The idea that early-life conditions can have lasting effects on an individual's fitness is widely accepted. One factor that influences the condition of early-life stages in animals is the number of youngsters competing for the same resources, such as food. Ulrika Candolin and colleagues from the University of Helsinki, Finland, had previously found that an increased number of embryos in three-spined stickleback (Gasterosteus aculeatus) nests decreases the hatching success of embryos. However, fathers can improve the embryos’ chances of hatching successfully by fanning their nests more, to give youngsters more oxygen and get rid of waste. Yet, the effects of an overcrowded nest after hatching, with an increased number of youngsters in the limited space, were unknown. To figure this out, the same team determined whether the number of embryos in three-spined stickleback nests would impact their ability to survive and grow after hatching.
Candolin and colleagues caught three-spined stickleback adults in the Baltic Sea off the coast of Finland and then, when the males were ready to mate, the team coaxed the fish into building tunnel-shaped nests with algae and sand by showing them a fertile female, before allowing them to mate with one, two or four females to vary the number of eggs in each nest. Then, the team allowed the embryos to grow in the nests while their fathers cared for them until they hatched. Three days after hatching, the researchers recorded how many of the embryos hatched successfully and measured their length. The researchers also separated a group of 20 hatchlings from each nest into individual tanks to develop with the same amount of space. Four weeks later, the team measured their length and survival, to look at the impact of the number of embryos packed into a nest later in life.
They found that the stickleback embryos from larger clutches, with more eggs crammed inside a nest, hatched 6 days earlier and were smaller than those from the smaller clutches that had more space while developing. The team also found that the proportion of eggs that hatched successfully was greater in the nests that were crowded with more eggs. However, 4 weeks after hatching, the youngsters from the larger clutches of eggs had caught up and there was no difference in their body length from that of youngsters that developed in smaller numbers. Unfortunately, the hatchlings from the larger clutches of eggs also had lower survival rates than those that had developed in smaller clutches, even though all the hatchlings were given the same amount of space to develop in. The lower survival rate occurred regardless of how much the fathers fanned their eggs, which meant they did not entirely neutralize the damaging effects of being raised in a higher density nest. The team suggested that this may explain why fathers eat some of their own embryos; it could be a strategy for maintaining an optimal number of youngsters in their nests to give the ones that survive the best start in life.
It seems that stickleback fathers must maintain a delicate balance between the number of offspring in their nests and the youngsters’ ability to survive. However, Candolin and colleagues point out the need for more research on the mechanisms that cause embryos to be smaller when they hatch and to hatch early, in addition to the impact that this may have later in life. Most importantly, they stressed that researchers should consider the conditions in which they raise embryos when investigating the impact of early-life experiences as they grow older.