440 ANlSnjAL REPORT SMITHSONIAN INSTITUTION, 1961 



breeding sites are uncovered; and Marshall and Roberts [11], find 

 that the cormorants {Phalacrocorax spp.) on Lake Victoria at 0°20' 

 N. are seasonal, breeding from May to December, and they come to 

 the conclusion that the determining cause is the greater frequency of 

 high winds between January and April, which destroy their flimsy 

 nests. 



Such reasons will not account for the general occurrence of 

 periodicity in equatorial animals. If no explanation can be found in 

 environmental changes, it may be suggested that the control lies 

 in the animals themselves, in endogenous rhythms. But, if the breed- 

 ing or migration is at the same date from year to year — and it seems 

 to be so in most species — it is hardly possible, as Baker points out, for 

 the control to be wholly by endogenous rhythm, for the period of the 

 rhythm would have to agree very exactly with the annual cycle; any 

 difference, however small, would mean that the time of breeding or 

 migration altered from year to year. I know of only one instance 

 in which the periodicity of a tropical animal is wholly due to an 

 endogenous rhythm. This is the case of the wide-awake or sooty 

 tern {Sterna fuscata)^ which nests on Ascension Island (8° S.). In 

 this bird the interval between nesting times is not a year but 9 to 10 

 months [12]. No environmental stimulus could give this result. 



Though the whole cause of the periodicity cannot be endogenous, 

 this does not mean that endogenous rhytlims play no part in its 

 causation. It may be that in many species the rhythm is at base 

 endogenous and is kept adjusted to the annual cycle by some external 

 stimulus of which we are at present ignorant. Such a stimulus might 

 be of almost any kind ; it would probably differ from species to species 

 and need not always be physical. Marshall and Williams, for in- 

 stance, suggest that the northward migration of the yellow wagtails 

 in Uganda is stimulated by the passage of birds of the same species 

 from farther south where they have been stimulated to migrate by 

 environmental stimuli. The rhythm of gonad growth in Uganda 

 would be endogenous and the birds would only respond to the stimula- 

 tion when the gonads were in the appropriate condition. Another 

 explanation of this example would seem to be that their migration 

 southward is determined by environmental stimulation in Europe 

 and the time of the northward migration by an endogenous rhythm 

 of gonad growth starting from the time of their arrival in Africa. 



In the many species that live all the year round in apparently in- 

 variable environments but yet are seasonal, it seems that there must 

 be some environmental stimulus, physical or other, that controls their 

 periodicity. For almost all of them we cannot say what the stimulus 

 is and we can only admit our ignorance. Clearly this is a subject on 

 which further work is needed. 



