Risk management of solitary and eusocial reproduction

TL;DR

This paper uses mathematical models to compare solitary and eusocial reproductive strategies in social insects, showing eusociality requires significant fitness benefits and solitary reproduction generally has lower extinction risk.

Contribution

It provides a quantitative framework for understanding the conditions favoring eusocial versus solitary reproduction in social insects.

Findings

Eusocial strategies need large fitness advantages to emerge.

Solitary reproduction is more resilient when its reproductive ratio exceeds a critical threshold.

Eusociality is a high risk, high reward reproductive strategy.

Abstract

Social insect colonies can be seen as a distinct form of biological organization because they function as superorganisms. Understanding how natural selection acts on the emergence and maintenance of these colonies remains a major question in evolutionary biology and ecology. Here, we explore this by using multi-type branching processes to calculate the basic reproductive ratios and the extinction probabilities for solitary versus eusocial reproductive strategies. In order to derive precise mathematical results, we use a simple haploid, asexual model. In general, we show that eusocial reproductive strategies are unlikely to materialize unless large fitness advantages are gained by the production of only a few workers. These fitness advantages are maximized through obligate rather than facultative eusocial strategies. Furthermore, we find that solitary reproduction is `unbeatable' as long as the solitary reproductive ratio exceeds a critical value. In these cases, no eusocial parameters exist that would reduce their probability of extinction. Our results help to explain why the number of solitary species exceeds that of eusocial ones: eusociality is a high risk, high reward strategy, while solitary reproduction is better at risk management.