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| Red-billed Quelea - male in breeding plumage Bernard DUPONT from FRANCE, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0> via Wikimedia Commons |
Background
The origins of this story—and its unanswered questions—go back to the 1930s. J.R. Baker (1900-1984) was working at Oxford in the 1920s on the control of sperm formation in crickets when he went on the first of several expeditions to the New Hebrides. There, amongst other observations, on the large proportion of intersex domestic pigs, for example, he began to ask the question of what controlled the seasonality of reproduction of birds and mammals—and plants—in the tropics where there may be little or no variation in the environment during the year. He therefore planned another expedition to Espiritu Santo, the largest island of the New Hebrides, now Vanuatu. To continue the work after other members of the expedition had to return to Oxford, he took Tom Harrisson and, in Sydney, recruited ‘Jock’ Marshall (1911-1967). Other members of the expedition were his first wife, Inezita*, his sister Geraldine and Terence F. Bird (about whom I have no further information).
I will not dwell on the relationship between Baker and Marshall here. It is well described in the notes edited by Marshall’s late widow, Jane, here. In short, it began very well but in later years deteriorated badly. Nevertheless it was Baker and Marshall whose names became associated with factors controlling seasonal breeding in the tropics and of putting the questions in an evolutionary framework.
At first glance Baker and Marshall made an unlikely combination. Indeed, I knew people who worked in Oxford being surprised that Baker, the quintessential don, had ever ventured to the tropics or had qualified for a pilot’s licence. But he was the great-nephew of Sir Samuel Baker, the explorer of the Nile and Central Africa. Marshall, only later to obtain a degree at Sydney and an Oxford D. Phil. with Baker was a self-confessed larrikin, still sorting himself out after shooting off his arm in an accident but helping at the museum in Sydney and going on long birding trips to various parts of Australia. They were, along with Mrs Baker, Terence Bird and Harrisson, as tough as old boots. And they needed to be for the conditions encountered on Espiritu Santo were rough. This is how Jane Marshall describes just part of it:
There is constant talk in both their diaries [Marshall and Harrisson] of each others sores and fevers, dysentery and leprosy among the natives, malaria and one of the government men with blackwater fever for the third time. Jock talked of jagged coral disguised in foliage always ready to gouge a new wound, blowflies that deposited maggots on their blankets which then entered their open flesh, mosquitos, ants, spiders, cockroaches so numerous they scuttled round their feet nibbling at toe-nails while they were eating; 'they flew from wall to wall, gradually reducing calendars, photographs and record papers to flimsy ribbons; they ate holes in the bellows of my camera and shaved the titles from our books.' It was no tropic idyll; a body-wearing, tearing climate; 'each day at meal times Tom and I sat with our scarred legs in kerosene buckets of lysol and hot water - and we did much of our work in the kerosene tins each evening closely examining the lesions and jealously regarding each other's progress.'
The expedition made detailed meteorological records as well as studying the reproductive organs of a number of animals and plants 'in an endeavour to determine the existence, periodicity and proximate causes of breeding seasons in one of the most uniform climates in the world.'
Baker drew together his survey of breeding seasons, including data from Espiritu Santo, in a chapter, The Evolution of Breeding Seasons, for a book to celebrate the 70th birthday of E.S. Goodrich published in 1938. On the wet tropics he wrote:
Occasional species may breed all the year round in certain places…but the general rule is for birds to have breeding seasons. The Oxford University Expedition to the New Hebrides…was struck by the seasonal behaviour of organisms in a little-changing climate. The climax was presented by the insectivorous bat, Miniopterus australis, the adult females of which all become pregnant once a year about the beginning of September, despite the constancy of climate and the fact that they hang all day in a dark and almost thermostatic cave.
Although the results of the expedition demonstrated seasonality of breeding in the wet tropics, ‘Baker and his colleagues were unable to show in the New Hebrides what factor or factors determined its onset, e.g. food supplies for the offspring, or more direct environmental factors such as rainfall, day length, light, etc.’
