Bat House Forum

[Terry Lobdell]

Here is a question for those of you who have little brown maternity colonies. In 2007 I observed 3 maternity colonies where the number of pups was about half of the number of mothers. Is this ratio normal?

The big brown maternity colony  of 15-16 mothers I've observed at my home in nw pa has had an equal number of pups to mothers the last 3 years. 

I was under the impression that my little brown maternity colonies should have produced one pup per mother.

These numbers have me puzzled especially after reading that wns has affected little browns but not big browns.

Could there be a possibility that some of my little brown mothers are malnourished or infertile? 

[IowaNate]

  Good observation Terry. My best records of my little brown nursery colonies at the Nature Center (granted these are from 5 years ago) had 98 adult bats leave one house, and the pink pup count was 35. This approximate 3 to 1 ratio has me baffled as well.

  Without being able to determine sex of every adult bat in the house or check their fertility, I would have to say that a few males and infertile females also occupy the houses...then again I have positively identified both big and little browns using these houses, and none of the big browns reared pups to my knowledge.

  Could it possibly be that males and infertile females do inhabit the same colonies as nursing mothers, but to a lesser extent? I know that most research shows that males and non-nursing females typically choose other roosts...but there has to be an explanation.

  With the wide range of temperatures that most large well built bat houses provide, I have to suspect that the sexes are intermingling more than they would in natural roosts. And now with more bat houses having access to other chambers within the house, the bats can freely move to their desired climate without having to exit the roost.

  I could be wrong, but I might be right...but one thing is for sure, this will take some serious research and bat sexing to find the truth. I wish I had the time and resources to check it out, but for now I can just make a speculation.

                                                         Nate

[Terry Lobdell]

Thanks Nate! That is exactly the kind of info I was hoping for.............now I know I'm not the only one with those kind of numbers in my little brown maternity colonies......it leads me to believe everything is probably o.k. with my bats.......

I wonder what kind of ratios Kent has in his little brown maternity colonies?

[kent borcherding]

I have some maternity colonies with higher ratio of mothers and pups.

Probably the best explanation is the females that are in the lower ratio of pups , is these are females that were born later and did not mature enough to mate in the hibernaculum .

These young females still followed the older females to the colony site , because of bats strong homing instinct.

If these younger females , not mated , did not home with the other females from this site , the size of the colony would decrease.

It is more important for little browns to have larger colonies to maintain higher temps in the maternity colony , the higher temps are very critical for the young little brown pups growth. Nature sure is amazing .

The one puzzle is in the warmer southern climates the males usually roost with the females ?

I almost never try to count pups in the bat houses until they are flying or beginning to fly , believe it can cause pups to fall.

The new pups will be gray colored and are easily counted when they are beginning to fly or flying , when they shed gray will be the normal golden brown color of the little brown.

An excellent book about bat reproduction is " Bats A Natural History "
ISBN 0-292-70752-5

It is out of print but your local library may have it. I do have some extra copies , don't know how many .

[Terry Lobdell]

Thanks Kent! I feel better now knowing there's probably nothing wrong with my maternity colonies!

[IowaNate]

 Very good theory Kent! And it would make perfect sense that a female born the season before wouldn't be sexually mature, yet they would have the homing instincts to return to the same nursery colony where she was born.