Hummingbirds and bats at the same Marginatocereus flower

A red flower in a cactus forest

The Tehuacán Valley in central Mexico supports numerous columnar cacti. In the landscape studied, Marginatocereus marginatus grew among plants whose flowers receive visits from nocturnal mammals. Its own flower was red and tubular, a form associated with nectar feeding by hummingbirds. Saleem Dar and colleagues therefore asked a specific question about how pollination was distributed across the day and night. They combined measurements of floral structure and function with animal observations and experiments separating daytime from nighttime visits. This combination allows the appearance of a flower to be compared with its actual pollination process [Dar et al. 2006].

Observations took place from March to May 1997 at two localities in the Zapotitlán de las Salinas valley, within the Tehuacán region. Most measurements came from Cerro Cutá, at approximately 1700 m elevation. The plants described had red flowers and corresponded to the southern form discussed in older literature. The findings retain this population scope. POWO currently accepts the name Marginatocereus marginatus, with Lophocereus marginatus and Pachycereus marginatus among its synonyms. Connecting these names helps locate biological studies of the same cactus in literature from different periods [Dar et al. 2006] [2-WWW].

Nighttime opening and morning activity

Flowers began opening at about 21.00 and became fully open between 1.30 and 2.30. Pollen was available approximately from 1.00 to 10.00. At the same time, the stigma, the pollen-receiving surface of the pistil, changed condition. A small proportion of the observed flowers showed signs of receptivity before dawn, while most did so by about 7.00. Activity of the male and female parts of the flower therefore extended across the night and morning. For an animal visitor, both access to the flower and the current state of its reproductive organs mattered [Dar et al. 2006].

The researchers followed individual flowers at regular intervals, recording opening, pollen availability and stigma turgidity. The last of these served as an indicator of receptivity, so the reported times describe an observed change in the appearance and hydration of the tissue. Most flowers were beginning to close or had already closed by about 13.00. All had closed by approximately 16.30. Each flower opened once. That single functional period nevertheless included two different groups of animals, allowing successive parts of one floral cycle to contribute to the opportunity for pollination [Dar et al. 2006].

Nectar as a reward that changes over time

Nectar was collected from flowers protected against animal visits. This measurement described accumulated secretion that, under unrestricted access, could already have been consumed by visitors. Volume was measured with a syringe and sugar concentration with a refractometer. Separating the two variables matters because equal volumes of solution can contain different amounts of sugar. In this experiment, both measurements contributed to a description of when food was available in the flowers. Concentration samples came from fewer flowers than the full set used to observe nectar accumulation [Dar et al. 2006].

Nectar volume was initially small at night. At about 3.30 it was 0.01 ml, while the greatest accumulation occurred between 9.15 and 10.15. During that interval, mean volume was 0.098 ml in a sample of six flowers. Mean sugar concentration in five flowers was 25.48%. Much of the reward was therefore available in the morning, while flowers remained open. Comparing the nectar schedule with pollen availability and stigma condition explains how nighttime opening can be combined with a substantial contribution from daytime visitors during the same reproductive period [Dar et al. 2006].

Pollen on an animal’s body

Three hummingbird species were observed by day, recorded in the paper as Amazilia violiceps, Cynanthus sordidus and Cynanthus latirostris. Nighttime visits involved Choeronycteris mexicana and bats identified in the study as Leptonycteris curasoae. The authors captured visitors in nets and collected pollen from their bodies using stained gelatin. They then compared the preparations with pollen taken directly from flowers. Reference material also included other cacti flowering in the same area [Dar et al. 2006].

Pollen of the studied cactus was found on the foreheads and throats of hummingbirds and bats. These locations connect the animal’s contact with surfaces that may touch a flower during feeding. A moth was also recorded visiting flowers, but pollen of Marginatocereus was not detected on it in the sampled material. Pollen grains confirm visitation and the carriage of material, while the next experimental stage evaluates the consequences for the plant. The researchers followed fruit formation and seed maturation after flowers had been made accessible in specified ways [Dar et al. 2006].

