Migratory Pollinators Program
LESSER LONG-NOSED BATS
Energetics
of Spring Migration
How long does it take lesser long-nosed bats to migrate
up to 1,200 km from Jalisco to Arizona each year, and what is the energetic cost
of such a trip? To obtain preliminary answers to this question, energetic calculations
can be made using information on the amount of fat individuals deposit prior to
migration, the mileage a bat can get from each gram of fat, the energetic cost
of flight, typical flight speed, and the energetic value of nectar needed. The
data for making these calculations, presented in Table
2, are based on data from Sahley et al. (1993), Fleming et al. (1996), Ceballos
et al. (1997), and Horner et al. (1998).
Stored fat is the fuel used by
birds and mammals during migration, and it is assumed that this is also true for
lesser long-nosed bats. Ceballos et al. (1997) reported that both males and females
of this species deposit fat in the late fall. Although the precise amounts of
fat deposited per individual have not yet been determined, 3 g of fat (i.e., about
13% of a bat's non-fat mass) is a reasonable estimate. Some migrating and hibernating
bats increase their mass by 25 percent through fat deposition in the fall (Fleming
and Eby 2001). The data in Table 2 can
be plugged into the following formula (from Ewing et al. 1970) to estimate the
maximum flight range that a lesser long-nosed bat can attain on a given amount
of fat:
| (g fat)(9.4Kcal/g fat)(flight speed in kph)(103) | |
| Maximum range = |
|
| (MRr)(20)(bat mass in g)(4.8Kcal/1 O2) |
where MRr = resting metabolic rate (in ml O2/g-h). At a flight speed of 40 kph, the maximum flight range for 3 g of fat is about 227 km; at a flight speed of 50kph, it is about 283 km. These calculations suggest that individuals need to make several refueling stops to replenish their fat during a migration of 1,200 km. If they deposit 6 g of fat prior to migration and at each refueling stop, and they fly an average of 50 kph (which is not unreasonable for these strong flying bats), their maximum flight range between refueling stops would be about 550 km. Thus, they would need to make two refueling stops between Jalisco and Arizona. Based on these calculations, we estimate lesser long-nosed bats stop two to four times to refuel during their spring migration. This migration probably costs them the energetic equivalent of about 13 grams of fat.
Thirteen grams of fat has an energetic value of 511 kJ, so a bat needs to acquire (minimally) this much energy from flowers to fuel the migration. Bats receive about 0.45 kJ per visit to a cactus flower (Horner et al. 1998), which translates into 1,136 flower visits to obtain 511 kJ of energy. Lesser long-nosed bats make about five visits per flower, so a bat will need to visit about 227 cactus flowers to fuel the trip from Jalisco to Arizona. A maternity roost located north of Puerto Peñasco contains about 100,000 adult females, some of which probably mate in the cave near Chamela.
 |
| This nearly barren lava flow in the Gran Desierto de Pinacate Natural Park contains a maternity roost of 100,000 lesser long-nosed bats. Extensive stands of saguaro and organ pipe cactus occur within 30 km. |
The above calculations suggest that it will take at least 10 million cactus flowers (assuming each flower is visited by 2-3 bats) to fuel the spring migration of the Pinacate maternity colony.
Bats can easily meet their energy needs in large stands of columnar cacti during the peak blooming season. It's more challenging in sparse stands or during off-peak blooming times. These stands are near Bahía de Los Angeles, Baja California.
Migration Duration
How long does it take female lesser long-nosed bats
to migrate from Jalisco to Arizona during the spring migration period? Current
data, and the above calculations, suggest that the trip is probably slow. Spring
migration could take as long as about 3.5 months, or it could be shorter if females
remain in Jalisco or elsewhere in central Mexico for some time after leaving the
mating cave before heading north.
Lesser long-nosed bats need to stop at least twice to replenish their fat deposits. Judging by how long they remain in a transient (i.e., non-maternity) roost near Bahía Kino, Sonora, before moving to their maternity roosts (about 3-4 weeks; Horner et al. 1998), females could spend ³ 1.5 months at refueling stops en route to Arizona. This means that actual (estimated) travel time, excluding stops, from Jalisco to southern Arizona is two months.
These bats may also have to expend considerable energy finding food each night at transient and maternity roosts, particularly when they arrive before peak cactus flowering or when flower densities are low. At a transient roost near Bahía Kino, the foraging radius was estimated at ³ 30 km in late March and early April (1998). If this estimate is accurate for bats at this roost, the energy costs of finding enough flowers each night to fuel the next leg of the journey is extremely high.
Effects on Floral
Resources
If lesser long-nosed bats rely heavily on certain kinds of flowers
to fuel their migrations, it is also likely that some of their food plants rely
on bat visits to maximize their reproductive success. Pollinator exclusion experiments
conducted in different parts of Mexico and Arizona indicate that there is significant
geographic variation in the extent to which plants with flowers that fit the "bat
pollination syndrome" (See Heithaus 1982) actually depend on bats for maximum
fruit set.
In the Tehuacan Valley of the State of Puebla, for example, bat pollination accounts for 100 percent of fruit set in three species of Neobuxbaumia columnar cacti (Valiente-Banuet et al. 1996, 1997). In contrast, in the Gulf coast region of the Sonoran Desert near Bahía Kino, bat pollination accounts for 25-90 percent of total fruit set in three species of columnar cactus (cardón, saguaro, organpipe; Fig. 1; Fleming et al. 1996). For the geographically widespread cactus, etcho, bats account for 100 percent fruit set near Chamela, Jalisco, but only seven percent near Alamos in southern Sonora (Valiente-Banuet et al. in preparation). Slauson (2000) conducted similar pollinator exclusion experiments with Agave palmeri in southeastern Arizona and found that bats (both L. curasoae and Choeronycteris mexicana) accounted for about 33 percent of total fruit set in areas near their roosts.
These studies indicate that the extinction of lesser long-nosed bats would have a strong negative effect on plant reproductive success in some parts of its range (e.g., in central and southern Mexico), but less of an impact in other areas (e.g., in Sonora and Arizona). Thus, the nature of the pollinator mutualism between lesser long-nosed bats and their food plants appears to vary latitudinally. It is highly symmetrical (i.e., both partners are mutually interdependent) in south-central Mexico, but is less symmetrical (i.e., bats are more dependent on plants than vice versa) in northwestern Mexico and southern Arizona.
CONCLUSIONS
Migration Corridors
- Based on carbon isotope and MtDNA
data, lesser long-nosed bats migrate north in the spring along two corridors,
a Coastal Lowland Route and an Inland Montane Route (along the foothills and western
flank of the Sierra Madre Occidental).
- Fleming's work shows that
Arizona is colonized by at least two genetically different waves of lesser-long
nosed bats each year that reside in different parts of Mexico during winter.
- Spring
migration takes 2.0 to 3.5 months and bats expend considerable energy replenishing
their fat deposits en route.
- The southward migration in fall follows an inland agave corridor. Duration of this fall migration is not known. Further research is necessary to determine if other corridors are utilized for the return south.
Floral Resources
- Lesser long-nosed bats use
CAM plants (several species of columnar cactus and paniculate agaves) to fuel
their spring and fall migrations.
- In south-central Mexico, bats and their food plants tend to be mutually interdependent, whereas in northwestern Mexico and southern Arizona, bats are more dependent on plants than vice versa.