activity-rest cycles of the Indian Weaver

Birds were indeed circadian was confirmed from the data

under LLdim, in which all the individuals exhibited activity

pattern with the mean (±SE) period of 23.7 ± 0.34 h, and

the phase of activity was maintained after environmental

synchronization. Secondly, their sensitivity to light as an

environmental zeitgeber was demonstrated by the delayed

phase shift in response to a single 4-h light pulse at ZT12 as

a result of single light perturbation. Here, the mean (±SE)

circadian period of activity rhythm was changed to

24.78 ± 0.47 h (Fig. 2b).

Return to 12L:12D re-established the bimodality in

activity pattern. The amplitude of activity at this time

appeared slightly lower, perhaps due to acclimation to the

caged situation, but the difference between two 12L con-

ditions was not significant (P [ 0.05; two-way ANOVA)

(Fig. 2a,c). The change to skeleton paradigm restricted

activity mainly to short light periods, perhaps due to

masking effects (Fig. 2d). In the following LLdim, two of

five birds appeared to have lost their circadian rhythm

(s[ 27 h), but three individuals still retained circadian

rhythmicity with the mean (±SE) period of 24.25 ± 0.41 h

(Fig. 2e). Whether the loss of circadian rhythmicity in

some individuals was due to exposure to the skeleton

photoperiod, needs to be investigated.

Photoperiodic induction in the Indian Weaver Bird:

role of pineal and melatonin

More than three decades ago, Balasubramanian and Saxena

(1973) reported the acceleration of testis growth, the

darkening of androgen-dependent beak color and the

yellow pigmentation of luteinizing hormone (LH)-depen-

dent plumage in pinealectomized birds held under naturally

increasing day lengths or exposed to artificial long day

lengths (18L:6D). More interestingly, when reproductively

mature Weaver Birds were pinealectomized and exposed to

short day lengths (9L:15D), a non-stimulatory photoperiod

which causes full testicular regression and concomitant

changes in secondary sexual characters, there was only a

Fig. 1 A schematic plot of the seasonal reproductive cycle of the

Indian Weaver Bird (Ploceus philippinus) based on information as

reported in the literature (Ali and Ripley 1974; Saxena 1964;

Thapliyal and Saxena 1964). Alphabets at the periphery indicate

months of the year. Note the temporal relationship between changes

in day lengths of late spring/summer and different reproductive

activities

J Ornithol (2007) 148 (Suppl 2):S601–S610 S603

123

transient and partial regression of testes; pinealectomized

individuals never fully regressed, lightened their beak to

straw color or de-pigmented their plumage to the non-

breeding (‘‘female like’’ or ‘‘henny’’) type. The results,

which clearly evidenced an inhibitory role of the pineal

gland on annual reproductive cycle of Indian Weaver

Birds, were significant and generated lots of interest since

they were inconsistent with the majority of results from

experiments done on birds since then. Therefore, in a

recent study reported elsewhere, Rani et al. (2005) have

investigated whether the pineal had indeed an inhibitory

role under short days. In this experiment, Indian Weaver

Birds were pinealectomized during different phases of the

annual cycle and exposed to short (8L:16D) and long day

lengths (16L:8D). The changes in testicular size and

pigmentation of beak and plumage were recorded at regular

intervals.

Pinealectomized and sham-operated Weaver Birds

(n = 6–8 each) were exposed to short day lengths (in late

September) when they had almost regressed testes [mean

(±SE) testis volume (TV) = 3.09 ± 0.45 mm3] but dark

pigmented beak [mean (±SE) score = 3.15 ± 0.13] and

nuptial plumage [mean (±SE) score = 3.00 ± 0.00, head;

5.00 ± 0.00, breast], and to long day lengths (in early

March) when they had small testes [mean (± SE) TV =

0.34 ± 0.07 mm3], unpigmented beak and ‘‘female-like’’

plumage.

