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The seasons in a tropical rain-forest (New Hebrides). Part 7 (Final part). Summary
and general conclusions, By JOHN13. BAKER,M.A., D.Sc. (Communicated by
Prof. E. S. Goodrich, F.R.S., F.L.S.) (From the Department of Zoology and
Comparative Anatomy, Oxford.)
(Wltll I Text-figure)
[Read 7 Jum 19451
Introduction.-In the study of the causes of breeding seasons, physiological
investigation alone cannot suffice : it niust be reinforced by observations made in
natural habitats. It is not sufficient to show experimentally that this or that
environmental change will start or stop reproduction in a certain organism: one
must try t o find what are the causes that start or stop it in nature. In the course of
evolution, animals have adapted themselves t o respond t o certain environmental
changes by breeding, and t o others by stopping breeding. Breeding seasons can
only be understood when these changes are known. It is not necessarily relevant to
the problem of breeding seasons t o discover what environmental changes can be
used artificially t o start or stop reproduction. For instance, one may prevent an
animal from breeding by depriving it of vitamin E ; but unless lack of this vitamin is
the cause of the cessation of breeding in nature, the knowledge gained by the experiment does not throw light on the adaptation of the organism t o its environment. In
fact, one might almost as well study animal locomotion by propelling the animal
mechanically or breaking its legs. These facts have tended t o be overlooked, and
purely physiological studies have been regarded as throwing direct light on the natural
mechanism by which breeding seasons are controlled (see Baker, 1938). Such
physiological studies may have great intrinsic interest and practical importance, but
may not illuminate the subject of breeding seasons.
One of the most interesting natural habitats in which t o investigate the breeding
seasons of animals is the tropical r&-forest, for here the environmental conditions
vary less during the year than in any other part of the land-surface of the world. A
study of the climates of tropical rain-forest regions all over the world shows that the
northern New Hebrides (in the western Pacific Ocean) have a particularly unvarying
climate. The mean temperature of the hottest month is not much more than 2" C.
higher than that of the coolest, and rain falls so abundantly throughout the year
that on the average of many years the driest month in the New Hebrides is wetter
than the wettest in most parts of England.
The study of the seasons in the rain-forest of the northern New Hebrides was
the main object of the Oxford University Expedition, 19334. The whole of the
expedition's work on this subject was done in the vicinity of Hog Harbour, in the
island of Espiritu Santo (15"15' S . ) . The members of the expedition were Mr. T. F.
Bird, Mr. A. J. Marshall, Mr. T. H. Harrisson, Mrs. Z. Baker, Miss I. Baker, and the
writer. All the members shared the work in the field, which started a t the beginning
of September 1933 and ended a t the beginning of September 1934. The results of
the expedition have been recorded in six papers in this journal, which are quoted in
the list of references a t the end of the present paper. The purpose of this paper is to
provide a summary of the results and t o draw general conclusions.
The expedition set itself two problems :(1) Do the vertebrates of this rain-forest region reproduce continuously, or do
they have breeding seasons ?
(2) If there are breeding seasons, can they be correlated with any environmental
changes ?
THE SEASONS IN A TROPICAL RAIN FOREST
249
A definite answer was obtained to tlic first question. It was not found possible
to answer the second question positively, but proof was obtained that certain
environmental factors do not control certain breeding seasons.
To prove that a spccies is not breeding at, a certain time demands intensive study.
The existence of breeding seasons has often been claimed because birds’ eggs have
been found a t one time of year and not at another. I n any study of the breeding
seasons of birds, nests must of course be searched for and recorded, but conclusions
must not be based solely on the success or failure of the search. Proof of the existence of breeding cycles can only be established by the dissection and microscopical
investigation of a considerable number of individuals in every month of the year.
An investigation of this kind is a large undertaking, and only a few species can be
studied in such detail ; but the results obtained are sure. Thc species chosen by
the expedition for intensive study were the following :A large fruit-bat, Pteropw geddiei.
An insectivorous bat, Miniopterus australis.
A passerine bird, Pachycephala pectoralis (Golden Whistler).
A small lizard (skink), Emoia cyanura.
