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Tewksbury and Nabhan (2001) are biologists who set out to test the hypothesis that capsaicin deters seed predators (rodents) from eating the fruits and seeds but not seed dispersers (birds). Deterring rodents should have a positive effect on plant fitness since rodents are thought to damage seeds. Seed dispersers should have a positive effect on plant fitness by dispersing seeds to new locations. There are two important predictions of this hypothesis: i) capsaicin deters rodents but not birds, ii) rodents reduce seed germination and birds have no effect or increase seed germination.

In their first lab experiment, the researchers tested the hypothesis that the presence of capsaicin in chili fruits deters rodents but not birds from consuming chilies. This was a simple food-choice experiment. They captured thrashers, packrats and cactus mice and brought them to the lab. In the experiment, individual animals were placed in cages and simultaneously given three different types of fruits: i) desert hackberries ii) mild chilies that lacked capsaicin (Capsicum chacoense) and iii) hot chilies (C. annuum var. glabriusculum). They then measured the percentage of each fruit that was eaten.

Each of the fruit types is considered a treatment; a combination of factors selected and regulated by the experiment- ers. The treatments in this experiment are summarized in Table 1 (next page). The hot chilies are the experimental treatment since they contain capsaicin, the feature expected to have an effect on fruit consumption. The other two treatments were controls, a positive control and a negative control. A control is an experimental group or treatment that serves as a reference to which a treatment group or treatment(s) can be compared. Hackberries were a positive control for feeding under laboratory conditions; even animals that had an aversion to chili-like fruits were likely to eat hackberries. The mild chilies were a negative control since they lacked capsaicin but were otherwise similar to chilies. If animals ate mild chilies, it would be possible to compare consumption of mild chilies to the consumption of hot chilies. A good control is as similar as possible to the treatment and should only differ in the factor of interest, in this case, capsaicin content.

For the negative control (mild chilies), fruits were chosen that were as similar as possible to the hot chilies but which lacked capsaicin. The mild chili was a mutant genotype
of another species, Capsicum chacoense (Figure 8), which doesn’t produce capsaicin. The hot and mild chilies are similar in size, color and nutritional content. Animals were presented with three fruit choices at the same time and the percentage of each fruit type consumed was recorded.

In the second lab experiment, the researchers tested the
hypothesis that consumption of chilies by rodents but
not birds reduces the survival of chili seeds. They fed
mild chilies to thrashers, packrats and cactus mice and
measured the percentage of seeds that germinated. They
also collected seeds from ripe fruits and measured germi-
nation of seeds that had not been ingested by any animals.
Since packrats and cactus mice won’t eat hot chilies, they
needed to use mild chilies, Capsicum chacoense, to measure the effect of fruit consumption on seed germination.

In addition to looking at the direct effect of consumption by birds on seeds, the researchers also assessed the effect of dispersal by thrashers on chili fitness. At their field site, the researchers found that dispersal of chili seeds by thrashers resulted in movement of seeds to locations that increased seedling survival and overall seedling fitness (Tewksbury & Nabhan, 2001).

As he continued his reading, Phil discovered that a gene, Pun1, was responsible for capsaicin production (Stewart et al., 2005). The Pun1 gene present in modern Capsicum species was present in the common ancestor of capsicum- containing species and inherited by all of the descendants of that ancestor.


Figure 8. Ripe and unripe from a wild chili plant (Capsicum chacoense) from the USDA, ARS plant germplasm collection. Credit: R. Jarret (USDA, ARS).

“Las alas del Diablo” by Brett C. Couch Page 4


Some individuals of some species have mutations that disrupt the Pun1 gene, resulting in mild chilies that don’t produce capsaicin (Stewart et al., 2005). Plants that don’t produce capsaicin have two defective copies of the Pun1gene; the version of the gene for capsaicin production is dominant to the mutant versions of the gene.

Researchers have also discovered why birds and mammals respond differently to capsaicin. In mammals, capsaicin stimulates nerves responsible for sensing heat, which results in a burning sensation (Mason et al., 1991; Jordt & Julius, 2002). The nerves from birds responsible for sensing heat are not stimulated by capsaicin; birds should not experience the burning sensation from consuming capsaicin (Mason et al., 1991; Jordt & Julius, 2002).

Table 1. Summary of treatments in Tewksbury and Nabhan’s 2001 feeding choice experiment.

Hackberry (Celtis pallida)

Mild Chili (Capsicum chacoense)

Hot Chili (Capsicum annuum var. glabriusculum)

Treatment type:

Positive control

Negative control

Experimental treatment

Purpose of the treatment:

Control for feeding under laboratory conditions and consumption of fruits other than chilies. Provides a comparison for consumption of chilies.

Control for consumption of chili-like fruits that lack capsaicin.
Provides a comparison for consumption of hot chilies.

The treatment contains the factor, capsaicin that is expected to affect fruit consumption.

Expected result if capsa- icin deters seed predators but not dispersers:

All animals should con- sume hackberries.

All animals should con- sume mild chilies.

Only birds should con- sume hot chilies.

Refer to the results of Tewksbury and Nabhan’s field experiments and observations; do not refer to the lab experiments from Part II. Provide at least two reasonable, alternative, biological explanations for why rodents did not eat chilies.

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