.ADAPTATIONS OF SURVIVAL VALUE 



Adults are involved in the lollowing manner. They breed after rain of 

 one inch or more at any time between March and September and only in 

 temporary water (Bragg, 1944-45). This means that at any one time and 

 place, several breeding periods may occur (as in the region studied most 

 intensively in 1948). During or immediately after any one rain, males may 

 call from any of several pools, but not all secure females. In 1948, for example, 

 twenty-nine calling males were counted in Pool E one night but no eggs were 

 laid. At the same time only two males were calling at Pool F, one of which 

 secured a female that laid eggs. At another time, forty to fifty males in a pool 

 failed to attract a single female, although several females were seen feeding 

 near the pool. Near by at other pools a clutch of eggs was produced here and 

 there. I have seen many other such instances. It would seem that the gonadal 

 or hormonal cycle in females is an irregular one in the sense that not all fe- 

 males are ready at the same time to produce eggs. Therefore, at the beginning 

 of each evaporation cycle in a given pool, some females are likely to be ready 

 to produce eggs if found by a male. This has the effect of staggering the 

 laying of eggs throughout the warm months, depending upon local rainfall. 

 and of allowing some pools to "rest" through one or more evaporation cycles 

 while organic matter inay be built up. 



Once tadpoles are in a pool their behavior depends upon the nature and 

 amount of the food supply. When they are doing nothing else they swim 

 continuously; usually their concentration is so great that they easily cover 

 the bottom of the pool. If the pool is of type 3, food is abundant and they eat 

 voraciously of what algae, plant debris, and animal matter is to be found, 

 seldom eating the bottom materials. If in a pool of type 1 or 2, the bottom 

 debris is the principal food source. When one tadpole finds a concentration 

 of food, others commonly are attracted, apparently by sight of its accelerated 

 feeding, and a great feeding aggregation is soon formed. Such aggregations 

 in S. hurterii apparently never change to cannibalistic feeding. 



In pools of type 1, where organic materials tend to be rare or inadec]uate. 

 as the tadpoles approach metamorphosis they become cannibalistic, especially 

 on the shrivelling tails of transforming individuals (Bragg, 1948). In pools 

 of all types, if the food source is adequate, the tadpoles do not show canni- 

 balism. However, as the water level falls dangerously low, they may form 

 a dense metamorphic aggregation (Bragg, 1944) which tends to conserve 

 the fast disappearing water by deepening a depression in the soft bottom, 

 thus draining water into the area containing the animals and at the same 

 time reducing materially the surface exposed to evaporation. Fifteen minutes 

 to an hour saved in this way may make all the difference between death and 

 survival of all tadpoles in a given place, so close is the race between develop- 

 ment and evaporation in many cases. 



In at least half of the instances, this race is lost and all of the tadpoles 

 die. But even in their deaths they serve their species by building organic 

 food for the next lot of tadpoles in the pools. This much the observations 

 seem to demonstrate. 



Whether or not some special substance is accumulated in the bodies of 



"5 