The dimensions of the left testis were measured by

unilateral laparotomy under local anesthesia as described

by Kumar et al. (2001), and from this TV was calculated

15

25

30

35

40

45

50

55

60

65

70

75

80

Day

sTime of the day

12L:12D (a)

LLdim (b)

12L:12D (c)

Light pulseZT12-16

1L:10D:1L:12D(d)

LLdim (e)

0 6 12 18 24

0 6 12 18 24

0 6 12 18 24

4 h light pulse

Hours

Hours

Hours

0 4 8 12

0 1 2 3 4 5 6 7

Days

Act

ivit

y co

un

ts/ m

inu

te

Days

a

b

c

d

e

0600 1800 0600 1800 0600 0600 1800 0600 1800 0600

Fig. 2 Left panels: double plotted activity recordings of two repre-

sentative male Indian Weaver Birds sequentially exposed to 12L:12D

(a), LLdim (b), 12L:12D (c), 1L:10D:1L:12D (d) and LLdim (e)

(L = 400 lux; D = \1 lux; LLdim = \1 lux). When in the first LLdim,

a single 4-h light pulse (400 lux) was introduced beginning at

zeitgeber time 12 (ZT12, ZT0 = lights on). Horizontal bar at the topshows clock time of the day: 0600 hours—lights ON of an LD cycle.

Right panels: mean (±SE, n = 5) activity profile of all birds in the

experiment at different times of the experiment, compare with

actograms on the left with corresponding figures (a–e). Note a

bimodal activity pattern under 12L:12D and light-restricted activity

under 1L:10:1L:12D. Also note the circadian period under LLdim after

12L:12D changed from (mean ± SE) 23.7 ± 0.34 h to 24.78 ± 0.47

after imposition of 4-h pulse demonstrating the sensitivity of the

circadian clock to single light perturbation


123

using the formula 4/3 p ab2, where a and b denote half of

the long and short axes, respectively. Beak color in males

changes over seasons from non-breeding straw to breeding

dark black. Similarly, the plumage color over the head and

breast changes from non-breeding ‘‘female-like’’ to

breeding (nuptial) bright yellow. We recorded these

changes using a subjective criterion, and considered them

an indicator of the endogenous levels of androgen (beak

color; Saxena and Thapliyal 1962) and LH (plumage color;

Thapliyal and Saxena 1961) levels. We scored both the

beak and plumage colors for better illustration of the

observations and statistical comparisons, as described by

Trivedi (2004; see also Fig. 3). Briefly, beak color was

scored on a scale of 0–5 as follows: 0 = straw in color (S),

1 = straw with a little tinge of blackness (ratio = SSS:B),

2 = slightly blackish (ratio = SS:B), 3 = straw and black in

approximately 50:50 patches (ratio = S:B), 4 = black with

very little straw patch left (ratio = S:BB), 5 = fully black

(B). Similarly, plumage color was scored on a scale of 1–3

[head: 1 = ‘‘female-like’’ feathers; 2 = mixed ‘‘female-

like’’ and yellow (nuptial) feathers; 3 = complete yellow

feathers] or 0–5 [breast: 0 = all ‘‘female-like’’ (no yellow)

feathers; 5 = all yellow (no female-like) feathers; every 0.5

increment in this scale (e.g., 0.5, 1.0 and so on] meant 10%

increase from ‘‘female-like’’ to nuptial feathers].

Figure 4b,d shows that, contrary to the expectation

based on the previous results of Balasubramanian and

Saxena (1973), there was no photostimulation under short

day lengths; instead, relatively larger testes of September

underwent testicular regression. Similarly, testes under-

went significant growth and regression under long day

lengths. The response of both the pinealectomized and

sham-operated birds to short or long day lengths was

essentially similar, except that under long days pinealec-

tomized birds had a faster rate and greater amplitude of the

photoperiodic induction. Changes in beak pigmentation

and plumage color (data not shown) also corresponded to

the effects found on testes. Birds darkened and lightened

their beak, respectively, as they grew, and regressed their

testes, confirming a previous finding of Thapliyal and

Saxena (1961). Also, there were increased numbers of

bright yellow feathers over the head and breast regions in

birds that were stimulated under long days, reflecting the

activity of the hypothalamo-hypophyseal (h-h) axis. A

study of Kumar et al. (2002) showing that exogenous

melatonin given via silastic implants to Weaver Birds in

late August failed to show an effect either on testes or on

androgen-dependent beak color supports the present

results.