The investigation was based on the dissection of 378 Pteropus geddiei, 542 Miniopterus australis, 319 Pachycephala pectoralis, and 520 Emoia cyanura.
Less intensive observations were also made on other insectivorous bats and on
the smaller fruit-bat, Pteropus eotinw. Mr. T. H. Harrison and Mr. A. J. Marshall
also made a general study of the reproduction of birds, which they propose to publish
when they are released from military duties.
Observations were also made on the flowering and fruiting of certain common
wild plants, namely :An orchid, Corymborchis veratrifolia.
A fig, Ficus eopiosa.
Wild Kava, Piper methysticum.
A deciduous tree, Garuga jloribunda.
A large leguminous tree, Cashnospermum austrule.
A rose-apple, Eugenia Richii.
A strand-tree, Barringtonia asiaticu.
A bush (Rubiaceae), Psychotria aneityensis.
-4 small herb, GPophih hwbncen.
Observations were also niaclc on the plant callcd by the natives ‘ nasukonru’.
This is probably the commonest angiosperm of the northern New Hebrides. Nearly
everywhere it is the dominant plant of the forest undergrowth. Curiously enough,
neither flower nor fruit of this species was seen by any member of the expedition
throughout the year, except that flower-buds were seen on one occasion in the high
mountains, far from the scene of the breeding-season investigation. The natives,
who are keen naturalists, state that they do not know the flower of this species,
which was doubtfully identified from our leaves as Procris peduwulata.
The plants were studied because it was thought that they might provide a sensitive
index of meteorological changes.
The animals and plants named in the two lists given above will generally be referied
to by their generic names only in this paper, for the sake of brevity.
The rncteorological changes during the year were studied in detail. Instrumenta
were exposed both in the standard way and also in the forest itself.
250
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A summay of the seasonal changes in weather and reproduction.
The contents of the six preceeding papers in this series are very briefly summarized
below, each month of the year being treated separately. Fig. 1 represents the main
facts of the breeding seasons graphically.
SEPTEMBER.
When the expedition began its work, the length of day (nearly 12 hours) was
increasing ; the daily period of sunshine averaged nearly six hours. The heaviest
rains had not started, and the mean barometric pressure was higher than in any
other month of the year. The temperatures shown by all the thermometers (by the
maximum and minimum in the Stevenson screen and the grass-minimum in the open,
as well as by the maximum, minimum and earth-thermometers in the forest) were
about half-way between the highest and lowest recorded during the year. The
temperature was to rise and the rainfall t o increase in succeeding months as the length
of day increased, and then all were to fall away until they began t o rise once more
before the year’s work was complete. Thus the expedition stafted work at the end
of the southern winter ; but it is absurd t o apply such a term as winter when the
climate was so hot and the annual temperature changes so small, and it is better to
say the end of the borepheuge (the period during which the sun has left its most
northerly point but is not yet overhead a t the equator). The notodune (southern
spring) started when the sun reached the equator a t the September equinox.
Two of the observed plants showed no change in their reproductive condition
throughout the year, and they need therefore only be mentioned once. These are
the common undergrowth herb, Geophila, which reproduced continuously, and the
‘ nasukonru ’, which, as has been stated above, never showed a flower or fruit in any
of the twelve months of observation. All th6 other observed plants except Corymborchis were in flower or fruit in September. Early in the month Garuga was devoid
of leaves. This large leafless tree had a remarkably incongruous appearance when the
luxuriantly green rain-forest was viewed from the sea. Later in the month leaves
appeared, and with them the flowers, which were not seen again during the year.
This is the most obviously seasonal plant in the nort-hern New Hebrides.
A few of the fruit-bats, Pteropus, were pregnant, but the main season for reproduction was over and the testes of the males were growing in preparation for the next.
The females of the insectivorous bat, Miniopterus, became pregnant a t the beginning
of this month (or possibly a t the very end of the preceding month). The epididymides
of the males were still crowded with spermatozoa. No eggs of the passerine bird,
Pachycephala, were found, though the testes of the males were near their maximum
size. About half the adults of Pachycephala were moulting to some extent. The
little lizards, Emoia, were breeding rapidly, the males having large testes and many
of the females being ovigerous.
OOTOBER.
