Seasonal Changes

Photo by Stan Tekiela

by Stan Tekiela
© NatureSmart

August 15, 2008

We are coming up on a very confusing time of year for many wild critters. It’s a time when the birds are getting ready to migrate and frogs are preparing to burrow into the earth for the long cold winter. But that’s not the confusing part. The confusing part is when these critters start to do something that they normally do only in spring—they start to give mating calls.

Each autumn this strange phenomenon happens. Birds such as the Northern Cardinal and American Robin perch up high in a tree and start to sing their breeding songs as if it were spring. If that’s not enough, several species of frog start to call from the shallow ponds. It’s like a mini-spring breeding season all over again. So why do they do it? Well it all has to do with something called gonadal recrudescence. Hold on, I know what you are thinking, don’t give up on this story just yet. Read on.

Each spring birds, mammals, reptiles and amphibians use environmental cues to indicate when it’s time to breed. More specifically they use the amount of daylight (sunshine) available each day, kind of like a luminous calendar. The amount of light in a day is called the photoperiod. In mammals the length of daylight is perceived through its eyes. In birds and amphibians the daylight is perceived or absorbed right through the skull and or body. Either way a photoperiod signal is sent to a gland in the critters brain called the hypothalamus which is connected by a thin stalk to the pituitary gland. The pituitary gland sends out gonadotrophic hormone into the body which acts upon the testes of males and the ovaries of the females putting the bird or amphibian into breeding condition. It’s like turning on a breeding switch.

During the breeding season the testes of male birds will grow several hundred times their non-breeding size and weight and become active in preparation of mating. A similar process happens in female birds. The ovary of the female bird swells and starts to produce ovum which is the beginning stages of egg development.

After the breeding season the testes and ovaries of these birds regress or shrink and become inactive and the breeding season is over for the year. It’s important that this happens after migration because the extra weight and size during migration would only serve to use up more energy and slow the bird down during migration.

However in late summer the photoperiod or the amount of available daylight matches the photoperiod of the spring breeding season and the critters hypothalamus picks up on this signal and is temporarily tricked into thinking its spring. This is where gonadal recrudescence occurs. Recrudescence means to break out anew after a dormant period and that is what happens. The testes of the males and the ovaries of the females temporarily start to swell and become active again. So for a short couple of days or maybe a week the birds and frogs are tricked into thinking it’s breeding season based on the available daylight (photoperiod) and the birds start to sing and the frogs begin to croak.

It doesn’t last long before the photoperiod shortens as the days shorten and the hypothalamus is no longer stimulated and the puitary gland stops sending out hormones to the testes and ovary and the bird returns to a non-breeding condition again.

So if you hear a bird singing from a tree or a frog calling from your local pond in late summer you can be thankful you are not as controlled by your hormones as the birds and frogs are. Or are you? Until next time…