Pollen from the same plant and from another plant

In some treatments, flowers remained covered throughout their opening period. This tested whether spontaneous movement of their own pollen led to fruit production. A separate group was hand-pollinated with fresh pollen from a flower on the same plant. Neither procedure produced fruit. Agreement between these outcomes supports the authors’ conclusion that the studied material was self-incompatible. Hand transfer separates delivery of pollen to the stigma from the subsequent success of the reproductive process. Access to pollen from another individual was important for seed production [Dar et al. 2006].

Hand cross-pollination involved applying fresh pollen from another plant. Six fruits developed from 21 flowers treated this way. Control flowers remained accessible to natural visitors throughout their opening period and produced 25 fruits from 55 flowers, or 45.4%. The authors placed these two treatments in the same group in the statistical comparison. Plants were followed until fruits and seeds matured. The final experimental measure was consequently a reproductive outcome, while the preceding observations of pollen and visits helped explain the route leading to it [Dar et al. 2006].

Fruit number and seed number per fruit describe two different components of that outcome. In the control, the mean was 102.12 seeds per fruit, with individual fruits containing from 43 to 171 seeds. These observations came from the 25 fruits that reached maturity. They describe variation among successful reproductive events. Assessment of the whole treatment must also retain information about flowers that produced no fruit. Separating these outcomes helps explain why the effectiveness of pollinator access was assessed primarily through the proportion of fruiting flowers, while counting mature seeds added detail to the picture of reproductive success [Dar et al. 2006].

The contributions of day and night

The central comparison involved uncovering flowers only during a specified part of the daily cycle. One group was accessible to daytime visitors and another to nighttime visitors. At the end of the relevant interval, the flowers were covered again. One fruit matured in the first group and two in the second. Access restricted to one visitor group was therefore associated with substantially less fruit production than unrestricted visits throughout the cycle. Either daytime or nighttime activity could lead to seed production, while the greatest proportion of fruiting flowers occurred when both opportunities remained available [Dar et al. 2006].

The result describes complementarity between two periods of access under the field conditions studied. The difference between treatments may involve both visit numbers and the successful delivery of suitable pollen. The experiment did not separately measure the outcome of every individual visit. In addition, daytime and nighttime sample sizes are reported inconsistently between the methods and the results table. The strongest basis for interpretation is therefore the direction of the difference and the actual number of fruits obtained. Both animal groups participated in pollination, and retaining the full opening period increased reproductive success in this observation [Dar et al. 2006].

Flexibility in the pollination system

The authors connected this arrangement with local pollinator availability. Hummingbirds were relatively scarce in the area, while cacti flowering at night could draw on the same pool of bats. A long opening period therefore enlarged the set of potential floral partners. The researchers described this as a system that safeguards reproduction when visitor availability varies. This ecological explanation rests on the experiment and its habitat context. Establishing the evolutionary history of such an arrangement requires comparisons among populations and related lineages, together with evidence of changes over longer periods [Dar et al. 2006].

Comparison of Pachycereus pecten-aboriginum populations shows how day and night contributions can vary within a species. On tropical Isla Pajarera, flowers opened in the evening, closed early in the morning and secreted nectar at night. In timed animal-access treatments, fruits formed after nocturnal visits, while the daytime treatment produced none. In the northern Sonoran population, flowers and nectar were also available during the day, and both visitation periods contributed to reproduction. Both populations were self-incompatible [Valiente-Banuet et al. 2004].

Valiente-Banuet and colleagues associated this difference with the year-round presence of nectar-feeding bats in the tropics and their migration and lower predictability in the north. This was an interpretation of a geographical comparison. For Marginatocereus, the shared feature is the function of prolonged flower availability, which enables use of partners active at different times. The contributions of hummingbirds at Tehuacán and daytime birds in Sonora remain specific to their respective systems. The comparison links opening time, reward secretion and the outcome of controlled pollinator access [Valiente-Banuet et al. 2004][Dar et al. 2006].

The example of Marginatocereus marginatus shows how floral structure works together with the timing of floral activity. The red tube, nighttime opening, morning increase in nectar and availability of compatible pollen are parts of one process. An observation of an animal at a flower becomes biologically more informative when it can be connected with contact with reproductive organs and subsequent fruit production. In the Tehuacán study, this combination of methods revealed the contributions of hummingbirds and bats. The resulting picture concerns particular red-flowered populations observed during one season and provides a basis for further comparisons of the species’ biology [Dar et al. 2006].