In this month, just after the beginning of the notodune or southern spring, there
was little change in the weather. Corymborchis came into flower and all the other
plants under obsorvation remained in flower or fruit *. A few females of Pteropus
wore still pregnant, and the testes of the males continued t o grow. The males of
Miniopterus had small testes, while the embryos in the females had made a large
advance in size. A single nest of Pachycephala containing eggs was found, evidently
produced by a straggler from the previous season. Emoia continued t o breed, and
indeed did so throughout the year, though not always a t the same rate.
* Gurzrga w m not observed during this month.
'PHP SZASONY I N A TROPICAL RAIN FOREST
251
NOVEMBER.
This month was marked, as usual, by the beginning of wetter weather, though it
must be remembered that there is no dry season in this part of the world. The sun
passed overhead on its way south on the 14th ; the temperatures shown by all the
thermometers rose. The observed plants (except Garuga) remained in flower or
fruit. No more pregnancy was found in Pteropus ; the testes were continuing t o
grow in readiness for the next season. The embryos of Miniopterus had once more
grown greatly. The testes of Pachycephalu a t last begen to decrease in size, and
nearly every adult of both sexes was moulting, at least in patches.
DECEMBER.
Rainfall inorcased to a daily avcrage of 18.3 mm., the highest for any month of
the year. The temperature remained high. The longest day of the year, on the
21st, extended to 13 hours 2 minutes. Most of the observed plants remained in
flower or fruit. Thc testes of Pteropus continued to grow. The embryos of Miniopterus increased to their maximum size and were born during this month (or perhaps
early the next). The epididymides of the males were now empty of spermatozoa.
Pachycephalu was in full moult ; the testes of the males dropped t o their minimum
weight, less than one-fortieth of what they were to reach a t their maximum in July,
and the majority contained no spermatozoa.
JANUARY.
The early part of the notopheuge or southern summer was hot and wet. The
mean temperature for the month in the Stevenson screen reached its highest figure
for the year (26.6"C.,=80" F.), but it is to be recollected that this was only 2.1" C.
more than the mean temperature of the coolest month. All the observed plants,
except Garuga, remained in flower or fruit. The testes of Pteropus reached their
maximum size. The females of Miniopterus had now given birth to their young.
The testes of Pachycephalu were just beginning to increase in size. .
FEBRUARY.
The weather remained hot and wet. The sun passed overhead on its way north
on the 8th. Our botanical observations were unfortunately incomplete during this
month, but the species on which we recorded observations were mostly still in flower
or fruit. The only striking change among the observed animals was that some of
the female Pteropus began to become pregnant.
MARCH.
The weather was still hot and wet. The monthly mean of barometric pressure
reached its lowest point this month, but this was only 0.23 inch less than its maximum
in September. The general flowering season was now a t an end : no flowers nor
fruit were seen on Corywbborchis, Castanosperrnurn, Eugenia, Barringtonia or Psychotria,
though Ficus still carried its figs and fruit was seen on Piper. There was no striking
change among the observed animals. Some more Pteropus became pregnant and the
testes of Pachycephala continued t o increase steadily in size. The lizard, EmOia, a t
last began to breed less rapidly.
APRIL.
The early part of the boredune or southern autunin brought a distinct change in
the weather : there was a sharp decrease in rainfall and a noticeable increase hi
saturation deficiency. The mean daily period of sunshine reached its maximum for
the year (6.9 hours), and the temperature remained high. I n the forest the maximum
and earth thermometers showed their highest monthly mean temperatures of the
year. The average cooling power of the atmosphere in the forest, as shown by the
dry katathermometer, reached its lowest figure for the year (3.5 millicalories per
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DR. J. R. BAKER ON
square centimetre per second). Nearly all the observed plants, including Ficus and
Piper, were now without flowers or fruit. The main pregnancy-season of Pteropus
began (though occasional females had been pregnant in the two previous months).
The testes of Paehycephala continued to increase steadily in size as the days grew
shorter.
SEPT. OCT. NOV DEG. JAN. FEE. MAR. APR. MAY JUNE JULY
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1 .-Graphs showing the niran wriglits of' tlics twtes of adult male8 of tlir observed animals
throughout tlia year (1938-4). Thr oitlinutes rt.piwrnt thr weights of both testes in
milligrams. The seasons when ox-a wcrr fertilized, etc.., are also indicated.
