Monarchs and temperature

[Paul Cherubini]

Dr. Taylor wrote:

> In addition to lipid reserves, the butterflies need water, even if it
> is only the condensation which forms on an adjacent butterfly.
> Degradation of the forest results a drying up of the area and it is
> probable that the dew point is reached less often.

On the other hand, we know there is a huge seven fold level
of variation in forest density and basal areas from one colony to
another -See the table below (copied from Calvert & Lawton, 1993).
In other words, the butterflies in Mexico can thrive within very
wide range of tree cover. 

Physical characteristics of the major 1985-86 overwintering
colonies [in Mexico] during the middle of the overwintering 
period.

MONARCH                            OYAMEL FIR DENSITY
COLONY   COLONY SIZE         TREES / ACRE       

Chincua          1.30 acres                 251 trees/acre     
El Rosario       5.08 acres                102 trees/acre     
Picacho             .66 acre                  175 trees/acre     
Chivati-Huacal  .44 acre                  127 trees/acre    
Cerro Pelon     1.21 acres                  36 trees/acre       
Herrada             .17 acre                  290 trees/acre       
Palomas             .37 acre                 188 trees/acre        

There is also published data that indicates the colonies 
closest to water, contrary to expectation, do not have the lowest 
rates of dry weight decline.

Calvert & Lawton (1993) found an inverse
relationship between lipid expenditure and distance to water.
They wrote:

"Picacho, one of closest colonies to water, had the
greatest rate of dry weight decline while Chincua's, also 
close to water, was intermediate. In contrast, Huacal and 
Palomas which were the greatest distance from water, had
the least rate of weight decline."

Dr. Taylor also wrote:

>The average lipid masses of the arriving butterflies
>probably varies from year to year due to the availability 
> of nectar during the fall, especially over the last 600 miles of 
>the journey. Monarchs are one of the few organisms that gain 
>in mass during the migration and this gain is likely to be
>critical to wintering success.

On the other hand, we know arriving butterflies in November
show little interest in nectaring despite abundant nectar supplies
at the overwintering sites.

For example, monarch biologist Dr. Karen Oberhauser wrote 
on the dplex-list Nov. 19, 1999:

"I just returned from an eight-day meeting in Angangueo,
Michoacan and spent time in both Chincua and Rosario
[monarch colonies]. There were many monarchs flying into 
the area, and there was a noticeable increase in colony size at
Chincua from 11/12 to 11/15, the two days we visited this 
colony. Interestingly, we saw no nectaring, despite the fact 
that there were many flowers in bloom."

Paul Cherubini, Placerville, Calif.

Calvert & Lawton 1993. Comparative Phenology of Variation
in Size, Weight, and Water Content of Eastern North American
Monarch Butterflies at Five Overwintering Sites in Mexico.
Biology and Conservation of the Monarch Butterfly, Natural
History Museum of Los Angeles.