Figure 4 (cf. a–d) also compares some of the results from

the two studies on Indian Weaver Birds: those of Bala-

subramanian and Saxena (1973) with Rani et al. (2005),

briefly described above. Clearly, the results from the study

of Rani et al. (2005) were only partially consistent with

those of Balasubramanian and Saxena (1973), as noted

above. However, we suggest that the effect of pinealectomy

on photoperiodic induction in the Indian Weaver Bird is

dependent on the phase of the birds’ annual gonadal cycle.

Also, a reproductive phase dependent effect of the pineal-

ectomy and of exogenous melatonin has been reported,

respectively, in Indian Jungle Bush Quail (Perdicula

asiatica; Haldar and Ghosh 1990) and Rose-ringed Para-

keet, (Psittacula krameri; Maitra and Dey 1992).

In another long-term experiment, we further addressed

on the issue of the role of pineal in photoperiodism of the

Indian Weaver Bird. We pinealectomized or sham operated

Weaver Birds (n = 5 each) in the first week of July 2004

when testes were still large. These birds were kept in an

outdoor aviary (Lucknow, India: 26.55�N, 80.59�E), and

thus experienced the changes in day length that would

occur in wild. Changes in testicular size and beak and

plumage pigmentation were measured over the next 15

Fig. 3 Pictures of head and

breast region of a male Indian

Weaver Bird showing plumage

and beak pigmentation during

their non-breeding (a, b),

transitory (c, d) and breeding (e,

f) stages. On the right hand sideare shown the scores we have

accorded for our study as

described in the text


123

months. Figure 5 shows the results. Testes regressed in

response to decreasing day lengths at the similar rate in

both the pinealectomized and sham-operated birds. With

the increasing day lengths of the following spring/summer,

testes began to recrudesce, but the reinitiation of testis

growth and regression was advanced by about a month in

the pinealectomised group (Fig. 5d). As with the previous

experiment, the changes in beak and plumage pigmentation

corresponded to the effects found on testes (Fig. 5a–c).

Thus, the removal of the pineal affected only the temporal

phasing of the testicular cycle, as suggested earlier by

Kumar et al. (2002) from their study on migratory Red-

headed Buntings (Emberiza bruniceps).

Circadian system in the Indian Weaver Bird:

role of pineal gland

Pineal is part of the avian circadian clock system (Kumar

et al. 2004). Also, as noted above, a direct effect of the

pinealectomy and/or melatonin administration has been

shown in regulation of several circadian rhythms (Gaston

and Menaker 1968; Gaston 1971; Binkley et al. 1971,

1972; Menaker and Zimmerman 1976; Turek et al. 1976;

Binkley and Mosher 1985; Gwinner et al. 1987, 1997;

Beldhuis et al. 1988; Gwinner 1989; Oshima et al. 1989;

Chabot and Menaker 1992; Lu and Cassone 1993a, b; Pohl

2000). Of all the overt markers of the circadian system, the

rhythm of locomotor activity is conveniently measurable

and is also reliable. Therefore, in an experiment, we

examined the role of pineal in regulation of circadian

activity rhythms of Indian Weaver Birds. A group of birds

procured during the preparatory phase of their annual cycle

(first week of January) were subjected to 12L:12D

(L = 50 ± 2 lux; D = \1.0 lux). After 2 weeks, they were

pinealectomized or sham-operated and retained in 12L:12D

for another 2 weeks. Then, they were released into LLdim

(\1.0 lux) for another 2 weeks. Their activity was recorded

continuously as described above.

Figure 6 shows the activity record of the representative

pinealectomized and sham-operated birds. Both birds

showed good entrainment to LD cycle, but thereafter under

LLdim the rhythmicity was lost after a few cycles in the

pinealectomized but not in the sham-operated birds. This

clearly provides evidence for the major role of the pineal

clock in the generation of circadian rhythmicity at least in

behavioral functions. This is consistent with the findings on

other songbirds in which a similar role of pineal is sug-

gested (summary in Kumar 2001). The pinealectomized

birds, however, did not become arrhythmic immediately.