THE SEASONS IN A TROPICAL RAIN FOREST
259
MAY.
The wcather remained relatively dry and hot. This month and the last were the
only ones in which the saturation deficiency of the atmosphere in the forest was
noticeably above the average for the year. The mean temperature in the forest now
reached its maximum for the year, considerably later than it was reached in the
Stevenson screen in the open. Several of the observed plants, which had been in a
non-reproductive phase in March or April, were now in flower or fruit again (Piper,
Eugenia, Barringtonia, and Psychotria). Most of the embryos of Pteropus were
considerably larger than in the previous month. The testes of Pachycephalu
continued to increase in size, and an egg was found in an oviduct of a single female.
The lizard, Emoia, now reached a low point in its breeding cycle : only two ovigerouh
females were observed during the month, and the mean weight of the testes of adults
had sunk to about one-third of the maximum reached in November and December.
JUNE.
June was relatively dry and somewhat cooler. The shortest day of the year, on
the Zlst, extended t o 11 hours 14 minutes. Corymborchis and Ficus started t o reproduce again. This and the preceding month witnessed a subsidiary flowering season
in a number of plants in which the main sewon was about September t o about January
or February. Most of the embryos of Pteropus were now larger again, though some
very small ones showed that eggs were still being fertilized ; but the mean weight of
the testes had greatly decreased from its maximum in January. The testes of
Miniopterus now at last started t o grow. Those of Pachycephala had nearly
reached their maximum. Young were found in a nest, but the main breeding season
had not yet started. Only a single ovigerous Emoia was found during the month,
and the testes of this species remained small.
JULY.
The early part of the borepheuge or southern winter was relatively dry and cool.
All the thermometers registered their lowest means of the year either this month or
the next. The cooling power of the atmosphere in the forest, as shown by the dry
katathermometer, reached its highest monthly mean of the year (5.27 millicalories
per square centimetre per second). The mean daily hours of sunshine reached
their minimum for the year (3.5 hours). Several of the observed plants were no
longer reproducing, their subsidiary flowering season being a t an end. Most of the
embryos of Pteropus were now large. A few very small embryos showed that
fertilization was still occurring sporadically although the testes had reached their
minimum size for the year. Most of the adult male Miniopterus now had their
epididymides once more crowded with spermatozoa. This month was the main
egg-season of Pachycephalu. The testes reached their maximum weight, more than
forty times the December minimum. 18 nests containing living eggs or young were
found, while only three such nests were found during the whole of the rest of the
year. Emoia was starting t o reproduce more rapidly in this and the succeeding
month.
AUGUST.
The weather remained relatiGely dry and cool. The earth thermometer and the
minimum therniometcr in the forest, as well as the minimum thermometer in the
Stevenson screen and the grass-minimum thermometer in the open all recordcd the
lowest means of the year. Two of the observed plants (Barringtonia and Psychotria)
started reproduction again, while most of the others were still without flowers or
fruit. The embryos of Pteropus reached their maximum size (two exceeded 80 grams
each), and most of them were probably born this month. Fertilization had now
ceased, for no more very carly pregnancies were seen, though there were quite a
number of medium-sized embryos which would probably not be born until October.
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DR. j,R. B A B R ON
The testes of Miniopterus now reached their maximum size, presumably in readiness
for fertilizing the females early next month as they did the year before. The epididymides were crowded with spermatozoa. The breeding of Pachycephalu was now
almost a t an end : only a single new nest was found, which contained young birds,
and the size of the testes was falling off.
