3. The Influence of External Factors on the Spawning Date and Migration of the Common Frog, Rana temporaria temporaria Linn.
- 1By statistical methods it is found that frogs are little affected by the weather which obtains at the time of spawning. There is a tendency for spawning to take place more in wet weather than at other times, and, on general grounds, severe frost would prevent activity, but weather in which night frosts occur is not avoided. Humidity is without action.
- 2The mean spawn date is influenced by the weather of a long period preceding it. The date on which frog-spawn is laid in March is much affected by the weather of February. Bain is most important, but temperature plays a part. The effect is quantitative—it is the total amount of rain which acts. An equation is developed showing the relation between the factors.
- 3The order in which spawn is deposited in different ponds in the same district is determined by features which are constant from one year to another, but is affected also by random fluctuations.
- 4Up to moderate altitudes, especially in districts subject to orographical rainfall, spawning is earlier the higher the pond above sea-level.
- 5Frogs spawn in a great variety of ponds, but those which have a stream flowing in or out are especially favoured.
- 6The migration of frogs is carried out independently of hydrotaxis, geotaxis, or voice. There is evidence that the smell of the pond is the directing factor.
- 7Frogs exercise a great deal of selection in their choice of a breeding pond. Ponds suitable for hibernation are left and journeys undertaken to other ponds in which they breed.
- 8The spot at which the eggs are laid is the same year after year. It is the scene of a silent gathering of frogs for some weeks prior to spawning, at least in some years. None of the obvious factors, such as shallowness or exposure to the sun, explains the selection of the site.
- 9The period in which this gathering is proceeding is distinguished by the term “pre-spawn season.” This is defined.
- 10A theory is put forward to explain the facts of observation, in which it is suggested that the determining factor in deciding the spawn date and the selection of site is a change in the algal flora at the site.
- 11Some confirmation of this view is obtained by analyses of the water of ponds for phosphate. Frogs appear to spawn at corresponding phases of phosphate changes in different ponds.
- 12By theoretical deduction it is shown that the date at which the largest number of ponds in a district have the significant change is indeterminate from the spawn observations. The development of spawning with time deduced from this theory bears a resemblance to the actual results of observation.
- 13It is suggested that some cases of discontinuous distribution in a small area may be the result of corresponding differences in the distribution of the algæ to which the amphibian is attached.