Abstracts

William M. Cook ¹ , Robert M. Timm ¹ and Dena E. Hyman ²

¹ Department of Ecology and Evolutionary Biology & Natural History Museum, Dyche Hall, University of Kansas, Lawrence, KS 66045 USA. Fax: (785) 864-5335. E-mail: wmcook@ku.edu.

² Department of Biology, University of Miami, Coral Gables, FL 33124 USA.

Received 18-VII-2000. Corrected 20-III-2001. Accepted 30-III-2001.

Key words: Dry forest, Liomys salvini, Oryzomys couesi, Sigmodon hispidus, swimming ability.

Via prior observations, morphology, and habitat preference, we predicted swimming ability in each species. Released rice rats and cotton rats swim briefly over water before disappearing into their native emergent vegetation; we expected that each could swim for at least a moderate period of time. While neither population in the study area has flattened tails or webbed feet, some mammals without obvious adaptations can still be very good swimmers (^{Blair 1939}, ^{Dagg and Windsor 1972}). In general, we expected rice rats and cotton rats to swim or float during short trials as if crossing short gaps between floating vegetation mats. We did not expect spiny pocket mice to be adept swimmers as they also lack obvious morphological adaptations, are primarily a xeric species, and have not been observed in the water at our site.

Animals in apparent distress were removed from the water prior to the end of the 90 s period. Swimming ability was evaluated by: 1) observed diving or floating behavior, 2) body inclination, 3) propulsive use of the tail, 4) leg positions while swimming, and 5) whether eyes were open underwater. We considered diving and swimming underwater, maintaining the body’s rear at the water’s surface, use of the eyes underwater, and tail undulation as useful swimming skills. Species were categorized as possessing superior or inferior swimming ability based on the array of skills demonstrated by the tested individuals.

As expected, spiny pocket mice gave little indication of swimming skill. All individuals kept their heads above water for some time, but this appeared to require considerable effort. No spiny pocket mouse dove or floated, and the animals were soon (1 min or less) oriented almost vertically in the water (Table 1).

All four legs were used to paddle, a technique that looked highly inefficient. No spiny pocket mice used the tail for propulsion, and they continually touched the edge of the container with their legs, seemingly trying to gain purchase. One animal (#8) had to be rescued 7 s before completion of the trial.

Hispid cotton rats, while clearly not as skilled swimmers as rice rats, appear competent to disperse at the water’s surface. Observations of released individuals indicate that cotton rats can pass readily between emergent vegetation patches. While our individuals used the tail for propulsion, they did not dive in trials or when released into the marsh; we cannot suggest that they use the underwater habitat. Our results are strongly consistent with prior evidence on swimming ability in Sigmodon

Given above-water structure associated with cattails and aquatic Palo Verde trees (Parkinsonia aculeata), Laguna rice rats and cotton rats may have nest substrate, forage, and cover available on the interior of the marsh. As both species were captured up to 50 m away from shore (the maximum distance attempted), the marsh interior may support self-sustaining populations. Because both species have been captured in all marsh microhabitats and appear to be habitat generalists, we suggest that rice rats’ use of the underwater environment may help to differentiate the two species. This is consistent with Esher et al.’s (1978) suggestion that advanced swimming skills in O. palustris contribute significantly to niche separation between it and cotton rats in Florida.

The third species found at Palo Verde, the spiny pocket mouse, is clearly a terrestrial animal. We did not expect spiny pocket mice to be adept swimmers, as they lack obvious morphological adaptations, and are well-known bur-rowers. This species was only captured on dry land at the edge of the marsh, and all individuals in trials could be considered poor swimmersat best. Spiny pocket mice do not use their tails while swimming, and their motions appear to be desperate and inefficient. This is consistent with ^{Harris and Petersen (1979)}, who described L. irroratus’ performance as "lacking," and noted that all heteromyids they tested had to be rescued to prevent drowning. Most spiny pocket mice are not associated with aquatic habitats, and their swimming ability appears merely sufficient to survive accidental immersion.

Finally, much of the emergent vegetation used by rice rats and cotton rats did not exist in Laguna Palo Verde as recently as 1985, so local rodents have much more available habitat than in the past. As marshes such as Palo Verde fill with silt or become overgrown by cattails (possibly due to human activity), we can expect the abundance and distribution of native rodents to change along with the structure of their habitats.

We thank the Ministerio del Ambiente y Energía and Sistema Nacional de Áreas de Conservación for allowing us to work at Parque Nacional Palo Verde, and especially Javier Guevara S. for facilitating research permits. Eugenio González, Deedra McClearn, and the Organization for Tropical Studies provided logistical support for this project. Particular thanks go to Daniel Ardia, Jenny Drnevich, Andrea Huberty, and Jeremy Zujko-Miller for field assistance.

Nosotros investigamos las habilidades de nado de tres ratones del bosque seco de Costa Rica (la rata arrocera de Coue, Oryzomys couesi, la rata algodonera híspida, Sigmodon hispidus, y el ratón espinoso, Liomys salvini) asociados a un gran pantano, Laguna Palo Verde, usando pruebas de nado de 90 s en un contenedor de plástico. La habilidad de nado fue evaluada observando el uso de las extremidades y cola en el agua, inclinación hacia la superficie y comportamiento de flotar y buceo. Las ratas arroceras pudieron flotar, nadar y bucear, sugiriendo que ellas pueden explotar los recursos en la superficie y bajo el agua. Las ratas algodoneras nadaron en la superficie del agua, pero fueron menos hábiles nadadoras que las rata arroceras. Los ratones espinosos se cansaron rápidamente y tuvieron dificultad para mantenerse en la superficie del agua. Los resultados sugieren que la habilidad de nado diferencial está relacionada con la distribución de las tres especies simpátricas dentro del pantano y los hábitats del bosque adyacentes.

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