The reader who has followed this summary of the seasonal changes will have
noticed that although the mean temperature of the hottest and coolest months only
differed by 2.1"C. and the longest day was only about 12 hour longer than the
shortest, and although there was a daily average of 3.9 mm. of rain even in the least
wet month, yet most of the terrestrial organisms showed marked seasonal changes
in their reproductive behaviour. Most of the observed plants had a long and a short
reproductive season during the year ,though the herb Geophila reproduced continuously
and Ficus and Piper nearly continuously, like h m i u m purpureum. in Britain. Of
the vertebrates observed, only the lizard, Emoia cyanura, reproduced continuously,
and even here there was almost a cessation during two months. (Emoia wemzeri
probably stopped reproduction altogether during one month in 1927.) The male
Pteropus retained spermatozoa in the epididymides throughout the year, but the
testes underwent a regular cyclical change in weight, and there was a distinct breeding
season, the females mostly becoming pregnant about March. Miniopterw showed
the sharpest possible breeding season. The evidence shows that all the females that
bore young became pregnant about the first few days of ,September : the fertilization
of ova was entirely c o n k e d to these few days of the year. (The fourth paper in
this series should be consulted for the evidence on which this statement is based.)
Afterwards the epididymides of the males lost all their spermatozoa. No animal
living in a strongly seasonal climate could have a more sharply defined breeding
season. This seems all the more extraordinary when it is remembered that this little
bat hangs throughout the period of daylight, till about ten minutes before sunset, in
a dark and almost thermostatic cave. Pachycephala, again, has as sharp a breeding
season as most British birds, with the greater part of the egg-laying crowded into
a period of about a month. At one time of year the testes are more than forty times
as heavy as they are at another, and when they are a t their minimum they contain
no spermatozoa.
Briefly, then, the mammals and bird studied are a t least as seasonal in their
reproductive behaviour as the mammals and birds of temperate climates. However,
the various animals reproduce a t times of year that bear no obvious relation t o the
breeding seasons of temperate lands. Thus the passerine bird breeds very early in
the southern ' winter ', while the insectivorous bat copulates not in the ' autumn ',
but a t the end of the southern ' winter ', and the development of the embryos proceeds
at once without any latent period.
Although the other fmit-bat, Pteropus eotinus, was not studied nearly so intensively as Pt. geddiei, suEcient data were obtained t o show that its breeding ==on
is approximately the same.
Are the breeding seasom annually recurrent 1
Since the Oxford University Expedition only rasted for 366 clays, it could not
collect definite evidence t o show whether the brtxding seasons of the animals studied
recur annually a t about the same time, or whether, on the contrary, they recur
irregularly in different years, or regularly a t periods exceeding twelve months. This
is a matter requiring careful consideration.
It is relevant t o note first the evidence presented by the phenomena of periodicity
in the plants. It has been mentioned that the deciduous tree, Garuga, was without
leaves at the beginning of September but regained them during that month. The
Rev. W. Anderson, who lives at Hog Harbour, kindly agreed t o make observations
over a period of years. He made reports in 1936, 1937, 1938, 1941 and 1942, as well
THE SEASONS IN' A TROPICAL RAIN FOREST
255
as a general statement in 1944. Garuga is clearly deciduous a t about the same
season annually. It usually begins to lose its leaves about the end of June and becomes
leafless in July. The leaves appear again in September. In 1942 the leaves were
lost about a month earlier than usual. Mr. Anderson attributed this to the very
dry weather.
Chcruga, then, has an annual season of leaf-fall. No data over a period of years
are available for other plants, but t h e forest-trees Erythrina indica (Papilionaceae)
and Alphitonia zizyphoides (Rhamnaceae) are evidently regularly periodical, for the
natives rely upon their flowering t o fix the time for the annual planting of yams and
sweet potatoes respectively.
The evidence that the reproductive periodicity of the animals is annual rests
mostly on indirect evidence, but the following ten arguments suggests strongly that!
it is :(1) The writer studied the weight of the testes of Pteropus a t Hog Harbour from
January to July 1927, and there was a very clo,se resemblance between the change
of weight during these months in 1927 and 1934. (See p. 132 of the third paper in
this series.)
(2) In 1927 the writer found that the embryos of Pteropus grew steadily from
February till July and were born late in that month or in August. This closely
corresponds with what happened in 1934.
(3) Pteropus was a t the tail-end of its pregnancy-season in September and October
1933 ; the following August the cycle had come round again and young were being
produced, while medium-sized embryos in some of the females showed that the
pregnancy-season would again straggle on till October.
(4) The graph showing the weight of the testes of Pteropus throughout the year
would join on smoothly t o another similar graph placed beyond it.