Test

is V

olu

me

(mm

3 )Te

stis

Vo

lum

e (m

m3 )

Test

is V

olu

me

(mm

3 )Te

stis

Vo

lum

e (m

m3 )

Fig. 4 Comparison of the

results from two studies done on

Indian Weaver Birds

investigating the role of pineal

in photoperiodic induction. aand c (adapted from

Balasubramanian and Saxena

1973); b and d (adapted from

Rani et al. 2005). Note the

difference in photoperiod used

in the two studies, but clearly

the results of Rani et al. (2005)

did not support an inhibitory

role of the pineal under short

days


123

Rather, the loss of circadian rhythmicity was gradual. This

indicated that other clocks (i.e., hypothalamic clock)

maintained rhythmicity for a few days in pinealectomized

birds, but the lack of the sustainment of circadian

rhythmicity for a longer period in these individuals clearly

suggested that the input from the pineal to the circadian

system was vital. In birds, the contribution from all the

components of the central clock system appears crucial for

the sustainment of circadian rhythmicity (Kumar et al.

2004). In a previous study, it was shown that the lesions of

the hypothalamic pacemaker resulted in severe impairment

of the rhythmicity in pineal intact House Sparrows

(Takahashi and Menaker 1982). Similarly, complete

removal of eyes (blinding) produced arrhythmicity in the

Japanese Quail (Underwood 1994). In all probability,

therefore, the pineal interacts with the hypothalamic oscil-

lator and possibly the retinal oscillator (Kumar et al. 2004).

Because the pineal is part of the avian timekeeping

system that regulates a variety of circadian rhythms

(Kumar 2001; Gwinner and Brandstatter 2001; Kumar

et al. 2004), its absence should disrupt the circadian

rhythms that helps distinguish Weaver Birds between short

and long days. However, the findings described above

suggest that in Weaver Birds pinealectomy disrupted cir-

cadian activity rhythm but not the photoperiodic time

measurement; pinealectomized birds continued showing

photoperiodic responses characteristic of short and long

days (cf. Figs. 4, 6). Studies on several other species

including European Starling (Sturnus vulgaris; Gwinner

and Dittami 1980), Spotted Munia (Lonchura punctulata;

Chandola-Saklani et al. 1988), Tree Sparrow (Spizella

arborea; Wilson 1991), Japanese Quail (Kumar et al.

1993), Black-headed Bunting (Emberiza melanocephala;

Kumar 1996; Kumar et al. 1996b) and Red-headed Bunting

(Kumar et al. 2002) also showed similar results.

Conclusion and perspective

The pineal clock regulates circadian rhythms, but appears

not to mediate the circadian rhythm-dependent photo-

periodic measurement in the Indian Weaver Bird. It is likely

that the circadian clock regulating photoperiodic induction

is different from the one that regulates other circadian

rhythms, e.g., rhythms in activity–rest. An earlier finding on

Japanese Quail also showed dissociation between circadian

rhythms of photoperiodically induced LH release and

locomotor activity rhythm (Kumar et al. 1993; Juss et al.

1995). A molecular assay of the hypothalamic clock

(median suprachiasmatic nucleus; mSCN) of Japanese

Quail also did not support the role of melatonin in avian

clock system. Given as a single injection or as implants

melatonin did not affect the expression of qper2, qper3 and

qclock, despite the fact that the Quail mSCN had detectable

levels and daily rhythmicity of both Mel 1a and Mel 1c

receptors (Yasuo et al. 2002). The rhythmicity of per2 in the

mSCN of House Sparrows is also not influenced by

0 3 7 11 15 19 23 27 35 39 443 47 51 55 59

0

15

30

45d)

50'tcO40'luJWeeks

Tes

tis v

olum

e (m

m3 )

0

1

2

3

4

5a)

xnip

mahsB

eak

colo

r sco

re

0

1

2

3b)

Hea

d co

lor s

core

0

1

2

3

4

5c)

