Insects and spiders less likely to survive in tropics, new research shows

24th October

Tropical insects and spiders left unprotected by their parents are far more likely to be killed by predators than bugs in colder climates.

An assassin bug guards its brood from predators.

New research by academics at the University of Hull and the University of Sao Paulo in Brazil shows the mortality of unguarded broods is significantly higher in warmer and wetter climates where predators and parasitoids are more active.

Appearing in the journal Biological Reviews, the research also shows the survival of broods left home alone increases significantly when parents build extra defences to protect their young when they are not around, such as silk egg sacs, sticky coatings or nests.

The results also help to explain why parental care is more common in tropical species.

James Gilbert, a lecturer in Zoology at the University of Hull involved in the study, said:

In arthropods, activity is linked to temperature – predators and parasitoids are more active in the tropics because it is hotter.
Parental care protects against these natural enemies, so in theory the benefits of this behaviour should be higher in tropical regions.
When we tested this prediction by comparing the mortality of guarded and unguarded broods in arthropods, we found that the mortality of unguarded broods was higher, and parental care was more beneficial in warmer, less seasonal environments.
The research backs up a hypothesis dating back more than 40 years that parental care in arthropods evolved in response to predation in tropical rainforests. While that’s been assumed to be true for many years, this research now provides evidence.

In the insect world, offspring attendance (remaining with young after hatching) is a common way of protecting broods.

Previous researchers have noted that experimentally removing arthropod parents of various species “condemns the offspring to death”, mostly due to attacks from predator and parasitoids - rogue insect robbers that hijack and consume eggs and larvae.

In the new research, entitled “Macroecology of Parental Care in Arthropods”, a team of experts from the University of Hull and the University of Sao Paulo in Brazil took these past authors’ data and systematically compared them, looking for a link between mortality and climate.

The researchers also looked at the impact of additional defences on mortality rates and found that where parental care and additional defences were combined, the differences in mortality between warm and wet and cold and dry climates largely disappeared.

Despite the threat posed by natural enemies, many parents occasionally have to leave their offspring temporarily unguarded, whether it’s to catch prey to eat themselves or get food for their offspring. Some arthropod species have developed backup defences to keep home-alone broods safe.

Mr Gilbert said:

The combined message is, tropical arthropod broods with no other defence than parental care are highly likely to be killed when left unattended by their parents, whereas in a temperate climate the eggs are less likely to be trashed.
However, if parents have extra defences, they can afford to leave their brood to go off and feed, even in tropical climates.
Although you don’t normally think about arthropods as caring parents, many of them love their children too.

The assassin bug

The assassin bug is very unusual in the insect world in that it’s the dad who guards the eggs from predators and parasitoids, while the mum goes off and lays more eggs.

Found in Uganda, assassin bugs are ambush predators that prey on other insects. The major threat to their eggs is from parasitoid wasps.

While the stay-at-home dads may sound like enlightened creatures, their behaviour has an ulterior motive: females of their species are attracted to males that already have eggs.

The assassin bug father doesn’t build any additional defences to allow it to go off and feed. Instead he sustains himself by eating some of his own eggs.

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