( 5 ) I n Miniopterus fertilization occurred during the first few days of September
1933 and a t no other time during the year ; in correspondence with thifi, the testes
attained their maximum size in August of the following year.
(6) A single nest of Pachycephalu, containing eggs, was found early in October
1933, but no nest thereafter till June 1934 ; this agrees well with the fact that the
main egg-season occurred in July, 1934. The October nest was presumably produced
by a straggler breeding later than most of the species.
(7) The testes of Pachycephala were large in September 1933 and again in August
1934.
(8) By a study of skins of Pachycephalu taken in the northern New Hebrides in
1926-7, Mayr (1932) found that January wa8 the season of full moult. This agrees
sufficiently well with the full moult season in 19334 (December : observations on
moulting were not made in January).
(9) The graph showing the weights of the testes of Emoia cyanura throughout
the year would join on smoothly to another similar graph placed beyond it.
(10) The writer studied the reproduction of the closely related Emoia werneri a t
Hog Harbour throughout 1927. The breeding cycle was closely similar to that of
E . cyanura in 19334, the only difference being that whereas E . cyanura nearly
stopped breeding in May, E . werneri appears to have done so altogether in the same
month. I n both cases reproduction was at, its height in November and December.
(See the sixth paper in this series.)
These arguments, taken together, almost prove that the breeding cycles are
annually recurrent. It is the writer's hope that he or some one else will have an
opportunity t o obtain conclusive evidence on this matter. To do this it would not
be necessary to spend a whole year at Hog Harbour : it would suffice to remain from
June or July till September or October. If Ptwopus showed large embryos in Jiily
256
DR. J. R. BAKER ON
and August, if Miniopterw females became pregnant in August or September, if
Pachycephah showed an egg-season about July, and if Emoia increased its reproductive activity during the period of study, the proof would be conclusive.
The causes of the breeding seasom.
The causes of breeding seasons may be analysed thus (Baker, 1938) :
Ultimate causes
internal rhythm
fioximate causes {external (environmental) factors*
The ultimate causes of breeding seasons are the reasons why it is beneficial for a
species t o breed only a t a certain time of year. I n temperate climates the young are
usually hatched or born a t a time of year that is obviously suited t o their welfare. It
is not possible t o suggest what may be the ultimate causes of breeding seasons in the
northern New Hebrides, for one cannot see why one season of the year should be
preferable t o another for the production of offspring. The young fruit-bats are born
about August and the insectivorous bats in December, while the young Pachycephala
hatch in July, and reproduction in the lizard, Emoia, reaches its maximum in November
and December. One can only suggest that if reproduction continued throughout
the year, the whole species might die out from the consequent elimination of the foodsupply.
The proximate causes of breeding seasons are the influences that act upon
individual animals and make them start and stop breeding a t certain times of year.
Although internal rhythm is an important auxiliary proximate cause of breeding
seasons in some organisms, yet it can never act altogether independently of environmental control. This was proved conclusively, by reductio ad absurdum, on pp. 517-8
of the second paper in this series. It is necessary t o consider what are the external
(environmental) changes to which the vertebrates of the northern New Hebrides
respond by starting or stopping breeding (or, in the case of Emoia, by breeding more
rapidly or more slowly).
If a graph showing the reproductive activity of Emoia throughout the year is
placed beside another showing the duration of daylight, as was done in the sixth paper
in this series, the resemblance is obvious ; for reproduction reaches its maximum in
the long days of November-December and its minimum in the short ones of MayJune. It must not be taken for granted, however, that this is a case of cause and
effect, for Pachycephala behaves in the opposite way. From their minimum size in
December the testes grow larger by regular monthly increments during the whole of
the period (January t o June) while the days are growing shorter ; and while the
days are growing longer from October t o December, the testes are becoming reduced
to less than one-fortieth of their maximum size. Thus it is certain that increase in
length of day is not the environmental factor that stimulates the growth of the
testes in Pachycephalu ; and the start of the egg-season coincides almost exactly
with the period of shortest days, at the time of year when the daily hours of sunshine
are also at their minimum for the year. In Pteropus, again, the time when most of
the females become pregnant is February and March, that is to say, part of the
period during which the days are getting shorter. Duration of daylight obviously
cannot influence the reproduction of Miniopterus, which only emerges from its dark
cave a t nightfall.
