Counting Texas Tortoises at Palo Alto Battlefield: The 2023 Survey

The Texas tortoise (Gopherus berlandieri) is a small, hard-to-spot tortoise of south Texas and three states of Mexico. It is threatened by widespread habitat loss and illegal collecting. Only one U.S. national park unit has it in good numbers: Palo Alto Battlefield National Historical Park in Brownsville, Texas. The tortoises there are spared the immediate threats facing those on unprotected land, but their numbers, health and movements still need watching to guide future management.

This tortoise was found in his natural habitat at Palo Alto Battlefield NHP during the 2023 NPS I&M annual monitoring event. NPS.

How the survey works

The National Park Service’s Gulf Coast Inventory and Monitoring Network began monitoring the park’s tortoises in 2008. It is a long-term mark-recapture study with two goals: to track the tortoises in seven permanent 4-hectare sampling units — how many are found per hour of searching, how they move, their body condition, and their sizes and sexes — and to analyze trends in abundance and survival across years.

All seven units sit on lomas: low ridges covered in thorny shrubs, prickly pear and mesquite, the tortoise’s favored habitat on the Texas coast.

A loma and the park’s other main habitats: sacahuiste prairie (Spartina spartinae), old streambeds with sea ox-eye daisy (Borrichia frutescens), and the ridge itself, with woody shrubs, prickly pear (Opuntia engelmannii) and mesquite (Prosopis glandulosa). Image from NPS’s page.

Each fall, over three days, every unit is searched twice, by two different teams, each spending eight person-hours per unit — about two hours for a team of four. Crews weigh each tortoise found, measure its shell, photograph it, record its sex and take a GPS location. A newly found tortoise is marked by drilling small holes in the edge plates of its shell in a unique numeric code, so it can be recognized if found again. The current protocol, version 3.0, dates from 2018.

A juvenile Texas tortoise after marking; the two holes in the front edge scutes are part of its ID code. NPS photo / GULN staff.

The 2023 trip

Thirteen people surveyed from October 30 to November 2, 2023. Besides the routine searches, crews made four bonus surveys of eight person-hours each in other high, suitable-looking parts of the park, to look for marked tortoises that might have moved outside the usual units.

An adult Texas tortoise (Gopherus berlandieri) at Palo Alto Battlefield National Historical Park. NPS photo / GULN staff.

Inside the units, 42 tortoises in 112 person-hours works out to 0.38 per person-hour — the second-highest yield since the 2018 protocol began.

Tortoises found per person-hour, by unit, Fall 2018 – Fall 2023. Image from NPS’s page.

About one in four tortoises found in the units was new — typical of past surveys. New adults keep turning up, which suggests tortoises drift in and out of the units even though their home ranges are small.

The tortoises are hard to find twice: only 9 of the 42 (21%) were found by both teams, only slightly more than in earlier surveys. That reflects how well this small, cryptic animal hides on the lomas.

Yields varied between units. Units 3 and 4 had 11 tortoises each, more than double any other; units 1, 5 and 7 had only three or four each, as they have in many recent years. Why is not fully understood — candidates include differences in plant life on the lomas, past farming, and invasion by non-native Guinea grass (Urochloa maxima).

Weather and detection

Yields have always fluctuated from year to year. Two factors are known to matter: temperature, which acts at once (tortoises shelter or go inactive when too hot or cold), and rainfall, which acts slowly through the plants — affecting food and how easily tortoises can be seen. The 2023 survey was unusually cold and dry.

Mean air temperature during surveys and rainfall beforehand, Fall 2018 – Fall 2023. Image from NPS’s page.

Cold made tortoises less active and harder to find — but drought and dying-back vegetation made the ground easier to see. Early satellite analyses partly support this: crews tend to find more tortoises when the park’s vegetation is drought-stressed and less green. Then again, higher counts might instead mean more tortoises had moved onto the monitored lomas at those times.

Notable observations

• Long moves. Since 2008 the network has recorded 673 movements between consecutive sightings, with a median of 47.9 m (157.2 ft). The project treats anything beyond its 95th percentile — more than 212 m (695.6 ft) — as a long move. Four tortoises made one in 2023; two crossed from one loma to another over a low, sparse old streambed that would likely block them in wet periods.
• Pairs. Five pairs were found within 2 m of each other, possibly courting: four male–female pairs and one pair of males.
• Deaths. Four dead tortoises were found — above the project’s average of two per trip, but not an alarming rate. Three had been marked before; the fourth was a juvenile.
• Long absences. Nine tortoises turned up after going unseen for five years or more — one after nine years, another after eleven.

Other wildlife

Crews also noted a diamondback rattlesnake, a Texas indigo snake, Schott’s whipsnake, Texas spiny and rose-bellied lizards, a yellow mud turtle, javelina, a southern plains woodrat, a striped skunk and a young eastern cottontail. Among non-natives were nilgai, seen regularly in the park in small herds, and a bearded dragon, apparently a released pet, which was taken back into captivity.

(A) A juvenile eastern cottontail; (B) a large male nilgai, a non-native species often seen during the surveys. NPS photo / GULN staff.

Why it matters

Development keeps spreading around the park, and Texas tortoise habitat across the region keeps shrinking. The park holds a sizable population, and long-term monitoring is how the Park Service will see how outside pressures affect it. The next survey was planned for fall 2024.

Sources

Based on "Texas Tortoise Monitoring at Palo Alto Battlefield National Historical Park: Results for 2023," Gulf Coast Inventory and Monitoring Network, National Park Service; a work of the United States government in the public domain. The source gives 2022’s shared-capture rate as 9 of 52 tortoises, or 18%, which do not agree, so that comparison is left out; it also spells the prickly pear “englemannii”, corrected here to engelmannii. Pictures an earlier version of this page left out were restored on 2026-09-26.

• The film and any audio are the National Park Service's own files, from the source page.