People are often afraid of spiders. Perhaps one of the most fearful moments is when they move fast unexpectedly, even though they might be trying to avoid humans. And the spider's ability to move fast or slow depends on the temperature of the environment, because they are ectothermic creatures. It is also known that changes in their body mass and their build can influence their moving speed. However, there is not much information on how the combination of environmental temperature and changes in body composition together influences how fast spiders move. This prompted scientists from various countries in Latin America and the USA to investigate how temperature and carrying an eggsac affects the ability of female Pardosa wolf spiders (Lycosidae) to move.

Damián Villaseñor-Amador from the Universidad Autónoma de Mexico, and researchers from Brazil, Chile, Argentina and the USA collected 86 female wolf spiders from the sub-Alpine zone of the Buena Vista peak in the Talamanca Mountain range, Costa Rica, where temperatures can be extremely high or low, and conditions can change suddenly. Thirty-six of the spiders were carrying eggsacs, which can weigh as much as 77% of their body mass, while the remaining females were eggsac-free. After transporting the spiders in individual tubes to a warehouse 22 km away, Villaseñor-Amador and colleagues placed half of the eggsac-carrying females in a cooler for 5 min, to chill the spider. Then, they monitored each spider as it ran around an arena with a leaf floor covered in clear plastic, marking the spider's locations on the plastic surface as the animal moved, before using a cotton string to track the markings and measuring the string length to calculate the distance travelled by the spider. Next, they placed the remaining egg-carrying females in a 30°C heater for 5 min to raise their temperature before recording the warmed spiders’ movements. The team then tracked the movements of the females that had no eggsacs at 7°C, 30°C and also at room temperature (13°C), in addition to recording how long all of the spiders (with and without eggsacs) kept moving until they paused for at least 5 s.

Not surprisingly, the wolf spiders moved faster when the environment was hotter; however, the largest spiders moved the slowest in hot temperatures, while the opposite occurred in cold temperatures. This may be because it is easier for the smallest wolf spiders to heat up to move faster in hot conditions. In contrast, at cold temperatures, the largest spiders may be more efficient at using their energy.

However, in contrast to expectation, the heavy burden carried by females with eggsacs did not affect their speed. Villaseñor-Amador and colleagues suggest that Pardosa wolf spiders may modify their body posture to compensate for the extra weight and drag that the eggsac generates. Alternatively, egg-carrying females may simply invest more energy in movement, allowing them to run as fast as unburdened females.

So, the extreme temperatures experienced by Pardosa wolf spiders in the Talamanca Mountains affect how fast the animals can move, yet their speed is unaltered when carrying an extremely heavy load, allowing them to escape predators as successfully as egg-free females, to ensure the future of the next generation.

Villaseñor-Amador
,
D.
,
Vilas-Bôas
M. P.
,
de Cerqueira
,
L.
,
Gómez
,
Q.
,
Zamora Cornejo
,
F.
,
Paulucci
,
J.
and
Escalante
,
I.
(
2023
).
Locomotor behaviour of tropical wolf spiders is affected by external temperature and body size, not load bearing
.
Anim. Behav
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