Although the daily duration of illumination can profoundly influence the reproduction of birds (Rowan, 1926) and mammals (Baker and Ranson, 1932 ; Bissonnette,
1932), yet there is a tendency t o exaggerate the part it plays in the control of breeding
seasons.
* External fartors are sometimes called ‘ exteroreptive ’ factors, but this is undesirable.
‘ Exteroceptive ’ means ’ wreptive of extmnal stimuli ’ and is rtpplirrchle only to thr organism.
not to the environment.
TIW SEASONS I N A TROPIObL RAIN FOREST
257
In most tropical countries there is a distinct dry season, and there is strong
evidence that the d a r t of the rainy season is a stimulant to the reproduction of many
birds ; but in the northern New Hebrides there is no dry season, and the breeding
season of Pachycephala falls in the less wet season of the year. The saturation
deficiency of the atmosphere in the forest was markedly less in April and May than
in the other months of the year, but this cannot be correlated with changes in the
reproductive cycles of any of the animals studied.
Pachycephala bred during the cooler part of the year, and the very short copulating
season of Minioptem fell also during this period. P t e r o p copulated during the
hotter part of the year, during which also the reproduction of Emoia reached its
maximum. The difference in mean temperature between the hottest and coolest
month was so small (2.1" C.) that it is not very easy to believe that it could be the
controlling factor. It is worth remembering, however, that the dry katathermomebr
showed considerable changes in the cooling power of the atmosphere in the forest
during the year. This varied from a mean of only 3.5 millicalories per square oentimetre per second in April to a mean of 5.27 millicalories in July, during the period of
the trade winds.
Pteropus is frugivorous and Minioptem, Pachycephala and Lygosoma insectivorous.
The expedition was not able to discover any change in food-supply that would be
likely to affect &production. Some of the fruits eaten by Pteropus are only available
during a part of the year.
The barometric pressure varied remarkably regularly during the year, but the
difference between the mean figures for the months of highest and lowest pressure
was only 0.23 inch, and this is scarcely likely to influence reproduction.
The facts suggest strongly that the breeding seasons of the vertebrates of the
northern New Hebrides are controlled by environmental factors of a totally
unsuspected kind, not affecting the sense-organs or meteorological instruments of
man. This conclusion is forced upon one above all by the astonishing behaviour of
the insectivorous bat, Minioptem, which hung all day in a dark and almost thermostatic cave situated in one of the most unvarying climates in the world, yet never
fertilized its eggs throughout the year of observation except during a few days at the
beginning of September, when most of the adult females suddenly become pregnant.
Ackmwledgenzents.-The expedition to the New Hebrides in 1933-4 was made
under the auspices of the Oxford University Exploration Club. Contributjons to
the funds of the expedition were made by the Royal Society, the Percy Sladen
Memorial Fund, the University of Oxford, and New College, Oxford. Before the
expedition left England valuable advice on meteorology was given by Prof. P. A.
Buxton and Mr. W. G. Kendrew. In the islands help was given especially by the
British and French Resident Commissioners, the Rev. W. Anderson, Mns. Anderson
and Mr. T. R. ff. Salisbury, as well as by many native friends, who were keen natural
historians. The expedition's vertebrates were identified by Mr. A. Fellows, Mr.
N. B. Kinnear and M i . H. W. Parker. The Keeper of the Department of Botany at
the British Museum (Natural History) took charge of the expedition's plants, which
were identified by Mr. J. E. Dandy, Mr. A. W. Exell, Mr. J. S. L. Gilmour, Prof. A.
Guillaumin, and Dr.G. Taylor. Prof. Guillaumin (1938) has published a flora of the
island of Espiritu Santo, based on the expedition's collectiuns. The results of the
expedition's work on breeding seasons were worked out in the Department of Zoology
and Comparative Anatomy at Oxford, by permission of Prof. E. S. Goodrich and with
the benefit of his advice. On behalf of the expedition the writer offers his thanks to
the above-mentioned persons and institutions for all their help. He expresses
particular gratitude to the Rev. W. Anderson and Mrs. Anderson for their kindness
and helpfulness during his visits to the New Hebrides in 1922-3, 1927 and 193.3-4,
and to the Percy Sladen Trustees for supporting all three expeditions.