To a great extent the work of Baker and Marshall on what happens in the tropics was overshadowed by research, begun by William Rowan (1891-1957) in Canada, which showed that seasonal change in daylength is the major factor controlling the onset of breeding in organisms that live nearer the poles. Not only was that phenomenon demonstrated but the neuro-hormonal pathways that control the seasonal growth and activity of the reproductive organs by that route could be, and were, worked out. There have also been suggestions that even relatively close to the equator daylength is actually important in some birds. At latitudes of up to 10° the variation throughout the year is less than 1 hour of daylight per day.
Baker largely moved on to cytology and its techniques, a number of which he had applied to the reproductive organs. The torch for studying the factors controlling seasonal breeding in birds in the tropics was carried by Marshall until his death in 1967. His general thesis was summed up by the late Brian Lofts in Marshall’s obituary:
He strongly believed that the seasonal reproductive cycles of most birds were based on an autonomous cycle synchronized by a spectrum of environmental stimuli. These he classified into either accelerators or inhibitors. He always stressed that there was no single universal environmental regulator and that different species had evolved a response to different environmental stimuli.
What those environmental stimuli are and how they work bring me to the reason for writing this article.
Rainfall
In his 1938 chapter Baker, after scouring the literature of naturalists, described a whole range of environments in which the onset of rain after a dry season, or, in some cases longer drought, was associated with the onset of breeding in birds, amphibians and reptiles. Many of these observations are now of course well known. For example:
The tendency of tropical African birds to breed whenever the rains start has been remarked by several naturalists, and it has been pointed out that those species which breed in the spring in the subtropical parts do so in the tropical regions whenever the rainy season happens to be.
It was Marshall who demonstrated experimentally ‘the importance of rainfall as a breeding “timer” in an equatorial species’. That species is Quelea quelea, the Red-billed Weaver or Dioch, the most numerous non-domesticated species of bird on earth. I have never seen a large flock of the size justifying the bird’s description as ‘Africa’s flying locust’ because of its devastating effect on seed crops. We have however, seen impressively large flocks flying to roost before dusk against the backdrop of spray clouds from the Victoria Falls, from the verandah of the eponymous hotel on the Zimbabwean side of the Zambezi. And yes, a glass was raised to Jock Marshall; indeed some of the flocks were seen through the bottom of a glass.
However, despite a continuing appearance of paper after paper discussing the effects of rainfall on the onset of breeding of birds and how that stimulus might be more or less important than, say, changes in temperature or some other environmental factor, nobody so far as I am aware, answered the question of how rainfall brings about the hormonal stimulus to the reproductive system. Baker signalled the importance of the mechanism or mechanisms involved noting:
The receptor whose stimulation causes certain animals to breed when it rains is unknown, nor is it clear whether the stimulant is the rain itself or the small saturation deficit of the atmosphere or the existence of ponds or floods. It has been suggested in the Cape Verde Islands it is the green vegetation resulting from the rain, rather than the rain itself…
All sorts of possibilities of what it is about rain that is physiologically important. Sight, smell (of wet earth for example), touch, atmospheric pressure, change in temperature…and so on The list of possibles seems endless. But it cannot be beyond the wit of man to devise experiments to find out. Even more observational studies that produce associations will not suffice; experiments are needed.
In this article, I have confined myself to mainly to birds since that was the main interest of Baker and Marshall. In a follow-up I will deal with amphibians since there is the possibility of a more convenient experimental approach to determining how rainfall stimulates breeding.
There may, of course, not be a single mechanism by which rainfall stimulates breeding even within a species, let alone between different species. But, more than 80 years since Baker wrote his chapter, it does seem remarkable that we still do not know the answer.
*Her affair with Richard Crossman, (1907-74), the future Labour MP and Minister caused a great scandal in Oxford. He is referred to tangentially in Baker’s Biographical Memoir: ‘Inezita Hilda having been captivated by the charm of the Vice-Warden of New College’. They married in 1937; she died in 1952.
Baker JR. 1938. The evolution of breeding systems. In, Evolution, edited by G.R. de Beer. Oxford: Clarendon Press.
Lofts, B. 1968. Professor Alan John Marshall, D.Sc., D. Phil., 1911-1967. Ibis 110, 206-207.
Willmer, E.N., Brunet, P.C.J. 1985. John Randal Baker. 23 October 1900-8 June 1984. Biographical Memoirs of Fellows of the Royal Society 31, 33-63
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