Bre

ast p

lum

age

colo

r

Fig. 5 Changes in beak and plumage (head and breast) pigmentation

and testicular growth in pinealectomized and sham-operated Indian

Weaver Birds kept under natural photoperiodic variations at Lucknow

(26.55N, 80.59E) for 15 months beginning from July 2004. Note the

advancement in testicular cycle and associated secondary sexual

characters in the pinealectomized birds


123

pinealectomy (Abraham et al. 2003). Interestingly, lesions

of the mSCN abolish circadian rhythms in locomotor

activity. It could be that melatonin acts at the translational

and not at the transcriptional level, and its role is more that

of amplifying the level of oscillations within the central

clock system rather than generating these oscillations. Thus,

as an output from the retina and/or the pineal gland, mela-

tonin exerts control over circadian functions, and absence of

the circadian melatonin output would lead to arrhythmicity.

It may also be probable that the pineal/melatonin might

have input to the site(s) other than the mSCN.

Although a direct effect of pineal and melatonin has not

been found in the majority of bird species investigated so

far, there may still be an indirect effect of them on the

seasonal cycles. For example, pineal melatonin may have

modulatory effects on synchronizing different phases of the

annual reproductive cycle with the environmental factors

such that they occur at the most profitable time of the year.

How this is achieved is unclear? One possibility is that the

absence of the pineal or of the rhythmic melatonin signal

enhances the sensitivity of the circadian system to light. As

a consequence of this, the amplitude of perceived LD cycle

is amplified which might alter the photoperiodic response

generally seen in terms of the gonadal growth–regression

cycle. Another compelling evidence for the role of pineal in

regulation of hypothalamo–hypophyseal–gonadal (h-h-g)

axis comes from the studies of Ubuka et al. (2005, 2006).

They have shown the effects of melatonin on GnIH

synthesis in male Japanese Quail. GnIH directly inhibits

gonadal development and maintenance by decreasing

gonadotropin synthesis and release (Tsutsui et al 2000;

Ubuka et al. 2006), and the expression of GnIH precursor

mRNA is photoperiodically controlled and influenced by

the melatonin (Ubuka et al. 2005). Further, testicular

regression is not caused by the decrease in GnRH-1 (one

form of gonadotropin-releasing hormone; Foster et al. 1988;

Dawson et al. 2001), which is considered controlling

reproductive functions in birds (Sharp et al. 1990). Ubuka

et al. (2005) suggest that gonadal regression instead occurs

due to increase in the expression and content of the GnIH. It

is therefore likely that the role of pineal/melatonin is more

pronounced in termination of the breeding season, which is

a much more critical event as far as temporal phasing of the

reproductive cycle in relation to the environment is con-

cerned. Incidentally, the majority of investigations done

thus far were focused on determining the role of pineal/

melatonin in photoperiodic induction of gonadal growth. In

view of this, it may be interesting to investigate if pineal/

melatonin is involved in regulation of annual reproductive

cycle of Indian Weaver Birds and of other species, as well

via its actions on processes along the h-h-g axis associated

with termination of the breeding season.

Acknowledgments The experiments described in this article con-

form to Indian laws, and were done using the facility exclusively

generated from the SERC research grant to V.K. by the Department of

Science and Technology (DST), Government of India. The recent

award of DST-IRHPA Center for Excellence on Biological Rhythm

Research which made available resources to us for the preparation of

this article is gratefully acknowledged. We thank both the reviewers

for their helpful comments which improved the manuscript.

Sham operated Pinealectomised

0900 2100 0900 2100 0900 2100 0900 2100 0900

Day

s

12L:12D

LLdim

Onset of

arrhythmicity

**

0900

Fig. 6 Double plotted activity

recordings of a representative

male Indian Weaver Bird

exposed first to 12L:12D and

then released into dim

continuous light (LLdim) in the

month of January, when birds

had small testes and non-

breeding type of beak and

plumage pigmentation. Bar at

the top: Time of day:

0900 hours—lights ON of

12L:12D. Note the abolition of

circadian rhythmicity after a

few cycles in pinealectomized

(right), but not in sham-operated

birds. Asterisk on the right

indicates when pinealectomy or

sham-operation was done. Data

are taken from Rani et al. (2005)


123

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