258
ON THE SEASONS IN' A TROPICAL RAIN FOREST
Summay.-The following is an attempt to summarize this paper, which is itself
a summary :(1) The vicinity of Hog Harbour, 15" S. in the New Hebrides, Pacific Ocean,
was chosen for a study of the breeding seasons of vertebrates in a particularly
unvarying tropical rain-forest region. The mean temperature of the hottest month
is only about 2" C. higher than that of the coolest, and there is no dry season (though
one part of the year is wetter than the other).
(2) The animals chosen for intensive study were a fruit-bat (Pteropus geddiei), an
insectivorous bat (Miniopterus australis), a passerine bird (Pachywphala pectoralis)
and a lizard (the skink, Emoia cyanura).
(3) Pteropus, Miniopterus and Pachycephulu each had one breeding season during
the year of observation. The main period of fertilization of ova was February and
March for Pteropus, early September for Miniopterus, and July for Pachycephala.
Emoia bred throughout the year, but much more rapidly during November and
December than during May and June.
(4) It is almost certain that the breeding seasons are annually recurrent.
( 5 ) It is certain that increasing length of day does not stimulate the growth of
the testes of Pachycephula. From their minimum size in December the testes grew
larger by regular monthly increments throughout the period (January to June)
during which the days were growing shorter ; and while the days were growing
longer from October to December, the testes were shrinking to less than one-fortieth
of their maximum size.
( 6 ) Miniopterus hangs throughout the period of daylight till about ten minutes
before sunset in a dark and almost thermostatic cave situated in one of the most
unvarying climates in the world, but it did not fertilize its eggs throughout the year
of observation except during a few days about the beginning of September, when
most of the adult females suddenly became pregnant.
(7) It seems probable that the breeding seasons of the animals investigated arc
regulated by some altogether unsuspected cause or causes.
REFERENCES.
The six previous papers in this series on ' The seasons in a tropical rain-forest
(New Hebrides) ' are here listed separately for convenience of reference :BAKER,J. R. & HARRISSON,
T. H. (1936). Part 1 . Meteorology. Journ. Linn. S O ~2001.
. XXXIX,
p. 443.
BAKER,J. R. & BAKER,I. (1936). Part 2. Botany. Ibid, XXXIX, p. 507.
BAKER,J. R. & BAKER,Z. (1936). Part 3. Fruit-bats (Pteropidae). Ibid, XL, p. 123.
BAKER,J. R. & BIRD,T. F. (1936). Part 4. Insectivorous bats (Vespertilionidar and Rhinolophidae). Ibid, XL, p. 143.
BAKER,J. R., MARSHALL,A. J. & HARRISSON,
T. H. (1940). Part 5 . Birds (Parhycephaln).
IMd,XLI, p. 50.
BAKER,
J. R. (1945). Part 6. Lizards (Emoia). In the press.
The following are also referred to in the prcsent paper :RAKER,J. R. (1938). Chapter on ' The evolution of breeding seasons ' in ' Evolution : essays on
aspects of evolutionary biology presented to Professor E. S. Goodrirh,' edited by G. R.
de Beer. Clarendon Press, Oxford.
BAKER,J. R. & RANSON,R. M. (1932). Factors affecting the breeding of the field mousc
(Microtus agrestis). Part 1 . Light. Proc. Roy. SOC.B, CX, p. 313.
BISSONNETTE,
T. H. (1932). Modifirationof mammalian Rexual cycles. Proc. Roy. SOC.B, cx,
p. 322.
GUILLAUMIN,A. (1938). A florula of the island of Espiritu Santo, one of the New Hebrides.
Journ. Linn. SOC.Bot. LI, p. 547.
ROWAN,W. (19%). On photoperiodism, rcprocliirtiw pcsriotlirity, and thr nnniial migrations of
I)irtls a l i d wrtnin fishrs. I'?m-. Ro.\fo7~Soc. Krrf. lfint. xxxvirr. 1). 147.